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Climate change: extremes of weather around the world
9 Aug 2010

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There have always been extremes of weather around the world but evidence suggests human influence is changing the odds.

Over the past week or so, Pakistan has been devastated by its worst floods for generations and Moscow has suffered under a blanket of smog after its hottest day in 130 years of records. What is causing these and other recent extreme weather events and are they linked to climate change?

Because of a rare meteorological pattern we can see a connection between extreme weather across Eurasia. Usually, the flow in the upper troposphere over northern India, the Himalayas and Pakistan is dominated by the monsoon anticyclone which pushes the sub-tropical jet to the north of the Tibetan Plateau. This prevents mid-latitude weather systems from penetrating very far south, unlike this year, when active weather systems have spread southwards into Pakistan. Here this has combined with the monsoon to produce record rainfall. The record-breaking high temperatures in Moscow, forest fires and damaged crops are another consequence, as was the excessive rain over China when the Three Gorges Dam almost reached capacity a few short weeks ago.

So are we seeing the effects of climate change in these extreme weather events? Analysing the observational data shows clearly that there has been a rise in the number of extremely warm temperatures recorded worldwide and that there have been increases in the number of heavy rainfall events in many regions over land. Evidence, including in India and China, that periods of heavy rain are getting heavier, is entirely consistent with our understanding of the physics of the atmosphere in which warmer air holds more moisture. Our climate change predictions support the emerging trend in observations and show a clear intensification of extreme rainfall events in a warmer world.

It can still be problematic to blame a specific individual extreme weather event on climate change, because there have always been extremes of weather around the world. However, if the likelihood of a particular extreme weather event has changed it is possible to say something. I and colleagues from Oxford University showed, in a paper we published in Nature, that the probability of the hot European temperatures in 2003 had very likely doubled as a result of human influence. While still relatively rare, the odds of such extreme events are rapidly shortening and could become considered the norm by the middle of this century.

For some other types of extreme weather there is a need for more research. For example, circulation changes could mean that some extreme weather events become less, not more likely under climate change. Better understanding of which extreme weather events are part of normal variations rather than of a developing pattern of climate change effects will help societies adapt to the challenges of ongoing climate change. Next week in Colorado, experts from the UK and US forecasting centres at the Met Office and NOAA will meet to consider how we can provide better information on the causes of extreme weather in near-real time.

Alongside continued efforts to advance our forecasting systems we are improving our monitoring of the climate to put extreme weather into a long-term context. Precise local information on the evolving climate and how it fits into the longer-term picture remains insufficient in many of the most vulnerable parts of the world. This is a challenge that will begin to be addressed next month, when scientists from around the world meet at the Met Office to start to develop a new observational record to help identify changing trends in extremes. There is no time to waste if we are going to equip societies to better cope with the severity of weather in a changing climate.

By Peter Stott, Head of climate monitoring and attribution at the Met Office


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Source: Guardian