Marine clouds that protect us from hothouse Earth conditions by reflecting sunlight back into space could break up and vanish if CO2 in the atmosphere triples, researchers warned Monday.
"Our results show that there are dangerous climate change thresholds of which we were unaware," Tapio Schneider, a scientist at the Jet Propulsion Laboratory in Pasadena, California and lead author of a study detailing the findings, told AFP.
So-called stratocumulus clouds cover about 20 percent of subtropical oceans, mostly near western seaboards such as the coasts of California, Mexico and Peru.
"When they disappear, Earth warms dramatically, by about eight degrees Celsius (14 degrees Fahrenheit) -- in addition to the global warming that comes from enhanced greenhouse concentrations alone," according to the study.
A temperature increase of that magnitude would melt polar ice and lift sea levels tens of metres.
The last time the planet was that hot, some 50 million years ago during the Eocene Epoch, crocodiles roamed the Arctic.
Even half that much warming would overwhelm humanity's capacity to adapt, scientists say.
A barely one-degree increase since the mid-19th century -- mostly in the last 50 years -- has been enough to worsen heatwaves, droughts, and flooding, along with cyclones engorged by rising seas.
The 2015 Paris climate treaty enjoins nations to cap the rise in temperatures at "well below" 2C.
A major UN report published in October, however, showed that even a 2C hike over the preindustrial benchmark will have dire impacts, such as the demise of shallow-water coral reefs that sustain a quarter of marine biodiversity.
Since manmade global warming began, CO2 concentration in the air has gone up nearly 45 percent, from 285 to 410 parts per million (ppm).
Using an innovative approach to modelling the behaviour of the stratus clouds, Schneider and his colleagues calculated that protective cloud cover could break up if CO2 levels reached 1200 ppm, though the "tipping point" might be somewhat higher.
Passing that threshold -- three times present levels -- may not be as far-fetched as it seems, scientists caution.
Even after 30 years of dire warnings about the consequences of climate change, global greenhouse gas emissions are still climbing year-on-year.
At current rates of CO2 pollution -- known as the "business as usual" scenario -- "the 1200 ppm value will be crossed by 2104," Malte Meinshausen, director of the Climate and Energy College at the University of Melbourne, told AFP, citing a study to be published later this year.
A second worry is that manmade global warming may trigger the release of CO2 and methane -- another powerful greenhouse gas -- from natural sources such as permafrost, overwhelming our efforts to reduce emissions.
The Eocene hothouse
The findings, published in Nature Geosciences, may solve a long-standing mystery about the early Eocene Epoch 50 million years ago, when temperatures were about 12C warmer than today.
Climate models indicate that CO2 levels would have had to rise above 4,000 ppm to explain that level of warming, but geological evidence shows concentrations only 25-to-50 percent that high.
This 1,000-to-2,000 ppm range, according to the new study, in precisely where planet-cooling clouds would disappear, boosting temperatures by up to 8C.
"A warming spike caused by the loss of stratus cloud decks could explain the appearance of the Eocene's hothouse climate," Schneider said.
The study also help plug a huge hole in current climate change models.
How low clouds over tropical oceans respond to global warming is probably the largest contributor to uncertainties in climate predictions.
Clouds have mostly "fallen through the computational cracks," said Schneider.
"The scales of motion in clouds are metres to tens-of-metres," he explained. "The computational meshes of climate models are tens to hundreds of kilometres wide."
To get around that problem, the researchers created a small-scale model or a typical area above a subtropical ocean, simulating the clouds and their turbulent, ever-changing motion on supercomputers.
In simulations, the cloud "decks" broke up when CO2 levels hit 1,200 ppm, and did not reform until the concentration of carbon dioxide dropped well below that threshold.
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