11.3: Evidence of Recent Climate Change - Geosciences

11.3: Evidence of Recent Climate Change - Geosciences

While climate has changed many times in the past (see chapter 14.5.1 and chapter 15.3), the scientific consensus is that human activity is causing the climate to change today more rapidly [11; 7]. While this seems like a new idea, it has been suggested for more than 75 years [12]. This section describes the evidence that scientists agree is most likely a result of anthropogenic climate change, or, human-caused climate change. For more information, watch this six-minute video on climate change by two professors at a North Carolina State University.

Global Temperature Rise

Since 1880, average global surface temperatures have trended upward and most of that warming has occurred since 1970 (see this NASA animation). Since the ocean is absorbing a lot of the additional trapped heat, surface temperatures include both land surface and ocean temperatures [13]. Changes in land surface or ocean surface temperatures can be expressed as temperature anomalies. A temperature anomaly is the difference in average temperature measurement from a predetermined datum. This datum is the average temperature of a particular date range, for example, 1951 to 1980. Another common datum is the last century (1900-2000). Therefore, an anomaly of 1.25 ℃ for 2015 (last century datum) means that the average temperature for 2015 was 1.25 ℃ greater than the 1900-2000 average. In 1950, the temperature anomaly was -0.28 ℃, so this is -0.28 ℃ lower than the 1900-2000 average [3]. These temperatures are annual average surface temperatures.

This video figure shows worldwide temperature changes since 1880. The more blue, the cooler; the more yellow and red, the warmer.

In addition to a rising average surface land temperature, the ocean has absorbed a lot of the heat (remember that the specific heat of water is unusually large). With oceans covering about 70% of the earth’s surface, there is a lot of opportunities to absorb energy. The ocean has been absorbing about 80% to 90% of the additional heat added due to human activities. As a result, the top 2,300 feet of the ocean has increased in temperature 0.3℉ since 1969 (external link to this 3-minute video by NASA JPL on heat capacity of the ocean) [3]. The reason the ocean has warmed less than the atmosphere, while still taking on most of the heat, is due to the very high specific heat of water, which means that water can absorb a lot of energy for a small temperature increase. In contrast, the atmosphere needs less energy to increase its temperature.

Some scientists suggest that anthropogenic greenhouse gases do not cause global warming since surface temperatures have not increased very much between 1998 and 2013, while greenhouse gas concentrations have continued to increase during that time period. However, since the oceans are absorbing most of the heat, decade-scale circulation changes (similar to La Niña) in the ocean push warmer water deeper under the surface [14; 15; 16]. Once the absorption and circulation of the ocean is accounted for and the heat added back into surface temperatures, then the temperature increases become apparent as shown in the above figure. Furthermore, this ocean heat storage is temporary, as reflected in the record-breaking warm years of 2014-2016. Indeed, with this temporary ocean storage effect, 15 of the first 16 years of the 21st century have been the hottest in recorded history.

Carbon Dioxide

Anthropogenic greenhouse gases, mostly carbon dioxide (CO2), have increased since the industrial revolution when the burning of fossil fuels dramatically increased. These levels are unprecedented in the last 800,000-year earth history as recorded in geologic sources such as ice cores. Carbon dioxide has increased by 40% since 1750 and the rate (or speed) of increase has been the fastest during the last decade [3; 6]. For example, since 1750, 2040 gigatons of CO2 have been added to the atmosphere, about 40% have remained in the atmosphere while the remaining 60% have been absorbed into the land (by plants and soil) or the oceans [6]. Indeed, during the lifetime of most young adults, the total atmosphere has increased by 50 ppm or 15%.

Charles Keeling, an oceanographer with Scripps Institution of Oceanography in San Diego, California was the first person to make regular measurements of atmospheric CO2. Using his methods, constant measurements of CO2 in the atmosphere have been made at the Mauna Loa Observatory on Hawaii since 1958. These measurements are published regularly by NASA at this website: Go there now to see the very latest measurement. Keeling’s measured values have been posted in a curve of increasing values called the Keeling Curve. This curve varies annually up and down from summer (when the plants in the Northern Hemisphere are using CO2) to winter when the plants are dormant, but shows a steady increase over the past several decades. This curve increases exponentially, not linearly indicating that the rate of increase of CO2 is itself increasing!

The following video shows how atmospheric CO2 has varied recently and also over the last 800,000 years as determined by many CO2 monitoring stations (shown on the insert map). It is also instructive to watch the CO2 variation of the Keeling portion of the video by latitude. This shows that most of the human sources of CO2 are in the Northern Hemisphere.

Melting Glaciers and Shrinking Sea Ice

Glaciers are ice on top of the land. Alpine glaciers, ice sheets, and sea ice are all melting. Explore melting glaciers at NASA’s interactive Global Ice Viewer). Satellites have recorded that Antarctica is melting at 118 gigatons per year and Greenland is melting at 281 gigatons per year (1 gigaton is over 2 trillion pounds). Almost all major alpine glaciers are shrinking, deflating, and retreating and the rate of ice mass loss is unprecedented (never observed before) since the 1940’s when quality records for most began. Before anthropogenic warming, glacial activity was variable with some retreating and some advancing [17]. The extent of spring snow cover has decreased. In addition, the extent of sea ice is shrinking. Sea ice is ice floating in the ocean (not on land like a glacier). Most sea ice is at the North Pole which is only occupied by the Arctic Ocean and sea ice [3; 6]. Below, the NOAA animation shows how perennial sea ice has declined from 1987 to 2015. The oldest ice is white and the youngest (seasonal) ice is dark blue. The amount of old ice has declined from 20% in 1985 to 3% in 2015.

Rising Sea-Level

Sea-level is rising 3.4 millimeters (0.13 inches) per year and has risen 0.19 meters (7.4 inches) from 1901 to 2010. This is thought largely to be from both the melting of glaciers and thermal expansion. Thermal expansion means that as objects such as solids, liquids, and gases heat up, they expand in volume. Since 1970, the melting of glaciers and thermal expansion account for 75% of the sea-level rise [6].

Classic video demonstration (30 seconds) on thermal expansion with brass ball and ring (North Carolina School of Science and Mathematics).

Ocean Acidification

Since 1750, about 40% of the new anthropogenic carbon dioxide has remained in the atmosphere. The remaining 60% gets absorbed by the ocean and vegetation. Therefore, the ocean has absorbed about 30% of new anthropogenic carbon dioxide. When carbon dioxide gets absorbed in the ocean, it creates carbonic acid which makes the ocean more acidic which has an impact on marine organisms that secrete calcium carbonate shells. Recall that hydrochloric acid reacts by effervescing with limestone rock made of calcite, which is calcium carbonate. Ocean acidification associated with climate change has been linked to the thinning of the carbonate walls of some sea snails (pteropods) and small protozoan zooplankton (foraminifera) and declining growth rates of corals [6]. Small animals like protozoan zooplankton are an important component in the marine ecosystem. Acidification combined with warmer temperature and lower oxygen levels is expected to have severe impacts on marine ecosystems and human-used fisheries, possibly affecting our ocean-derived food sources [6].


Extreme Weather Events

Occurrence and intensity of extreme weather events such as hurricanes, precipitation, and heatwaves are increasing [3; 6]. Since the 1980s, hurricanes, which are generated from warm ocean water, have increased in frequency, intensity, and duration and connections to a warmer climate are likely. Since 1910, average precipitation has increased by 10% in the contiguous United States, and much of this increase is associated with heavy precipitation events like storms [18]. However, the distribution is not even and more precipitation is projected for the northern United States while less precipitation is projected for the already dry southwest [3]. Further, heatwaves have increased and rising temperatures are already affecting crop yields in northern latitudes [6]. Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events [19].


3. Lindsey, R. Climate and Earth’s Energy Budget : Feature Articles. (2009). Available at: (Accessed: 14th September 2016)

6. Pachauri, R. K. et al. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. (IPCC, 2014).

7. Oreskes, N. The scientific consensus on climate change. Science 306, 1686–1686 (2004).

11. Earle, S. Physical geology OER textbook. (BC Campus OpenEd, 2015).

12. Callendar, G. S. The artificial production of carbon dioxide and its influence on temperature. Q.J.R. Meteorol. Soc. 64, 223–240 (1938).

13. Hansen, J., Sato, M., Kharecha, P. & others. Earth’s energy imbalance and implications. Atmospheric (2011).

14. Foster, G. & Rahmstorf, S. Global temperature evolution 1979–2010. Environ. Res. Lett. 6, 044022 (2011).

15. Kosaka, Y. & Xie, S.-P. Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature 501, 403–407 (2013).

16. Easterling, D. R. & Wehner, M. F. Is the climate warming or cooling? Geophys. Lett. 36, (2009).

17. Zemp, M. et al. Historically unprecedented global glacier decline in the early 21st century. J. Glaciol. 61, 745–762 (2015).

18. Karl, T. & Knight, R. W. Secular trends of precipitation amount, frequency, and intensity in the United States. Bull. Am. Soc. 79, 231–241 (1998).

19. Santer, B. D. et al. Identification of human-induced changes in atmospheric moisture content. Proc. Natl. Acad. Sci. U. A. 104, 15248–15253 (2007).

No one questions the link between smoking and cancer, because the science was settled in the 1960s after more than 50 years of research.

We can think of the state of human activities and climate change as no different than smoking and cancer.

In fact, we are as confident that humans cause climate change as we are that smoking causes cancer.

Scientists have no doubt that humans are causing global warming.

How much of climate change is natural? How much is man-made?

Power station in South Africa. Credit: Therina Groenewald/Shutterstock

How much climate change is natural? How much is man-made?

As someone who has been working on climate change detection and its causes for over 20 years I was both surprised and not surprised that I was asked to write on this topic by The Conversation. For nearly all climate scientists, the case is proven that humans are the overwhelming cause of the long-term changes in the climate that we are observing. And that this case should be closed.

Despite this, climate denialists continue to receive prominence in some media which can lead people into thinking that man-made climate change is still in question. So it's worth going back over the science to remind ourselves just how much has already been established.

Successive reports by the Intergovernmental Panel on Climate Change – mandated by the United Nations to assess scientific evidence on climate change—have evaluated the causes of climate change. The most recent special report on global warming of 1.5 degrees confirms that the observed changes in global and regional climate over the last 50 or so years are almost entirely due to human influence on the climate system and not due to natural causes.

