Catastrophic floods not only destroy crops, houses and lives, as observed in recent months around the world. They also remind societies of their fundamental vulnerability to the forces of nature. Whether humans have in turn exacerbated the forces of nature by emitting greenhouse gases to the atmosphere has been an open question. Two new studies affirm. One reveals that human-induced emissions have contributed to the observed intensification of extreme rainfall events over parts of the Northern Hemisphere1. Another quantifies the impact of anthropogenic emissions on the risk of the floods2 experienced in the UK in autumn 2000.

A range of potentially damaging climatic extremes — storms, droughts, heatwaves and extreme precipitation — have been projected to become more frequent as global warming progresses. But the rarity of these extreme events makes it hard to detect trends in their frequency, determine causes, or quantify a potential contribution from anthropogenic greenhouse gas emissions. This feat was achieved3 for the European heatwave in 2003, but not for relatively small-scale events, such as the bouts of extreme precipitation that lead to flooding. After a flood, however, it is much clearer who has been affected in monetary terms. Those people might be tempted to seek compensation.

A quantifiable change in flood risk that can be pinpointed to industrial greenhouse gas emissions could mean that they have a case. Climate change litigation — the idea that the people that lose property in a global-warming-related catastrophe could sue companies that emit greenhouse gases4 — depends on scientists' ability to prove a causal link between individual natural disasters and a slowly changing climate. Such attribution is possible only in a probabilistic sense. For example, it is very likely that the flood risk was raised by more than 20% in England and Wales in 2000 because of human-made greenhouse gas emissions2.

But a risk increase by as little as 20%, at a 90% level of probability, may not be deemed sufficient in a courtroom to claim compensation. And it is not entirely clear yet how the probabilistic nature of the argument would work in legal frameworks that were originally built around proof of causation. Nor is it obvious how the blame could be distributed between various emitters of greenhouse gases. That said, it seems that the science is far enough advanced for the legal profession to become interested.

When eight US states and New York City filed a lawsuit in 2004 against five US power companies for contributing to global warming, the complaint was initially dismissed. However, the case has been reopened on appeal, and is awaiting a decision from the Supreme Court at present. If the complaint is successful, emitters will consider trading or simply reducing carbon dioxide emissions more seriously. The law may yet come to the aid of climate stabilization.