EAPS

Professor Schneider (Professor of Environmental Science and Engineering at CalTech) will give a special lecture for the week of the Climate Symposium.

Title:
Possible stratocumulus surprises in warm climates and geoengineering implications

Abstract:
Stratocumulus cloud decks over subtropical oceans are vulnerable to changes in longwave cooling: They are sustained by longwave cooling at their cloud tops, which drives turbulent air motions from the cloud tops downward and thereby couples stratocumulus decks to their moisture supply at the surface. This longwave cooling weakens as greenhouse gases (GHG) accumulate in the atmosphere, raising questions about how stratocumulus clouds will respond to increased GHG levels. Yet stratocumulus clouds are notoriously poorly captured in current climate models, making it difficult to study their response to increasing GHG levels in global models.

We invert the standard approach in climate modeling: Instead of simulating the large-scale dynamics of the atmosphere explicitly while representing the important smaller-scale dynamics of clouds semi-empirically, as is common, we simulate the dynamics of clouds explicitly and represented large-scale dynamics semi-empirically. In idealized global-warming simulations, we show that subtropical marine stratocumulus decks can become unstable and break up at CO2 concentrations above around 1,200~ppm, triggering global warming of up to about 8 K in addition to the warming that arose from the elevated CO2 concentrations before the clouds broke up. We also show that the same instability and breakup of stratocumulus clouds can still occur in a solar geoengineering scenario, in which GHG concentrations rise while their top-of-atmosphere effect is offset by manipulating the amount of sunlight Earth absorbs. This highlights a previously unrecognized risk of solar geoengineering, stemming from the direct effect of GHG on clouds.