As carbon dioxide accumulates in the atmosphere, the Earth will get hotter. But exactly how much warming will result from a certain increase in CO2 is under study. The relationship between CO2 and warming, known as climate sensitivity, determines what future we should expect as CO2 levels continue to climb.
New research led by the 91探花 analyzes the most recent ice age, when a large swath of North America was covered in ice, to better understand the relationship between CO2 and global temperature. It finds that while most future warming estimates remain unchanged, the absolute worst-case scenario is unlikely.
The open-access was published April 17 in Science Advances.
鈥淭he main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections,鈥� said lead author , a 91探花doctoral student in atmospheric sciences. 鈥淏y looking at how much colder Earth was in the ancient past with lower levels of greenhouse gases, we can estimate how much warmer the current climate will get with higher levels of greenhouse gases.鈥�
The new paper doesn鈥檛 change the best-case warming scenario from doubling CO2 鈥� about 2 degrees Celsius average temperature increase worldwide 鈥� or the most likely estimate, which is about 3 degrees Celsius. But it reduces the worst-case scenario for doubling of CO2 by a full degree, from 5 degrees Celsius to 4 degrees Celsius. (For reference, CO2 is currently at 425 ppm, or about 1.5 times preindustrial levels, and unless emissions drop is headed toward double preindustrial levels before the end of this century.)
As our planet heads toward a doubling of CO2, the authors caution that the recent decades are not a good predictor of the future under global warming. Shorter-term climate cycles and atmospheric pollution鈥檚 effects are just some reasons that recent trends can鈥檛 reliably predict the rest of this century.
鈥淭he spatial pattern of global warming in the most recent 40 years doesn’t look like the long-term pattern we expect in the future 鈥� the recent past is a bad analog for future global warming,鈥� said senior author , a 91探花associate professor of atmospheric sciences and of oceanography.
Instead, the new study focused on a period 21,000 years ago, known as the Last Glacial Maximum, when Earth was on average 6 degrees Celsius cooler than today. Ice core records show that atmospheric CO2 then was less than half of today鈥檚 levels, at about 190 parts per million.
鈥淭he paleoclimate record includes long periods that were on average much warmer or colder than the current climate, and we know that there were big climate forcings from ice sheets and greenhouse gases during those periods,鈥� Cooper said. 鈥淚f we know roughly what the past temperature changes were and what caused them, then we know what to expect in the future.鈥�
Researchers including co-author , a 91探花professor of atmospheric sciences, have created new statistical modeling techniques that allow paleoclimate records to be assimilated into computer models of Earth鈥檚 climate, similar to today鈥檚 weather forecasting models. The result is more realistic temperature maps from previous millennia.
For the new study the authors combined prehistoric climate records 鈥� including ocean sediments, ice cores, and preserved pollen 鈥� with computer models of Earth鈥檚 climate to simulate the weather of the Last Glacial Maximum. When much of North America was covered with ice, the ice sheet didn鈥檛 just cool the planet by reflecting summer sunlight off the continents, as previous studies had considered.
By altering wind patterns and ocean currents, the ice sheet also caused the northern Pacific and Atlantic oceans to become especially cold and cloudy. Analysis in the new study shows that these cloud changes over the oceans compounded the glacier鈥檚 global cooling effects by reflecting even more sunlight.
In short, the study shows that CO2 played a smaller role in setting ice age temperatures than previously estimated. The flipside is that the most dire predictions for warming from rising CO2 are less likely over coming decades.
鈥淭his paper allows us to produce more confident predictions because it really brings down the upper end of future warming, and says that the most extreme scenario is less likely,鈥� Armour said. 鈥淚t doesn’t really change the lower end, or the average estimate, which remain consistent with all the other lines of evidence.鈥�
The research was funded by the National Science Foundation, the Department of Defense鈥檚 National Defense Science and Engineering Graduate Fellowship, the Alfred P. Sloan Foundation, the National Oceanic and Atmospheric Administration and the European Union鈥檚 Horizon 2020 program. Other co-authors are Jessica Tierney at the University of Arizona; Matthew Osman at the University of Cambridge in the U.K.; Cristian Proistosescu and Philip Chmielowiec at the University of Illinois Urbana-Champaign; Yue Dong at the University of Colorado; Natalie Burls at George Mason University; Timothy Andrews at the U.K. Met Office Hadley Centre; Daniel Amrhein and Jiang Zhu at the NSF National Center for Atmospheric Research in Boulder; Wenhao Dong at the University Corporation for Atmospheric Research in Boulder and Geophysical Fluid Dynamics Laboratory; and Yi Ming at Boston College.
For more information, contact Cooper at vcooper@uw.edu or Armour at karmour@uw.edu.