Flood, drought, fire = extreme weather = climate change?

  • 11 Mar 2011, 12:50
  • Robin

The United States based National Oceanic and Atmospheric Administration (NOAA) released research on Wednesday which found that the 2010 Russian heatwave was primarily due to a natural atmospheric process called "atmospheric blocking". It suggested that while a contribution from climate change could not be ruled out, natural variability played a much larger role.

The study was the latest to have examined the relationship between a specific extreme weather event and increasing greenhouse gases in the atmosphere.

Climate scientists are well aware of the pitfalls inherent in this kind of work - and the statement that "a single event cannot be attributed to climate change" is almost axiomatic in the public discussion around climate science. As the science blog Real Climate put it in 2005, following Hurricane Katrina:

"For a single event, regardless of how extreme, such attribution is fundamentally impossible. We only have one Earth, and it will follow only one of an infinite number of possible weather sequences. It is impossible to know whether or not this event would have taken place if we had not increased the concentration of greenhouse gases in the atmosphere as much as we have."

This argument, however, has come under increasing pressure in 2010, as media interest in a series of unusual weather events around the world intensifies.

2010 was the joint warmest year on record. Australia experienced cyclones and flooding; Pakistan saw flooding; there was drought in the Amazon; the Northern hemisphere had an unusually cold winter whilst countries around the world set new record temperature highs.

The problems inherent in looking at particular events does not mean that science has nothing to say. Climatologist Francis Zwiers put the answer well in his written evidence to the US Senate earlier this week:

"… individual extreme events cannot be ascribed to human influence on the climate system in the sense that the event could not have occurred if it were not for human influence. It is, however, possible to assess how human influence on climate may be "loading the weather dice", making some events more likely, and others less likely."

Zwiers cited two studies published in Nature last month, both of which took the "loaded dice" approach to addressing the link between climate change and flooding.The first (Min et al 2011) found that the probability of intense rainfall on any given day has increased by 7% over the last 50 years, across the whole of the Northern Hemisphere.

The second (Pall et al 2011) found that the likelihood of a single event - the UK flooding of 2000 - was significantly increased by human-induced climate change. Zwiers also discussed research by the same Oxford team in 2004, which estimated that it is highly likely that human influence had at least doubled the risk of a heatwave like the one that occurred in Europe in 2003.

Predictably, the NOAA study into the Russian heatwave has prompted some triumphalism from climate sceptics, with the prominent website Watts Up With That stating

"Maybe this will be a lesson to those in the MSM and eco blogland who immediately jump on every newsworthy weather event, with no supporting evidence, attributing it to "global warming", "climate change" or "climate disruption"…"

It is true that statements made immediately after any extreme weather event should be treated with caution. Producing scientific research is a slow process (the two papers on flooding were submitted to Nature in March 2010), and this is a tricky area.

The WUWT statement ignores the second conclusion from the NOAA research - that while the scientists could not attribute the intensity of this particular heat wave to climate change, they found that extreme heat waves are likely to become increasingly frequent in the region in coming decades - with the risk rising "from less than one percent in 2010, to 10 percent or more by the end of this century."

It looks as though conversations like this may be continuing into the future.

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