In an era of dire climate records the US and South Asia floods won't be the last
Thursday 31 August 2017 09.47 BST
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.
This means the skies fill more quickly and have more to dump. In Harvey’s case, the surface temperature in the Gulf of Mexico is more than a degree higher than 30 years ago.
Yes, the storm surge was greater because sea levels have risen 20cm as a result of more than 100 years of human-related global warming. This has melted glaciers and thermally expanded the volume of seawater.
As the rain in Texas moved towards the 120cm US record set in 1978, the nation’s meteorologists have had to introduce a new colour for their charts. It may not be the last revision.
“For large countries like the United States, we can expect further rainfall records – and not just for hurricanes,” said Friederike Otto, deputy director of the Environmental Change Institute at the University of Oxford. This is part of a wider trend. “For the globe, we’ll see heat and extreme rainfall records fall for the foreseeable future,” she predicted.
She cautioned that the situation is likely to be different from country to country. Many factors are involved, but human impact on the climate has added to the tendency for more severe droughts and fiercer storms.
High tides have added to the unusually harsh monsoon flooding in India and Bangladesh that has killed about 1,000 people in recent weeks and forced millions from their homes.
Climatologists are able to attribute with growing accuracy the impact of human emissions on extreme weather events, but much remains uncertain.
A key focus now is whether climate change is connected to the “stalling” of storms. In the US, hurricanes usually move inland and diminish in power as they get further from the sea. Harvey, however, was stationary for several days – which is the main factor in its rainfall record.
Scientists have said this may be the single biggest question posed by Harvey. “I’m not aware of anyone asking this before. I’m not sure anyone would have predicted this kind of event,” said Tim Palmer a Royal Society research professor at the University of Oxford.
Researchers have recently identified a slowdown of atmospheric summer circulation in the mid-latitudes as a result of strong warming in the Arctic. But Palmer said such studies of pressure patterns need more powerful analytical tools, including supercomputers.
In the US, however, such research has become highly politicised. President Donald Trump claims climate change is a myth invented by China. He has announced that the US will pull out of the Paris climate treaty and cut funding for related research.
“It shouldn’t be a political matter to try to understand how much more frequent events like Harvey will become in the future,” said Palmer. “It appalls me how basic science has become embroiled in politics like this.”