What is climate change?

First we should perhaps ask what we mean by climate change. The Intergovernmental Panel on Climate Change defines climate change as: "A change in the state of the climate that can be identified by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer."

The causes of climate change can be any combination of:

  • Internal variability in the climate system, when various components of the climate system—like the atmosphere and ocean—vary on their own to cause fluctuations in climatic conditions, such as temperature or rainfall. These internally-driven changes generally happen over decades or longer shorter variations such as those related to El Niño fall in the bracket of climate variability, not climate change.
  • Natural external causes such as increases or decreases in volcanic activity or solar radiation. For example, every 11 years or so, the Sun's magnetic field completely flips and this can cause small fluctuations in global temperature, up to about 0.2 degrees. On longer time scales—tens to hundreds of millions of years—geological processes can drive changes in the climate, due to shifting continents and mountain building.
  • Human influence through greenhouse gases (gases that trap heat in the atmosphere such as carbon dioxide and methane), other particles released into the air (which absorb or reflect sunlight such as soot and aerosols) and land-use change (which affects how much sunlight is absorbed on land surfaces and also how much carbon dioxide and methane is absorbed and released by vegetation and soils).

What changes have been detected?

The Intergovernmental Panel on Climate Change's recent report showed that, on average, the global surface air temperature has risen by 1°C since the beginning of significant industrialization (which roughly started in the 1850s). And it is increasing at ever faster rates, currently 0.2°C per decade, because the concentrations of greenhouse gases in the atmosphere have themselves been increasing ever faster.

The oceans are warming as well. In fact, about 90% of the extra heat trapped in the atmosphere by greenhouse gases is being absorbed by the oceans.

A warmer atmosphere and oceans are causing dramatic changes, including steep decreases in Arctic summer sea ice which is profoundly impacting arctic marine ecosystems, increasing sea level rise which is inundating low lying coastal areas such as Pacific island atolls, and an increasing frequency of many climate extremes such as drought and heavy rain, as well as disasters where climate is an important driver, such as wildfire, flooding and landslides.

Multiple lines of evidence, using different methods, show that human influence is the only plausible explanation for the patterns and magnitude of changes that have been detected.

This human influence is largely due to our activities that release greenhouse gases, such as carbon dioxide and methane, as well sunlight absorbing soot. The main sources of these warming gases and particles are fossil fuel burning, cement production, land cover change (especially deforestation) and agriculture.

Most of us will struggle to pick up slow changes in the climate. We feel climate change largely through how it affects weather from day-to-day, season-to-season and year-to-year.

The weather we experience arises from dynamic processes in the atmosphere, and interactions between the atmosphere, the oceans and the land surface. Human influence on the broader climate system acts on these processes so that the weather today is different in many ways from how it would have been.

One way we can more clearly see climate change is by looking at severe weather events. A branch of climate science, called extreme event or weather attribution, looks at memorable weather events and estimates the extent of human influence on the severity of these events. It uses weather models run with and without measured greenhouse gases to estimate how individual weather events would have been different in a world without climate change.

As of early 2019, nearly 70% of weather events that have been assessed in this way were shown to have had their likelihood and/or magnitude increased by human influence on climate. In a world without global warming, these events would have been less severe. Some 10% of the studies showed a reduction in likelihood, while for the remaining 20% global warming has not had a discernible effect. For example, one study showed that human influence on climate had increased the likelihood of the 2015-2018 drought that afflicted Cape Town in South Africa by a factor of three.

Adapting to a changing climate

Weather extremes underlie many of the hazards that damage society and the natural environment we depend upon. As global warming has progressed, so have the frequency and intensity of these hazards, and the damage they cause.

Minimising the impacts of these hazards, and having mechanisms in place to recover quickly from the impacts, is the aim of climate adaptation, as recently reported by the Global Commission on Adaptation.

As the Commission explains, investing in adaptation makes sense from economic, social and ethical perspectives. And as we know that climate change is caused by humans, society cannot use "lack of evidence" on its cause as an excuse for inaction any more.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Climate variability

The Earth has experienced extended periods of warming and cooling in the past, with previous research suggesting these periods were synchronized across the entire planet. A research team led by Neukom set out to test whether or not this was true, using a database of temperature recordings that extends back 2,000 years.

Thermometer recordings of Earth's temperature only extend back 150 years, which makes obtaining historical climate records a little difficult. However, the PAGES 2k consortium offers proxy recordings, using data obtained from tree rings, glacier ice, lake sediment, coral skeletons, mollusks and other biological and geological archives, which keep an accurate record of previous temperatures.

The full database contains 692 records from 648 locations across the globe, allowing Neukom and colleagues to assess how the climate changed annually across the entire planet.

The researchers focused on several time periods, two epochs in particular: a warmer period known as the Medieval Climatic Anomaly, occurring between 950 and 1250, and the Little Ice Age, a cooler period that followed between 1400 and 1800. The team discovered the rate of warming and cooling in these periods didn't occur simultaneously across the globe, with only 40% of the Earth's surface reaching maximum temperatures at the same time.

Global mean temperature increase (red) and temperature decrease (blue) are displayed over the last 2,000 years. The green line signifies the maximum expected warming rate -- if humans didn't impact the climate.

The black line indicates direct temperature measurements made since the Industrial Revolution.

When they looked at the entire period, from 1 AD to 2000, they discovered the warmest period on record for 98% of the globe (only leaving out a slither of Antarctica) was the second half of the 20th century.

"It is significant that this study has shown that such prolonged warming, over 98 percent of the globe, has never occurred previously," said Fiona Armstrong, executive director of the Climate and Health Alliance. "This research is a wake-up call that we are heating the planet, and we will suffer increasing impacts if we fail to take action to reduce emissions very, very soon."

The final line in the paper is perhaps, the most telling.

"Against this regional framing, perhaps our most striking result is the exceptional spatiotemporal coherence during the warming of the twentieth century. This result provides further evidence of the unprecedented nature of anthropogenic global warming in the context of the past 2,000 years."

Basically, we're turning the heat up on the planet in a way we've never seen before.

"There is absolutely no doubt that humans are fundamentally changing Earth's climate," said Nerilie Abram, an Earth sciences researcher at the Australian National University.

There are some drawbacks to the research using the PAGES 2k proxy temperature data which can't be overcome, making it harder to draw direct comparisons across the entire two millennia.

However, in a companion paper, Neukom and his team detail seven different methods to perform statistical analyses of global average temperatures over the 2,000-year period, pulling data from different datasets. The methods all lined up with very little variation, lending more credence to the warming trends the team has seen. The work, which includes comparing the new climate reconstructions with existing climate simulations, reveals that volcanic eruptions played a major role in the swings in temperature we see before 1800. After 1850, greenhouse gas emissions dominate.

Recent climate extremes have driven unprecedented changes in the deep ocean

Credit: CC0 Public Domain

New measurements reveal a surprising increase in the amount of dense water sinking near Antarctica, following 50 years of decline.

Dense water formed near Antarctica, known as Antarctic Bottom Water, supplies oxygen to the deep ocean. Bottom water also forms part of the global network of ocean currents that influences climate by storing heat and carbon dioxide in the ocean. Changes in bottom water formation can therefore impact global climate and deep ocean ecosystems.

The study, led by Dr. Alessandro Silvano from the University of Southampton and CSIRO and published in the journal Nature Geoscience, documents an increase in the supply of bottom water to the deep Indian and Pacific Oceans. "Over the past 50 years of oceanographic campaigns we have seen a reduction in the amount of dense water reaching the deep ocean," Dr. Silvano said. "This trend was mysteriously interrupted in 2018."

The research found that unusual wind patterns near Antarctica changed ocean currents in the Ross Sea where bottom water is formed. The changes in wind and currents increased the amount of ocean cooling and freezing. The extra cooling and freezing, in turn, increased the density of water that sinks into the deep ocean, producing additional dense bottom water.

"We found that an unusual combination of two climate phenomena drove the renewal of bottom water formation: an extreme El Niño event occurring at the same time as stronger and southward-shifted westerly winds," said Dr. Silvano. "These results show how remote forcing can influence Antarctic processes and climate."

Co-author Annie Foppert, from the Australian Antarctic Program Partnership and CSIRO's Centre for Southern Hemisphere Oceans Research, said, "Evidence suggests the gradual decline in bottom water formation over the past five decades probably resulted from increased melt of the Antarctic Ice Sheet. The surprising rebound in recent years shows how extreme climate events can temporarily reverse long-term trends in Antarctic climate."

"In the future, we expect the accelerating melt of the Antarctic Ice Sheet to reduce the formation of bottom water" said Dr. Silvano. "But climate extremes like those that drove the recent rebound in bottom water formation are also projected to become more common if greenhouse gas emission by human activities continue at current rates."

"Further work is needed to understand how these competing factors will affect bottom water formation in a warming climate."

Why Apocalyptic Claims About Climate Change Are Wrong

Climate scientists are speaking out against grossly exaggerated claims about global warming.

Environmental journalists and advocates have in recent weeks made a number of apocalyptic predictions about the impact of climate change. Bill McKibben suggested climate-driven fires in Australia had made koalas “functionally extinct.” Extinction Rebellion said “Billions will die” and “Life on Earth is dying.” Vice claimed the “collapse of civilization may have already begun.”

Few have underscored the threat more than student climate activist Greta Thunberg and Green New Deal sponsor Rep. Alexandria Ocasio-Cortez. The latter said, “The world is going to end in 12 years if we don't address climate change.” Says Thunberg in her new book, “Around 2030 we will be in a position to set off an irreversible chain reaction beyond human control that will lead to the end of our civilization as we know it.”

Sometimes, scientists themselves make apocalyptic claims. “It’s difficult to see how we could accommodate a billion people or even half of that,” if Earth warms four degrees, said one earlier this year. “The potential for multi-breadbasket failure is increasing,” said another. If sea levels rise as much as the Intergovernmental Panel on Climate Change predicts, another scientist said, “It will be an unmanageable problem.”

