Based on global climate models and multiple hypotheses, scientists expected a 50-year drying trend to continue unabated into the 21st century, but a new study in Nature Climate Change shows that the trend has reversed.
The Indian summer monsoon (ISM) delivers about 80% of the Indian subcontinent’s annual precipitation and thus impacts the livelihood of more than 1/5 of the world’s population. The timing (typically June – September) and location of the ISM has far-reaching impacts—including floods and droughts—on farming and many other economic activities, and on the availability of water for people and livestock. These factors are of particular concern for north central India, which experienced a significant reduction in summer monsoon rainfall in the second half of the 20th century, resulting in degraded water security and widespread socioeconomic impacts. Global climate models and multiple hypotheses projected that this drying trend would continue unabated into the 21st century, but a new study in Nature Climate Change based on observational data shows that the trend has reversed.
Prediction of long-term monsoonal rainfall variations is critical to securing water supplies and planning agricultural and other economic activities in monsoonal regions. Enhanced knowledge about such variations can help scientists to improve current Earth-system models to more accurately project future climate change.
Using data obtained from ground-based and satellite measurements, researchers at the MIT Joint Program on the Science and Policy of Global Change find that since 2002, monsoon rainfall has increased in north central India at a rate of 1.34 millimeters per day per decade. They ascribe the revival of ISM precipitation to a combination of stronger warming over the Indian continent and slower rates of warming over the Indian Ocean. Today’s global climate models—and several hypotheses that have been proposed to explain the long-lasting drying trend in the second half of the 20th century—fail to capture the observed rainfall revival and corresponding trends in the underlying land-ocean temperature differences. In addition, the researchers find that this trend reversal of the Indian summer monsoon significantly mismatches the overall behavior of Northern Hemispheric monsoonal systems, raising the question of whether conditions unique to India, such as high concentrations of anthropogenic aerosols, are causing this phenomenon.
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This research was supported by the U.S. National Science Foundation, U.S. Department of Energy Office of Science, and the National Research Foundation of Singapore.
Jin, Q. and C. Wang, 2017: A revival of Indian summer monsoon rainfall since 2002, Nature Climate Change, doi: 10.1038/NCLIMATE3348
Photo: Monsoon rains combined with a tropical depression to leave portions of India’s Orissa state flooded starting on September 22, 2007, said the International Federation of Red Cross and Red Crescent Societies (IFRC). By September 27, every river system except the Mahanadi was flooded. This image, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite on September 30, 2007, shows significant flooding in northeastern Orissa, one of the hardest-hit regions of the state.