The 17tn US gallons of rain (roughly 26m Olympic swimming pools) dumped on Texas by Hurricane Harvey has set a new high for a tropical system in the US, but it is unlikely to last long as rising man-made emissions push the global climate deeper into uncharted territory.
Images of flooded streets in Texas are mirrored by scenes of inundated communities in India and Bangladesh, the recent mudslides in Sierra Leone and last month’s deadly overflow of a Yangtze tributary in China. In part, these calamities are seasonal. In part, the impact depends on local factors. But scientists tell us such extremes are likely to become more common and more devastating as a result of rising global temperatures and increasingly intense rainfall.
Our planet is in an era of unwelcome records. For each of the past three years, temperatures have hit peaks not seen since the birth of meteorology, and probably not for more than 110,000 years. The amount of carbon dioxide in the air is at its highest level in 4m years.
This does not cause storms like Harvey – there have always been storms and hurricanes at this time of year along the Gulf of Mexico – but it makes them wetter and more powerful.
As the seas warm, they evaporate more easily and provide energy to storm fronts. As the air above them warms, it holds more water vapour. For every half a degree celsius in warming, there is about a 3% increase in atmospheric moisture content. Scientists call this the Clausius-Clapeyron equation.