Apocalyptic statements like these have real-world impacts. In September, a group of British psychologists said children are increasingly suffering from anxiety from the frightening discourse around climate change. In October, an activist with Extinction Rebellion (”XR”) — an environmental group founded in 2018 to commit civil disobedience to draw awareness to the threat its founders and supporters say climate change poses to human existence — and a videographer, were kicked and beaten in a London Tube station by angry commuters. And last week, an XR co-founder said a genocide like the Holocaust was “happening again, on a far greater scale, and in plain sight” from climate change.

Climate change is an issue I care passionately about and have dedicated a significant portion of my life to addressing. I have been politically active on the issue for over 20 years and have researched and written about it for 17 years. Over the last four years, my organization, Environmental Progress, has worked with some of the world’s leading climate scientists to prevent carbon emissions from rising. So far, we’ve helped prevent emissions increasing the equivalent of adding 24 million cars to the road.

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I also care about getting the facts and science right and have in recent months corrected inaccurate and apocalyptic news media coverage of fires in the Amazon and fires in California, both of which have been improperly presented as resulting primarily from climate change.

Journalists and activists alike have an obligation to describe environmental problems honestly and accurately, even if they fear doing so will reduce their news value or salience with the public. There is good evidence that the catastrophist framing of climate change is self-defeating because it alienates and polarizes many people. And exaggerating climate change risks distracting us from other important issues including ones we might have more near-term control over.

I feel the need to say this up-front because I want the issues I’m about to raise to be taken seriously and not dismissed by those who label as “climate deniers” or “climate delayers” anyone who pushes back against exaggeration.

With that out of the way, let’s look whether the science supports what’s being said.

First, no credible scientific body has ever said climate change threatens the collapse of civilization much less the extinction of the human species. “‘Our children are going to die in the next 10 to 20 years.’ What’s the scientific basis for these claims?” BBC’s Andrew Neil asked a visibly uncomfortable XR spokesperson last month.

“These claims have been disputed, admittedly,” she said. “There are some scientists who are agreeing and some who are saying it’s not true. But the overall issue is that these deaths are going to happen.”

“But most scientists don’t agree with this,” said Neil. “I looked through IPCC reports and see no reference to billions of people going to die, or children in 20 years. How would they die?”

“Mass migration around the world already taking place due to prolonged drought in countries, particularly in South Asia. There are wildfires in Indonesia, the Amazon rainforest, Siberia, the Arctic,” she said.

But in saying so, the XR spokesperson had grossly misrepresented the science. “There is robust evidence of disasters displacing people worldwide,” notes IPCC, “but limited evidence that climate change or sea-level rise is the direct cause”

What about “mass migration”? “The majority of resultant population movements tend to occur within the borders of affected countries," says IPCC.

It’s not like climate doesn’t matter. It’s that climate change is outweighed by other factors. Earlier this year, researchers found that climate “has affected organized armed conflict within countries. However, other drivers, such as low socioeconomic development and low capabilities of the state, are judged to be substantially more influential.”

Last January, after climate scientists criticized Rep. Ocasio-Cortez for saying the world would end in 12 years, her spokesperson said "We can quibble about the phraseology, whether it's existential or cataclysmic.” He added, “We're seeing lots of [climate change-related] problems that are already impacting lives."

That last part may be true, but it’s also true that economic development has made us less vulnerable, which is why there was a 99.7% decline in the death toll from natural disasters since its peak in 1931.

In 1931, 3.7 million people died from natural disasters. In 2018, just 11,000 did. And that decline occurred over a period when the global population quadrupled.

What about sea level rise? IPCC estimates sea level could rise two feet (0.6 meters) by 2100. Does that sound apocalyptic or even “unmanageable”?

Consider that one-third of the Netherlands is below sea level, and some areas are seven meters below sea level. You might object that Netherlands is rich while Bangladesh is poor. But the Netherlands adapted to living below sea level 400 years ago. Technology has improved a bit since then.

What about claims of crop failure, famine, and mass death? That’s science fiction, not science. Humans today produce enough food for 10 billion people, or 25% more than we need, and scientific bodies predict increases in that share, not declines.

The United Nations Food and Agriculture Organization (FAO) forecasts crop yields increasing 30% by 2050. And the poorest parts of the world, like sub-Saharan Africa, are expected to see increases of 80 to 90%.

Nobody is suggesting climate change won’t negatively impact crop yields. It could. But such declines should be put in perspective. Wheat yields increased 100 to 300% around the world since the 1960s, while a study of 30 models found that yields would decline by 6% for every one degree Celsius increase in temperature.

Rates of future yield growth depend far more on whether poor nations get access to tractors, irrigation, and fertilizer than on climate change, says FAO.

All of this helps explain why IPCC anticipates climate change will have a modest impact on economic growth. By 2100, IPCC projects the global economy will be 300 to 500% larger than it is today. Both IPCC and the Nobel-winning Yale economist, William Nordhaus, predict that warming of 2.5°C and 4°C would reduce gross domestic product (GDP) by 2% and 5% over that same period.

Does this mean we shouldn’t worry about climate change? Not at all.

One of the reasons I work on climate change is because I worry about the impact it could have on endangered species. Climate change may threaten one million species globally and half of all mammals, reptiles, and amphibians in diverse places like the Albertine Rift in central Africa, home to the endangered mountain gorilla.

But it’s not the case that “we’re putting our own survival in danger” through extinctions, as Elizabeth Kolbert claimed in her book, Sixth Extinction. As tragic as animal extinctions are, they do not threaten human civilization. If we want to save endangered species, we need to do so because we care about wildlife for spiritual, ethical, or aesthetic reasons, not survival ones.

And exaggerating the risk, and suggesting climate change is more important than things like habitat destruction, are counterproductive.

For example, Australia’s fires are not driving koalas extinct, as Bill McKibben suggested. The main scientific body that tracks the species, the International Union for the Conservation of Nature, or IUCN, labels the koala “vulnerable,” which is one level less threatened than “endangered,” two levels less than “critically endangered,” and three less than “extinct” in the wild.

Should we worry about koalas? Absolutely! They are amazing animals and their numbers have declined to around 300,000. But they face far bigger threats such as the destruction of habitat, disease, bushfires, and invasive species.

Think of it this way. The climate could change dramatically — and we could still save koalas. Conversely, the climate could change only modestly — and koalas could still go extinct.

The monomaniacal focus on climate distracts our attention from other threats to koalas and opportunities for protecting them, like protecting and expanding their habitat.

As for fire, one of Australia’s leading scientists on the issue says, “Bushfire losses can be explained by the increasing exposure of dwellings to fire-prone bushlands. No other influences need be invoked. So even if climate change had played some small role in modulating recent bushfires, and we cannot rule this out, any such effects on risk to property are clearly swamped by the changes in exposure.”

Nor are the fires solely due to drought, which is common in Australia, and exceptional this year. “Climate change is playing its role here,” said Richard Thornton of the Bushfire and Natural Hazards Cooperative Research Centre in Australia, “but it's not the cause of these fires."

The same is true for fires in the United States. In 2017, scientists modeled 37 different regions and found “humans may not only influence fire regimes but their presence can actually override, or swamp out, the effects of climate.” Of the 10 variables that influence fire, “none were as significant… as the anthropogenic variables,” such as building homes near, and managing fires and wood fuel growth within, forests.

Climate scientists are starting to push back against exaggerations by activists, journalists, and other scientists.

“While many species are threatened with extinction,” said Stanford’s Ken Caldeira, “climate change does not threaten human extinction. I would not like to see us motivating people to do the right thing by making them believe something that is false.”

I asked the Australian climate scientist Tom Wigley what he thought of the claim that climate change threatens civilization. “It really does bother me because it’s wrong,” he said. “All these young people have been misinformed. And partly it’s Greta Thunberg’s fault. Not deliberately. But she’s wrong.”

But don’t scientists and activists need to exaggerate in order to get the public’s attention?

“I’m reminded of what [late Stanford University climate scientist] Steve Schneider used to say,” Wigley replied. “He used to say that as a scientist, we shouldn’t really be concerned about the way we slant things in communicating with people out on the street who might need a little push in a certain direction to realize that this is a serious problem. Steve didn’t have any qualms about speaking in that biased way. I don’t quite agree with that.”

Wigley started working on climate science full-time in 1975 and created one of the first climate models (MAGICC) in 1987. It remains one of the main climate models in use today.

“When I talk to the general public,” he said, “I point out some of the things that might make projections of warming less and the things that might make them more. I always try to present both sides.”

Part of what bothers me about the apocalyptic rhetoric by climate activists is that it is often accompanied by demands that poor nations be denied the cheap sources of energy they need to develop. I have found that many scientists share my concerns.

“If you want to minimize carbon dioxide in the atmosphere in 2070 you might want to accelerate the burning of coal in India today,” MIT climate scientist Kerry Emanuel said.

“It doesn’t sound like it makes sense. Coal is terrible for carbon. But it’s by burning a lot of coal that they make themselves wealthier, and by making themselves wealthier they have fewer children, and you don’t have as many people burning carbon, you might be better off in 2070.”

Emanuel and Wigley say the extreme rhetoric is making political agreement on climate change harder.

“You’ve got to come up with some kind of middle ground where you do reasonable things to mitigate the risk and try at the same time to lift people out of poverty and make them more resilient,” said Emanuel. “We shouldn’t be forced to choose between lifting people out of poverty and doing something for the climate.”

Happily, there is a plenty of middle ground between climate apocalypse and climate denial.

What Climate Science Tells Us About Temperature Trends

This article expands on claims about global temperature trends made in Ronald Bailey's article in the January 2020 issue of Reason, "Climate Change: How Lucky Do You Feel?" for readers who are keen to dive deeper into the topic. (The print article is currently only available to subscribers.)

I began my time on the climate change beat as a skeptic. After attending the 1992 Earth Summit in Rio de Janeiro where the United Nations Framework Convention on Climate Change was negotiated, I noted in a Reason article that by signing the treaty "United States is officially buying into the notion that 'global warming' is a serious environmental problem" even while "more and more scientific evidence accumulates showing that the threat of global warming is overblown." I was simply unconvinced that the available data demonstrated the need for the kind of radical intervention activists were proposing.

