The acid test of a scientific theory is whether it makes predictions that eventually come true. So consider this old prediction, from a pair of researchers in Australia and New Zealand. They were summarising the results of then-primitive computerised forecasts about global warming: “The available evidence suggests that a warmer world is likely to experience an increase in the frequency of heavy precipitation events, associated with a more intense hydrological cycle and the increased water-holding capacity of a warmer atmosphere.” That was published in 1995, and it was based on research going back to the 1980s. Fast forward to 2014. In the National Climate Assessment, published last week, researchers in the United States reported that “large increases in heavy precipitation have occurred in the Northeast, Midwest and Great Plains, where heavy downpours have frequently led to runoff that exceeded the capacity of storm drains and levees, and caused flooding events and accelerated erosion”. The future, it would seem, has arrived. Climate is a difficult branch of science, full of ambiguities and uncertainties. But scientists can justly claim to have demonstrated some predictive skill about many of the potential implications of the human release of greenhouse gases. Their track record goes back to 1896, when a Swede named Svante Arrhenius predicted that emissions of carbon dioxide would cause the planet to warm. It took 80 years to be sure he was right. People in the Florida Panhandle recently had to dodge flash floods after 2 feet of rain fell in 26 hours. Torrential rains caused a Washington state hillside to collapse earlier this year. Tumultuous rainstorms and floods overwhelmed Colorado last year, and sudden floods swept through Nashville, Tennessee in 2010, and Atlanta in 2009. What led the researchers to expect this long before it actually happened? In the US, the increase in water vapour has been on the order of 3 per cent or 4 per cent since the 1970s (most of the human-caused global warming has occurred since then). That may not sound like a big jump, but the effect is enormous. Two leading scientists, Kevin E Trenberth at the National Centre for Atmospheric Research and David R Easterling at the National Oceanic and Atmospheric Administration, ran some calculations and agreed that the warming has, on average, put more than 1 trillion gallons of extra water into the air. That extra water has to fall as rain or snow. But from the elementary physics, it was long unclear whether this would mean more rainy days overall, or more intense rains, or both. “It rains harder than it used to,” said Trenberth, who could not resist adding: “When it rains, it pours.”
