INTERVIEW: Global warming explains flooding, says German scientist

By Simone Humml Jan 16, 2011, 20:43 GMT

Berlin - Floods around the globe this month are partly just vagaries of the weather, but are also partly the outcome of global warming, a German climate expert warns.

Last year and 2005 were the planet's warmest years since daily records began, suggesting increasingly hotter and wetter weather globally.

Stefan Rahmstorf of the Potsdam Research Institute on Climate Effects (PIK) spoke to the German Press Agency dpa on Sunday.

dpa: Is global warming causing the major floods, as has been the case in Pakistan, Colombia, South-East Asia, Australia and most recently Brazil?

Rahmstorf: Extreme weather events are always a combination of the vagaries of weather and longer-term climate changes, in this case global warming. It's true that the frequency of extreme weather events has increased. A series of studies prove it.

Germany's weather service has confirmed this is true of both high temperatures and high precipitation.

dpa: How does warming lead to floods?

Rahmstorf: That's fairly simple as a matter of physics. In 2010 we had the warmest year globally since the start of weather records, first equal with 2005.

A warmer climate increases precipitation because most of the moisture in the air comes from the oceans by way of evaporation. So if the sea gets warmer, you'll always get more rain too.

And 2010 was at the same time the wettest year, meaning with the most rain worldwide since precipitation records began 100 years ago.

dpa: Is flooding in Europe connected to this?

Rahmstorf: You can never distinguish these factors in a single extreme event. But it is clear that the frequency of such extreme events is on the rise, and moreover, in the past decade there have been extremes on a whole new scale.

dpa: For example?

Rahmstorf: Last summer's heatwave in Russia brought July temperatures that were well above any previous readings. The Pakistan floods followed rain of an intensity never recorded before there. The same applies in Australia currently.

dpa: But aren't the floods in Australia and Brazil caused by the La Nina effect, in which oscillating currents in the Southern Pacific Ocean cause weather changes in the tropics and southern hemisphere?

Rahmstorf: Yes, of course its La Nina that's the direct cause. But the alternation between El Nino and La Nina is cyclical, happening every three to seven years, so it doesn't explain why the recent precipitation has set a 100-year record.

Some scientists have been suggesting the swings of El Nino and La Nina are growing more powerful. Right now we've got an extreme La Nina. Back in 1998 it was the most powerful El Nino.

But personally, I think it's too early to say that this has been proved in a really robust way.