But I stayed on the beat, closely following the progress of scientific study and policy debate. By 2005, following significant corrections to the satellite data record, I declared in Reason that "We're All Global Warmers Now." And in 2006 I concluded that "I now believe that balance of evidence shows that global warming could well be a significant problem."

In the years since 2007, I have remained largely sanguine, joining the many who noted that global temperature at the beginning of this century rose at a considerably lower rate than that projected by computer climate models. I was generally persuaded by researchers who predicted a sedate pace of increase, with temperatures unlikely to rise much above 1.5 degrees Celsius over the 19th century average. In this scenario, the world might get a bit warmer, but people and societies have proven themselves up to the task of adapting to such changes in the past and fundamentally the process of lifting hundreds of millions of poor people out of abject poverty through technological progress and economic growth fueled by coal, gas, and oil can safely continue unabated.

But as research continued, a number of possible scenarios have emerged. For example, some people read the scientific evidence as suggesting that man-made climate change is not greatly impacting people now, but might become a bigger problem toward the end of this century. Basically current weather—droughts, rainstorms, snowfall, and hurricanes–– cannot now be distinguished from natural variations in climate. However, as the temperature increases computer climate models project that future droughts will last longer, rainstorms fiercer, snowfall less, and hurricanes stronger. In addition, coastal flooding of major cities will become more common as sea level rises. These changes in climate will put the property and lives of children and grandchildren at greater risk. Computer models combining climate and economic components calculate that endeavoring now to slow warming would cost about the same as later efforts to adapt to a somewhat hotter world. Let's call this the somewhat worried scenario.

Another set of people note that temperature increases have apparently resumed a steady march upwards after a slow-down at the beginning of this century. They parse the results of recent studies that conclude that climate change is already causing deleterious impacts, e.g., heat waves both on land and in the oceans are becoming more common, the extent of Arctic sea ice is steeply declining, and glaciers , ice sheets , and permafrost are melting. The sanguine conclusion that future warming will proceed slowly and not rise much above 1.5 degrees Celsius by the end of this century appears to be too optimistic . If greenhouse gas emissions continue unabated, average global temperature looks to be on track to reach 1.8 degrees Celsius in 50 years and continue rising beyond 2 degrees Celsius by 2100. This trajectory significantly increases the risk that things could go badly wrong. This is the really worried scenario.

Spurred by current alarums, I spent the summer reading and reviewing recent findings of climate science to see if my belief that the somewhat worried scenario as the more likely outcome remains justified. Climate science is a massive enterprise involving research into a vast array of topics including atmospheric physics, ocean and atmospheric currents, solar irradiance, adjustments in temperature records, the effects of atmospheric aerosols, how forests and fields react to rising carbon dioxide, trends in cloudiness, heat storage in the deep oceans, changes in glaciers and sea ice, to name just a few. A simple Google Scholar search using the terms climate change and global warming returns more than 2.6 and 1.7 million results each. Just searching glaciers and climate change returns 124,000 results.

Researchers use complicated computer climate models to analyze all these data to make projections about what might happen to the climate in the future. My reporting strategy has been to take seriously what I believe to be the principal objections made by researchers who argue on scientific grounds that panic is unwarranted. I also assume that everyone is acting in good faith. What follows is based on what I hope is a fair reading of the recent scientific literature on climate change and communications with various well-known climate change researchers.

Ice Age Climate Change

To decide how worried we should be, we need to go back much further than 1992. Starting about 2.6 million years ago the Earth began experiencing ice ages lasting between 80,000 and 120,000 years. The w orld's most recent glacial period began about 110,000 years ago.

Most researchers believe that variations in Earth's orbital path around the Sun is the pacemaker of the great ice ages. Ice ages end when wobbles in Earth's orbit increase the sunlight heating the vast continental glaciers that form in the northern hemisphere. These orbital shifts initiate a feedback loop in which the warming oceans release of large amounts of carbon dioxide into the atmosphere which in turn further boosts global temperatures. Higher temperatures increase atmospheric water vapor which further boosts warming that melts more ice and snow cover. Less snow and ice enables the growth of darker vegetation which absorbs more heat and so forth.

At the height of the last glacial maximum 19,000 years ago atmospheric concentrations of carbon dioxide stood at only about 180 parts per million . The level of atmospheric carbon dioxide increased to around 280 parts per million by the late 18th century. This chain of feedbacks eventually produced a rise in global average surface temperature of about 4 degrees Celsius . That's the difference between the last ice age in which glaciers covered about one-third of the Earth's total land area and today when only 10 percent of the land area is icebound.

As a result of human activities, the level of carbon dioxide in the atmosphere has risen to about 415 parts per million now . The annual rate of increase in atmospheric carbon dioxide during the past 60 years is about 100 times faster than the rate of increase that occurred at the end of the last ice age. How much this increase is responsible for having warmed the planet over the last century, along with how much more warming will result if carbon dioxide concentrations continue to rise, is the central issue in climate change science.

Just Add Carbon Dioxide

Of course, the sun powers the Earth's climate. A bout 30 percent of solar energy is directly reflected back into space by bright clouds, atmospheric particles, and sea ice and snow. The remaining 70 percent is absorbed. The air and surface re-emit this energy largely as infrared rays that are invisible to us but we feel as heat .

The nitrogen and oxygen molecules that make up 99 percent of the atmosphere are transparent to both incoming sunlight and outgoing infrared rays. However, water vapor, carbon dioxide, methane, nitrous oxide, and ozone are opaque to many wavelengths of infrared energy. These greenhouse gas molecules block some escaping heat and re-emit it downward toward the surface. So instead of the Earth's average temperature being 18 degrees Celsius below zero, it is 15 degrees Celsius above freezing. This extra heating is the natural greenhouse effect .

NASA climate researcher Andrew Lacis and his colleagues contend that carbon dioxide is the key to greenhouse warming on Earth. Why? Because at current temperatures carbon dioxide and other trace greenhouse gases such as ozone, nitrous oxide, methane, and chlorofluorocarbons do not condense out of the atmosphere. Overall, these noncondensing greenhouse gases account for about 25 percent of the Earth's greenhouse effect. They sustain temperatures that initiate water vapor and cloud feedbacks that generate the remaining 75 percent of the current greenhouse effect. Lacis and his colleagues suggest that if all atmospheric carbon dioxide were somehow removed most of the water vapor would freeze out and the Earth would plunge into an icebound state.

Princeton physicist and lately resigned Trump administration National Security Council member William Happer has long questioned the magnitude of carbon dioxide's effect with respect to warming the atmosphere. In fact, Happer is the co-founder and former president of the nonprofit CO2 Coalition established in 2015 for the "purpose of educating thought leaders, policy makers, and the public about the important contribution made by carbon dioxide to our lives and the economy." His 2014 article, " Why Has Global Warming Paused? " in the International Journal of Modern Physics A , Happer argued that climate scientists had gotten crucial spectroscopic details of how atmospheric carbon dioxide absorbs infrared energy badly wrong. As a result, he asserts, a doubling of atmospheric carbon dioxide would likely warm the planet by only about 1.4 degrees Celsius. If the effect of carbon dioxide on temperatures was indeed constrained to that comparatively low value man-made global warming would probably not constitute a significant problem for humanity and the biosphere.

In 2016, NASA Langley Research Center atmospheric scientist Martin Mlynczak and his colleagues analyzed Happer's claims in a Geophysical Research Letters article and found , "Overall, the spectroscopic uncertainty in present-day carbon dioxide radiative forcing is less than one percent, indicating a robust foundation in our understanding of how rising carbon dioxide warms the climate system." In other words, the details of how carbon dioxide absorbs and re-emits heat are accurately known and unfortunately imply that future temperatures will be considerably higher than Happer calculated them to be.

Another related claim sometimes made is the effect of carbon dioxide on the climate is saturated, that is, the amount of carbon dioxide in the atmosphere is already absorbing re-emitting about as much heat as it can. Consequently, increasing the amount of carbon dioxide in the atmosphere won't much increase the average temperature of the globe. But is this so?

This claim is based on the fact in the current climate era that, as Princeton University climatologist Syukuro Manabe in a 2019 review article " Role of greenhouse gas in climate change ," notes, "surface temperature increases by approximately 1.3 degrees C in response to the doubling of atmospheric CO 2 concentration not only from 150 ppm [parts per million] to 300 ppm but also from 300 ppm to 600 ppm." To get a further increase of 1.3 degrees Celsius would require doubling atmospheric CO2 concentration to 1200 ppm. A metaphorical way of thinking about this issue is to visualize that the atmosphere consists of layers and as each layer fills up with enough carbon dioxide to absorb all the heat that it can, the extra heat radiates to the next layer that then absorbs it and re-emits it, and so forth. Consequently, the effect of CO2 on temperatures does decline but it does not saturate at levels relevant to future climate change.

Again, an increase of 1.3 degrees Celsius due to doubling carbon dioxide doesn't seem too alarming. "It is much smaller than 2.3 degrees C that we got in the presence of water vapour feedback," notes Manabe. Researchers find under current climate conditions that "water vapour exerts strong a positive feedback effect that magnifies the surface temperature change by a factor of ∼1.8." A warmer atmosphere evaporates and holds more water vapor which again is the chief greenhouse gas. Just as predicted, water vapor in the atmosphere is increasing as average global temperatures rise. Citing satellite data, a 2018 article in Earth and Space Science reported , " The record clearly shows that the amount of vapor in the atmosphere has been increasing at a rate of about 1.5% per decade over the last 30 years as the planet warms."

Evidence Tampering?

Researchers have devised various records to track changes in global average temperatures. These include surface records incorporating thermometer readings on land and at sea remote sensing of atmospheric trends using satellites, and climate reanalyses to calculate temperature trends for two meters above the surface.

All temperature records must be adjusted since all have experienced changes that affect the accuracy of their raw data. For example, surface temperature records are affected by changes in thermometers, locations of weather stations, time of day shifts in measurements, urban heat island effects, shipboard versus buoy sampling and so forth. Satellite data must be adjusted for changes in sensors and sensor calibration, sensor deterioration over time, and make corrections for orbital drift and decay. Climate reanalysis combines weather computer models with vast compilations of historical weather data derived from surface thermometers, weather balloons, aircraft, ships, buoys, and satellites. The goal of assimilating and analyzing these data is to create past weather patterns in order to detect changes in climate over time. Since climate reanalyses incorporate data from a wide variety of sources they must be adjusted when biases are identified in those data.

