Carbon Dioxide Could Make Clouds Vanish, Raising Global Temperatures by 8 C

Increasing concentrations of atmospheric carbon dioxide could lead to the disappearance of stratocumulus cloud decks. This could result in global temperatures increasing by 8 degrees Celsius—on top of warming from greenhouse gas emissions.

In a paper published in Nature Geoscience, researchers have modeled the impact atmospheric CO2 has on stratocumulus clouds. These clouds are characterized as being large, rounded masses that form in groups—or decks. They are found at a lower height compared to other types of cloud, normally sitting below 6,600 feet.

As the researchers point out, these clouds cover about 20 percent of the low-latitude oceans and are known to cool the Earth by shading huge portions of its surface from the Sun. Despite playing a role in limiting global warming, little is known about how they will react to the climate change predicted as a result of greenhouse gas emissions.

Stratocumulus clouds are different to other clouds, as instead of being sustained by heating from the surface of the Earth they are maintained by cooling at the top. This makes them more vulnerable to increasing levels of greenhouse gasses in the atmosphere.

In the study, scientists tried to assess how these clouds will react to warming in the future. The team created models that represent the atmospheric section of the subtropics where stratocumulus clouds are found. They simulated cloud motions over this patch of Earth using supercomputers. Findings showed the clouds became unstable when CO2 levels exceeded 1,200 ppm (parts per million). At this point, the clouds vanished and only returned when CO2 levels were considerably lower.

Once the clouds had disappeared, the models showed a huge spike in surface warming, with global temperatures increasing by 8 C.

At present, the level of CO2 in the atmosphere is around 410 ppm. However, it is rising fast and—if emissions continue at their current pace—will reach the 1,200 threshold at some point in the next century.

"I think and hope that technological changes will slow carbon emissions so that we do not actually reach such high CO2 concentrations," study author Tapio Schneider, from the California Institute of Technology, said in a statement. "But our results show that there are dangerous climate change thresholds that we had been unaware of."

As well as providing an insight into future warming, the researchers also say the findings help shed light on the last major period of global warming, some 56 million years ago. During this time there was a major spike in CO2 emissions and the global temperature got so warm the Arctic was ice-free. How and why this happened has been a long-standing mystery—and one that the latest findings could go some way to explaining.

The study follows research that shows current CO2 emissions are at their highest level since the last period of warming—the Paleocene-Eocene Thermal Maximum (PETM). At the peak of the PETM, global temperatures were about 23 C, which is 7 degrees higher than they are now.