Rising emissions could one day trigger a tipping point that causes certain clouds to disappear, creating a 'hothouse' Earth scenario

Aleksandar Georgiev/Getty ImagesIf warming continues, subtropical stratocumulus clouds could disappear all together.
  • Clouds blanket 70% of the planet. These floating collections of ice crystals and water droplets also serve as natural shade for the Earth, reflecting sunlight to keep air and ocean temperatures down.
  • Stratocumulus clouds are particularly important to shading the open ocean, especially in the subtropics.
  • Researchers used a simulation to show that, if greenhouse gas emissions continue to warm the planet unabated, these clouds could disappear in the next 150 years.
  • The absence of these clouds could lead to another 14 degrees Fahrenheit increase in global temperatures, creating a ‘hothouse Earth’ climate.

If you’re sailing the subtropical open ocean and look up, it’s likely that you’ll be floating under swaths of lumpy, grey, and occasionally drizzling stratocumulus clouds.

These large bands of clouds – called cloud decks – help shade the Earth from the sun’s rays and increase the planet’s albedo, or its ability to reflect the sun’s energy and radiation back into space.

But if the planet continues to warm, these clouds may not be around for very long.

A new study simulated that, under “business as usual” conditions of rising carbon dioxide emissions that heat up the atmosphere, subtropical stratocumulus clouds could disappear in the next 100 to 150 years.

When the clouds are gone, the simulation shows, the Earth’s temperature could skyrocket as much as 8 degrees Celsius (14 degrees Fahrenheit).

This could force the planet into a “hothouse” climate akin to what the world looked like 56 million years ago, when the oceans boiled and rose, animals fled to the tropical North Pole, and severe storms battered a planet that warmed up over a short time.

The absence of these clouds could lead to a 20 degree Fahrenheit increase in global temperatures, creating a ‘hothouse’ climate on Earth

Stratocumulus clouds are unique among their peers – their cloud decks are sustained by cool air flitting over the top, and they sweat their excess heat back into the cooler atmosphere.

But if rising carbon dioxide levels trap more heat in the surrounding atmosphere, stratocumulus clouds can’t keep cool, Tapio Schneider, a scientist in the California Institute of Technology’s climate dynamics group and lead author of the new study, told Business Insider. That could cause to cloud decks break up into smaller cumulus clouds that don’t do much in the way of shade or reflecting sunshine back into space.

Using a supercomputer, Schneider’s group simulated how different concentrations of atmospheric carbon would impact the longevity of stratocumulus clouds. When the simulation hit 1,200 ppm (roughly three times the amount of carbon dioxide in the atmosphere today), these critical clouds vanished.

While 1,200 ppm seems impossible high, it’s a level that “business-as-usual” greenhouse-gas emissions could get us to in a century or so. Once the tipping point is reached, those clouds are gone for good until carbon dioxide emissions are reduced.

Ultimately, the planet is at risk of warming a total of 12 degrees Celsius (20 degrees Fahrenheit) in the next 150 years if stratocumulus clouds reach this tipping point, Schneider’s simulation shows.

Schneider said that the cloud decks would come back once carbon dioxide levels decreased below the tipping point mark. But he added that even if this occurred, it wouldn’t do much to reverse the effects of the heat wave on melting ice sheets and rising sea-levels that would have already happened.

Once clouds go away, the simulated climate “goes over a cliff,” Kerry Emanuel, a climate scientist at the Massachusetts Institute of Technology, told Quartz.

The simulation could solve a 56 million year old mystery

Schneider thinks this cloud tipping point could shed light on a mystery that’s long baffled researchers. Nearly 56 million years ago, the Earth experienced a “hothouse climate” in which there was a heat spike of 11 degrees Fahrenheit over 20,000 years. During that time, oceans hit 100-plus degrees Fahrenheit temperatures and the Arctic melted.

In order for the planet to have warmed that quickly, climate models show that atmospheric carbon dioxide levels would have had to be twice as high as scientists have found evidence for in the geologic record.


Read More:
World at risk of heading toward irreversible ‘hothouse’ state

Perhaps clouds were the missing piece of the puzzle.

If carbon dioxide levels reached the tipping point at which stratocumulus clouds disappeared, and the clouds’ absence contributed to additional warming, that might explain why the models and the evidence don’t match up.

“This has been a conundrum in climate modelling,” Schneider said. “But this could be that mechanism that explains how we had a climate that warm.”

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