What can be said about the recent extreme weather and climate change?
Image - jono-haysom-brisbane (note)
Flooding in Brisbane. CC Jono Haysom – flickr
Over the last month, the world has seen extremes of weather with the coldest December on record in the UK, and widespread destruction caused by flooding in Australia and Brazil. Sceptic commentators have been quick to claim the US/EU cold snap as proof that AGW is a myth, while arguments have sparked up about what, if anything, can be said about the links between extreme weather events like cold spells, flash-flooding and climate change.
Cold winters
There have been suggestions that abnormally cold weather in the US and Europe could be linked to the shrinking of Arctic sea ice. James E Overland, an Arctic expert based at the American NOAA published a peer-reviewed paper in 2010 called “Hot Arctic-Cold Continents: Hemispheric Impacts of Arctic Change” which discusses the cold winter of 2009-10 alongside a climate pattern called the North Atlantic Oscillation (NAO).
The NAO is a cycle of shifts in atmospheric pressure, measured by the difference between air pressure over Iceland and the Azores. A positive NAO value indicates there will be a cold Arctic and warmer continents, while a (rarer) negative NAO can have the opposite effect.
Discussing Overland’s study, Dr Jeff Masters elaborates:
“The Arctic is normally dominated by low pressure in winter, and a “Polar Vortex” of counter-clockwise circulating winds develops surrounding the North Pole. However, during the winter of 2009-2010, high pressure replaced low pressure over the Arctic, and the Polar Vortex weakened and even reversed at times, with a clockwise flow of air replacing the usual counter-clockwise flow of air around the pole. This unusual flow pattern allowed cold air to spill southwards and be replaced by warm air moving poleward.”
2009-10 was the strongest NAO negative value ever measured, and the only the fourth time since records began that the vortex actually spun inversely. It was the resultant “southward spill” of cold air that was responsible for one of the UK’s coldest winters in decades. Meanwhile, warm air rushed north and record warm winter spells for Canada and Greenland were seen.
Overland’s thesis is that the loss of Arctic sea ice was the principal factor for these “freak” occurrences. But other oscillations, weather patterns like El Nina/La Nino and natural chaotic variability also played a part, he concludes, and the study actually concludes that a strong correlation between Arctic sea ice loss and a negative NAO cannot yet be drawn.
Overland’s theory is echoed by an analysis of the recent severe winters in Northern Europe by Petoukhov & Semenov (of the Potsdam Institute for Climate Impact Research and Leibniz Institute of Marine Sciences respectively). The pair have similar thesis to Overland, but are more confident of the links between cold weather and a warming planet. They say, “Our results imply that several recent severe winters do not conflict the global warming picture but rather supplement it.”
Of course, one or two peer-reviewed papers don’t settle the issue. NCAR Scientist Kevin Trenberth has in the past argued that climate change is having a systemic effect on weather patterns throughout the atmosphere:
I’m sure you’ve probably heard â?¦ “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago
But does his very preliminary view of the cold weather link to arctic sea ice loss?
I am aware of some German work that suggests the cold outbreak pattern might somehow be stimulated by reduced Arctic Sea Ice. I have not seen the study but count me skeptical.
Rain and floods
Elsewhere, torrential rain has recently devastated Queensland, Australia and an area 40km north of Rio de Janeiro, Brazil. Despite their geographical disparities, the catastrophes bear some similarities. Both regions experienced the heavy rainfalls following severe droughts. Author and Blogger Joseph Romm has suggested that ” this climate-whipsawing from mega-drought to mega-flood will become increasingly common as human emissions intensify the hydrological cycle”.
Yet the specific causes of the rainfall are related to different weather events. The storm near Rio drew its power from the sea, which was much warmer than usual for December.
The molecular activity of a warm sea is greater than a cold one, making evaporation and rainfall more likely. On his Wunderground blog, Jeff Masters calculated that ” sea surface temperatures â?¦ along the Brazilian shore nearest the disaster area â?¦ were the second warmest on record since 1900.”
The rain in Australia meanwhile appears to have been caused by a major La Nina event, according to Climate Signals and the UK Met Office.
Unlike the storms in Brazil, La Nina is associated with cool Pacific sea temperatures. Indeed, as Climate Signals reports, “Temperatures below the surface [were] up to 4 °C below normal in central and eastern parts comparable to the La Niña event of 1988”. During La Nina, atmospheric patterns such as Pacific trade winds combine with cool sea temperatures, making tropical cyclones more frequent in northern Australia. 2010 saw one of the strongest La Nina’s on record.
The Met Office say the extreme rain in Australia is consistent with computer modelling forecasts of La Nina and there is no evidence to link them to climate change. Professor Neville Nicholls, a meteorologist and one of the lead authors of the 2007 IPCC report agrees, and notes that scientifically, it is very hard to make a link to any one weather event and climate change.
But, he adds, when we start to look at long-term patterns of weather and the join the dots between the numerous anomalies, things are different. He reflects, “Putting them together, you really have to strain credibility to say it has nothing to do with climate change.”