Some skeptics allege that the official climate research groups that compile surface temperature records adjust the data to make global warming trends seem greater than they are. A recent example is the June 2019 claim by geologist Tony Heller, who runs the contrarian website Real Climate Science, that he had identified "yet another round of spectacular data tampering by NASA and NOAA. Cooling the past and warming the present." Heller focused particularly on the adjustments made to NASA Goddard Institute for Space Studies (GISS) global land surface temperature trends.

One general method used by climate scientists of adjust temperature records, explains Berkeley Earth climate data scientist Zeke Hausfather (now at Breakthrough Institute) is statistical homogenization. Researchers compare each weather station to all of its nearby neighbors and look for changes that are local to one station, but not found at any others in the area. A sharp sustained jump to either lower or higher temperatures at a particular station generally indicates a change such as a shift in location or a switch in instrumentation. The records of such out-of-line stations are then adjusted to bring it back in line with its neighboring stations.

In general, temperatures increase more rapidly over land compared to the oceans because of the oceans' greater capacity to absorb heat and ability to get rid of extra heat through evaporation. Heller is right that raw land station adjustments by NOAA/NASA have increased overall land warming by about 16 percent between 1880 and 2016. On the other hand, NOAA/NASA adjustments of raw sea temperature data to take account of the shift from measuring ocean temperatures using buckets and intakes aboard ships to a widely deployed network of automatic buoys reduced the amount of warming in past. The adjustments result in about 36 percent less warming since 1880 than in the raw temperature data . When taken together the NOAA/NASA adjustments to land and ocean data actually reduce , rather than increase, the trend of warming experienced globally over the past century. Adjustments that overall reduce the amount of warming seen in the past suggest that climatologists are not fiddling with temperature data in order to create or exaggerate global warming.

It's Definitely Getting Hotter

The latest global temperature trends are compiled in the State of the Climate in 2018 report published in August 2019 by the American Meteorological Society. Since 1979, the surface records from NASA's Goddard Institute for Space Studies (GISS) report an increase of +0.18 C per decade. The both the Hadley Centre of the U.K. Met Office (HadCRUT) and the U.S. National Climatic Data Center finds a rise of +0.17 C per decade and the Japan Meteorological Agency shows an increase of +0.14 C per decade. In other words, according to surface records the planet has warmed by between 0.7 and 0.55 degrees Celsius in the last 40 years, a difference of 0.15 degrees Celsius.

Back in 2010 University of California, Berkeley physicist and self-proclaimed climate change skeptic Richard Muller founded the nonprofit Berkeley Earth Surface Temperature project aimed at independently checking the temperature trends devised by other research groups. To do so, the Berkeley Earth team created and analyzed a merged dataset by combining 1.6 billion temperature reports from 16 pre-existing data archives derived from nearly 40,000 unique weather stations using raw data whenever possible. In 2013, Berkeley Earth reported a rise in average world land temperature of approximately 1.5 degrees Celsius in the past 250 years and about 0.9 degrees in the past 50 years. In their 2018 report the group finds that since 1980, the overall global trend (land and sea) is +0.19 C per decade and has changed little during this period. Basically, it is slightly higher than the other surface temperature records.

The European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-I reanalysis calculates since 1979 an increase of global average temperature in the lower troposphere (surface to 6 miles up) of +0.14 C per decade the ECMWF's updated ERA-5 reanalysis reckons a per decade increase of +0.16 C. The Japanese 55-Year Reanalysis (JRA-55) estimates an increase of +0.17 C per decade and NASA's Modern Era Retrospective Analysis for Research and Applications Version 2 (MERRA-2) reports a rise of +0.16 C per decade. The differences in global temperature increase since 1979 between the reanalyses is even narrower ranging from 0.66 to 0.55 degrees Celsius.

The State of the Climate in 2018 reports satellite data temperature trends since 1979 (when satellite measurements began) for the lower troposphere. The University of Alabama in Huntsville (UAH) trend is +0.13 per decade, while Remote Sensing Systems' (RSS) trend is +0.20 degrees per decade. The difference in the two long-term lower tropospheric trends is more substantial. UAH reports warming since 1979 of only about 0.51 degrees whereas the RSS finds an increase of 0.78 degrees Celsius, a substantial difference of 0.27 degrees Celsius.

Which temperature records should be considered as the more accurate is hotly disputed among climate scientists. For example, atmospheric scientist John Christy, one of the developers of the UAH satellite dataset, asserts that it is more accurate because unlike the RSS record it removes spurious warming measurements that occurred as the orbits of a couple of NOAA satellites decayed around the turn of the 21 st century. In addition, Christy argues that the UAH temperature dataset has been validated through comparing it to weather balloon temperature data.

It is notable that the four satellite datasets all based on the same raw data find very different global temperature trends. For example, in the lower atmosphere RSS reports about 60 percent more warming than does UAH. Lawrence Livermore National Laboratory climate scientist Stephen Po-Chedley who has helped to develop a different satellite temperature record at the University of Washington observes, "These records are useful, but have substantial uncertainty." The "structural uncertainty" in the satellite records occurs Po-Chedley explains "because researchers use different approaches to remove known biases that affect long-term trends. No method is perfect, which leads to widely varying estimates of atmospheric warming."

Carl Mears, one of the developers of the RSS satellite dataset, disputes claims that the satellite and radiosonde temperature records are more accurate than the surface temperature record. "I consider this to be unlikely (even though I developed one of the satellite records) as indicated by the smaller spread in trends in the surface record than in the satellite record," he states .

The UAH record is something of an outlier with respect to the surface, reanalyses and other satellite records. Of course, that does not mean that it's wrong, but everyone must take into account the balance of the evidence when considering what the rate of global warming has been.

Are Climate Models Running Too Hot?

The differences between the UAH's relatively lower tropospheric temperature increase trend and the generally higher surface trend increases is at the center of a fierce debate over how man-made global warming will play out in the course of this century. The chief researchers who have developed and oversee the UAH satellite dataset are atmospheric scientists John Christy and Roy Spencer. While both acknowledge that adding carbon dioxide to the atmosphere likely does contribute to some warming, they doubt that future climate change will produce an "uninhabitable earth."

Christy and his colleagues argue in a 2018 article that the mid-tropospheric temperature observations in the crucial tropics are way lower than those projected by most computer climate models. Christy summarized his results in a 2019 report, "The Tropical Skies: Falsifying climate alarm," for the U.K.-based Global Warming Policy Foundation. Christy notes that most climate models project significant warming in the tropical troposphere between latitudes 20 degrees north to 20 degrees south of the equator at 30,000 and 40,000 feet.

Christy argues that this missing "tropical hotspot" shows that "the consensus of the models fails the test to match the real-world observations by a significant margin." At a 2017 congressional hearing, Christy had earlier testified , "As such, the average of the models is considered to be untruthful in representing the recent decades of climate variation and change, and thus would be inappropriate for use in predicting future changes in the climate or for related policy decisions."

Christy notes that the average of 102 climate model simulations project tropical troposphere temperature trend of +0.328 C/decade. In the State of the Climate in 2018 report the four decadal satellite tropospheric trends are: UAH +0.12 C RSS +0.17 C NOAA +0.22 C, and UW +0.16 C. In addition, the average for the reanalyses is +0.14 C. In other words, the tropical troposphere in the models are warming about two to three times faster than the actual temperatures in the tropical troposphere. On its face, this difference between model projections and temperature data makes Christy's point that the climate models are getting a very important feature related to future global warming badly wrong. Christy's research was cited in an August 2019 op-ed, " The Great Failure of the Climate Models ," in The Washington Examiner by climatologist Patrick Michaels and climate statistician Caleb Stewart Rossiter who are now both associated with the CO2 Coalition.

In a 2017 Journal of Climate article, Lawrence Livermore climate researcher Benjamin Santer and his colleagues acknowledged that "model–data differences in the vertical structure of atmospheric temperature change in the deep tropics—is a long-standing scientific concern."Santer and his colleagues published one such effort to address these scientific concerns in a 2017 Nature Geosciences article. In that article, they suggested that the differences between projections and empirical trends occurred due to a combination of fickle natural climate variability, uncertainties in satellite temperature datasets, and sporadic external effects such as cooling from volcanic eruptions that could not be included in the model simulations. Even so, the article concluded , "Our analysis is unlikely to reconcile divergent schools of thought regarding the causes of differences between modelled and observed warming rates in the early twenty-first century." As the ongoing research pursued by Christy and his colleagues shows, divergent schools of thought have indeed not reconciled.

Greenhouse theory predicts that warming at the surface will be amplified in the troposphere due to increased evaporation and convection. Basically, warmer air tends to rise. Climate model calculations project an overall tropospheric warming that is 1.2 times faster than at the surface. In the tropics, where most of the moisture is, the amplification factor is larger , about 1.4 to 1.6.

It is worth noting that Christy is comparing actual tropical temperature trends to modeled temperature trends. Lawrence Livermore atmospheric scientist Stephen Po-Chedley counters that "the model amplification should compare the model surface trend with the model atmospheric temperature trend. And the observed amplification should be the observed surface trend with the observed atmospheric temperature trend." He adds, "When models have sea surface temperatures that are forced to match the observations, the atmospheric warming in those model simulations matches the satellite record."

So let's go to the data. The first column in the table below contains tropical decadal sea surface temperature trends since 1979 between latitudes 20 N and 20 S as measured by four different research groups. In the second column are the actual satellite tropospheric trends over the same region as measured by four different research groups. Averaging the tropical sea surface temperatures yields a rate of increase of about +0.11 C per decade. Multiplying that average by the 1.5 tropospheric warming amplification factor used by the climate models yields a projected increase in tropospheric temperatures of +0.165 C per decade. This is basically in line with the increase of nearly +0.17 C per decade derived from averaging the four tropospheric temperature trends.

When actual surface data are taken into account, the tropical tropospheric temperature trend rises as pretty much as the models project. On the other hand, it is evident that the models are projecting higher tropical surface temperature trends than have been the case.

