Stratocumulus clouds may not be the showiest puffs in the International Cloud Atlas, but they are the workhorses of the atmosphere. The low, flat decks of clouds—also known as marine layers—cover over 20 percent of subtropical oceans and reflect about 30 percent of the sun’s light, keeping the planet much cooler than it would otherwise be. But a new climate model suggests that rising carbon dioxide concentrations in the atmosphere could disrupt stratocumulus formation, leading to dramatic rises in Earth’s surface temperature, as much as 14 degrees Fahrenheit.
Joel Achenbach at The Washington Post reports that clouds are an important but frustrating part of climate modeling. Depending on their location, type and quantity, they can either trap heat or help reflect it. Accurately modeling cloud behavior, however, takes a lot of computing power and the air currents that sustain clouds are too small to add into global climate models.
That’s why researchers decided to simplify things, modeling a five-by-five kilometer section of cloud above California’s subtropical ocean on a supercomputer. As they increased the concentration of CO2 in their models, they saw a surprising effect. At levels over 1,200 parts per million of carbon dioxide, the stratocumulus clouds were no longer able to form their large, flat, reflective sheets—instead breaking into puffier clouds. Emiliano Rodriguez Mega at Nature reports that’s because to maintain their shape, stratocumulus clouds need to continuously radiate heat into the upper atmosphere. If the air temperature gets too warm, they can no longer do this and break apart. The paper appears in the journal Nature Geosciences.
Currently, global CO2 levels are at 410 ppm, up from about 280 ppm before the start of the Industrial Revolution. While exceeding 1,200 ppm sounds unlikely, it’s where the atmosphere is headed in about a century at humanity’s current pace of carbon pollution. “I think and hope that technological changes will slow carbon emissions so that we do not actually reach such high CO2 concentrations,” lead author Tapio Schneider of the Jet Propulsion Laboratory at Caltech says in a press release. “But our results show that there are dangerous climate change thresholds that we had been unaware of.”
Schneider says the 1,200 ppm threshold for the cloud breakup is just a rough estimate. And because so many elements of the climate model were simplified in the new model, Matthew Huber, a palaeoclimatologist at Purdue University, tells Mega at Nature it’s hard to say with certainty how accurate the new cloud model may be.
But the findings are not pie in the cloudless sky.“It’s not bonkers,” Andrew Ackerman, a cloud researcher at NASA’s Goddard Institute for Space Studies not involve in the study tells Mega. “The underlying mechanism is totally plausible.”
If the model holds true, it could explain a strange period in Earth’s past known as the Paleocene Eocene Thermal Maximum about 55 million years ago. During that period, the world warmed so much that the Arctic melted and was even home to crocodiles. For such a dramatic event to occur, current climate models say carbon dioxide levels would have to reach 4,000 ppm, which is about twice the CO2 levels researchers have found in the geologic record. However, if rising CO2 led to the loss of stratocumulus clouds, that could explain the unusual heat spike. The ebb and flow of clouds could also help explain other unusual heat spikes in Earth’s climate history.
“Schneider and co-authors have cracked open Pandora’s box of potential climate surprises,” Huber tells Natalie Wolchover at Quanta Magazine. “All of a sudden this enormous sensitivity that is apparent from past climates isn’t something that’s just in the past. It becomes a vision of the future.”