Highlight of climate sensitivity in new climate models is considered less acceptable

A recent analysis of the latest generation of climate models – known as a CMIP6 – provides a warning about the interpretation of climate simulations, as scientists develop more sensitive and sophisticated projections of how the earth will respond to increasing levels of carbon dioxide in the atmosphere.

Researchers at Princeton University and the University of Miami have 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 provide a plausible scenario for the future. climate of the earth. .

These models overestimate the global cooling effect resulting from interactions between clouds and aerosols and project that clouds will moderate greenhouse gas emissions – especially in the northern hemisphere – far beyond what climate records show, the researchers report in the magazine Geophysical research letters.

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

These findings are potentially important when it comes to climate change policy, explained co-author Gabriel Vecchi, a professor of geosciences at Princeton, and the High Meadows Environmental Institute and lead researcher at CMI. Because models with higher climate sensitivity predict greater warming due to greenhouse gas emissions, they also project more serious and threatening consequences such as more extreme sea level rise and heat waves.

The high climate sensitivity models predict an increase in the world average temperature of 2 to 6 degrees Celsius below the current carbon dioxide levels. The current scientific consensus is that the increase should be kept below 2 degrees to prevent catastrophic consequences. The Paris Agreement of 2016 sets the threshold at 1.5 degrees Celsius.

“A higher climate sensitivity will obviously necessitate much more aggressive carbon mitigation,” Vecchi said. “Society will have to reduce carbon emissions much faster to achieve the goals of the Paris Agreement and keep global warming below 2 degrees Celsius. Reducing climate uncertainty is helping us to develop a more reliable and accurate strategy to tackle climate change. “

The researchers found that the high and low climate sensitivity models correspond to the world temperatures observed during the 20th century. However, the higher sensitivity models contain a stronger cooling effect of aerosol-cloud interaction that compensates for the greater heating due to greenhouse gases. Furthermore, the models have aerosol emissions that occur mainly in the northern hemisphere, which do not correspond to observations.

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

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

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

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

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