If climate change was a game, we'd have racked up quite a score. A fresh study suggests that humans are responsible for a hefty number of today's extreme hot days and rainstorms.
Weather extremes, such as a Russian heatwave in 2010 and a drought in Texas in 2011, have been blamed on climate change before – but the attribution of individual events to it is still hotly debated.
So Erich Fischer and Reto Knutti at the Institute for Atmospheric and Climate Science in Zurich, Switzerland, took a bird's-eye view of how human activity is changing the planet. Using 25 different climate models, they calculated how the odds of unusual events – such as a 1-in-100 day temperature high or a 1-in-10,000 day rainfall event – have changed with the rise in global temperatures.
Their results show that global warming of 0.85 °C since the industrial revolution has had a powerful effect. Climate change is now responsible for 75 per cent of our extreme highs in temperature and 18 per cent of extreme rainfall, according to the data. The rarer a particular event, the more likely that warming is the cause, they say.
"A 1-in-10,000 day heat event is something that's only expected to happen every 30 years. But in a global-warming world, it's turned into a 4-in-10,000 day event. Three of those hot days – or 75 per cent – would never have happened if global warming wasn't around," says Fischer.
Tipping the balance
Even though warming isn't the only cause of extreme weather, it is increasingly tipping the scales in its favour, says Fischer. "Some people argue that these things have happened before," he says. "Well, yes, they have. But they had been far less frequent."
Fischer and Knutti also looked at what would happen if the climate warmed by 2 °C, a threshold that many scientists warn would be dangerous to exceed. In a 2 °C world, global warming would be responsible for four of every 10 extreme rainstorms, they find.
In a companion essay, Peter Stott at the Met Office Hadley Centre in Exeter, UK, suggests that this statistic should be a "sobering thought" for policy-makers looking to protect their cities and countries.
The numbers align with what climate scientists already suspected, says Francis Zwiers, a statistician at the University of Victoria in British Columbia, Canada. But they can help us when we wonder whether an unusually hot day is the result of global warming, or just chance.
"You can turn that question into an answer," he says. "We shouldn't be quite so surprised when what was formerly a once-in-a-20-year event occurs. We should be expecting it."
Journal reference: Nature Climate Change, DOI:10.1038/nclimate2617
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