IMAGE: The researchers found that models with lower climate sensitivity were more in tune with observed temperature differences, particularly between the northern and southern hemispheres. THE GRAPHIC SHOWS CHANGES IN THE YEAR… View More CREDIT: PICTURE BY CHENGGONG WANG, PROGRAM IN ATMOSPHERIC AND OCEAN SCIENCES, PRINCETON UNIVERSITY
A recent analysis of the latest generation of climate models – known as CMIP6 – offers a cautionary story for interpreting climate simulations as scientists develop more sensitive and nuanced projections of how the earth will react to increasing levels of carbon dioxide in the atmosphere.
Researchers at Princeton University and the University of Miami reported that newer models with high “climate sensitivity” – meaning they predict much more global warming for the same atmospheric carbon dioxide levels as other models – are not a plausible scenario for Earth’s future climate .
These models exaggerate the global cooling effect that results from interactions between clouds and aerosols and assume that clouds will moderate warming caused by greenhouse gases – especially in the northern hemisphere – much more than climate records actually show, the researchers reported in the journal Geophysical Research Letters.
Instead, the researchers found that models with lower climate sensitivity better match the observed temperature differences between the northern and southern hemispheres and therefore more accurately represent projected climate change than the newer models. The study was supported by the Carbon Mitigation Initiative (CMI) of the High Meadows Environmental Institute (HMEI) in Princeton.
These findings are potentially important for climate policy, said co-author Gabriel Vecchi, professor of earth sciences at Princeton and High Meadows Environmental Institutes and principal researcher for CMI. Since models with higher climate sensitivity predict more warming from greenhouse gas emissions, they also predict worse – and imminent – consequences such as more extreme sea level rise and heat waves.
The models with high climate sensitivity predict an increase in the global average temperature from 2 to 6 degrees Celsius below the current carbon dioxide values. The current scientific consensus is that the rise must be kept below 2 degrees to avoid catastrophic effects. The 2016 Paris Agreement sets the threshold at 1.5 degrees Celsius.
“Obviously, higher climate sensitivity would require much more aggressive carbon abatement,” Vecchi said. “Society would have to reduce CO2 emissions much faster in order to achieve the goals of the Paris Agreement and keep global warming below 2 degrees Celsius. Reducing uncertainty about climate sensitivity helps us develop a more reliable and accurate strategy for dealing with climate change. “
The researchers found that both the high and low climate sensitivity models correspond to global temperatures observed in the 20th century. However, the models with higher sensitivity contain a stronger cooling effect due to the interaction between aerosol and cloud, which offsets the increased warming due to greenhouse gases. In addition, the models show aerosol emissions that mainly occur in the northern hemisphere, which is inconsistent with the observations.
“Our results remind us that we should be careful about a model result, even if the models accurately reflect past global warming,” said first author Chenggong Wang, Ph.D. Candidate in Princeton’s Atmospheric and Ocean Sciences Program. “We show that the global average hides important details about the patterns of temperature change.”
In addition to key findings, the study helps shed light on how clouds can mitigate warming both in models and in the real world on a large and small scale.
“Clouds can add to global warming and cause warming to accelerate rapidly over the next century,” said co-author Wenchang Yang, associate research scholar in Geosciences at Princeton. “In short, improving our understanding and ability to correctly simulate clouds is really key to making more reliable predictions about the future.”
Scientists at Princeton and other institutions have recently focused on the effects of clouds on climate change. Related research includes two articles by Amilcare Porporato, Thomas J. Wu ’94 of Princeton, Professor of Civil and Environmental Engineering and the High Meadows Environmental Institute, and a member of the CMI leadership team reporting on the future effects of heat-induced clouds on solar energy and how climate models underestimate the cooling effect of the daily cloud cycle.
“Understanding how clouds modulate climate change is at the forefront of climate research,” said co-author Brian Soden, professor of atmospheric science at the University of Miami. “It is encouraging that, as this study shows, there are still many treasures that we can use from historical climate observations to refine the interpretations we get from the global change in mean temperature.”
The paper “Compensation between cloud feedback and aerosol-cloud interaction in CMIP6 models” was published in the February 28 issue of Geophysical Research Letters. The research was supported by the National Oceanic and Atmospheric Administration (grants NA20OAR4310393 and NA18OAR4310418) and the Carbon Mitigation Initiative of Princeton University’s High Meadows Environmental Institute (HMEI).