Warming behind floods

By Vibha Varshney

Studies prove greenhouse gases cause frequent intense rainfall

Two recent studies have, for the first time, pinned down greenhouse gas (GHG) emissions as a cause for frequent extreme weather events like rainfall and floods.

In one study, researchers from the UK and Switzerland analysed one of the worst floods in Europe to understand if GHG emissions triggered the disaster. The floods in October 2000 in the UK had damaged properties worth US $2.1 billion.

The researchers, led by Myles Allen at the University of Oxford, asked volunteers to create thousands of probable weather conditions for that year simulating scenarios with GHG emissions and where no GHG was emitted in the 20th century. They used the results to estimate river runoff under different conditions. GHG emissions increased the risk of floods by over 20 per cent, they found.

In another study, researchers from Canada and the UK concluded that GHG emissions have been responsible for the frequent heavy downpours in the Northern Hemisphere in recent decades. The team, led by Gabriele Hegerl of University of Edinburgh in the UK, collected daily rainfall data between 1951 and 1999 from 6,000 stations across the Northern Hemisphere. They then prepared a model that simulated weather conditions influenced by GHG emissions. They found that GHG emissions were responsible for the intense rainfall in two-thirds of the region.

The researchers are, however, a bit reticent about linking their findings to extreme weather events in general. Francis Zwiers, one of the authors of the study from Canada’s Department of Environment, said it requires more research to establish that human activities are actually responsible for frequent extreme weather events across the globe. For example, concurrent severe floods in Australia, Brazil and Sri Lanka early this year were because of the cooling of the equatorial Pacific Ocean. Global warming could have compounded the effect. But this is difficult to prove, Zwiers added.

M Rajeevan of the National Atmospheric Research Laboratory in Andhra Pradesh said the studies are significant as they relate the observed increase in extreme events to a rise in sea-surface temperatures, which is attributed to human activities.

“One of the problems that people have with climate change is it is a victimless crime. The impact is hypothetical; nobody gets affected by a small rise in temperature. The studies offer a scientific base to differentiate between climate change-induced extreme events and bad weather. This will help quantify the impact of climate change on people,” says Myles Allen.