Are High-End High-Sensitivity Climate Models' Predictions Accurate?

Newer models with a high "climate sensitivity" - meaning they predict much greater global warming from the same levels of atmospheric carbon dioxide as other models - do not provide a plausible scenario of Earth's future climate according to researchers from Princeton University and the University of Miami.

According to the researchers, these models exaggerate the global cooling effect caused by cloud-aerosol interactions, implying that clouds will mitigate greenhouse-gas-induced warming much more than climate records show, according to the researchers, who published their findings in the journal Geophysical Research Letters.

Accuracy and Consistency

Instead, the researchers discovered that models with lower climate sensitivity are more consistent with observed temperature differences between the northern and southern hemispheres and, as a result, are more accurate depictions of projected climate change than newer models. The Carbon Mitigation Initiative (CMI), based at Princeton's High Meadows Environmental Institute, helped fund the research (HMEI).

According to co-author Gabriel Vecchi, a Princeton professor of geosciences and the High Meadows Environmental Institute and principal investigator in CMI, these findings could significantly impact climate-change policy. Models with higher climate sensitivity predict greater warming from greenhouse gas emissions, as well as more dire - and imminent - consequences like more extreme sea-level rise and heatwaves.

Conditions

High-climate-sensitivity models predict a 2 to 6 degree Celsius increase in global average temperature under current carbon dioxide levels. To avoid catastrophic consequences, scientists agree that the increase should be kept below 2 degrees. The Paris Agreement of 2016 set a limit of 1.5 degrees Celsius for global warming.

"Clearly, a higher climate sensitivity necessitates much more aggressive carbon mitigation," Vecchi said. "To meet the Paris Agreement's goals of keeping global warming below 2 degrees Celsius, society would need to drastically reduce carbon emissions. Reducing the uncertainty in climate sensitivity allows us to develop a more reliable and accurate climate change strategy."

The researchers discovered that both high and low climate-sensitivity models accurately predicted global temperatures during the twentieth century. On the other hand, the higher-sensitivity models include a stronger cooling effect from aerosol-cloud interaction, which balances out the increased warming from greenhouse gases. Furthermore, the models predict that aerosol emissions will be concentrated in the northern hemisphere, which contradicts observations.

Findings

"Our findings remind us to be careful when interpreting model effects, even though the models correctly reflect past global warming," said first author Chenggong Wang, a Ph.D. nominee in Princeton's Program in Atmospheric and Oceanic Sciences. "We prove that the global trend obscures crucial data about climate shift trends."

In addition to the main findings, the study sheds light on how clouds can moderate warming on large and small scales in models and the real world.

Scientists at Princeton and other universities have recently focused on the impact of clouds on climate change. Two papers by Amilcare Porporato, a member of the CMI leadership team and Princeton's Thomas J. Wu '94 Professor of Civil and Environmental Engineering and the High Meadows Environmental Institute, reported on the future effect of heat-induced clouds on solar power and how climate models underestimate the cooling effect of the daily cloud cycle.