Human activity prods major weather events


Human culpability in floods and droughts appears more certain, according to new research. Amy Middleton reports.


Human activity has contributed to increasing severe weather events.
Busa Photography/Getty Images

Human-induced climate change could be responsible for a series of extreme weather events over the past decade, suggests a new study.

The study, by Michael Mann of Pennsylvania State University and colleagues, claims warming temperatures are interrupting the flow of Rossby waves – giant currents of air, sometimes called jet streams, that carry heat and moisture around the atmosphere.

Mann’s team focused on Rossby waves that circle the Earth’s northern hemisphere, urged on by the contrasting temperatures between the Arctic and the Equator. When they get held up because of anomalous heat spikes, the precipitation traffic jam can cause flooding and drought in surrounding regions.

The paper, published in Scientific Reports, suggests warming temperatures in the Arctic caused by greenhouse gas emissions are interrupting the jet streams, in turn causing extreme knock-on effects.

The researchers looked for catalysts for a series of extreme global weather events recorded over the past 13 years.

"The unprecedented 2016 California drought, the 2011 US heat-wave and 2010 Pakistan flood, as well as the 2003 European hot spell, all belong to a most worrying series of extremes," explains Mann. "

The increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate change effect.”

The team gathered satellite data and global temperature records from the past 150 years, and used computer-simulated climate modelling to measure the impact of human-induced climate change on airstream stalling.

“In data from computer simulations as well as observations, we identify changes that favour unusually persistent, extreme meanders of the jet stream that support such extreme weather events,” says Mann.

“Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity."

This influence can lead to ongoing extreme weather, as co-author Stefan Rahmstorf, at the Potsdam Institute for Climate Impact Research in Germany explains: "If the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, or lasting rains can lead to flooding.”

The data also showed a particular increase in this pattern over the past 40 years.

"The more frequent persistent and meandering jet stream states seem to be a relatively recent phenomenon, which makes it even more relevant," says co-author Dim Coumou from the at University in Amsterdam in the Netherlands.

"We certainly need to further investigate this – there is some good evidence, but also many open questions.”

The researchers say the data adds to existing evidence that limiting greenhouse gas emissions could minimise the risks associated with extreme weather events.

  1. http://www.nature.com/articles/srep45242
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