Amidst worrisome indications that human-induced climate change is affecting the monsoon over India, research just published adds to a body of evidence showing that extremes in rainfall are increasing.
“Our analyses indicate a shift in the recent period towards more intense wet spells and more frequent but less intense dry spells,” say a team of researchers from Stanford University in the U.S. in a Nature Climate Change paper.
The scientists examined daily rainfall data for the peak monsoon months of July and August over a large swathe across central India. This region gets heavy rains during those months and also has considerable day-to-day variability in rainfall.
They evaluated how the characteristics of wet spells, with days of heavy rain, and dry spells, with little or no rain, had changed between two 30-year periods, 1951 to 1980 and 1981 to 2011.
The average daily rainfall for July and August taken together had declined over the region in the latter period. Day-to-day fluctuations in rain had increased, with days of light and heavy rain becoming more frequent.
Morover, when compared to 1951-1980, the intensity of rainfall during wet spells was significantly higher during 1981-2011.
At the same time, dry spells had become 27 per cent more frequent during the latter period, which had twice as many years with three or more dry spells as the former.
The changes seen in 1981-2011 were “outside the range of natural variability observed in 1951-1980, a period when there was relatively less warming,” noted Deepti Singh, a graduate student at Stanford University and first author of the paper, in an email. A continuation of those trends could “imply increased flooding risk in parts of [that] region and also can have substantial impacts on farmers that still primarily depend on rainfed agriculture.”
Several papers in recent years have drawn attention to the increasing intensity of heavy rainfall events over central India, commented M. Rajeevan, an atmospheric scientist and adviser to the Ministry of Earth Sciences in Delhi, who has published work on extreme rainfall events in the country.
The Nature Climate Change paper showed that the frequency of dry spells too was going up, which had implications for agriculture and water resource management, he told this correspondent.
The paper’s findings were consistent with climate model simulations, which show that extreme events, with too much or too little rain during the monsoon, would increase as the climate warmed. However, more detailed studies will be needed to establish that the observed increases in extreme events were indeed due to climate change brought about by human action and not part of a multi-decadal cycle of natural variability, Dr. Rajeevan added.
“It is clear that the farmers have to start adjusting to changing wet and dry spells, and resort to rainwater harvesting to tide themselves through the increasing dry spells,” remarked Raghuram Murtugudde, a professor in the University of Maryland's Department of Atmospheric and Oceanic Sciences in the U.S.
More importantly, there should be greater emphasis on forecasting the wet and dry spells during a monsoon, not just the total seasonal rainfall, he said in an email. The seasonal totals were important for water resource management and also correlated with total food production.
But farmers cared about distribution of rainfall within the season after monsoon onset had occurred.
“Subsistence farmers who don’t get irrigation water need the wet and dry spell information to survive,” added Prof. Murtugudde.