IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical EnvironmentFAQ 3.2 | Has Climate Change Affected Individual Extreme Events?A changing climate can be expected to lead to changes in climate and weather extremes. But it is challenging to associate a singleextreme event with a specific cause such as increasing greenhouse gases because a wide range of extreme events could occur even inan unchanging climate, and because extreme events are usually caused by a combination of factors. Despite this, it may be possible tomake an attribution statement about a specific weather event by attributing the changed probability of its occurrence to a particularcause. For example, it has been estimated that human influences have more than doubled the probability of a very hot European summerlike that of 2003.Recent years have seen many extreme events including the extremely hot summer in parts of Europe in 2003 and 2010, and the intenseNorth Atlantic hurricane seasons of 2004 and 2005. Can the increased atmospheric concentrations of greenhouse gases be consideredthe ‘cause’ of such extreme events? That is, could we say these events would not have occurred if CO 2 had remained at pre-industrialconcentrations? For instance, the monthly mean November temperature averaged across the state of New South Wales in Australia forNovember 2009 is about 3.5 standard deviations warmer than the 1950-2008 mean, suggesting that the chance of such a temperatureoccurring in the 1950-2008 climate (assuming a stationary climate) is quite low. Is this event, therefore, an indication of a changingclimate? In the CRUTEM3V global land surface temperature data set, about one in every 900 monthly mean temperatures observedbetween 1900 and 1949 lies more than 3.5 standard deviations above the corresponding monthly mean temperature for 1950-2008. 1Since global temperature was lower in the first half of the 20th century, this clearly indicates that an extreme warm event as rare as theNovember 2009 temperature in any specific location could have occurred in the past, even if its occurrence in recent times is more probable.A second complicating issue is that extreme events usually result from a combination of factors, and this will make it difficult to attributean extreme to a single causal factor. The hot 2003 European summer was associated with a persistent high-pressure system (which ledto clear skies and thus more solar energy received at the surface) and too-dry soil (which meant that less solar energy was used forevaporation, leaving more energy to heat the soil). Another example is that hurricane genesis requires weak vertical wind shear, as wellas very warm sea surface temperatures. Since some factors, but not others, may be affected by a specific cause such as increasinggreenhouse gas concentrations, it is difficult to separate the human influence on a single, specific extreme event from other factorsinfluencing the extreme.Nevertheless, climate models can sometimes be used to identify if specific factors are changing the likelihood of the occurrence ofextreme events. In the case of the 2003 European heat wave, a model experiment indicated that human influences more than doubledthe likelihood of having a summer in Europe as hot as that of 2003, as discussed in the AR4. The value of such a probability-basedapproach – “Does human influence change the likelihood of an event?” – is that it can be used to estimate the influence of externalfactors, such as increases in greenhouse gases, on the frequency of specific types of events, such as heat waves or cold extremes. Thesame likelihood-based approach has been used to examine anthropogenic greenhouse gas contribution to flood probability.The discussion above relates to an individual, specific occurrence of an extreme event (e.g., a single heat wave). For the reasons outlinedabove it remains very difficult to attribute any individual event to greenhouse gas-induced warming (even if physical reasoning or modelexperiments suggest such an extreme may be more likely in a changed climate). On the other hand, a long-term trend in an extreme(e.g., heat wave occurrences) is a different matter. It is certainly feasible to test whether such a trend is likely to have resulted fromanthropogenic influences on the climate, just as a global warming trend can be assessed to determine its likely cause.____________1 We used the CRUTEM3V land surface temperature data. We limit our calculation to grid points with long-term observations, requiring at least 50 non-missing valuesduring 1950-2008 for a calendar month and a grid point to be included. A standard deviation is computed for the period 1950-2008. We then count the number ofoccurrences when the temperature anomaly during 1900-1949 relative to 1950-2008 mean is greater than 3.5 standard deviations, and compare it with the totalnumber of observations for the grid and month in that period. The ratio of these two numbers is 0.00107.by Working Groups I and II in the AR4. It provides detailed guidance onthe procedures that include two main approaches to attribute a changein climate to causes. One is single-step attribution, which involvesassessments that attribute an observed change within a system to anexternal forcing based on explicitly modelling the response of thevariable to the external forcings. The alternate procedure is multi-stepattribution, which combines an assessment that attributes an observedchange in a variable of interest to a change in climate, with a separateassessment that attributes the change to external forcings. Attributionof changes in climate extremes has some unique issues. Observed data127