New climate models with high climate sensitivity are not plausible

A recent analysis of the latest generation of climate models – known as CMIP6 – provides a caveat on interpreting climate simulations as scientists develop more sensitive and sophisticated projections of how the Earth will react to levels. increasing amounts of carbon dioxide in the atmosphere.

Researchers from Princeton University and the University of Miami reported that newer models with high “climate sensitivity” – meaning they predict much greater global warming from the same levels of atmospheric carbon dioxide as other models – do not. not provide a plausible scenario of Earth’s future climate.

These models overestimate the global cooling effect that results from interactions between clouds and aerosols and project that clouds will moderate the warming induced by greenhouse gases – especially in the northern hemisphere – much more than what the climate records actually show, the researchers reported in the journal. Geophysical research letters.

The researchers found that models with lower climate sensitivity are more consistent with observed temperature differences, especially between the northern and southern hemispheres. The graph shows the changes in the annual global average surface temperature (a) and the temperature difference between the northern and southern hemispheres (b) from 1850 to 2000. The red line represents models with high climate sensitivity, while the line blue represents models with low climate sensitivity. The black line shows the observed temperature fluctuations collected by NASA’s Goddard Institute for Space Studies Surface Temperature Analysis Project, which follow the blue line more closely with respect to interhemispheric temperature. Gray backgrounds indicate years when the difference between the high and low climate sensitivity models is significant. Credit: Image by Chenggong Wang, Atmospheric and Oceanic Sciences Program, Princeton University

Instead, the researchers found that models with lower climate sensitivity are more consistent with the observed temperature differences between the northern and southern hemispheres and, therefore, are more accurate representations of projected climate change than the models. more recent. The study was supported by the Carbon Mitigation Initiative (CMI) based at the High Meadows Environmental Institute (HMEI) in Princeton.

These findings are potentially important for climate change policy, explained co-author Gabriel Vecchi, professor of geosciences at Princeton and the High Meadows Environmental Institute and senior researcher at CMI. Because models with higher climate sensitivity predict greater warming in greenhouse gas emissions, they also predict more serious – and imminent – consequences such as more extreme sea level rise and waves. heat.

Highly sensitive climate models predict global average temperature increase of 2-6 degrees Celsius under running carbon dioxide levels. The current scientific consensus is that the increase should be kept below 2 degrees to avoid catastrophic effects. The 2016 Paris Agreement sets the threshold at 1.5 degrees Celsius.

“Higher climate sensitivity would obviously require much more aggressive carbon mitigation,” Vecchi said. “The company would need to reduce its carbon emissions much faster to meet the Paris Agreement targets and keep global warming below 2 degrees Celsius. Reducing the uncertainty of climate sensitivity helps us develop a more reliable and precise strategy for dealing with climate change. “

The researchers found that the models of high and low climate sensitivity correspond to global temperatures observed during the 20th century. Models of higher sensitivity, however, include a stronger cooling effect of the aerosol-cloud interaction which compensates for the greater warming due to greenhouse gases. In addition, the models have aerosol emissions occurring mainly in the northern hemisphere, which is not consistent with the observations.

“Our results remind us that we need to be careful about the outcome of a model, even if the models accurately represent past global warming,” said first author Chenggong Wang, PhD. candidate for Princeton’s program in atmospheric and oceanic sciences. “We show that the global average hides important details about the patterns of temperature change.”

In addition to the main findings, the study helps shed light on how clouds can moderate warming both in models and in the real world on a large and small scale.

“Clouds can amplify global warming and cause warming to accelerate rapidly over the next century,” said co-author Wenchang Yang, associate geoscience researcher at Princeton. “In short, improving our understanding and our ability to correctly simulate clouds is really the key to more reliable predictions of the future. “

Scientists at Princeton and other institutions have recently focused on the effect of clouds on climate change. Related research includes two papers by Amilcare Porporato, Professor Thomas J. Wu ’94 of Civil and Environmental Engineering from Princeton and the High Meadows Environmental Institute and a member of the CMI leadership team, which reported on the future effect of heat-induced clouds on solar energy. power 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 sciences at the University of Miami. “It is encouraging that, as this study shows, there are still many treasures that we can exploit from historical climate observations that are helping to refine the interpretations we are getting of the global average temperature change. “

Reference: “Compensation Between Cloud Feedback and Aerosol-Cloud Interaction in CMIP6 Models” by Chenggong Wang, Brian J. Soden, Wenchang Yang and Gabriel A. Vecchi, January 25, 2021, Geophysical research letters.
DOI: 10.1029 / 2020GL091024

The article appeared in the February 28 edition of Geophysical research letters. The research was supported by the National Oceanic and Atmospheric Administration (grants NA20OAR4310393 and NA18OAR4310418) and the Carbon Mitigation Initiative based at the High Meadows Environmental Institute (HMEI) at Princeton University.