A recent report on climate simulations show that global warming could break up stratocumulus clouds [nature.com], letting in more energy as High CO2 levels break up startocumulus cloud decks [quantamagazine.org], once the levels rise above 1,200 ppm. Stratocumulus provide no precipitation but do cover about 20% of the low-latitude oceans and are especially prevalent in the subtropics, cooling by providing shade. If they disappear then, according to calculations, the added sunlight hitting the ground or ocean would increase temperatures by over 8°C.
Now, new findings reported today [nature.com] in the journal Nature Geoscience make the case that the effects of cloud loss are dramatic enough to explain ancient warming episodes like the PETM — and to precipitate future disaster. Climate physicists at the California Institute of Technology performed a state-of-the-art simulation of stratocumulus clouds, the low-lying, blankety kind that have by far the largest cooling effect on the planet. The simulation revealed a tipping point: a level of warming at which stratocumulus clouds break up altogether. The disappearance occurs when the concentration of CO2 in the simulated atmosphere reaches 1,200 parts per million — a level that fossil fuel burning could push us past in about a century, under “business-as-usual” emissions scenarios. In the simulation, when the tipping point is breached, Earth’s temperature soars 8 degrees Celsius, in addition to the 4 degrees of warming or more caused by the CO2 directly.
Once clouds go away, the simulated climate “goes over a cliff,” said Kerry Emanuel [mit.edu], a climate scientist at the Massachusetts Institute of Technology. A leading authority on atmospheric physics, Emanuel called the new findings “very plausible,” though, as he noted, scientists must now make an effort to independently replicate the work.
To imagine 12 degrees of warming, think of crocodiles swimming in the Arctic and of the scorched, mostly lifeless equatorial regions during the PETM. If carbon emissions aren’t curbed quickly enough and the tipping point is breached, “that would be truly devastating climate change,” said Caltech’s Tapio Schneider [climate-dynamics.org], who performed the new simulation with Colleen Kaul [climate-dynamics.org] and Kyle Pressel [climate-dynamics.org].
Huber said the stratocumulus tipping point helps explain the volatility that’s evident in the paleoclimate record. He thinks it might be one of many unknown instabilities in Earth’s climate. “Schneider and co-authors have cracked open Pandora’s box of potential climate surprises,” he said, adding that, as the mechanisms behind vanishing clouds become clear, “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.”