High level of climate sensitivity in new climate models considered less 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 at Princeton University and the University of Miami have reported that newer models with high “climate sensitivity” – that is, they predict much greater global warming from same levels of atmospheric carbon dioxide as other models – do 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 predict that clouds will moderate the warming induced by greenhouse gases – especially in the northern hemisphere – much more than climate records show, the researchers reported in the journal. Geophysical research letters.

Instead, the researchers found that models with low 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 more recent models. . 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 High Meadows Environmental Institute and senior researcher at CMI. Because the more climate-sensitive models predict greater warming from greenhouse gas emissions, they also project more dire – and imminent – consequences such as more extreme sea level rise and sea waves. heat.

Highly sensitive climate models predict an increase in the global average temperature of 2 to 6 degrees Celsius below current levels of carbon dioxide. 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. “Society should reduce carbon emissions much faster to meet Paris Agreement goals and keep global warming below 2 degrees Celsius. Reducing uncertainty in climate sensitivity helps us strategize more reliable and more precise in 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. Higher sensitivity models, however, include a stronger cooling effect of the aerosol-cloud interaction which compensates for the greater warming from 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 faithfully represent past global warming,” said lead author Chenggong Wang, a Ph.D. candidate for Princeton’s science program. atmospheric and oceanic. “We show that the global average hides important details about the patterns of temperature change.”

In addition to the main findings, the study provides insight into 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 can 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 High Meadows Environmental Institute and a member of the CMI leadership team, who reported on the future effect of heat-induced clouds on solar energy. power and how climate models underestimate the cooling effect of the daily cloudy 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.”

The article, “Compensation Between Cloud Feedback and Aerosol-Cloud Interaction in CMIP6 Models”, was published in the February 28 edition of Geophysical research letters