A group of climate experts, including Prof James Hansen, claim recent heat waves and extreme summers can be attributed to global warming and not random chance.
The group outline this view in a new study, published in PNAS, which compared recent temperatures to a 1951-1980 base period. Their findings revealed that extremely hot summers—those with temperatures 3 standard deviations greater than the mean temperature in the base period—occurred much more frequently in the past several years than during the base period, when they were practically absent.
This warming could potentially make extremely hot summers the norm and possibly contribute to extreme droughts and floods, according to the authors. They use the analogy that the increasing global temperatures have “loaded” the the “climate dice”, increasing the chances of extreme weather events.
“It follows that we can state, with a high degree of confidence, that extreme anomalies such as those in Texas and Oklahoma in 2011 and Moscow in 2010 were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small,” the authors write.
The lead author on the paper, James Hansen, is well known for his research in climate change and was the first highlight the issue to the US Congress in 1988.
Our colleagues at the UK SMC collected the following expert commentary. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476; firstname.lastname@example.org).
Dr Peter Stott, Head of Climate Monitoring and Attribution at the MET Office, said:
“As far as how climate change is changing the statistics of extreme weather around the world, this new paper is broadly in line with previous work including work that we published in 2008 showing that ”hot summers which were infrequent 20–40 years ago are now much more common and that our projections indicate that the current sharp rise in incidence of hot summers is likely to continue.”
“However there are problems with saying, as James Hansen does, that the Moscow 2010 and Texas 2011 heatwaves ‘were caused by global warming, because their likelihood was negligible prior to the recent rapid global warming.’ 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. Both of these heatwaves were associated with unusual circulation characteristics, and a large part of the explanation for both lies in that – La Nina conditions for Texas 2011 and an unusual blocking high pressure system for Moscow 2010. But recent 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. As part of our ACE initiative (Attribution of Climate-related Events), whose next meeting is in Oxford in September, we are seeking to develop the science needed to quantify better the changing odds of extreme weather events.”
Dr Myles Allen, Head of the Climate Dynamics Group in the Department of Physics at the University of Oxford, said:
“The results of Hansen et al are broadly in line with recent similar papers showing substantial increases in the probability of extreme heatwaves as a result of the warming that has occurred since the 1960s, but their interpretation goes further – further than many scientists are comfortable with. It is doubly misleading to say “we can state, with a high degree of confidence, that [recent heatwaves] were a consequence of global warming because their likelihood in the absence of global warming was exceedingly small.” First, it is very hard to quantify with any confidence the absolute probability of a weather event occurring in a hypothetical pristine climate: the public understand that extreme weather events happen, and the rarer and more extreme the event, the harder it is to quantify the odds of it happening in any given year.
The second problem is that this statement implies 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. And we can quantify how odds are changing as a consequence of external factors with much greater confidence than we can quantify what the odds were in the first place.
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.”
Bob Ward, Policy and Communications Director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science, said:
“This important new study shows that extreme heatwaves have become more widespread across the world over the past 30 years. This provides proof that the undeniable increase in global average temperature over the past 30 years has been accompanied by an increase in the occurrence of heatwaves. This differs from other recent studies which have calculated how climate change increased the probability of an individual extreme weather event, like loading a dice. But these other studies have not offered the same level of proof because you cannot prove that a dice is loaded just by throwing a six once. This study is important because we know that extreme heat can be fatal, and the 2003 European heatwave, for instance, killed about 70,000 people.
“The paper also points out that this increase in extreme heatwaves, which are still comparatively rare even if they are more frequent than 30 years ago, can only be detected on a global scale and not by looking in isolation at tiny parts of the Earth’s surface, such as the United Kingdom or the United States. So-called climate change ‘sceptics’, in the attempt to mislead the public, point to the recent cool summer weather in the UK as evidence that global warming does not exist or is nothing to worry about. But proper scientific assessment of the climate means carrying out robust statistical analysis of long-term trends instead of just looking at the weather outside the window. As the authors of this study point out, the public need to be aware of the consequences that climate change is already having, such as the increase in extreme heatwaves, and should not be fooled into thinking they are just distant theoretical possibilities.”