A group of climate scientists from the Potsdam Institute for Climate Impact Research (PIK) in Germany has used data from the last great Ice Age to conclude that a doubling of carbon dioxide concentration in the atmosphere would cause a global temperature increase of around 3 ºC. This is in line with previous estimates using other methods, mainly computer models.
Ice Ages are caused by changes in the Earth’s orbit, but there are multiple factors that make Ice Ages so cold, including carbon dioxide concentration. The PIK team combined modern climate models with a multitude of data about Ice Age climate to disentangle the different factors. The results are published this month in the journal Climate Dynamics.
In their study, the scientists accounted for model uncertainties by creating a set of 1,000 climate model versions—each one with a somewhat different behaviour of clouds, ocean currents or various other uncertain processes and feedbacks affecting climate. With each of these 1,000 models, a global warming scenario with doubled carbon dioxide concentration was computed.
As expected, the amount of global warming differed substantially between different model versions—as, in fact, it does between different climate models developed by different research groups.
All model versions were then tasked to simulate the climate of the last great Ice Age. Models that are too sensitive to carbon dioxide would tend to simulate an Ice Age that is too cold, and vice versa. After a careful analysis of all sources of uncertainty, the researchers concluded that a climate sensitivity of less than 1.2º C or greater than 4.3º C would be inconsistent with what we know about the great Ice Age, with the most likely value being near 3 ºC.
This assessment of climate sensitivity is a simple measure of how much climate would warm in the long run if CO2 concentration was doubled, and should not be confused with the global warming at a given time (say, the year 2100), which depends on the actual time evolution of the CO2concentration and on other climate factors.
One positive result is that we can practically rule out an extremely high climate sensitivity of six, seven or even more degrees Celsius, which some colleagues have speculated about.—Thomas Schneider von Deimling, lead author of the study
Uncertainty about the future climatic effect of carbon dioxide emissions consists of two parts: the future carbon dioxide concentration given current levels of emissions, and the amount of warming that can be expected from a given carbon dioxide concentration.
The new PIK study addressed the second part. An earlier study from PIK published in Geophysical Research Letters looked at the first issue, also with the help of past climate data. Taking the findings of both studies together, the results suggest that current estimates of the most likely future global warming may be somewhat too low—not because the climate sensitivity has been underestimated, but because the carbon cycle could act to amplify the warming later this century.
“Climate sensitivity estimated from ensemble simulations of glacial climate”; Thomas Schneider von Deimling, Hermann Held, Andrey Ganopolski and Stefan Rahmstorf; Climate Dynamics (2006), Vol. 27, 149-163
“Positive feedback between global warming and atmospheric CO2 concentration inferred from past climate change”; Scheffer, M., Brovkin, V., and Cox, P.; Geophysical Research Letters, vol. 33, doi:10.1029/2005gl025044, 2006