The European heatwave of 2003: was it merely a rare meteorological event or a first glimpse of climate change to come? Probably both, is the answer, and the anthropogenic contribution can be quantified.
The European summer of 2003 was characterized by highly anomalous meteorological conditions1, and was extremely hot and dry2,3. In the northern parts of the continent, the summer was perceived as beautiful and warm. But in central and southwestern Europe, the heat was prolonged and intense, and the economic and societal consequences were disastrous (as described in Box 1).
Given the heatwave's severe repercussions, the question has arisen whether the summer of 2003 is evidence of man-made climate change. On page 610 of this issue, Stott, Stone and Allen4 take a major step towards answering this difficult question. Previous studies had found that recent changes in the European summer climate were consistent with climate-change scenarios5,6, but there had been no attempts at a rigorous attribution of cause and effect. Indeed, because the atmosphere is a chaotic dynamical system, it is impossible to attribute — in a causal sense — an individual episode of extreme weather to changes in atmospheric composition. Nevertheless, it is feasible to estimate the probability or risk of occurrence of a certain weather event under natural and modified climatic conditions. This is the avenue taken by Stott and colleagues.
Using one of the leading global climate models available, the authors derive the probability distributions of European summer temperatures for two sets of climate simulations, each covering the period since 1900. The first set accounts for the past effects on climate that were due to variations in solar and volcanic activity, as well as to man-made influences (including increases in greenhouse-gas concentrations). The second set mimics a natural climate by prescribing natural factors alone. Stott and colleagues then calculate the changed risk of extremely hot summers that is attributable to past anthropogenic emissions of greenhouse gases, using a comparison of observed and simulated summer temperatures to account for uncertainties in man-made warming and natural variability. They find, at a confidence level of greater than 90%, that more than half of the risk of 2003-like extreme European summers is attributable to human influences on the climate system.
Methodologically, Stott and colleagues4 use an approach developed for detecting global climate change and attributing causes to the changes identified. There is long experience with such studies, all of which find that a significant anthropogenic contribution is required to explain the observed global climate records of the past 30–50 years7,8. The new study fits into these results, as the probability of extreme heatwaves must change as mean temperatures increase. The details of the analysis are rather complex. But the basic interpretation of the main result is comparatively straightforward: anthropogenic warming shifts the statistical distribution of summer temperatures towards warmer conditions, and this has a dramatic impact on the chance of temperatures exceeding some threshold out in the upper tail of the temperature distribution.
What about the limitations of the new work? We will mention two. First, Stott et al. address the whole summer of 2003 (and not the extreme heatwave in early August), and all of continental and southern Europe (not the much smaller central European region where the heatwave was most intense). Consideration of shorter-term and smaller-scale heatwaves will require higher computational resolution9, and will need to take the complexities of land-surface processes into consideration1,3. Accounting for these factors is a challenge. Second, representing natural climate variability is a general difficulty in studies attempting to attribute causes to particular effects. Stott et al. show that their model appropriately represents the spectrum of continental-scale European climate variability on interannual to interdecadal timescales. But more detailed studies will be needed to corroborate this conclusion, as there are large uncertainties in the estimates of natural climate variability-derived from both models and observations8.
Nonetheless, Stott and colleagues' work constitutes a breakthrough: it is the first successful attempt to detect man-made influence on a specific extreme climatic event. Such events are among the most notable features of a changing climate, not least given their impact on human affairs. Another article in this issue, by Allen and Lord (page 551)10, discusses how refined analyses might lead to liability claims for costs incurred by climatic shifts. The advent of such ‘attribution studies’ might profoundly affect the course of international negotiations on ways to mitigate, adapt to and ultimately pay for the consequences of climate change.
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