| 英文摘要: | Climate change includes not only changes in mean climate but also in weather extremes. For a few prominent heatwaves and heavy precipitation events a human contribution to their occurrence has been demonstrated1, 2, 3, 4, 5. Here we apply a similar framework but estimate what fraction of all globally occurring heavy precipitation and hot extremes is attributable to warming. We show that at the present-day warming of 0.85 °C about 18% of the moderate daily precipitation extremes over land are attributable to the observed temperature increase since pre-industrial times, which in turn primarily results from human influence6. For 2 °C of warming the fraction of precipitation extremes attributable to human influence rises to about 40%. Likewise, today about 75% of the moderate daily hot extremes over land are attributable to warming. It is the most rare and extreme events for which the largest fraction is anthropogenic, and that contribution increases nonlinearly with further warming. The approach introduced here is robust owing to its global perspective, less sensitive to model biases than alternative methods and informative for mitigation policy, and thereby complementary to single-event attribution. Combined with information on vulnerability and exposure, it serves as a scientific basis for assessment of global risk from extreme weather, the discussion of mitigation targets, and liability considerations. Significant trends in temperature and precipitation extremes over the recent decades have been observed7, 8, 9, 10 and attributed to human influence11, 12, 13, 14, 15. Although none of these extreme events was exclusively anthropogenic in a deterministic sense, climate change has changed their odds, which can be expressed as a change in the fraction of attributable risk (FAR; refs 2, 16). The FAR framework has been used to quantify the human influence on the occurrence of individual recent heat waves and dry spells1, 2, 3, 4, 17 and heavy precipitation and flooding events5. Although the framework is effective, the underlying model experiments often have to be designed specifically for certain events. Thus, the FAR estimates for the 2003 European heatwave are only valid for the observed anomaly over the specific area, but do not apply to a similar event occurring further east. Here we extend the FAR framework from individual observed events to global scales. Thereby we address the question of what fraction of extremes occurring globally is attributable to human influence. We use the two metrics ‘probability ratio (PR)’ and FAR (ref. 2), defined as PR = P1/P0 and FAR = 1 − (P0/P1), respectively, where P0 is the probability of exceeding a certain quantile during the pre-industrial control period and P1 the probability of exceeding it, for example, in present-day climate (see Methods). In simple words, PR is the factor by which the probability of an event has changed, and FAR indicates the fraction attributable to humans. ‘Fraction of events’ throughout the text should be interpreted as an anthropogenic contribution to the probability of such events, rather than some events being anthropogenic and some not. We base our estimates on well-defined percentiles of daily temperature and precipitation derived from long pre-industrial control runs of 25 CMIP5 models (see models in Supplementary Table 1). In response to increasing global temperatures, models project more heavy precipitation days, as illustrated by histograms aggregating daily precipitation (Fig. 1) across Northern Europe and North America (see Methods). The simulated occurrence of heavy precipitation days under present-day warming of 0.85 °C (blue lines) is only slightly higher than in pre-industrial conditions. At a warming of 2 °C (red lines) the probability of the most extreme cases, exceeding the pre-industrial 99.99%-quantile, increases by about a factor of 1.5 to 3 depending on region and model (lower panels). This implies that on average across the area an event expected once every 10,000 days (about 30 years), in pre-industrial conditions, is expected every 10 to 20 years at a 2 °C warming. The wet tail of the precipitation distribution becomes fatter; thus, the PR increases most rapidly for the most intense and rarest events (Fig. 1) at the expense of days with moderate, low or no precipitation. This is consistent with the finding that in some cases mean precipitation decreases (primarily owing to large-scale circulation change), whereas extreme precipitation increases owing to increased water-holding capacity of warmer air18.
| http://www.nature.com/nclimate/journal/v5/n6/full/nclimate2617.html
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