Land-cover change and the cooling influence of evaporation: Dr. Julia Pongratz
Land use change is known to substantially influence the exchange of energy between a land surface and the atmosphere. For example, when we irrigate a field or when we reforest agricultural land, then usually more energy is transferred in the form of the latent heat flux, that is evaporation, and less in the form of sensible heat.
Now, we know that more evaporation will decrease local temperatures due to the effect of evaporative cooling, but it’s not clear if it would affect global temperatures because the local cooling may be compensated when the water vapor condenses elsewhere in the atmosphere and releases heat there.
So in a study led by my colleague George Ban-Weiss, we wanted to see if these shifts from sensible to latent heat could actually have an effect on global climate, not merely local one. And we did so in an idealized study using a global climate model.
In these simulations, we find that an increased evaporative flux indeed decreases global temperatures. The reason is that, on its way through the atmosphere, the evaporative flux causes more formation of low clouds, and these clouds reflect more sunlight back to space therefore cooling global climate.
So our simulation suggests that shifts towards more latent heat as can be caused by irrigation or by reforestation could not only cool local climate but actually global climate.