Rising carbon emissions will drive the clouds from the sky

The clouds may disappear from the skies within the next century, which would cause temperatures to leap by as much as 12C and force most of humanity to move to the polar ice caps.

That’s according to a state-of-the-art supercomputer simulation that shows a feedback loop between global warming and cloud loss that could push Earth’s climate past a disastrous tipping point.

As bne IntelliNews reported, one of the consequences of global warming is the increase in rainfall that has already lead to torrential rain and flash flooding around the globe this disaster season. Water vapour is a greenhouse gas (GHG) and as temperatures rise more water evaporates and adds to the warming affect in an alarming positive feedback loop.

However, as the share of CO₂ in the atmosphere builds up, that will prevent the clouds from forming, starting another positive, and potentially catastrophic, feedback loop. And this has already happened once in the Earth’s long history, causing a mass extinction event.

Physicists have struggled since the 1960s to understand how global warming will affect the many different kinds of clouds, and how that will influence global warming in turn. How rising temperatures will affect cloud cover, which is responsible for cooling the planet by several degrees, is one of the biggest unknowns in the climate crisis riddle.

Clouds currently cover about two-thirds of the planet, but computer simulations 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, and runaway warming, according to a paper “Possible climate transitions from breakup of stratocumulus decks under greenhouse warming” published in Nature Geoscience by Tapio Schneider, Colleen M. Kaul and Kyle G. Pressel a few years ago.

“Stratocumulus clouds cover 20% of the low-latitude oceans and are especially prevalent in the subtropics. They cool the Earth by shading large portions of its surface from sunlight,” the paper said. “In the simulations, stratocumulus decks become unstable and break up into scattered clouds when CO₂ levels rise above 1,200 ppm… Once the stratocumulus decks have broken up, they only re-form once CO₂ concentrations drop substantially below the level at which the instability first occurred. Climate transitions that arise from this instability may have contributed importantly to hothouse climates and abrupt climate changes in the geological past.”

One of those past events was, until now, a still inexplicably brief, cataclysmic hot spell 56mn years ago, now known as the Paleocene-Eocene Thermal Maximum (PETM). After heat-trapping carbon leaked into the sky from an unknown source, the planet rapidly gained an additional 6 degrees. The oceans turned jacuzzi-hot near the equator and there were mass extinctions worldwide. Temperatures at the two poles were subtropical.

“Such transitions to a much warmer climate may also occur in the future if CO₂ levels continue to rise,” the cloud break-up paper concluded.

But there is still some way to go before the world reaches this point. Cloud disappearance occurs when the concentration of CO₂ in the simulated atmosphere reaches 1,200 parts per million, but as bne IntelliNews reported recently in a look at the world’s carbon budget, the current concentration is 427 ppm as of April – a 7% increase since the Paris Agreement -- although it continues to rise at an alarming rate.

Emissions of the three most damaging GHGs are already at an all-time high, but one potential step-up event that could dramatically increase the amount of carbon in the atmosphere is that Russia’s permafrost is melting and once the ground reaches zero degrees – that will happen sometime in the next 10-30 years – gigatonnes of primoradial CO₂ that have been frozen for millennia will be released into the atmosphere in one go.

Once clouds disappear, the computer-simulated climate “goes over a cliff,” Kerry Emanuel, a climate scientist at the Massachusetts Institute of Technology, told Quanta magazine in an interview. Losing the clouds is a catastrophic tipping point that could see temperatures increase by an enormous 12C. Most of the equatorial regions would therefore become uninhabitable and all of humanity would be crowded into the polar regions.

The planet will recover, but the PETM episode lasted 200,000 years.