Study using paleoclimate data suggests climate sensitivity to CO2 doubling may be less severe than projected
A new study, funded by the National Science Foundation’s Paleoclimate Program and published online this week in the journal Science, suggests that the rate of global warming from doubling of atmospheric carbon dioxide from pre-industrial times may be less than the most extreme estimates of some previous studies and may be less severe than that projected by the Intergovernmental Panel on Climate Change (IPCC) report in 2007.
The team combined extensive sea and land surface temperature reconstructions from the Last Glacial Maximum (LGM) with a climate model of intermediate complexity to estimate the equilibrium climate sensitivity for a doubling of atmospheric carbon dioxide concentrations (ECS2xC) from preindustrial times. (Climate sensitivity is the change in global mean surface air temperature caused by radiative forcing of Earth’s radiative balance at the top of the atmosphere with respect to a given reference state.)
Their results showed an estimated lower median temperature increase (2.3 K) and reduced uncertainty (1.7 to 2.6 K 66% probability). Assuming paleoclimatic constraints apply to the future as predicted by their model, the researchers concluded, their results imply lower probability of imminent extreme climatic change than previously thought.
The experiments collectively favor sensitivities between 1 and 3 K. However, we cannot exclude the possibility that the analysis is sensitive to uncertainties or statistical assumptions not considered here, and the underestimated land/sea contrast in the model, which leads to the difference between land and ocean based estimates of ECS2xC, remains an important caveat.
Our uncertainty analysis is not complete and does not explicitly consider uncertainties in radiative forcing due to ice sheet extent or different vegetation distributions. Our limited model ensemble does not scan the full parameter range, neglecting, for example, possible variations in shortwave radiation due to clouds. Non-linear cloud feedbacks in different complex models make the relation between LGM and 2×CO2 derived climate sensitivity more ambiguous than apparent in our simplified model ensemble. More work, in which these and other uncertainties are considered, will be required for a more complete assessment.
In summary, using a spatially extensive network of paleoclimate observations in combination with a climate model we find that climate sensitivities larger than 6 K are implausible, and that both the most likely value and the uncertainty range are smaller than previously thought. This demonstrates that paleoclimate data provide efficient constraints to reduce the uncertainty of future climate projections.—Schmittner et al.
The authors emphasize that global warming is real and that increases in atmospheric CO2 will have multiple serious impacts. However, the most extreme projections of temperature increases from the doubling of CO2 are unlikely, according to their work.
Many previous climate sensitivity studies have looked at the past only from 1850 through today, and not fully integrated paleoclimate data, especially on a global scale. When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago—which is referred to as the Last Glacial Maximum—and compare it with climate model simulations of that period, you get a much different picture.
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.—Andreas Schmittner, Oregon State University researcher and lead author
Scientists have struggled for years trying to quantify climate sensitivity—how the Earth will respond to projected increases of atmospheric carbon dioxide. The 2007 IPCC report estimated that the air near the surface of the Earth would warm on average by 2 to 4.5 °C with a doubling of atmospheric CO2 from pre-industrial standards. The mean, or expected value increase in the IPCC estimates was 3.0 degrees; most climate model studies use the doubling of CO2 as a basic index.
Some previous studies have calculated the impacts could be much more severe—as much as 10 degrees or higher with a doubling of CO2. Studies based on data going back only to 1850 are affected by large uncertainties in the effects of dust and other small particles in the air that reflect sunlight and can influence clouds, known as “aerosol forcing,” or by the absorption of heat by the oceans, the researchers say.
To lower the degree of uncertainty, Schmittner and his colleagues used a climate model with more data and found that there are constraints that preclude very high levels of climate sensitivity.
The researchers compiled land and ocean surface temperature reconstructions from the Last Glacial Maximum and created a global map of those temperatures. During this time, atmospheric CO2 was about a third less than before the Industrial Revolution, and levels of methane and nitrous oxide were much lower. Because much of the northern latitudes were covered in ice and snow, sea levels were lower, the climate was drier (less precipitation), and there was more dust in the air. All these factors, which contributed to cooling the Earth’s surface, were included in their climate model simulations.
The new data changed the assessment of climate models in many ways, said Schmittner. The researchers’ reconstruction of temperatures has greater spatial coverage and showed less cooling during the Ice Age than most previous studies.
High sensitivity climate models—more than 6 degrees—suggest that the low levels of atmospheric CO2 during the Last Glacial Maximum would result in a runaway effect that would have left the Earth completely ice-covered.
Clearly, that didn’t happen. Though the Earth then was covered by much more ice and snow than it is today, the ice sheets didn’t extend beyond latitudes of about 40 degrees, and the tropics and subtropics were largely ice-free—except at high altitudes. These high-sensitivity models overestimate cooling.—Andreas Schmittner
On the other hand, models with low climate sensitivity—less than 1.3 degrees—underestimate the cooling almost everywhere at the Last Glacial Maximum, the researchers say. The closest match, with a much lower degree of uncertainty than most other studies, suggests climate sensitivity is about 2.4 degrees.
Reconstructing sea and land surface temperatures from 21,000 years ago is a complex task involving the examination of ices cores, bore holes, fossils of marine and terrestrial organisms, seafloor sediments and other factors. Sediment cores, for example, contain different biological assemblages found in different temperature regimes and can be used to infer past temperatures based on analogs in modern ocean conditions.
When we first looked at the paleoclimatic data, I was struck by the small cooling of the ocean. On average, the ocean was only about two degrees (Celsius) cooler than it is today, yet the planet was completely different—huge ice sheets over North America and northern Europe, more sea ice and snow, different vegetation, lower sea levels and more dust in the air. It shows that even very small changes in the ocean’s surface temperature can have an enormous impact elsewhere, particularly over land areas at mid- to high-latitudes.—Andreas Schmittner
Schmittner said continued unabated fossil fuel use could lead to similar warming of the sea surface as reconstruction shows happened between the Last Glacial Maximum and today.
Hence, drastic changes over land can be expected. However, our study implies that we still have time to prevent that from happening, if we make a concerted effort to change course soon.—Andreas Schmittner
Other authors on the study include Peter Clark and Alan Mix of OSU; Nathan Urban, Princeton University; Jeremy Shakun, Harvard University; Natalie Mahowald, Cornell University; Patrick Bartlein, University of Oregon; and Antoni Rosell-Mele, University of Barcelona.
Andreas Schmittner, Nathan M. Urban, Jeremy D. Shakun, Natalie M. Mahowald, Peter U. Clark, Patrick J. Bartlein, Alan C. Mix, and Antoni Rosell-Melé (2011) Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum. Science doi: 10.1126/science.1203513