Rainfall responds to climate change

Michael Previdi in Environmental Research Web: The amount of moisture that the atmosphere can hold varies strongly with temperature. For each degree of global warming, the amount of moisture that is stored in the atmosphere increases by about 7%. This higher moisture content does not imply a corresponding increase in rainfall, however, and climate models instead predict that global-mean precipitation will increase at a much slower rate of only 1–3% per degree of global warming. The reason for this is that changes in global rainfall are controlled, not by the change in atmospheric moisture with temperature, but by changes in atmospheric, radiative-energy loss with temperature.

As the atmosphere warms, it emits more long-wave energy, according to the Stefan–Boltzmann law of blackbody radiation. Because the heat capacity of the atmosphere is negligible, we need to compensate for this additional loss of long-wave energy by gaining energy through latent heating, which occurs when water vapour condenses in the atmosphere to form clouds and eventually precipitation. For this reason, changes in global-mean precipitation will follow changes in atmospheric, radiative-energy loss. However, this radiative-energy loss does not simply depend on temperature but on several other factors as well.

New research is now shedding light on factors other than temperature that will control global rainfall changes in the 21st century. One important factor is the increase in atmospheric humidity. As water vapour concentrations increase, the atmosphere radiates long-wave energy into space less effectively, and also absorbs a greater amount of incoming short-wave energy (sunlight). Another important factor is the increase in man-made airborne pollutants, in particular greenhouse gases, such as carbon dioxide and soot-aerosol particles, both of which act to reduce radiative-energy loss from the atmosphere. Finally, cloud changes, though more uncertain, are likely to reinforce radiative perturbations caused by increases in humidity, carbon dioxide and soot….

[Michael Previdi is an associate research scientist at the Lamont–Doherty Earth Observatory of Columbia University. His work focuses on understanding fundamental problems in climate dynamics, including changes in the atmospheric water cycle, and climate sensitivity to radiative forcing.]

Fogg Dam Conservation Reserve is one of several reserves in the lower lower Adelaide River catchment in the Northern Territory, shown here during a thunderstorm. Shot by Bidgee, Wikimedia Commons, under the Creative Commons Attribution 3.0 Unported license