Clouds play a significant role in the heating and cooling of the planet. They warm the Earth’s surface by absorbing the sun’s infrared radiation, and they cool it by reflecting sunlight back into space. Clouds are at the core of the water cycle, forming when warm air rises and cools, condensing around dust grains and other particles in the atmosphere, then releasing their heavy loads in the form of precipitation. Clouds cover two-thirds of the planet's surface at any given time, and their behavior helps create the conditions that make life as we know it possible.
And they could be in trouble. That's the dramatic conclusion drawn by a new study.
Thanks to computer simulations of cloud behavior, we know that clouds dissipate as CO2 concentrations rise. A dramatic loss in cloud cover could create a climate feedback loop. But there are a lot of unanswered questions here, and we don’t know is precisely how clouds, or the lack of them, will impact the progression of climate change.
In an written for Quanta Magazine, Natalie Wolchover writes, “For decades, clouds have been seen as by far the biggest source of uncertainty over how severe global warming will be—other than what society will do to reduce carbon emissions.” When you consider the influence of “heat, evaporation, turbulence, radiation, wind, geography” and ocean currents on clouds, it becomes difficult to accurately predict how the confluence of all these interactive and ever-changing forces will play out. Clouds are, in fact, the one factor that climate research centers around the world cannot agree upon when building models to predict a global rise in temperature.
A new suite of models, by climate physicists at CalTech, may change all that. The study focused on the “breakup of stratocumulus decks” over low-latitude oceans in response to greenhouse warming. The scientists chose stratocumulus clouds for their simulation because they are thick and they shade large areas in the sub-tropics, providing significant cooling for the planet.
Their simulation found that the clouds break apart when CO2 levels rise above 1,200 parts per million. The study goes on to explain that the absence of these cloud decks in the simulations “triggers a surface warming of about 8 K globally [that's 8 degrees Kelvin, equal to 14 degrees Fahrenheit] and 10 K in the subtropics” on top of greenhouse warming. Once the decks have broken up, CO2 concentrations would need to drop well below 1,200 ppm for the clouds to reestablish themselves.
Today, atmospheric CO2 concentrations measure above 410 ppm. According to the least aggressive climate models, that’s slated to double preindustrial levels, reaching 560 ppm by 2050. If we continue on our current emissions trajectory, CO2 levels would reach 1,200 ppm in a hundred years. Translate all this into temperatures, and we’re looking at an increase of 2 degrees Celsius by 2050 relative to preindustrial times. That’s a best-case scenario. More aggressive models predict double that in the same amount of time.
What’s the discrepancy between the best- and worst-case models? The latter account for a loss of cloud coverage.
Earth has experienced drastic temperature increases in its ancient past. Of note, the Paleocene-Eocene Thermal Maximum (PETM), which occurred 56 million years ago and resulted in a six-degree rise. The event, also stimulated by heat-trapping gases, was far more extreme in reality than theoretical models would suggest. Now we have new insights as to why: Those PETM models, like some of today’s milder 2-degree-increase predictions, didn’t factor in the absence of a cooling blanket of clouds.
“I think and hope that technological changes will slow carbon emissions so that we do not actually reach such high CO2 concentrations,” says Tapio Schneider, Professor of Environmental Science and Engineering and senior research scientist at NASA’s Jet Propulsion Laboratory at Caltech, and lead author of the study. “But our new results show that there are dangerous climate change thresholds that we had been unaware of.”
The upside: We can plan for those dangerous thresholds. Scientists estimate that the planet can absorb about 2 billion tons of carbon annually on top of what’s naturally recycled through Earth’s natural systems. If we can reduce our carbon emissions to 2 billion tons per year with advances in renewable energy, innovative agricultural practices, and new human settlement strategies, we can stop global warming.