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New study linking hot summers to climate change: scientists react

  • 08 Aug 2012, 15:00
  • Verity Payne

New research linking hot summers to climate change from climate scientist and activist James Hansen has prompted quite a response from the media and other scientists. So what does the paper really tell us, and what are the arguments about?

You've may have heard the so-called loaded dice analogy for climate change. The idea comes from a 1988 research paper by Hansen and colleagues, where they posited the notion of a die with two red faces representing hot summer temperatures. As climate change progresses, they wrote, there might be three or even four red faces on the dice, loading the odds in favour of more frequent hot summers and heatwaves.

Now new research from Hansen and colleagues provides evidence that the climate dice have indeed become loaded over the last 30 years. The research comes in the wake of a summer of unusual weather events - a US summer heatwave, warm weather over Greenland, not to mention the wettest British June on record. So this research has provided fresh material for a media and blogosphere already pretty fixated on extreme weather and its precise links to climate change.

In this latest research paper Hansen and colleagues studied average summer temperatures over land areas from the last 30 years as archived in the NASA Goddard Institute for Space Studies (GISS) temperature record, compared to the average summer temperature for land areas for the previous 30 year period (1951 to 1980).

We take a look at how Hansen's new paper compares to other research on climate change and heatwaves, and the responses of other climate scientists to this new research.

Heatwaves becoming more frequent

Hansen and colleagues find that Earth's land areas have become more likely to be subject to hot summers over the last 30 years, and that extremely hot summers - experienced by less than one per cent of Earth's land area between 1951 and 1980 - now cover as much as ten per cent of Northern Hemisphere land area each year.

These basic findings are very similar to previous research in this area. The IPCC's Special Report into Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) - considered to be the best current synthesis on extreme weather and climate change - found from observations since 1950:

"It is very likely that there has been [...] an overall increase in the number of warm days and nights, at the global scale, that is, for most land areas with sufficient data [...] In many (but not all) regions over the globe with sufficient data, there is medium confidence that the length or number of warm spells or heat waves has increased."

The SREX review also suggests that in the future:

"It is virtually certain that increases in the frequency and magnitude of warm daily temperature extremes and decreases in cold extremes will occur in the 21st century at the global scale. It is very likely that the length, frequency, and/or intensity of warm spells or heat waves will increase over most land areas."

So Hansen and colleagues' paper agrees with previous research suggesting that warmer summers are becoming more common and this trend is likely to continue.

Would recent heatwaves have occurred in the absence of global warming?

Hansen and colleagues also conclude in their paper that the heatwaves in Texas 2011, Moscow 2010, and France 2003 "almost certainly would not have occurred in the absence of global warming". They argue that the occurrence of such extremely hot summers was negligible between 1951 and 1980, so that sort of weather is essentially the emergence of a new category.

This interpretation is the point on which Hansen and colleagues seem to differ from most previous research in this area, and that has prompted criticism from other climate attribution experts.

Martin Hoerling, research meteorologist at the US National Oceanographic and Atmospheric Administration's Earth Systems Research Laboratory, tells Live Science blog that Hansen and colleagues' interpretation is "flawed scientifically". Peter Stott, Head of Climate Monitoring and Attribution at the UK Met Office seems to agree:

"While we can provide evidence that the risk of heatwaves has increased, we cannot say that the chances of such heatwaves were negligible before global warming set in."

This, Stott says, is because the heatwaves in Moscow 2010 and Texas 2011 were both associated with unusual large-scale weather patterns. The Moscow heatwave involved a blocking high pressure system, and Texas was affected by a La Niña in 2011.

Says Stott:

"[R]ecent research, not cited by Hansen's paper, has shown that anthropogenic climate change has increased the odds of record breaking temperatures when such unusual weather patterns are set up. Further research is needed to understand how climate change could be affecting such aspects of climate variability as the position of the jet stream. Therefore it is the interplay of variability and climate change that counts."

Hansen and colleagues do mention the effect of large-scale weather patterns in their paper, saying:

"Certainly the locations of extreme anomalies in any given case depend on specific weather patterns. However, blocking patterns and La Niñas have always been common, yet the large areas of extreme warming have come into existence only with large global warming. Today's extreme anomalies occur as a result of simultaneous contributions of specific weather patterns and global warming."

So Hansen and colleagues concede that weather patterns play a role in extreme heat events, but they argue that the extreme heat events wouldn't have happened in the absence of climate change.

Hoerling disagrees, telling Live Science:

"The weather patterns responsible for most of today's heat waves would have happened regardless of human-induced climate change"

Hoerling also suggests that reducing atmospheric carbon dioxide to the level Hansen says is safe "would not eradicate heat waves."

Another problem with Hansen and colleagues' interpretation, according to Myles Allen of the University of Oxford, is that it:

"[I]mplies that until we do see weather events that would have been exceedingly unlikely without global warming, we are not seeing the consequences of global warming in the weather. This is incorrect. The odds of getting a six with an unloaded dice are not exceedingly small. But this doesn't mean we can't increase them by loading the dice, and if you roll a six with a loaded dice, there is clearly a sense in which that outcome was affected by the loading even though there was a good chance that it might have happened anyway."

Once again the range in scientific expert opinion on the links of extreme weather to climate change has been thrust into the spotlight in something of a media circus. But is it really appropriate to have these sorts of discussions playing out in the media? After all, what is the public to think about this?

Says Allen:

"I understand there are many 'climate communicators' who feel probability is too abstract, and that we need to be able to point to damaging weather events that would not have occurred without human influence on climate for the public to take this issue seriously, but I think this underestimates the public. People understand risk much better than we give them credit for."

Realistically, all that scientists are able to ask is whether the odds of a particular weather event have been altered over the last century as the world's average temperature has risen.

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