by Judith Curry
The latest issue of the Bulletin of American Meteorological Society has published a collection of papers that illustrate different methodologies for attributing causes of recent extreme weather events.
Attribution of extreme events shortly after their occurrence stretches the current state-of-theart of climate change assessment. To help foster the growth of this science, this article illustrates some approaches to answering questions about the role of human factors, and the relative role of different natural factors, for six specific extreme weather or climate events of 2011. – TC Petersen, PA Stott, S. Herring
The collection of papers can be found online at this [link]. From the abstract:
Not every event is linked to climate change. The rainfall associated with the devastating Thailand floods can be explained by climate variability. But long-term warming played a part in the others. While La Niña contributed to the failure of the rains in the Horn of Africa, an increased frequency of such droughts there was linked to warming in the Western Pacific– Indian Ocean warm pool. Europe’s record warm temperatures would probably not have been as unusual if the high temperatures had been caused only by the atmospheric flow regime without any long-term warming.
Calculating how the odds of a particular extreme event have changed provides a means of quantifying the influence of climate change on the event. The heatwave that affected Texas has become distinctly more likely than 40 years ago. In the same vein, the likelihood of very warm November temperatures in the UK has increased substantially since the 1960s.
Comparing climate model simulations with and without human factors shows that the cold UK winter of 2010/2011 has become about half as likely as a result of human influence on climate, illustrating that some extreme events are becoming less likely due to climate change.
The titles and authors of the papers are (note: there are no abstracts for the individual papers):
Introduction – Peter Stott et al.
Historical context – Francis Zwiers et al.
The absence of a role of climate change in the 2011 Thailand floods – Geert Jan van Oldenborgh
Exceptional warming in the western Pacific-Indian warm pool has contributed to more frequent droughts in Africa – Chris Funk et al.
Did human influence on climate make the Texas drought more probable? – David Rupp et al.
Contribution of atmospheric circulation to the remarkable European temperatures of 2011 – Julien Cattiaux et al.
Have the odds of warm November temperatures and of cold December temperatures in central England changed? – N. Massey et al.
Lengthened odds of the cold UK winter of 2010/2011 attributable to human influence – Nikolaos Christidis and Peter Stott
Conclusions – Peter Stott et al.
Some excerpts from the Conclusions:
The section on historical context summarizes the evidence that human influence has affected trends and long-term behavior of temperature and pre- cipitation extremes around the globe, thus altering the types and frequencies of punches for which our boxer must train. This is to be anticipated from theo- retical expectations of a warmer world. The recent IPCC SREX report concluded that “it is likely that anthropogenic influences have led to warming of extreme daily minimum and maximum temperature at the global scale” and that “there is medium confidence that anthropogenic influences have contributed to intensification of extreme pre- cipitation at the global scale.” But even if human influence is making a particular type of event more likely on average, because of natural variability it does not necessarily follow that its likelihood is greater every year. So while it has been argued that in the anthropocene all extreme weather or climate events that occur are altered by human influence on climate (Trenberth 2011), and although it is difficult to prove that a particular extreme weather or climate event was not in some way influenced by climate change, this does not mean that climate change can be blamed for every extreme weather or climate event. After all, there has always been extreme weather.
The contributions in this article examining some of the specific extreme weather or climate events of 2011 demonstrate the importance of understanding the interplay of natural climate variability and anthropogenic climate change on their occurrence. We should not expect that climate change plays the major role in every extreme weather or climate event and indeed the rainfall associated with the devastating Thailand floods was not especially unusual. In this case, nonclimatic factors such as changes in land use and water management probably played a bigger role in the disaster. Thus attribution of the impacts of weather-related events to climate variability and change requires careful consideration of possible confounding factors not related to climate (Hegerl et al. 2010).
While much work remains to be done in attribution science, to develop better observational datasets, to improve methodologies, to make further progress in understanding and to assess and improve climate models, the contribu- tions in this article demonstrate the potential that already exists for meaningful assessments of the connection between specific extreme weather or climate events that occurred in a particular year and climate change. Whether readers react with excitement at the possibilities already demonstrated, or with irritation at the gaps and limitations still present, our hope as editors is that this initial selection of investigations encourages further de- velopment of the capability to produce timely and reliable assessments of recent extreme weather or climate events. Such an enterprise is much further advanced for climate monitoring—as shown by the maturity of the annual State of the Climate report —but even there important uncertainties exist and new assessments of past years will emerge, just as they will for attribution as understanding develops. By developing the scientific underpinning, the ability to put recent extreme weather or climate events into the longer- term context of climate change should improve as each year goes by.
JC comments: This is an interesting collection of papers, and the synthesis in the Introduction, Historical Context, and Conclusions is valuable. While I have some quibbles with the methods used in a few of the studies, this collection reflects a maturing of the extreme event attribution field in terms of examining the historical record, comparing regional simulations from models with observations, observed atmospheric circulation patterns and sea surface temperature anomalies, and consideration of confounding factors. All of these factors should be considered when attempting to explain the causes of an extreme event, and whether AGW played a role in increasing the odds of the event.