May 4, 2013
Rain And Drought Will Increase Due To Warming: NASA
April Flowers for redOrbit.com - Your Universe Online
Global warming may increase the risk for extreme rainfall and drought, a new NASA-led modeling study predicts. The study, to be published in the journal Geophysical Research Letters, demonstrates for the first time how rising carbon dioxide concentrations could affect the entire range of rainfall types on Earth.
"In response to carbon dioxide-induced warming, the global water cycle undergoes a gigantic competition for moisture resulting in a global pattern of increased heavy rain, decreased moderate rain, and prolonged droughts in certain regions," said William Lau of NASA's Goddard Space Flight Center.
According to the model projections, for every 1 degree Fahrenheit of carbon-induced warming we can expect global increases in heavy rainfall of 3.9 percent. Light rain will also increase by one percent, across the globe. Because moderate rainfall is projected to decrease by 1.4 percent, total global rainfall is not expected to change much.
Scientists define heavy rainfall as months that receive an average of more than about 0.35 of an inch per day, and light rainfall as months that receive an average of less than 0.01 of an inch per day. The definition of moderate rainfall is months that receive an average of between about 0.04 to 0.09 of an inch per day.
The tropical zones around the equator, especially in the Pacific Ocean and Asian monsoon regions, are projected to see the most significant increases in heavy rainfall. According to the study, some regions outside the tropics may have no rainfall at all.
For every degree Fahrenheit of warming, the models predict the length of periods with no rain will increase by 2.6 percent globally. Northern Hemisphere areas that will be the most affected include the deserts and arid regions of the southwest United States, Mexico, North Africa, the Middle East, Pakistan, and northwestern China. Drought becomes more likely in Southern Hemisphere regions such as South Africa, northwestern Australia, coastal Central America and northeastern Brazil.
"Large changes in moderate rainfall, as well as prolonged no-rain events, can have the most impact on society because they occur in regions where most people live," Lau said. "Ironically, the regions of heavier rainfall, except for the Asian monsoon, may have the smallest societal impact because they usually occur over the ocean."
The team based their analysis on simulations lasting 140 years on all 14 computer models. The models began their simulations with carbon dioxide concentrations at pre-industrial levels of around 280 parts per million (ppm). This concentration was increased by 1 percent each year, which is consistent with a “business as usual” trajectory of the greenhouse gas as described by the United Nations' Intergovernmental Panel on Climate Change.
The researchers calculated statistics on the rainfall responses for a 27-year control period, starting at the beginning of the simulation. They also calculated statistics for 27-year periods around the time of doubling and tripling concentrations.
Predictions for how much rainfall will occur at any one location as the climate warms are not very reliable, the team found.
"But if we look at the entire spectrum of rainfall types we see all the models agree in a very fundamental way -- projecting more heavy rain, less moderate rain events, and prolonged droughts," Lau said.