A powerful new model could make global warming estimates less vague

“A powerful new model could make global warming estimates less vague”
MIT Technology Review, April 24, 2019
Clean Energy, Climate Change
by Mallory Pickett

“One of the biggest sources of climate uncertainty is how clouds will behave. Caltech physicist Tapio Schneider is trying to give us some answers.”

The severity and speed of climate change will depend on the quantity of greenhouse gases we emit into the sky, but also on how sensitive the climate is to those gases.

One uncertainty is how clouds will respond as the atmosphere heats up. Tapio Schneider, a climate scientist at Caltech, and his colleagues from Caltech, the Naval Postgraduate School, JPL, and MIT are building a climate model that will use machine learning, powerful computing, and petabytes of data to help resolve some of the unknowns around how, why, and where clouds form, produce precipitation, or dissipate. The goal: to cut the uncertainty in current predictions of carbon dioxide’s impact on the planet in half.

Science journalist Mallory Pickett sat down with Schneider to find out how his research will do this, and why it matters.

How much uncertainty is there in current climate models?
There is a measurement called “climate sensitivity.” It’s the global mean surface temperature increase that you get after doubling CO2 concentrations and letting the system equilibrate. With current climate models, the climate sensitivity for doubling CO2 ranges somewhere between two degrees [Celsius] warming up to five degrees warming.

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Why the uncertainty?
The single biggest contributor is uncertainties about clouds, and specifically about low clouds in the tropics. Low clouds over tropical oceans reflect sunlight because they are white, and this cools the Earth. We don’t know if we’ll get more or fewer of them as it warms, and that’s the key uncertainty in climate predictions.

One other important piece is how much carbon is being taken out by the biosphere. Right now only about half the carbon that humans emit ends up in the atmosphere. The rest is taken up by oceans and the land biosphere, and we don’t quite know where it goes.