America’s rainfall patterns are changing with the global climate − and making catastrophic flood damage even more costly.
LONDON, 21 January, 2021 − Climate change alone has cost the United States a total of $73 billion in flood damage in the last 30 years.
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The figure is significant: floods are an expensive fact of life. But Californian scientists are now sure that more than one-third of the costs of US floods must be attributed to the global heating driven by human use of fossil fuels.
The news comes hard on the heels of a second finding: that over the last century, the count of what hydrologists call “extreme streamflow” events in Canada and the US has increased significantly. This confirms that droughts are on the increase − and so are floods.
Such findings matter to engineers and city planners, and to insurers, and each resolves some long-standing uncertainties.
Because floods and droughts are part of the challenge of living close to a constant flow of water, researchers have never been too sure whether costly floods are on the increase or are just more obvious because population growth and urban spread mean that more people with more expensive property are increasingly at risk.
“The shifts towards more extreme events are especially important given the age of our dams, bridges and roads”
A new study in the Proceedings of the National Academy of Sciences settles the matter of rising costs: researchers looked at 6,600 reports of flood damage and rainfall data between 1988 and 2017 and then applied sophisticated mathematical techniques to tease out the contribution from higher precipitation driven by higher average temperatures, driven in turn by ever-higher ratios of carbon dioxide in the atmosphere.
They decided that of the $199bn flood damage costs during those years, human-triggered climate change could account for 36%.
“The fact that extreme precipitation has been increasing and will likely increase is well-known, but what effect that has had on financial damages has been uncertain,” said Frances Davenport, of Stanford University.
“What we find is that, even in states where long-term mean precipitation hasn’t changed, in most cases the wettest events have intensified, increasing the financial damages relative to what would have occurred without the changes in precipitation.”
Higher temperatures mean more evaporation. Higher temperatures mean a bigger burden of water vapour in the atmosphere. So higher rainfall is inevitable.
But repeated studies have found this will happen unevenly: those places already rainy will see more rain. Other regions can expect longer, more intense dry spells.
A second team of US researchers reports in the journal Science Advances that they looked at streamflow data from 541 North American stations since 1910.
They found that in the US west and south-east, the frequency of “extreme low-flow events” has been on the increase, particularly during summer and autumn. In zones where rivers were likely to be fed by melting snow, there was a discernible rise in flood events. Once again, this is a finding with practical consequences.
“The shifts towards more extreme events are especially important given the age of our dams, bridges and roads,” said Evan Dethier, of Dartmouth College in New Hampshire. “The changes to river flows that we found are important for those who manage or depend on this type of infrastructure.” − Climate News Network