Print

Print


Here in Johannesburg (and South Africa as a whole), our summer has been
unusually hot. No word from the weather guys yet, but I believe it may be
one of the consistently hottest ever recorded. On the Celsius scale, we
should have average temps over December, Jan and Feb of a range between
24-26 degrees –it’s been consistently two to five degrees higher, sometimes
as much as eight. I had personal experience of how temps like these will
affect agriculture – planted three sets of Swiss chard seed one after the
other, only to have the seedlings die – each time during just one very hot
afternoon.


Mandi


Weather records due to climate change: A game with loaded dice


The past decade has been one of unprecedented weather extremes. Scientists
of the Potsdam Institute for Climate Impact Research (PIK) in Germany argue
that the high incidence of extremes is not merely accidental. From the many
single events a pattern emerges. At least for extreme rainfall and heat
waves the link with human-caused global warming is clear, the scientists
show in a new analysis of scientific evidence in the journal Nature Climate
Change. Less clear is the link between warming and storms, despite the
observed increase in the intensity of hurricanes.

In 2011 alone, the US was hit by 14 extreme weather events which caused
damages exceeding one billion dollars each – in several states the months of
January to October were the wettest ever recorded. Japan also registered
record rainfalls, while the Yangtze river basin in China suffered a record
drought. Similar record-breaking events occurred also in previous years. In
2010, Western Russia experienced the hottest summer in centuries, while in
Pakistan and Australia record-breaking amounts of rain fell. 2003 saw
Europe´s hottest summer in at least half a millennium. And in 2002, the
weather station of Zinnwald-Georgenfeld measured more rain in one day than
ever before recorded anywhere in Germany – what followed was the worst
flooding of the Elbe river for centuries.

"A question of probabilities"

"The question is whether these weather extremes are coincidental or a result
of climate change," says Dim Coumou, lead author of the article. "Global
warming can generally not be proven to cause individual extreme events – but
in the sum of events the link to climate change becomes clear." This is what
his analysis of data and published studies shows. "It is not a question of
yes or no, but a question of probabilities," Coumou explains. The recent
high incidence of weather records is no longer normal, he says.

"It´s like a game with loaded dice," says Coumou. "A six can appear every
now and then, and you never know when it happens. But now it appears much
more often, because we have changed the dice." The past week illustrates
this: between March 13th and 19th alone, historical heat records were
exceeded in more than a thousand places in North America.

Three pillars: basic physics, statistical analysis and computer simulations

The scientists base their analysis on three pillars: basic physics,
statistical analysis and computer simulations. Elementary physical
principles already suggest that a warming of the atmosphere leads to more
extremes. For example, warm air can hold more moisture until it rains out.
Secondly, clear statistical trends can be found in temperature and
precipitation data, the scientists explain. And thirdly, detailed computer
simulations also confirm the relation between warming and records in both
temperature and precipitation.

With warmer ocean temperatures, tropical storms – called typhoons or
hurricanes, depending on the region – should increase in intensity but not
in number, according to the current state of knowledge. In the past decade,
several record-breaking storms occurred, for example hurricane Wilma in
2004. But the dependencies are complex and not yet fully understood. The
observed strong increase in the intensity of tropical storms in the North
Atlantic between 1980 and 2005, for example, could be caused not just by
surface warming but by a cooling of the upper atmosphere. Furthermore, there
are questions about the precision and reliability of historic storm data.

Overall, cold extremes decrease with global warming, the scientists found.
But this does not compensate for the increase in heat extremes.

Climatic warming can turn an extreme event into a record-breaking event

"Single weather extremes are often related to regional processes, like a
blocking high pressure system or natural phenomena like El Niño," says
Stefan Rahmstorf, co-author of the article and chair of the Earth System
Analysis department at PIK. "These are complex processes that we are
investigating further. But now these processes unfold against the background
of climatic warming. That can turn an extreme event into a record-breaking
event."

###

Article: Coumou, D., Rahmstorf, S. (2012): A Decade of Weather Extremes.
Nature Climate Change [DOI: 10.1038/NCLIMATE1452]

Weblink to the article once it is published:
http://dx.doi.org/10.1038/NCLIMATE1452