Does increased evaporation lead to global cooling? Ken Caldeira
People who live in New York City have a sense that Central Park is helping to keep the city cool. And it’s true that the evaporation of the trees and lakes is taking energy away from the city that would otherwise go into heating up the city. And this energy evaporates water and goes into the atmosphere.
So the question is, does this evaporation produce only a local cooling, helping to keep New York City cool? Or, does it also contribute to a global cooling? And there are some reasons to think it might not contribute to a global cooling because we know that water vapor in the atmosphere is a greenhouse gas that traps outgoing heat radiation and so perhaps this water vapor in the atmosphere would cause the earth to heat up.
We also know that, at some place, this water is going to condense and form either rain or snow. And where this water vapor condenses, it will cause a local heating. So the effect of evaporating in one place and condensing somewhere else is really to transfer energy from one place to another and not really to remove energy from the whole earth’s global climate system.
The question we wanted to address in this study was, does local evaporation contribute only to local cooling? Or, does it really contribute also to a global cooling? My former post-doc George Ban-Weiss led the study in which we, in an idealized way, caused an increase in evaporation everywhere, and we took the energy from this evaporation from sensible heating increasing in the temperature of the lower atmosphere.
What we found was that this evaporation caused an increase in low cloudiness. These low clouds are white and cause sunlight to be reflected to space. So for each unit of energy that went into evaporating water, approximately three-quarters of a unit of energy of sunlight was reflected back to space. And this increased reflection of sunlight changed the global energy balance and caused a net cooling.
So the conclusion of our study was that, in general, evaporating water causes not only a local cooling but also contributes to global cooling.