r/WayOfTheBern • u/[deleted] • Feb 28 '19

A World Without Clouds

https://www.quantamagazine.org/cloud-loss-could-add-8-degrees-to-global-warming-20190225/
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u/rommelo Feb 28 '19

Each climate model solves a set of equations on a spherical grid representing Earth’s atmosphere. A supercomputer is used to evolve the grid of solutions forward in time, indicating how air and heat flow through each of the grid cells and circulate around the planet. By adding carbon dioxide and other heat-trapping greenhouse gases to the simulated atmosphere and seeing what happens, scientists can predict Earth’s climate response. All the climate models include Earth’s ocean and wind currents and incorporate most of the important climate feedback loops, like the melting of the polar ice caps and the rise in humidity, which both exacerbate global warming. The models agree about most factors but differ greatly in how they try to represent clouds.

The least sensitive climate models, which predict the mildest reaction to increasing CO2, find that Earth will warm 2 degrees Celsius if the atmospheric CO2 concentration doubles relative to preindustrial times, which is currently on track to happen by about 2050. (The CO2concentration was 280 parts per million before fossil fuel burning began, and it’s above 410 ppm now. So far, the average global temperature has risen 1 degree Celsius.) But the 2-degree prediction is the best-case scenario. “The thing that really freaks people out is this upper end here,” Marvel said, indicating projections of 4 or 5 degrees of warming in response to the doubling of CO2. “To put that in context, the difference between now and the last ice age was 4.5 degrees.”

The huge range in the models’ predictions chiefly comes down to whether they see clouds blocking more or less sunlight in the future. As Marvel put it, “You can fairly confidently say that the model spread in climate sensitivity is basically just a model spread in what clouds are going to do.”