One of the great mysteries of climate change is finally being solved


For over a decade, the greatest scientific uncertainty about how the planet will respond to warming has not been how much carbon dioxide will be absorbed by the ocean or absorbed by trees. It comes from clouds instead.

The fluffy, whimsical clusters of water droplets floating in the air have puzzled climate scientists and models alike for some time. Scientists have long known that the world could get even warmer or a little cooler, depending on how clouds respond to warming. You just didn’t know which one.

But in recent years, scientists have begun pinpointing exactly how clouds will change shape and location in the rapidly warming world. The result is good news for science – but not good news for mankind.

“We found evidence of the amplifying effect of clouds on global warming,” said Paulo Ceppi, climate scientist at Imperial College London.

Scientists have long known that clouds have two main influences on global climate. First, clouds are reflective – their white surfaces reflect the sun’s rays away from the earth, creating a cooling effect. (If the planet were suddenly free of those fluffy umbrellas, the planet would be about five times hotter than even the most catastrophic global warming projections.) But clouds also produce a warming effect — certain types of clouds insulate Earth’s radiation and keep the planet similarly warm such as carbon dioxide released when fossil fuels are burned.

Which effect is stronger depends on the type of cloud. Cirrus clouds—high, fine clouds visible in the distant atmosphere on relatively clear days—absorb and trap more radiation, warming the Earth. Stratus, or stratocumulus clouds — thick, fluffy clouds that often hover over the ocean on cloudy days — reflect more sunlight and cool the Earth.

Exactly how these two factors will balance out as the world warms is uncertain. This is mainly because while clouds can look gigantic – when you fly through them in an airplane or when you look at them from the ground – they are formed on a microscopic level when water vapor condenses around a dust particle or droplet. As a result, they are essentially impossible to model in the large standard climate models. (Clouds form at the micrometer level, while the models most climate scientists use divide the world into hundreds of blocks kilometers wide.)

“We’re having a really hard time simulating with any degree of accuracy how clouds actually behave in the real world,” said Timothy Myers, a postdoctoral researcher at Lawrence Livermore National Laboratory in California.

But in recent years, scientists have gained increasing clarity about what will happen to the clouds – and what is already happening – as the planet warms.

First, the tall, thin cirrus clouds that trap Earth’s radiation are expected to shift up the atmosphere into zones of lower temperatures. Thanks to a complicated relationship between clouds and Earth’s radiation, this will increase the amount of radiation that the cirrus clouds in the atmosphere will trap. “As they rise, their greenhouse effect, or warming effect on Earth, tends to increase,” Myers said.

This result has been known for about a decade and suggests clouds are likely strengthen global warming. But only in recent years have researchers also discovered that the number of low-lying stratus or stratocumulus clouds is expected to decrease as the planet continues to warm. A study in the journal Nature Climate Change used satellite observations to find out how cloud formation is affected by sea temperatures, wind speed, humidity and other factors — and then analyzed how those factors will change as the world warms.

“We concluded that as the ocean warms, the low clouds over the oceans clear up,” said Myers, one of the authors of the study. This means there will be fewer clouds to reflect sunlight and cool the Earth – and the change in low cloud will also increase global warming.

Another article in the Proceedings of the National Academy of Sciences found a similar result, also using observational techniques. Research based on high-resolution models — which are better able to model cloud formation than larger-scale general climate models — has also found that clouds are likely to increase global warming.

Researchers have also begun to understand how clouds are affected by certain changes beyond warming – such as reductions in artificial aerosols in the atmosphere. Clouds form around particles such as aerosols suspended in the atmosphere; It’s therefore possible that low clouds would have receded even more had it not been for human-caused air pollution. According to another study published last month in the Proceedings of the National Academy of Sciences, sulfate aerosols have driven cloud formation, masking some of the global warming that has already occurred. “There is a possibility that by eliminating air pollution, we will unmask global warming,” said Casey Wall, a postdoctoral researcher at the University of Oslo.

Taken together, these new findings have helped scientists estimate how much the planet will warm if atmospheric carbon dioxide emissions doubled from pre-industrial levels. (Before the Industrial Revolution, CO2 concentration was about 280 parts per million, or ppm; now it has reached 412 ppm and is continuing to rise.) Scientists once estimated that the temperature would rise between 1.5 and 4 degrees if CO2 levels would reach 560ppm Celsius – a range that spans a “still very livable planet” to “nearly apocalyptic levels of warming”.

Much of this uncertainty stems from the cloud issue. Because clouds already have such a large impact on Earth’s climate, even small changes in clouds as the world warms can have large impacts on future temperature changes.

The new cloud research shows that the lower estimates for warming are highly unlikely. Instead, the latest papers estimate that CO2 concentrations of 560 ppm would likely result in warming of at least 3 or 3.5 degrees.

That doesn’t mean the world will definitely reach 3 degrees warming – if countries continue to switch to clean energy, CO2 in the atmosphere could be stabilized at levels well below 560ppm. But it means the most optimistic estimates of how warming will play out have been taken off the table.

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