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Cherries in the rain

27 Nov 2019 | Science Notes

Global warming isn’t making it rain harder in the United States. Or less. Or both. So says a recent paper in the Journal of Hydrology that took issue with a claim in the 2017 US National Climate Assessment that greenhouse gases were behind an upward trend in American 20th century extreme precipitation trends. Using data from cities across the US Southeast and Pacific Coast regions the authors found that, yes, over the time interval the Assessment report examined, extreme precipitation went up. But when they extended the data back in the 1800s the trends went away. And when they focused on the last 40 years, instead of speeding up, the trends slowed down or reversed. And when they looked at evidence about drought conditions over the past 2,000 years, the 20th century changes looked unexceptional. It almost sounds like the government report was cherry-picking the interval to find the biggest trend. Nahhhhh, they wouldn't do that would they?

We've already reported on the fact that our own Environment Ministry projects increased extreme precipitation due to greenhouse gases even though they admit having no evidence it has happened to date. Maybe they just need to be a bit more creative in their statistics. In the case of their US counterparts, the authors of the new study noted that you can get an apparent upward trend by selecting just the right start date and end date. But when they looked at longer or shorter intervals, the trends aren't there. As was also found in the study of global precipitation data by York University scientist William van Wijngaarden (see our video on urban flooding).

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