November 19, 2018, by Tim Radford
Hurricane Harvey leaves its mark on Houston, August, 2017. Image: By Jill Carlson (jillcarlson.org), via Wikimedia Commons
Once again, US government scientists warn that hurricane and flood hazard is amplified by a warming world. But worse storms are caused by big cities too.
LONDON, 19 November, 2018 – Worse storms are on the way, as many Americans know all too well. Hurricane Katrina was the costliest natural disaster ever to hit the US: it blew ashore over New Orleans in August 2005 to claim at least 1,833 lives and wreak economic damage worth, in today’s prices, $160bn.
And however bad it was, climate change made it worse. Because of global warming up to that point, up to 9% more rain fell over the city, some of it to sweep away the river defences and precipitate disastrous flooding.
A second study, also in Nature, warns: big cities make bad storms even worse. Urbanisation – all those roads, pavements, rooftops and so on – multiplies the risk of flooding on average 21-fold. The growth of Houston in Texas left a city at the mercy of Hurricane Harvey in 2017: the scale of flooding was without precedent.
The research is based on computer modelling of the impact of overall planetary warming – around 1°C in the past century – on local sea and coastal conditions.
Rising economic harm
Warmer atmospheres hold more water. With each 1°C rise, the capacity to absorb moisture increases by 7% , so in a warmer world storms will be wetter. With higher temperatures, storms are likely to be more ferocious. Researchers have repeatedly warned that because of these simple principles, as global temperatures rise, the US faces ever bigger economic losses each succeeding hurricane season.
Water that falls on forest or wetland or coastal savannah is at least partly absorbed. Hard rain that hits tarmacadam and concrete could swiftly become a flash flood. So the latest study is a confirmation of much previous research.
“Efforts to build flood mitigation strategies must use an improved understanding of the multiple processes in place”
And although President Trump has condemned climate change science as a hoax devised by the Chinese, and announced a withdrawal from the Paris Agreement signed by 195 nations to limit global warming to if possible less than 2°C by 2100, the confirmation of greater climate change danger once again comes from a US government research base, the Lawrence Berkeley National Laboratory.
Christina Patricola, of the laboratory’s climate division, reports in Nature that she and a colleague chose 15 tropical cyclones that have occurred in the last decade in the Atlantic, the Pacific and the Indian Oceans, and then built computer simulations of those storms while changing factors such as air and ocean temperatures, humidity, and the greenhouse gas concentrations that dictate overall planetary temperatures.
The two scientists looked at the effects of climate change so far, and the shape of storms to come. They found that warming hitherto has made rainfall between 5% and 10% more intense, but may not have so far made much difference to overall hurricane windspeeds.
But if the climate continues to warm – and it could warm by 3°C or more this century, as ever greater combustion of fossil fuels puts ever more carbon dioxide into the atmosphere – peak wind speeds could increase by up to 25 knots or very nearly 50 kilometres per hour.
The scientists also found that future rainfall in such storms could increase by between 15% and 35%. And the same computer models that predict windier, wetter storms tomorrow accurately predicted the pattern of the storms that had already happened. “The fact that almost all of the 15 tropical cyclones responded in a similar way gives confidence to the results,” Dr Patricola said.
In a companion study, scientists from US universities looked at the other component of the Hurricane Harvey disaster in 2017: the changes in the city of Houston itself.
Between 25 and 30 August, Harvey dumped 1.3 metres of rain on the metropolis. Between 2000 and 2011, Houston had the largest urban growth and the fifth largest population growth in the entire US. That is, it became a bigger target, with a greater area of paving and sealed surfaces to channel the flowing water.
Slower and wetter
The changing contour of the city helped increase atmospheric drag, slowing the passage of the hurricane and delaying it for long enough to drop even more rain. And then the surface of asphalt and concrete made conditions worse.
So, the researchers concluded, the new building made the risk of catastrophic flooding somewhere between hardly at all and up to 90 times more likely, depending on which part of the city they were looking at. Altogether, the risk of more flooding on the scale of Harvey had increased 21-fold.
The message is that coastal cities must plan for the worst and keep planning. Hurricane winds and rainfall are going to intensify in the future. Cities will keep on growing as human numbers increase.
“Planning must take into account the compounded nature of these risks,” they conclude, “and efforts to build flood mitigation strategies must use an improved understanding of the multiple processes in place.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.