Natalie Wolchover writes:

Clouds currently cover about two-thirds of the planet at any moment. But computer simulations of clouds have begun to suggest that as the Earth warms, clouds become scarcer. With fewer white surfaces reflecting sunlight back to space, the Earth gets even warmer, leading to more cloud loss. This feedback loop causes warming to spiral out of control.

For decades, rough calculations have suggested that cloud loss could significantly impact climate, but this concern remained speculative until the last few years, when observations and simulations of clouds improved to the point where researchers could amass convincing evidence.

Now, new findings reported today 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 [the Paleocene-Eocene Thermal Maximum, a cataclysmic hot spell 56 million years ago] — 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, a climate scientist at the Massachusetts Institute of Technology. [Continue reading…]