Observations show that the 50-year drying trend (weakening of India summer monsoon) in north central India reversed in the past decade. The finding indicates improved water security and reduced socioeconomic impacts for the region in coming decades.

Background: 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.

Main point: Based on global climate models and multiple hypotheses, scientists expected this drying trend to continue unabated into the 21st century, but a new study in Nature Climate Change shows that the trend has reversed.

Findings: 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.

Significance: 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 also help scientists to improve current Earth-system models to more accurately project future climate change.