Intense rainfall & global warming

Print edition : August 07, 2015

RECORD-BREAKING heavy rainfall events, which result in high-impact flooding events, have been increasing in the past 30 years. Before 1980, multi-decadal fluctuations in extreme rainfall events were explained by natural variability. Now, a team of scientists of the Potsdam Institute for Climate Impact Research, Germany, has observed a clear upward trend in the past few decades towards more unprecedented daily rainfall events. They find the worldwide increase to be consistent with rising global temperatures, which are caused by greenhouse gas (GHG) emissions from burning fossil fuels.

Extreme rainfall in Pakistan in 2010 caused devastating flooding that killed hundreds and led to a cholera outbreak. There were the three so-called “once-in-a-century” flooding events in Germany, all of which happened in just a couple of years starting from 1997. Post-turn of the century, India too has been witness to several intense rainfall events: Mumbai in 2005, Uttarakhand in 2013 and Jammu and Kashmir in 2014. “In all of these places, the amount of rain pouring down in one day broke local records, and while each of these individual events was caused by a number of different factors, we find a clear overall upward trend for these unprecedented hazards,” said Jascha Lehmann, the lead author.

An advanced statistical analysis of rainfall data from the years 1901 to 2010 shows that over 1980-2010 there were 12 per cent more of these events than expected in a scenario without global warming. The increase in 2010 reached even 26 per cent, according to Lehmann. The scientists found distinct patterns in this increase, with generally wet regions seeing an over-proportional increase and drier regions less so. In South-East Asian countries, the observed increase in record-breaking rainfall events is as high as 56 per cent, in Europe 31 per cent and in Central U.S. 24 per cent. In contrast, some regions experienced a significant decrease of record-breaking daily rainfall events. In the Mediterranean, the reduction is 27 per cent and in West U.S. 21 per cent. Both regions are at risk for severe droughts, the researchers point out.

The scientists compared their findings to existing knowledge about how much more water can be stored in the atmosphere when temperatures rise. This additional moisture can be released during short-term heavy rainfall events. The scientists showed that the observed increase in unprecedented heavy rainfall events generally fit with this thermodynamically expected increase under global warming. “One out of 10 record-breaking rainfall events observed globally in the past 30 years can only be explained if the long-term warming is taken into account,” said co-author Dim Coumou. “For the last year studied, 2010, it is even one event out of four, as the trend is upward”.

Building on previous work on extreme precipitation, it is the first to study worldwide observational data of record-breaking daily rainfall events in this context.