In an August 2019 Geophysical Research Letters article, MIT climate scientist Alexandre Tuel sought to explain the differences between recent model and satellite tropospheric warming rates notes that the climate models' projections for the rate of tropical surface warming since 1979 between latitudes 30 N and 30 S is about +0.19 C per decade.

The average rate of tropical surface warming between latitudes 20 N and 20 S for the climate models cited by the IPCC is +0.21 C per decade. Applying the amplification factors of 1.4 to 1.6 yields a projected tropical troposphere temperature increase of +0.27 C to +0.30 C per decade and +0.29 C and +0.34 C per decade respectively. Those figures are about the same as the projected model rate for the tropical troposphere cited by Christy. As Tuel concludes, "The key to explaining recent tropical troposphere temperatures trends lies in understanding why tropical sea surface temperature trends are smaller in models than observations."

So what is going on with the model projections for tropical sea surface temperatures? The mismatch arises chiefly in the vast Pacific Ocean. Generally speaking, due to upwelling colder water the eastern part of the Pacific near South America remains much cooler than the western part near the Philippines and Indonesia (except during El Niño events).

In a June 2019 Nature Climate Change study Columbia University climate researcher Richard Seager and his colleagues note that the models project that rising greenhouse gases will warm the colder east reducing the temperature differences between east and west. However, 60 years of temperature data have actually found that the opposite is occurring, the east is getting cooler and the west is warming up. Seager's team finds that increases in greenhouse gases are having the effect of boosting temperatures in the already warm west which in turn strengthens the winds in the east that intensifies the upwelling of colder water in the east. Seager points out that pattern is akin to La Niña events and will likely drive La Niña-like climate trends worldwide including "drying in East Africa, southwest North America and southeast South America, and wetting in Southeast Asia, Northeast Brazil and the Sahel."

In his Global Warming Policy Foundation report, Christy pointedly observes that the preliminary tropospheric temperature trends are even hotter in the set of 42 climate models whose outputs will be used in the Sixth Assessment Report (AR6) issued by the Intergovernmental Panel on Climate Change in 2021. Although MIT's Tuel has not yet had time to analyze the new model outputs, he says, "I wouldn't be surprised that systematic sea surface temperature biases like the Pacific cold tongue have not been corrected" in that set of models.

Christy and Michaels are certainly right when they point out that the models get tropical tropospheric temperature trends wrong, but the source of the models' error apparently lies in the oceans, not in the skies. The upshot, as Seager notes , is that there is an "urgent need to improve how well state-of-the-art models simulate the tropical Pacific so that the next generation of models can more reliably simulate how it responds to rising GHGs [greenhouse gases]."

Early 20th Century Warming

In their op-ed Michaels and Rossiter note, "Globally averaged thermometers show two periods of warming since 1900: a half-degree from natural causes in the first half of the 20th century, before there was an increase in industrial carbon dioxide that was enough to produce it, and another half-degree in the last quarter of the century." Their implication is that the current warming could be largely natural as well. It is worth noting that the earlier warming (

0.3–0.4 degrees C) was actually about a third to a half of the warming since the 1970s (

In addition, an August 2019 article in the Journal of Climate by Oxford University climate data scientist Karsten Haustein and his colleagues analyzed the evolution of temperature trends during the 20 th century. They concluded that the early warming and mid-century cooling interludes could be almost entirely explained once the effects of rising aerosol pollutants, periodic volcanic eruptions, and spurious warming in some sea surface temperature records were accounted for. If they are right, warming due to accumulation of greenhouse gases has been proceeding for more than a century and is speeding up. Of course, it's early days, so it remains to be seen if these results stand the test of time and further analysis.

The Global Warming Hiatus

The increase in average global temperature appeared to slow down dramatically between 1998 and 2015 even as greenhouse gases continued steadily to accumulate in the atmosphere. The IPCC's 2014 Synthesis Report acknowledged that the rate of surface warming since 1998 had been only 0.05 degrees Celsius per decade, which is considerably lower than the 0.12 degrees Celsius per decade rate observed since 1951. This "hiatus" was seen as evidence by skeptics (and reported by me) that climate model projections of fast and dangerous man-made warming were way overblown. For nearly a decade most climate researchers ignored the hiatus handwaving that warming would soon resume as projected. Eventually, the mismatch could no longer be ignored. Perplexed researchers sought to explain the slowdown in articles that placed the blame on a range of possibilities spanning from changes in solar radiation and stratospheric water vapor to burying excess heat in the deep oceans and natural internal variations in climate. By 2016, researchers had published nearly 200 peer-reviewed studies on the topic.

In the course of this research, many climate scientists came to realize that comparing the lower global temperature trend to the climate model average was obscuring the fact that many of the models actually produced internal climate variability with slowdowns very much like the hiatus. In fact, global climate model runs indicated that internal variability in ocean temperatures and heat uptake can mask long-term man-made warming for periods lasting more than a decade. As discussed above, that seems to be what caused the divergence between model and observed tropical temperature trends. In addition, updates and corrections to surface temperature records later made it clear that warming largely unnoticed had actually continued more or less unabated.

In fact, research by University of Exeter climate data scientist Femke Nijsse and her colleagues published in the July 2019 Nature Climate Change counter-intuitively finds that " high-sensitivity climates, as well as having a higher chance of rapid decadal warming, are also more likely to have had historical 'hiatus' periods than lower-sensitivity climates." By high sensitivity, Nijsse means that average global temperature could potentially increase by +0.7 C in just one decade. If she is right, the early 21 st century hiatus could literally be the cooler calm before the warming storm.

In any case, the hiatus came to an end when a super El Nino event in the Pacific Ocean substantially boosted global temperatures making 2016 hottest year since more or less accurate instrumental records started being kept in the 19 th century. Even in the lower trending UAH dataset, 2016 edged out 1998 by +0.02 C to become the warmest year in that 38-year satellite record. Christy did observe that "because the margin of error is about 0.10 C, this would technically be a statistical tie, with a higher probability that 2016 was warmer than 1998."

During the hiatus period, Christy argued that the climate models were clearly wrong because their projections were warming the bulk atmosphere at about twice the rate reported by satellite and balloon temperature trend observations. However, the 2016 El Nino event pushed the model projections and observational temperature trends more or less into alignment. In November, University of Guelph economist and frequent Christy scientific collaborator Ross McKitrick asserted , "The El Nino disguised the model-observational discrepancy for a few years, but it's coming back." McKitrick evidently expects that as the effects of the last El Nino ebb it will become undeniable by around 2030 that the models are projecting much too much warming.

On the other hand, in an April 2019 International Journal of Climatology article, a team of Chinese atmospheric scientists try to figure out how the long-term warming trend affected both the 1998 and 2015/2016 super El Ninos and what that suggests about future warming. Using five different surface datasets they calculate that in 1998 the El Nino event added +0.18 C to the long-term warming trend whereas in 2016, that El Nino event added just +0.06 to the long-term warming trend. In other words, it took a lot less heat to boost the 2015/2016 El Nino to slightly above the level of the 1998 El Nino. They report that their analysis " implies that warmer years like 2014–2016 may occur more frequently in the near future. We conclude that the so-called warming hiatus has faded away." If these researchers are right, future El Ninos may well temporarily boost global temperature trends above the model projections. In which case McKitrick's expectations that model results and observational trends will again significantly diverge over the coming decade are likely to be disappointed.

The record warmth of 2016 has so far not been exceeded, but surface temperature records report that nine of the 10 warmest years have occurred since 2005, with the last five years comprising the five hottest .

Rising Seas

One possible consequence of man-made global warming is that the melting of glaciers and the Greenland and Antarctic ice sheets will boost sea level and inundate coastal cities. It is generally agreed that the oceans over the past century have risen by an average of about 7 to 8 inches. Former Georgia Tech climatologist Judith Curry issued in November 2018 a special report, Sea Level and Climate Change . Curry concluded that recent changes in sea level are within the range of natural variability over the past several thousand years and there is not yet any convincing evidence of sea-level rise associated with human-caused global warming.

The IPCC's AR5 report suggested that average sea level rose by 7.5 inches between 1901 and 2010. The IPCC also reported that sea level very likely rose at a rate of about 1.7 millimeters (0.07 inch) per year between 1901 and 2010, but had accelerated to 3.2 millimeters (0.13 inch) between 1993 and 2010. If the rate does not increase, that would imply that sea level would rise by an average of 10 inches by 2100. In fact, that is the IPCC's low-end estimate while its high-end projection is nearly 39 inches depending on how much extra carbon dioxide is emitted into the atmosphere during the rest of this century.

A February 2018 study in the Proceedings of the National Academy of Sciences based on satellite altimeter data reported that sea-level rise at 3 millimeters per year has accelerated at a rate of +0.084 millimeters (about 1/20th the thickness of a penny*) since 1993. If sea level continues to change at this rate and acceleration, the researchers estimate that average sea-level rise by 2100 will be closer to 24 inches than 10 inches in 2100.

Curry counters, however, that the calibrations to the satellite altimeter data are far larger than the resulting changes in global mean sea level reported in that study. Be that as it may, another study in Nature Climate Change published in August 2019, found "persistent acceleration in global sea-level rise since the 1960s." The new study reports that sea-level rise has been accelerating at a rate of 0.06 millimeters per year since 1960, bolstering the earlier finding that sea level increase is accelerating. All things considered, Curry concludes that "values exceeding 2 feet are increasingly weakly justified." In other words, Curry also accepts that sea level could possibly rise about three times more than it did over the last century.

All Models Are Wrong

So even though the models appear essentially OK with respect to their tropical troposphere projections once actual sea surface temperatures are inputted, do their mistaken Pacific Ocean surface temperature projections invalidate them? The Science and Environmental Policy Project headed by climate change skeptic Kenneth Haapala "questions the use of models for public policy unless the models have been appropriately verified and validated. No matter how elaborate, the results from numerical models that are not thoroughly tested against hard evidence are speculative and cannot be relied upon." So what would count as validating climate models?

One commonplace notion is that scientific validation is achieved only when researchers develop a hypothesis and then design experiments to test it. If the experimental data contradict the hypothesis, it is rejected (or at least reformulated). Climate science however is an observational, not an experimental, science. In a sense, climate models are gigantic hypotheses, but the empirical data with which to check their predictions lies in the future.

