Record-breaking heavy rainfall events increased under global warming

http://www.eurekalert.org/pub_releases/2015-07/pifc-rhr070815.php

Public Release: 8-Jul-2015
Potsdam Institute for Climate Impact Research (PIK)

Heavy rainfall events setting ever new records have been increasing strikingly in the past thirty years. While before 1980, multi-decadal fluctuations in extreme rainfall events are explained by natural variability, a team of scientists of the Potsdam Institute for Climate Impact Research detected 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 emissions from burning fossil fuels. Short-term torrential rains can lead to high-impact floodings.

Extreme rainfall in Pakistan 2010 caused devastating flooding which killed hundreds and lead to a cholera outbreak. Other examples of record-breaking precipitation events in the period studied include rainstorms in Texas in the US, 2010, which caused dozens of flash-floods. And no less than three so-called 'once-in-a-century' flooding events in Germany all happened in just a couple of years, starting 1997. "In all of these places, the amount of rain pouring down in one day broke local records - and while each of these individual events has been caused by a number of different factors, we find a clear overall upward trend for these unprecedented hazards", says lead-author Jascha Lehmann.

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While a statistical analysis of course cannot provide direct physical cause-effect relations, the scientists compared their findings to existing knowledge about how much more water can be stored in the atmosphere when temperatures rise, as given by the well-known Clausius-Clapeyron equation. This additional moisture can be released during short-term heavy rainfall events. The scientists show that the observed increase in unprecedented heavy rainfall events generally fits with this thermodynamically expected increase under global warming.

"One out of ten record-breaking rainfall events observed globally in the past thirty years can only be explained if the long-term warming is taken into account," says co-author Dim Coumou. "For the last year studied, 2010, it is even one event out of four, as the trend is upward".

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"The pronounced recent increase in record-breaking rainfall events is of course worrying," Coumou concludes. "Yet since it is consistent with human-caused global warming, it can also be curbed if greenhouse gas emissions from fossil fuels are substantially reduced."