As carbon dioxide (CO2) levels rise in the atmosphere, the Earth will continue to warm. However, the exact relationship between CO2 and temperature, known as climate sensitivity, is still being studied. A new study led by the University of Washington has analyzed the most recent ice age to better understand this relationship, finding that while most future warming estimates remain unchanged, the absolute worst-case scenario is less likely.
Narrowing the Range of Climate Sensitivity
“The main contribution from our study is narrowing the estimate of climate sensitivity, improving our ability to make future warming projections,” said lead author Vince Cooper, a UW doctoral student in atmospheric sciences. The study doesn’t change the best-case warming scenario from doubling CO2 (about 2 degrees Celsius) or the most likely estimate (about 3 degrees Celsius). However, it reduces the worst-case scenario for doubling of CO2 by a full degree, from 5 degrees Celsius to 4 degrees Celsius.
Looking to the Past to Predict the Future
The researchers focused on the Last Glacial Maximum, a period 21,000 years ago when Earth was on average 6 degrees Celsius cooler than today, and atmospheric CO2 levels were less than half of today’s levels. “If we know roughly what the past temperature changes were and what caused them, then we know what to expect in the future,” Cooper said.
The Role of Ice Sheets and Cloud Changes
Using new statistical modeling techniques, the researchers combined prehistoric climate records with computer models to simulate the weather of the Last Glacial Maximum. They found that when much of North America was covered with ice, the ice sheet not only cooled the planet by reflecting sunlight but also altered wind patterns and ocean currents, causing the northern Pacific and Atlantic oceans to become especially cold and cloudy. These cloud changes over the oceans compounded the glacier’s global cooling effects by reflecting even more sunlight.
CO2’s Smaller Role in Ice Age Temperatures
The study shows that CO2 played a smaller role in setting ice age temperatures than previously estimated. Consequently, the most dire predictions for warming from rising CO2 are less likely over coming decades. “This 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,” said senior author Kyle Armour, a UW associate professor of atmospheric sciences and of oceanography.
Collaborative Research and Funding
The research involved collaboration among scientists from various institutions, including the University of Arizona, University of Cambridge, University of Illinois Urbana-Champaign, University of Colorado, George Mason University, UK Met Office Hadley Centre, NSF National Center for Atmospheric Research, University Corporation for Atmospheric Research, Geophysical Fluid Dynamics Laboratory, and Boston College. The study was funded by the National Science Foundation, Department of Defense, Alfred P. Sloan Foundation, National Oceanic and Atmospheric Administration, and the European Union’s Horizon 2020 program.