Swiss Federal Institute of Technology environmental philosopher Christoph Baumberger and his colleagues address in their 2017 WIREs Climate Change article the issue of building confidence in climate model projections. They note that the most common way to evaluate climate models is to assess their empirical accuracy (how well model results fit past observations), robustness (how well they match the outputs of other models), and coherence with background knowledge (the support of model equations by basic theories). Nevertheless, they acknowledge that these three assessment criteria "neither individually nor collectively constitute sufficient conditions in a strict logical sense for a model's adequacy for long-term projections."

With respect to the adequacy of climate models (or of any other models for that matter), keep firmly in mind British statistician George Box's aphorism , "All models are wrong, but some are useful." Climate models certainly serve the heuristic function of helping climate researchers to better understand over time the feedback effects of the mind-bogglingly complicated interconnections between the atmosphere, the oceans, and the land. But how have they done with global warming projections?

Fairly well it turns out, according to a forthcoming evaluation by climate data scientist Zeke Hausfather and his colleagues of the projected warming trends in 17 different historical climate models published between 1970 and 2007. In their analysis the researchers also took into account mismatches between actual carbon dioxide emissions and other factors (effects of volcanic eruptions) that the modelers could not anticipate in order to assess the performance of the models' physics. The result was that 14 of the 17 model forecasts were consistent with the trends in the range of five different observational surface temperature time series.

Many critics have pointed out this notable emissions trajectories mismatch. Actual economic growth patterns during the past decades strongly suggest that future emissions will more closely track those projected by the more moderate IPCC scenarios and that the specific scenario featuring high emissions is exceedingly implausible . In the high emissions scenario, energy efficiency and carbon intensity (the amount of carbon dioxide emitted per dollar of GDP) gains that have been advancing for decades stall and the global energy system improbably re-carbonizes rapidly as it burns ever more coal, natural gas, and oil. Unfortunately, in many climate science studies and in popular reporting , outputs based on the high emissions scenario have been often treated as plausible business-as-usual projections instead of dubious worst-cases. One hopes that more credible socioeconomic and emissions scenarios will be developed as inputs for the next round of climate modeling to be used for the IPCC's upcoming Sixth Assessment Report.

In July, a so-far non–peer reviewed study by several young climate researchers at MIT report similar results when they assessed the projections of the 15 climate models used in the IPCC's Second Assessment Report (SAR) back in 1995. Their study aims to "probe the relationship between model hindcast skill and model forecast skill." In other words, do models that get trends in the past right also tend to get future trends right?

In order to figure out how well SAR models projected "future" temperature trends, the MIT researchers compare the model projections made back in 1995 to the observed global warming temperature trends between 1990 and 2018. They find that multi-model mean of the models "accurately reproduces the observed global-mean warming over a 1920-1990 hindcast period and accurately projects the observed global-mean warming over the 1990-2018 nowcast period." On that basis they boldly conclude, "Climate change mitigation has now been delayed long enough for the first projections of anthropogenic global warming to be borne out in observations, dismissing claims that models are too inaccurate to be useful and reinforcing calls for climate action." They do dryly observe that whether increasingly complicated modern models will prove to be more accurate "is yet to be determined." As we shall see below this may be a live issue with the set of models being used for the IPCC's Sixth Assessment Report in 2021.

While past performance is no guarantee of future results, at least with respect to projecting global average temperature trends, these historical climate models appear to have met the confidence building tests of empirical accuracy, robustness, and background knowledge coherence. In other words, they have proven useful.

The Cloud Wildcard

About 30 percent of incoming sunlight is reflected back into space with bright clouds being responsible for somewhere around two-thirds of that albedo effect. In other words, clouds generally tend to cool the earth. However, high thin cirrus clouds don't reflect much sunlight but they do slow the emission of heat back into space, thus they tend to warm the planet. In the current climate, clouds reflect more sunlight than they absorb and re-emit as heat downward toward the surface, so that on balance the earth is cooler because it has clouds than it would be than if it had no clouds.

In his 2018 lecture " The Role of Clouds in Climate ," NASA Goddard Institute for Space Science atmospheric scientist Anthony Del Genio notes, "It has often simplistically been assumed that clouds will offset greenhouse gas-induced climate change, based on the logic that warming evaporates more water from the ocean, which causes more clouds to form, which increases the albedo, which offsets the warming." However, most computer climate models project total cloud climate feedbacks ranging from near-neutral to strongly positive. What's going on?

How clouds will react to warming is one of the largest feedback uncertainties with respect to future climate change. The processes that form clouds are below the spatial resolution of climate models so researchers make estimates of how much sunlight they reflect and how much they absorb and then input those values into the models. The balance between cloud reflection and absorption matters a lot.

Researchers at the Pacific Northwest National Laboratory for example in a 2004 Journal of Applied Meteorology article noted that "a 4% increase in the area of the globe covered by marine stratocumulus clouds would offset the predicted 2–3 [degree C] rise in global temperature due to a doubling of atmospheric carbon dioxide." Marine stratocumulus clouds commonly form over cold ocean waters off the west coasts of continents . They are generally thin low clouds and cover more of the Earth's surface than any other cloud type making them extremely important for Earth's energy balance, primarily through their reflection of solar radiation.

On the other hand, wispy cirrus clouds that occur up to 20 kilometers above the surface let sunlight through but absorb and reflect infrared back downward to heat the surface. In 2001, MIT climatologist Richard Lindzen and his colleagues pointed to evidence in a Bulletin of the American Meteorological Society that cirrus clouds over the tropics tended to dissipate as temperatures increased. Such a process would serve as a negative feedback that, according to Lindzen and his colleagues, "would more than cancel all the positive feedbacks in the more sensitive current climate models." They likened this process to " an adaptive infrared iris that opens and closes in order to control the Outgoing Longwave Radiation in response to changes in surface temperature in a manner similar to the way in which an eye's iris opens and closes in response to changing light levels."

On the other hand, newer research suggests that rising temperatures will tend to dissipate low marine stratocumulus clouds which would generate a positive feedback that increases warming. In addition, changes in where clouds are located have big feedback effects. Climate models predict and preliminary satellite data finds that mid-latitude storm tracks (and their clouds) are retreating poleward, subtropical dry zones (deserts) are expanding, and the height to the highest cloud tops are rising. All three processes tend to increase global warming. "The primary drivers of these cloud changes appear to be increasing greenhouse gas concentrations and a recovery from volcanic radiative cooling," conclude Scripps Institution of Oceanography climatologist Joel Norris and his colleagues. "These results indicate that the cloud changes most consistently predicted by global climate models are currently occurring in nature."

Two different groups have lately revisited Lindzen's iris effect. One team in 2017 reported finding that increased sea surface temperatures boosted precipitation over the tropics. This, in turn, tended to reduce cirrus cloud cover allowing more infrared to escape into space which resulted in cooling. More recently, another group in 2019 analyzing trends in the western Pacific found that increasing sea surface temperatures tended to increase the amount of cirrus cloud cover slightly, generating a positive warming feedback.

Even though the details of how changes in clouds will affect future climate are still unsettled, Del Genio argues, "It is implausible that clouds could substantially offset greenhouse warming at this point in history." Why? "There is just no plausible physical mechanism that we can point to that would do that, nor is there any evidence in data that such a mechanism exists, nor is there any way one can possibly explain the observed warming of the past 60-70 years if that is the case," he explains.

More worryingly recent climate model research suggests that high atmospheric concentrations of carbon dioxide (1200 parts per million) could yield a tipping point in which cooling stratocumulus clouds are vastly dissipated. Such a break-up of low-level clouds would trigger a surface warming of about 8 C globally and 10 C in the subtropics. This scenario was bolstered by a September 2019 study in Science Advances seeking to simulate the climate of the Paleocene-Eocene Thermal Maximum (PETM) some 56 million years ago. Geological evidence indicates during the PETM that carbon dioxide levels were around 1,000 parts per million and that the Earth's surface was then at least 14 degrees Celsius warmer on average than it is now. The poles were ice-free. The research suggests that increases in carbon dioxide during the PETM produced a feedback process that greatly reduced low-level clouds which in turn further substantially boosted surface temperatures.

Ultimately Del Genio observes, "We think that clouds are likely to be a positive feedback, but we are not yet sure whether they are a small or large positive feedback. They could even be neutral. Many of the most recent climate models are predicting a fairly large cloud feedback (our GISS model is not one of them), but the jury is out on whether that is a reasonable result or not."

The Most Important Number

Scientific American in 2015 called equilibrium climate sensitivity "the most important number in climate change." Equilibrium climate sensitivity is conventionally defined as the increase in Earth's average surface temperature that would occur if carbon dioxide concentrations in the atmosphere were doubled and the climate system was given enough time to reach an equilibrium state. In 1979, the Charney Report from the U.S. National Academy of Sciences first conjectured that ECS was likely somewhere between 1.5 C and 4.5 C per doubling of CO2. The Intergovernmental Panel on Climate Change's Fifth Assessment Report (AR5) published in 2013 concluded that ECS is likely to be 1.5 C to 4.5 C. That is, nearly four decades later, the best estimate of sensitivity is largely the same.

Since the Charney report, climate researchers have reported more than 150 estimates of equilibrium climate sensitivity (ECS). Although the AR5 report did not offer a best estimate for ECS, the average for the models used in that report is 3.2 C. Just in 2018, statistician Nicholas Lewis and climatologist Judith Curry published in the Journal of Climate a median ECS estimate of 1.66 C with a range of 1.15–2.7 C.This is well below the IPCC's range and about half of the model average.

However, Texas A&M climate scientist Andrew Dessler and his colleagues also in 2018 estimated in the Journal of Geophysical Research: Atmospheres that median ECS was 3.3 C and likely ranged between 2.4 to 4.6 C. They added, "We see no evidence to support low ECS (values less than 2C) suggested by other analyses. Our analysis provides no support for the bottom of the Intergovernmental Panel on Climate Change's range. " Another group of researchers associated with MIT estimated in 2006 that upper bound of ECS could be as high as 8.9 C. That figure is basically twice the temperature increase that ended the last ice age.

