Europe’s weather could flip from droughts to floods every year as climate change kicks in, according to scientists who have modelled the mechanisms behind the continent’s most recent bouts of extreme weather.
In the summer of 2003, an intense heatwave across Europe was responsible for the deaths of up to 35,000 people and dried up many rivers. Yet in 2002, central Europe was awash with water after a massive river burst its banks.
Both events have been attributed to a peculiar phenomenon in which a “planetary wave” pins a particular weather system in one place. The drought in 2003 was triggered by trapped high pressure, while the flood of 2002 happened after a region of low pressure became pinned.
Planetary waves are gentle pressure oscillations in the atmosphere, set up by the rotation of the Earth. They usually roll gently around the planet, carrying weather systems with them.
But when the wavelength of the planetary wave fits an exact number of times around the circumference of the Earth, the peaks and troughs of the pressure oscillation overlap on each revolution, so they are fixed in place and grow in strength.
Alternate years 
This “resonance” will become more frequent if the wavelength of the planetary waves shortens, as is expected to happen as the planet warms, warns Vladimir Petoukhov, of the Potsdam Institute for Climate Impact Research in Germany.
He fed predictions of the Earth’s future temperature into a computer model of the waves. While resonance now is rare, he calculates that within two decades, it could happen as often as every year.
“Europe could flip between flood and drought conditions in alternate years,” says John Schellnhuber, research director of the Tyndall Centre for Climate Change Research in Norwich, UK, and a former colleague of Petoukhov.
Although the results are preliminary, Petoukhov says that the effect might occur in other continents too.
The results were presented at the American Association for the Advancement of Science conference in Seattle, Washington, and are being reviewed by the journal Climate Dynamics.
-chris tivnan [email protected]