Climate extremes: progress and future directions

This Special Issue of the International Journal of Climatology arose from two similar symposia held by the International Union of Geophysics and Geodesics (IUGG) and American Geophysical Union (AGU) in 2007. The IUGG session ‘Extreme Weather and Climate Events: Past Occurrences and Future Likelihoods’, jointly run with the International Association of Meteorology and Atmospheric Science (IAMAS), was held in Perugia, Italy in July 2007 and the AGU session ‘Extreme Weather and Climate Events: Observed and Projected Future Changes’ was held at the annual AGU Fall meeting in San Francisco, USA in December 2007. The motivation behind both symposia was the vulnerability of communities and ecosystems to climate variability and the likelihood that it will depend more on the change in intensity and frequency of extreme events than on changes in mean climate (Lynch and Brunner, 2007). For this reason the study of climate extremes in a changing climate has come to the fore in recent years but primarily since the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report (SAR) in 1995. The SAR concluded that although there was no evidence globally that extreme weather events or climate variability had increased, data and analyses were ‘poor and not comprehensive’ inspite of changes in extreme weather events observed in some regions where sufficient data were available (Nicholls et al., 1996). There was also little information or conclusive evidence on what effect anthropogenic climate change had had, or would have, on climate extremes. One reason for such ambiguity was that while there were studies of regional changes in climate extremes (such as evidenced by this Special Issue), the lack of consistency in the definition of extremes between analyses meant that it was impossible to provide a comprehensive global picture. These ambiguities in the SAR conclusions led to a number of workshops and globally co-ordinated efforts, which have made significant progress in our analysis of extremes (Nicholls and Alexander, 2007). In addition, extreme events in recent years have not only been extremely damaging but also in some cases have caused unprecedented mortality such as the extreme European heat of the 2003 summer (Foullet et al., 2007) and the intense hurricane season of 2005 (Trenberth and Shea, 2006) and have been a catalyst for more urgent analysis of the relationship between extreme weather and climate change. The impacts of most extremes are typically felt at a local or regional scale; so regional studies of climate extremes are of the highest priority for most countries for assessing potential climate impacts. However, given that climate change signals in climate extremes are difficult to detect at a regional scale, to understand fully how the climate varies and the extent to which humans have influenced the climate system requires a global approach. This in turn requires a consistent approach for analysis. Groups such as the World Meteorological Organisation (WMO) CCl/CLIVAR/JCOMM Expert Team on Climate Change Detection and Indices (ETCCDI), the European Climate Assessment (ECA) and the Asia–Pacific Network (APN), in addition to their primary aim of filling in data gaps, have aimed to provide a framework for defining and analysing observed climate extremes so that the results from different countries can be combined seamlessly. It was early realized that countries were more likely to exchange information on seasonal and/or annual climate indices e.g. heatwave duration, heavy precipitation events than they were to release raw daily or sub-daily meteorological observations. In addition, temperature and precipitation were the most widely available long-term climate variables; so most global studies have focused on analysis of these data. The first study to attempt a global analysis of temperature and precipitation extremes under the auspices of the ETCCDI was that of Frich et al. (2002) who showed that there had been significant changes in extreme climate indices, such as reductions in frost days and increases in warm nights and heavy rainfall events over the last 50 years. This approach has also been pivotal in ‘data mining’ in regions where previously little or no data had been readily available, developing ongoing capacity in these data-sparse regions and enhancing international collaboration (Peterson and Manton, 2008). Modelling groups have also now taken a similar approach through the Joint Scientific Committee (JSC)/CLIVAR Working Group on Coupled Models so that observations and model output can be compared consistently. The study by Tebaldi et al. (2006) was the first to use the multi-model approach to assess potential future changes in climate extremes showing that the twenty-first century would bring global changes in temperature extremes consistent with a warming climate. While that study also showed that global changes in precipitation extremes were consistent with a wetter world with greater precipitation intensity, the consensus and significance amongst the models were weaker when regional patterns were considered. This indicates the importance of combining global results with more regionally relevant studies to assess the impacts of these changes. These and other multi-national efforts such as that of Alexander et al. (2006) meant that by the time of the IPCC’s Third Assessment Report (TAR) and Fourth