Wednesday, August 24, 2005

Is extreme weather down to climate change? - BBC

The important conclusion of this report is as follows:
We can't blame any "specific" drought/flood/hurricane/typhoon on global warming.
After all, all these forms of natural disasters have already occured in the past decades/centuries/milleniums !
However, on average, more extreme weather conditions have been observed.


Is extreme weather down to climate change?
With fires raging through southern Europe - a region experiencing its worst drought for decades - and some parts of the continent submerged by floods, it is tempting to ascribe such extreme weather to the effects of global warming.

The wildfires are confounding attempts to contain them
But climate change researchers are reluctant to make such links.
"You can say that due to the Earth getting warmer there will be on average more extreme events," said Malcolm Haylock, of the University of East Anglia's Climate Research Unit, UK, "but you can't attribute any specific event to climate change."
Dozens of wildfires have been raging out of control across Portugal, confounding attempts to contain them.
Portugal, like other areas of southern Europe and North Africa has been experiencing searing heat and drought this summer.
Meanwhile, floods have brought chaos to a large swathe of central Switzerland, triggering landslides and cutting roads and railway lines.
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.
Many climate scientists now believe the data points to global temperatures rising by about two tenths of a degree C per decade for the foreseeable future.
But as far as the droughts and floods are concerned, climate scientists have found it more difficult to find long-term trends in rainfall.

Climate models can be used to predict future climate variationEuropean 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," said Dr Haylock.
The 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.
But 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 changes in the North Atlantic Oscillation cannot be linked to human-induced climate change.
Scientists simply don't have the long-term measurements to say either way and have not been able to tease out any long-term trends in rainfall on a global scale.
However, computer models suggest that, as the climate gets hotter over the coming decades, the available water in the landmass 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."
As for the fires in Portugal, observers point out that poor land management and arson have also played their part in the devastation.


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