Sounding alarm bells for Indian cities, scientists have warned that short bursts of intense rainfall are likely to increase with global warming.
The international Paris Agreement to reduce greenhouse emissions causing global warming aims to keep global temperature increase to below 2 degrees.
The study also found that increases in three-hour extremes were almost twice that of daily precipitation for the same warming scenarios. For every degree of warming, one can generally expect a 6% to 7% increase in daily rainfall, says Vimal Mishra, a scientist with IIT-Gandhinagar who co-authored the study. That’s because the atmosphere’s capacity to hold water increases when temperature increases. But short-duration precipitation seems to be more sensitive to warming. “In most places, we found that sub-daily precipitation extremes are increasing faster than daily extremes—that it’s more sensitive to warming,” Mishra says.
Interestingly, another study last year found that daily rainfall extremes did not increase beyond a certain level with rising temperatures-—but three-hour extremes did. One reason for that, says IIT-Bombay professor Subimal Ghosh, could be that there isn’t enough moisture in the air most of the time to maintain that level of precipitation for a whole day. “If rainfall is coming down at 100mm an hour, there is enough moisture supply in the atmosphere for three hours of that, but not for 24 hours,” says Ghosh, His study also found that short-duration high-intensity rainfall events are intensifying with warming, leading to “extreme streamflow” in smaller catchments like urban ones. This trend is critical, he says, because sudden bursts of rain are more likely to lead to flash floods that leave little time for evacuation or warning.
The trend is also risky for cities because outdated storm water drains and concretisation prevent drainage, leading to flooding of the kind seen on Sunday. Mumbai is currently expanding its storm water drain capacity from 25 mm of rainfall an hour to 50mm an hour, but what it needs now are systems that can handle more than 100 mm of rainfall in an hour, said Kapil Gupta, an urban drainage expert at IIT Bombay.
Gupta, who advised the National Disaster Management Authority on new urban flooding guidelines, recommends increasing capacity for a climate change scenario, keeping rivers and drains clear, and building underground storage tanks to absorb extra water. “We need to upgrade our systems to account for increasing extremes,” he said.