Study: Climate change to produce extreme rains

A new study observes a link between warmer temperatures and more intense downpours.

Commuters brave July's torrential rain at Oxford Street underground station in London.

I live in the Boston area, and the weather around here has been wacky all summer, with otherwise sunny days peppered with sudden torrential downpours, thunderstorms, and the odd flash flood.

Nobody knows if this particular summer is just a fluke or the beginning of a trend, but according to a recent study, some of us had better get used to it.

Using satellite data, a study conducted at the University of Miami and the University of Reading in Great Britain has found a distinct statistical link between warmer temperatures and extreme downpours, with heavy rains increasing during warmer periods and decreasing during colder ones.

What's more, according to the study, published in Science last week, "theobserved amplification of rainfall extremes is found to be largerthan predicted by models, implying that projections of futurechanges in rainfall extremes due to anthropogenic global warmingmay be underestimated."

The team looked at satellite images of rainfall over tropical oceans from 1988 to 2004. They found that, during El Niño – the periodic warming of tropical Pacific waters – heavy rains were more frequent, and light rains less so. In other words, during times of warmer temperatures, when it rained, it poured.

It has long been assumed that higher temperatures tend to produce heavier downpours – the reasoning is that warmer air can hold more moisture, increasing the intensity of the rainfall – and this link has been factored into computer climate models. This new study is the first one to directly observe the relationship between rainfall and temperature, and, as the Telegraph explains, it suggests that the computer models were selling the rainfall short:

Changes in heavy rainfall seem to keep pace with atmospheric moisture which rises by around 7 per cent for each ºC of warming. Based on computer models, this could mean an increase in the intensity of heavy rainfall of around 10 per cent by 2050.
However, the observed increase in extreme downpours appears to be larger than the increases predicted by current computer simulations, suggesting that predicted changes in rainfall due to global warming may be underestimated, either because of flawed measurements or because computer models lack some key understanding, for instance of the action of aerosol particles in the atmosphere.

The researchers say it is difficult to be precise about the underestimate and add that it is crucial to determine the cause for this discrepancy as soon as possible in order to accurately understand the implications of global warming and its effects on the water cycle.

The effects of these increased heavy rains will be felt most in the tropics. Reuters quotes one on the study's authors, Richard Allan of the University of Reading, who says that flash floods could affect vulnerable farming communities in Asia, Latin America, and Africa.

"Flash flooding can cause damage to settlements and societies. It can contaminate ground water, drinking supplies, with potential health effects," Allan said.
"The spread of disease can be impacted by heavy rainfall," he added. "Very intense rainfall can destroy crops. There are also possibilities of enhanced erosion, degradation of soil."
Scientists say it is hard to link climate change to one-off events such as floods in the U.S. Midwest that damaged millions of acres of cropland in June.

National Geographic reports that intense rainfall could potentially hasten the spread of illnesses thought to be more common in flooded areas.

But others are skeptical that the link between increased temperatures and precipitation is not as firmly established as the authors assert. The Miami Herald quotes Isaac Held, a senior research scientist with the National Oceanographic and Atmospheric Administration, who calls the study "a stretch."

''The satellite data is not of long enough duration to look at any global warming trends,'' Held said. "It looks at El Niño conditions only, and El Niño is not a perfect analog for global warming.''
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