It is evident that the three-day rainfall amounts in the spring of 2016 in France were rare events in the present-day climate. They occur roughly every hundred years in the Loire region, and are even more unusual in the Seine basin. Nevertheless, the different climate models produce consistent and thus robust results. They show that, because of climate change, the probability of regional events of at least the same intensity as 2016 has increased by factors of 2 (Loire) and 2.3 (Seine) in comparison with a world without climate change. They also show that the probabilities of less extreme events have also increased as a result of climate change. The fact that these enhanced probabilities can be attributed to climate change means that such events will occur even more often in the future.
Such attribution studies of selected weather extremes such as heatwaves, droughts and intense precipitation have been conducted regularly since 2011, generally on the basis of models. At the end of the year subsequent to the event, these studies are published in special supplements to the Bulletin of the American Meteorological Society. Climate change was found to have influenced the frequency or intensity of 65% of the more than 100 events studied so far, while no influence could be demonstrated for 35%. This illustrates how climate change is already having a significant impact on extreme events.
Because of the time lag factor, however, these studies fail to meet the reasonable criterion of rapid attribution mentioned above. For this reason, for a few years now, articles containing an attribution analysis have been submitted to specialist journals within a few weeks of an event (rapid attribution) – the study on the floods in France outlined above, for example, was online just three weeks after the event. A further recent example is the torrential rain and flooding that occurred in August 2016 in Louisiana, particularly in the Baton Rouge area, where one place experienced just under 650 litres/m2 of rain over a period of three days.
A little over three weeks after the event, a study (van der Wiel et al., HESSD, 2016) appeared online, stating that an extreme event of this kind now occurs roughly every 30 years in the central Gulf Coast region of the USA, and has become more frequent by a factor of at least 1.4 as a result of climate change. Similar studies have also been published for a number of heat and extreme precipitation events in recent years.