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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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