Although it’s clear that man-made greenhouse gas emissions are causing unprecedented levels of warming across the world, attributing specific weather events to this is notoriously difficult. A new study published in Nature Climate Change has attempted to do just that: Using a large ensemble of climate change models, scientists have concluded that the catastrophic winter 2013/2014 floods in southern England were directly attributable to human activity.
The total rainfall across the U.K. during that period was 163 percent above the 1961-1990 average. Within southeastern and central England itself, the total rainfall was up to 267 percent above average. At the time, an anomalous eastwards extension of the jet stream, an air current that has a direct influence over weather systems, was observed. This was followed by several low-pressure systems persisting off the west U.K. coast that caused surprisingly long-lasting rainstorms.
The subsequent flooding caused $646 million (£451 million) in insurance losses, one of the highest in history. In order to see whether or not anthropogenic emissions had a hand in causing this, the team led by the University of Oxford decided to model the climatic conditions at the time, with and without human activity factored in.
In order to run such a complex series of algorithms, the citizen-science project “weather@home” was initiated. Over 134,354 simulations on volunteers’ personal computers were run, which looked into the local and regional sea surface temperatures, atmospheric circulations, air temperatures, and precipitation rates over winter 2013/2014.
Comparing the “Actual Conditions” model to the “Natural” simulations – those without human activity – it was clear to see that only the former produced conditions akin to those observed over winter 2013/2014.
As atmospheric temperatures become warmer, an increasing amount of water vapor can be held in the lower atmosphere. These warmer temperatures ultimately cause greater precipitation rates during storm events. As the simulations showed, thanks to anthropogenic warming, the atmosphere above southern England in the winter of 2013/2014 was loaded with water vapor.
In terms of the atmospheric circulation, 26 out of 31 days in January 2014 (the month of peak rainfall) were considered abnormal – the highest divergence from the norm since 1871. High Atlantic sea surface temperatures, induced by anthropogenic warming, caused the jet stream to be “bent” and strengthened on its way east. This allowed it to deflect more low-pressure, stormy systems towards south England.
All in all, these key factors conspired to produce record-setting rainfall, which caused peak flow within the Thames and extremely water-saturated land. This study claims that such a one-in-100 year event was 43 percent more likely to happen under real-world conditions than in the “Natural” simulations. Two-thirds of this risk increase was due to temperature changes, whereas a third was due to atmospheric circulation changes, both of which are directly linked to anthropogenic warming.
Professor Rob Lamb, a co-author of the paper and researcher at Lancaster University said in a statement that “overall… there is a substantial chance of more properties having been placed at flood risk because of past greenhouse gas emissions.”
This study comes shortly after another groundbreaking piece of climate change research, which used the most up-to-date computer simulations to estimate that two-thirds of dangerous climate phenomena, between 1971 and 2010, are linked to human activity.