As Earth’s climate warms, most scientists agree that there will be less frequent rain but more extreme precipitation—heavy downpours that dump abnormally large quantities of rain, sometimes causing dangerous floods. Just how much these events will increase is hard to say, however, and various climate models make conflicting predictions.
One reason for the disagreement is the challenge of accurately modeling the complex physics of convection, which is the movement of heat and moisture through the atmosphere. Convection helps to form the towering, heaped cumulus clouds that produce thunderstorms.
Previous studies have shown that changes in the temperature of the ocean’s surface strongly affect whether convection remains diffuse, scattering clouds evenly across the globe, or forms just a few large clusters of storms. Scientists describe these patterns of convection as “organized,” with regular, distinct loci around which storms such as tropical cyclones tend to coalesce, or “disorganized,” with more scattered precipitation events.
To examine how such changes in convention will affect rainfall, Pendergrass et al. applied different surface temperatures to a simplified “aquaplanet” model of Earth’s climate, which eliminated variables such as land and which distributed incoming sunlight evenly over the planet’s watery surface. They started with a “cold” Earth, with a sea surface temperature of around 285 K, and gradually ramped up to a “hot” Earth scenario, stopping at roughly 307 K.
Rather than increasing steadily with warming, the frequency of extreme precipitation followed a surprising pattern. Colder conditions produced patchy areas of heavy rainfall, but as temperatures climbed to around 290 K, a more even, “disorganized” distribution emerged. At roughly 303 K, the model abruptly produced large clusters of exceptionally heavy rainfall that dwarfed those in both the cold and warm scenarios.
By isolating different inputs to the model, the authors found that an increase in the vertical velocity—the moisture-bearing air that rushes upward from Earth’s surface—largely accounted for the rapid shift in the hotter scenario. This finding suggests that scientists should look not only to rising temperatures but to shifts in atmospheric circulation when attempting to predict the likelihood of extreme precipitation events. (Geophysical Research Letters, doi:10.1002/2016GL071285, 2016)
—Emily Underwood, Freelance Writer