The ECS estimates in the lower range generally are derived from analyzing historical temperature observations. University of Reading climate modeler Jonathan Gregory and his colleagues published a study in October arguing that the historical temperature data upon which they are based may be skewed downward by, among other things, an anomalously cooler historic period due to internal climate variability along with the additional cooling effects of industrial aerosol pollutants and volcanic eruptions. However, independent climate research statistician Nicholas Lewis recently countered that Gregory and his colleagues used flawed statistical methods to obtain their results. Time will tell how this shakes out.

There is great socioeconomic value in pinning down ECS. The larger that ECS is, the faster temperatures will increase and the higher they will go. The upshot is that the higher that ECS is, the worse the effects of climate change are liable to be. Conversely, the smaller ECS is, the slower that temperatures will rise and the lower they will go. A smaller ECS would mean that humanity has more time to address and adapt to future climate change. It is worth noting that the ECS values used in the historical models evaluated by Hausfather and his colleagues fit within the IPCC's AR5 range.

Researchers relying on three strands of evidence that include increased understanding of climate feedbacks, the historical climate record, and the paleoclimate record find that they are together pointing toward a narrower span of plausible ECS boundaries. These analyses are converging on a likely ECS ranging between 2.2 to 3.4 C and further indicate a very likely ECS range of between 2 C to 4 C. If this research proves out, this is good news since it would strongly imply that the higher and much more catastrophic ECS projections are improbable.

But hold on, some preliminary ECS estimates from the set of 42 next-generation climate models that the IPCC will be referencing in its 2021 Sixth Assessment Report (AR6) are considerably more worrisome. Currently several of those models are reporting an ECS of 5 degrees Celsius or hotter . The researchers, who are not at all sure about why their models are producing these results, are probing further to see if the high estimates will stand after deeper scrutiny.

Over at RealClimate NASA Goddard Institute for Space Studies director Gavin Schmidt urges caution before accepting these preliminary model results with respect to ECS. "Why might these numbers be wrong?," he asks. "Well, the independent constraints from the historical changes since the 19th C, or from paleo-climate or from emergent constraints in [earlier climate] models collectively suggest lower numbers (classically 2 to 4.5ºC) and new assessments of these constraints are likely to confirm it."

In fact, as noted above the latest assessments of ECS based on historical, paleoclimate, and feedback data have narrowed the range of estimates considerably below these new model outputs. "For all these constraints to be wrong, a lot of things have to fall out just right (forcings at the LGM [last glacial maximum] would have to be wrong by a factor of two, asymmetries between cooling and warming might need to be larger than we think, pattern effects need to be very important etc.)," points out Schmidt. "That seems unlikely."


Assuming that the new much higher ECS estimates do happily turn out to be wrong, the earlier ECS estimates still suggest that it is unlikely that humanity can avoid substantial climate change if the atmospheric concentration of carbon dioxide doubles over the pre-industrial level of 280 ppm, that is, 560 ppm. In recent years, carbon dioxide has been increasing in the atmosphere at an annual rate just under 3 parts per million (ppm) reaching 415 ppm this year. If that rate of increase continues, it will take about 50 years to reach 560 ppm.

So, what would the average global temperature be around 2070 when atmospheric carbon dioxide has doubled? This is where another quantity, transient climate response (TCR), becomes relevant. TCR is generally defined as what the average global temperature would be when carbon dioxide atmospheric concentrations growing at 1 percent per year reaches the doubling point over a period of about 70 years. The average TCR is 1.8 C in models cited in the IPCC's AR5 report. Not surprisingly, the lower the ECS is calculated to be, the lower the TCR will be. For example, Lewis and Curry calculated their median TCR as 1.2 C (range 0.9 to 1.7 C).

Considering that the planet has already warmed by about a degree Celsius as the atmospheric carbon dioxide concentration rose by 45 percent, lower TCR estimates seem unlikely. Assuming that global warming proceeds at the NOAA rate of +0.17 C per decad e that adds up to an increase of around +0.85 C by 2070. Since average global temperatures have increased by 1 C since the late 19th century an additional +0.85 would more or less match the climate model TCR average of 1.8 degrees per doubling of carbon dioxide. Of course, the warming wouldn't stop then.

"Is it too late (to stop dangerous climate change)?," asks University of Cambridge climate researcher Mike Hulme in his October editorial introducing a special issue of the journal WIREs Climate Change devoted to the question. Given how long I have been reporting on climate change, I identify with his world-weary observation, "There is a long history of climate deadlines being set publicly by commentators, politicians and campaigners&hellipand then of those deadlines passing with the threat unrealized."

Hulme pointedly notes that "deadline-ism" as embodied in the Green New Deal "does not do justice to what we know scientifically about climate change." Climate change prediction science reports "a range of possible values for future global warming. It is as false scientifically to say that the climate future will be catastrophic as it is to say with certainty that it will be merely lukewarm." He adds, "Neither is there a cliff edge to fall over in 2030 or at 1.5 degrees C of warming."

Continued economic growth and technological progress would surely help future generations to handle many—even most—of the problems caused by climate change. At the same time, the speed and severity at which the earth now appears to be warming makes the wait-and-see approach increasingly risky.

Will climate change be apocalyptic? Probably not, but the possibility is not zero. So just how lucky do you feel? Frankly, after reviewing recent scientific evidence, I'm not feeling nearly as lucky as I once did.

*CORRECTION: The original version of this article overstated the fraction of a penny thickness represented by +0.084 millimeter rate of increase in rising sea level.

After 40 years, researchers finally see Earth’s climate destiny more clearly

It seems like such a simple question: How hot is Earth going to get? Yet for 40 years, climate scientists have repeated the same unsatisfying answer: If humans double atmospheric carbon dioxide (CO2) from preindustrial levels, the planet will eventually warm between 1.5°C and 4.5°C—a temperature range that encompasses everything from a merely troubling rise to a catastrophic one.

Now, in a landmark effort, a team of 25 scientists has significantly narrowed the bounds on this critical factor, known as climate sensitivity. The assessment, conducted under the World Climate Research Programme (WCRP) and publishing this week in Reviews of Geophysics, relies on three strands of evidence: trends indicated by contemporary warming, the latest understanding of the feedback effects that can slow or accelerate climate change, and lessons from ancient climates. They support a likely warming range of between 2.6°C and 3.9°C, says Steven Sherwood, one of the study’s lead authors and a climate scientist at the University of New South Wales. “This is the number that really controls how bad global warming is going to be.”

The new study is the payoff of decades of advances in climate science, says James Hansen, the famed retired NASA climate scientist who helped craft the first sensitivity range in 1979. “It is an impressive, comprehensive study, and I am not just saying that because I agree with the result. Whoever shepherded this deserves our gratitude.”

Humanity has already emitted enough CO2 to be halfway to the doubling point of 560 parts per million, and many emissions scenarios have the planet reaching that threshold by 2060. The report underscores the risks of that course: It rules out the milder levels of warming sometimes invoked by those who would avoid emissions cuts. “For folks hoping for something better, those hopes are less grounded in reality,” says David Victor, a climate policy researcher at the University of California, San Diego, who was not part of the study.

The WCRP sensitivity estimate is designed to be used by the United Nations’s Intergovernmental Panel on Climate Change (IPCC) when it publishes its next major report in 2021 or 2022. The estimate will also inform projections for sea-level rise, economic damage, and much else. A clearer picture of those consequences could do much to spur local governments to cut emissions and adapt to warming, says Diana Reckien, a climate planning expert at the University of Twente. “The decreasing uncertainty could potentially motivate more jurisdictions to act.”

The study dispels uncertainty introduced by the latest climate models. Models have historically been used to estimate sensitivity, beginning in 1979, with the world’s first comprehensive assessment of CO2-driven climate change. That summer, at a meeting in Woods Hole, Massachusetts, led by Jule Charney, scientists produced a paper, known ever since as the Charney report, that predicted between 1.5°C and 4.5°C warming for a CO2 doubling. Those numbers—based in part on a model Hansen had developed—stuck around far longer than anyone imagined: The latest IPCC report, from 2013, gave the same range.

Recent models suggest the range might even go higher. They run hot, some predicting warming of more than 5°C for a CO2 doubling, apparently because of the way they render clouds, especially over the Southern Ocean. Yet these high-end models struggle to accurately recreate the climate of the 20th century, undermining their credibility. Such models play only a supporting role in the new assessment, says Robert Kopp, a climate scientist at Rutgers University, New Brunswick, who was not involved in the effort. “We now have enough independent lines of evidence that we don’t need to use the climate models as their own line.”

The WCRP study arose out of a 2015 workshop at Schloss Ringberg, a castle in the Bavarian Alps. Many participants were dissatisfied with the IPCC process and wanted to look at how physical mechanisms might set the boundaries of the sensitivity range. “Work on the ends, rather than on the middle,” says Bjorn Stevens, a cloud scientist at the Max Planck Institute for Meteorology, who edited the WCRP report with Sandrine Bony of the Pierre Simon Laplace Institute. Sherwood and Mark Webb, a climate scientist at the United Kingdom’s Met Office, agreed to lead the effort.

The first line of evidence they considered was modern-day warming. Since record keeping began in the 1800s, average surface temperatures have risen by 1.1°C. Continuing that trend into the future would lead to warming on the lower end of the range. But recent observations have shown the planet is not warming uniformly in particular, warming has barely touched parts of the eastern Pacific Ocean and Southern Ocean, where cold, deep waters well up and absorb heat. Eventually, models and paleoclimate records suggest, these waters will warm—not only eliminating a heat sink, but also spurring the formation of clouds above them that will trap more heat. Adjusting the temperature projections for this fact rules out low-sensitivity estimates, says Kate Marvel, a climate scientist at NASA’s Goddard Institute for Space Studies.


We wish to thank M. Gerasomov of Icefield Inc. for expert drilling, and Qin Xiang, Longjiangping, Shao Wenzhang, S. Whitlow and others for their hard work in the field. The project is supported by Atmospheric Sciences, the U. S. National Science Foundation, and the Chinese Academy of Sciences (Major Programs KZ951-A1-402 and KZ951–A1–204), the Ministry of Science and Technology (Climbing Projects 95–yu-40 and 95–yu-24) and the National Natural Science Foundation of China (49871022).