Significant, risky, global warming still projected

New research takes some of the most dire global warming projections off the table. A study published last week in the journal Science concludes that the more extreme climate change scenarios, which involve temperature increases of up to 10°F are implausible. Instead, the study finds, we are likely in the midst of a more manageable, but still potentially dangerous, shift in the planet’s climate.

In fact, this study’s projections still warm the planet to the highest levels in human history.

According to the study, if atmospheric concentrations of carbon dioxide (CO2), the most important global warming gas, were to double, the global average surface temperature would increase by between 3 to 4.7°F, with a median value of 4.1°F (or 1.7 to 2.6°C, with a median value of 2.3°C). This is a narrower range, with a lower upper estimate, than other studies have concluded.

In the new study researchers led by Andreas Schmittner from Oregon State University focused on the climate during the last ice age, about 19,000 to 23,000 years ago, to estimate how sensitive the climate system is to changing amounts of CO2 (in the ice age scenario, a reduction of CO2 helped cool the globe).

What they found was that climate models run with a high degree of climate sensitivity overestimated the cooling that occurred during the time period, depicting a planet completely encased in ice, when in fact evidence shows the tropics and sub-tropics were largely-ice free. Also, models with low climate sensitivity underestimated the cooling during that period.

“If these paleoclimatic constraints apply to the future, as predicted by our model, the results imply less probability of extreme climatic change than previously thought,” Schmittner stated in a press release. He cautioned that climate change is nonetheless likely to have major consequences, even if it is not as extreme.

Atmospheric level of carbon dioxide have grown to about 390 ppm compared to preindustrial levels of around 270-280 ppm. (Climate Central)

A useful way to think about CO2 and other greenhouse gases comes from climate scientist Richard Alley of Penn State University, who is one of the foremost experts in paleoclimatology. He refers to carbon dioxide as the “biggest control knob” of the earth’s climate.

Right now, human activities, such as burning coal for energy, are adding greenhouse gases to the atmosphere at such a rapid rate that we’re well on our way to doubling the amount of CO2 in the atmosphere compared to the preindustrial era - from 280 parts per million back then to about 389 parts per million now.

Before trumpeting the study’s conclusions as an indication that climate change concerns are overblown, as some climate change skeptics have done, it’s important to keep in mind that this is one of many studies that have examined the planet’s likely response to a buildup in greenhouse gases. In fact, the question of exactly how warm the planet would get if the atmospheric concentration of CO2 were to double is one that researchers have tackled for centuries.

In 1896, Swedish physical chemist Svante Arrhenius used longhand algebra to arrive at his conclusions regarding the sensitivity of the climate system. In his insightful book “The Discovery of Global Warming,” science historian Spencer Weart describes Arrhenius’s contributions:

The prize sought by Arrhenius was the solution to the riddle of the ice ages. He focused on a decrease in CO2 as a possible cause of cooling, and found that cutting the level in half could indeed bring an ice age. But he also took the trouble to estimate what might happen if the amount of gas in the atmosphere, at some distant time in the past or future, was double its present value. He computed that would bring roughly 5 or 6°C of global warming.

This result is not far from the range that scientists would compute a century later using vastly better models - the current estimate is that a doubling of CO2 will bring some 3 degrees of warming, give or take a degree or two. Did Arrhenius end up in the same range by sheer luck? Partly, but not entirely. In the sort of simple physics and chemistry calculations where Arrhenius had made his name, you can expect to come out roughly right if you address a powerful physical effect in a straightforward way, starting with decent data…

Other noteworthy findings on climate sensitivity include the 1979 Charney Report from the National Academy of Sciences, which included a climate sensitivity estimate of 1.5 to 4.5°C. The most recent assessment from the U.N. Intergovernmental Panel on Climate Change (IPCC) contained the following language on climate sensitivity:

Progress since the TAR [Third Assessment Report] enables an assessment that climate sensitivity is likely to be in the range of 2 to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.

The IPCC mentioned several sources of uncertainty behind these estimates, including changes in cloud cover and type that could act to amplify or dampen global warming.

The new study also contains these and other uncertainties, including the fact that the dataset the researchers used only included 26 percent of the planet’s surface. In addition, the new study found a surprisingly small temperature difference between average ocean temperatures during the last glacial maximum and today -- just 2.6°C. This is not consistent with other studies, as the website Skeptical Science points out (in addition to the study’s authors themselves).

At the end of the day, this new study is another contribution to the long history of research on this topic, and may not even be much of a departure from the previous consensus view. Rather than a sharp break with the IPCC’s 2007 findings, over at Eric Berger’s “SciGuy” blog, Texas A&M University climate scientist Andrew Dessler says he views the new study as largely consistent with previous climate sensitivity estimates.

“Ordinarily, when something is published in Science, you expect it to be a significant advance/revision of our prior knowledge. I’m not sure that’s the case here,” Dessler stated. “My sense is that most scientists consider the very high end of the sensitivity range (greater than 4°C) to be pretty unlikely (although it cannot be ruled out), and the most likely value for climate sensitivity is around, probably slightly below, 3°C.”

Additional perspectives: Climate Progress | RealClimate