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IPCC AR6: Flooding, unspun edition

25 Aug 2021 | Science Notes

The best way to respond to a new IPCC report is to tune out all the press coverage and look at the actual document. But it seems to be the last thing almost anyone thinks to do. So over the next few weeks we will provide excerpts of the text of the new IPCC AR6 WG1 draft report on the big topics of the day, especially apocalyptic extreme weather and the like. Last week it was precipitation and this week it’s floods. The new report begins its discussion in Section 11.5.2 by noting that their Fifth Assessment Report in 2014 (AR5 to insiders) and other IPCC reports have all concluded that trends in flooding in specific regions around the world are too varied to conclude anything about a global trend. But now the extra evidence collected over the past few years leads them to conclude... the same. All that yelling about increasing floods worldwide proving global warming is on us turns out to be hooey, not because the rise in flooding doesn’t prove it but because it’s not happening.

Herewith the IPCC report text verbatim (with some citations removed for clarity:

Peak flow trends are characterized by high regional variability and lack overall statistical significance of a decrease or an increase over the globe as a whole. Of more than 3500 streamflow stations in the US, central and northern Europe, Africa, Brazil, and Australia, 7.1% stations showed a significant increase and 11.9% stations showed a significant decrease in annual maximum peak flow during 1961-2005 (Do et al., 2017).

On a continental scale, a decrease seems to dominate in Africa and Australia, an increase in the Amazon, and trends are spatially variable in other continents. In Europe, flow trends have large spatial differences, but there appears to be a pattern of increase in northwestern Europe and a decrease in southern and eastern Europe in annual peak flow during 1960-2000. In North America, peak flow has increased in the northeast US and decreased in the southwest US. There are important changes in the seasonality of peak flows in regions where snowmelt dominates, such as northern North America and northern Europe, corresponding to strong winter and spring warming.

In summary, the seasonality of floods has changed in cold regions where snowmelt dominates the flow regime in response to warming (high confidence). Confidence about peak flow trends over past decades on the global scale is low, but there are regions experiencing increases, including parts of Asia, southern South America, the northeast USA, northwestern Europe, and the Amazon, and regions experiencing decreases, including parts of the Mediterranean, Australia, Africa, and the southwestern USA.

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