Caldeira Lab

Climate forcing and response to idealized changes in surface latent and sensible heat

George A. Ban-Weiss, Govindasamy Bala, Long Cao, Julia Pongratz & Ken Caldeira

Evaporation of water causes local cooling, but that water must condense somewhere else, and wherever it condenses there will be local heating. Previously, it was not known whether this evaporation and condensation of water, on average, causes the Earth to cool globally or whether it simply represents a transfer of energy from one place to another. This study showed for the first time that, because increased evaporation tends to create more low clouds that reflect sunlight to space, on average, evaporation of water causes global and not merely local cooling.

Ban-Weiss, G. A., G. Bala, L. Cao, J. Pongratz, and K. Caldeira, 2011: Climate forcing and response to idealized changes in surface latent and sensible heat. Environmental Research Letters, 6, 034032, doi:10.1088/1748-9326/6/3/034032.


Julia Pongratz - click to read the video transcript

Ken Caldeira - click to read the video transcript



Land use and land cover changes affect the partitioning of latent and sensible heat, which impacts the broader climate system. Increased latent heat flux to the atmosphere has a local cooling influence known as ‘evaporative cooling’, but this energy will be released back to the atmosphere wherever the water condenses. However, the extent to which local evaporative cooling provides a global cooling influence has not been well characterized. Here, we perform a highly idealized set of climate model simulations aimed at understanding the effects that changes in the balance between surface sensible and latent heating have on the global climate system. We find that globally adding a uniform 1 W m−2 source of latent heat flux along with a uniform 1 W m−2 sink of sensible heat leads to a decrease in global mean surface air temperature of 0.54 ± 0.04 K. This occurs largely as a consequence of planetary albedo increases associated with an increase in low elevation cloudiness caused by increased evaporation. Thus, our model results indicate that, on average, when latent heating replaces sensible heating, global, and not merely local, surface temperatures decrease.


Radiative forcing (RF) caused by historical deforestation: The negative RF (cooling effect) of an increase in surface reflectivity has been overwhelmed in most agriculturally important regions by the positive RF (warming effect) of CO2 emissions, increasing their potential to counteract global warming when reversed to their natural forested state.