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Clouds have an amplifying effect on global warming

For climatologists, one of the biggest unknowns in predicting anthropogenic climate change is the effect of clouds. Now, a new study led by researchers at Imperial College London suggests that cloud feedback amplifies global heating. 

One of the goals of climatology is to model Earth’s climate and simulate how it may change. With the complex effects of anthropogenic climate change looming, one of the most important indicators that experts can use to predict global warming is known as “climate sensitivity.” 

This particular index tells us how the planet’s thermostat will adjust when we double our pre-industrial carbon dioxide level of 240 parts per million (ppm). The higher that number, the warmer the planet gets. 

For those keeping score at home, we’re currently sitting at 420 ppm, with that number increasing about 2.5 ppm every year, according to the American Meteorological Society’s 2019 State of the Climate.

“The value of the climate sensitivity is highly uncertain, and this translates into uncertainty in future global warming projections and in the remaining ‘carbon budget’ – how much we can emit before we reach common targets of 1.5°C or 2°C of global warming,” explained study co-author Dr. Paulo Ceppi.

Clouds that float closer to the ground tend to have a cooling effect, because they keep sunlight from directly reaching the Earth. Clouds higher up in the atmosphere let sunlight hit the ground, while also trapping energy emitted from our planet – enhancing the greenhouse effect. 

In collaboration with study co-author Dr. Peer Nowack, Dr. Ceppi looked at temperature and cloud cover data from satellite measurements. They used this data in a statistical learning analysis, a machine learning program that attempts to predict behavior from raw data. The program tells the researchers how clouds will react to a warming climate. 

“There’s been a growing amount of evidence that clouds probably have an amplifying effect on global warming,” said Dr. Nowack. “However, our new approach allowed us for the first time to derive a global value for this feedback effect using only the highest quality satellite data as our preferred line of evidence.”

The prognosis is not promising. The study found with more than 97.5 percent probability that clouds will exacerbate the warming of the planet by enhancing the greenhouse effect and by reflecting less sunlight. According to the researchers’ model, this will ultimately raise the global mean temperature by 3.2°C.

“Our paper makes a major step towards narrowing the most important uncertainty factor in climate sensitivity projections,” said Dr. Nowack. “As such, our work also highlights a new pathway in which machine learning methods can help us constrain the key remaining uncertainty factors in climate science.” 

The experts hope that by specifying those constraints, they can better estimate the future effects of global warming.

The study is published in the journal Proceedings of the National Academy of Sciences.

By Alex Ruger, Staff Writer

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