Compared with previous generations, current Earth system models predict that Earth’s climate is more sensitive to carbon dioxide. Where does the increased sensitivity come from?

Earth system models represent our best attempts at simulating the global climate. They are far from perfect, but they’re improving all the time, and they allow researchers to ask interesting what-if questions about Earth’s future. Pressing among these questions is, How much hotter would it get if we instantaneously doubled atmospheric carbon dioxide (CO 2 )?

This measure—the increase in global average surface temperature with a doubling of CO 2 —is known as equilibrium climate sensitivity, and it is heavily debated among scientists. State-of-the-art global climate models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) have equilibrium climate sensitivities ranging from 1.8°C to 5.6°C. On average, these sensitivities are nearly 0.6°C higher than the average predictions from the previous generation of models (i.e., CMIP5). In a new study, Zelinka et al. search for the source of the change.

Ultimately, the scientists traced the increased sensitivity to changes in how clouds respond to greenhouse warming in the southern middle latitudes (30°S–50°S). Specifically, in the latest models, both cloud areal coverage and water content decrease at these latitudes. With fewer bright white clouds reflecting sunlight back into space, the planet absorbs more energy and gets hotter, and even fewer clouds form. This amplifying feedback, the researchers argue, is responsible for the bulk of the increased climate sensitivity seen in the current models.

The next question the researchers asked was what was causing the model ensemble to predict stronger extratropical cloud loss. The answer lies in how modeled clouds respond to their environment. Specifically, in previous models, warmer ocean surface temperatures led to increased cloud cover and water content. In the latest models, this sensitivity has decreased substantially. This, in turn, may be the result of models better representing the mixture of ice crystals and liquid droplets making up clouds at subfreezing temperatures. Larger proportions of liquid relative to ice have previously been shown to lead to stronger amplifying cloud feedbacks upon warming.

The researchers stress that the latest models still need to be better evaluated using real-world observations, but if they simulate nature more faithfully than earlier models, the suggestion is that Earth’s climate is even more sensitive to increases in CO 2 than previously thought. (Geophysical Research Letters, https://doi.org/10.1029/2019GL085782, 2020)

—David Shultz, Science Writer