Is extreme weather due to climate change?

25 July 2007
BBC

With parts of Europe baking in a heatwave, while parts of England are experiencing their worst flooding for 60 years, it is tempting to ascribe this extreme weather to climate change.

Parts of Europe have been sweltering in record temperaturesBut climate scientists are reluctant to make this link.

"You can say that due to the Earth getting warmer there will be on average more extreme events," said Dr Malcolm Haylock, an expert on climate extremes, "but you can't attribute any specific event to climate change."

This month, hundreds of people have died in a heatwave that has swept across south-eastern Europe. Wildfires have raged across Greece, confounding attempts to contain them.

Meanwhile, severe floods have brought chaos to parts of England, forcing hundreds of homes to be evacuated.

Growing consensus

There is a growing consensus, based on past climate records and other data, that greenhouse gas emissions are warming the Earth's climate.

Homes around England have been evacuated following the floodsMany climate scientists now believe the data points to global temperatures rising by between 1.1C and 6.4C by the end of this century.

But as far as the droughts and floods are concerned, climate scientists have found it more difficult to attribute long-term trends in rainfall to human activities.

European weather is affected by a climate system called the North Atlantic Oscillation. This describes changes in atmospheric pressure at sea level as measured over Iceland and over the Azores.

"Over the last 50 years or so, there's been a trend to lower pressures over Iceland and higher pressures over the Azores in winter, although this trend has reduced in recent years," said Dr Haylock, who works for re-insurer PartnerRe in Zurich, Switzerland, and is formerly of the University of East Anglia's Climatic Research Unit, UK.

Firefighters have struggled to hold back the flames in GreeceThe impact of this climate system reaches from the upper atmosphere to the bottom of the ocean.

But its most obvious impact over the last half century is a trend towards drier conditions in southern Europe and more extreme rainfall in northern Europe during winter.

Its effects during other seasons, such as summer, are not as clear. Local weather systems seem to play a larger role here.

Computer models

Dr Haylock said that recent changes in the North Atlantic Oscillation could not be linked to human-induced climate change.

Scientists simply do not have the long-term measurements to say either way.

Climate models can be used to predict future climate variationOn the other hand, there is a growing consensus that the recent changes in the North Atlantic Oscillation will continue in the future, leading to winter storms in Europe tending to move further north and drier conditions in southern Europe.

Computer models suggest that, as the climate gets hotter over the coming decades, the available water in the landmass of Europe may be reduced. This may in turn have knock on effects for global temperatures.

"When we run these climate models for future years, we find we were getting very, very hot days. These were so hot, they can't be explained just by more carbon dioxide in the atmosphere," said Dr Haylock.

"Water on the ground cools the atmosphere around it a lot, and once this has dried out, the temperatures just accelerate. So there is some concern that these hot days may become more frequent over the next decade, but that is still uncertain."

http://news.bbc.co.uk/1/hi/sci/tech/6915309.stm