Cloudy, with a chance of fewer clouds

Thick, beautiful, and potentially vulnerable: stratocumulus clouds.

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Elevated levels of carbon dioxide in the upper atmosphere could trigger the break-up of huge blankets of stratocumulus clouds, resulting in an additional eight degrees Celsius global warming, modelling suggests.

Stratocumulus clouds blanket about 20% of the Earth and play an important role in maintaining the planet’s energy balance. They are sustained through a cooling process occurring at their upper levels, meaning that they are potentially vulnerable to the effects of increasing carbon dioxide levels.

The effects of atmospheric changes on cloud structures are difficult to determine in climate models because of their relatively small size. However, researchers led by Tapio Schneider of the California Institute of Technology, US, have constructed high-resolution simulations that succeed in resolving likely outcomes for the vast stratocumulus spreads.

And the news is not good. Beyond a certain level of carbon dioxide concentration the clouds will start to fracture, allowing more sunlight to directly impact the planet surface, boosting the global mean surface temperature.

Schneider and colleagues suggest that the break-up of stratocumulus cloud decks may represent a so far underestimated driver of major climate change. They suggest that the phenomenenon may have contributed to past hothouse climates, such as the Eocene, 50 million years ago.

Fortunately, however, if commitments by most countries in the world to cut greenhouse gas emissions in line with the Paris Agreement are honoured, the modelling predictions may never be put to the test.

The researchers pin the tipping point carbon dioxide concentration for stratocumulus break up at 1200 parts per million (ppm). Current levels hover around 400ppm.

The research is published in the journal Nature Geoscience.

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