Science measures humanity's mark on extreme weather

Global warming since the Industrial Revolution is responsible for about three-quarters of certain heat extremes today, and nearly a fifth of unusually heavy downpours, according to a new study.

Erich Fischer and Reto Knutti from the Institute for Atmospheric and Climate Science in Zurich, used simulations and modelling to compare weather extremes from the pre-industrial and modern eras in a bid to determine the risk attributable to warming caused by mankind.

Accumulated warming of 0.85 degrees Celsius (1.53 degrees Fahrenheit) to date was responsible for about 18 percent of "moderate daily extremes" for rainfall and 75 percent for heatwaves today, they estimated.

These "moderate" extremes are events that would ordinarily happen once in three years, as opposed to "very extreme" events that would occur about once in 30 years, Fischer explained.

"We find that what used to be a 1-in-3 year hot event occurs roughly 4-5 times in three years in today's climate," he told AFP by email.

"We then conclude that 3-4 of these occurrences that had not occurred in the pre-industrial world, are attributable to... global warming."

The study was published in the journal Nature Climate Change.

Scientists say the global average surface temperature rose 0.85 C from 1880 to 2012, mainly as a result of humans pumping Earth-warming greenhouse gases into the atmosphere.

And while the UN has targeted 2 C as the warming limit, experts predict that 4-5 C is more likely on current carbon emissions -- causing not only changes in average global climate but also extreme weather events.

Fischer and Knutti found that with warming of 2 C, about 40 percent of rainfall extremes will be "attributable to human influence".

"The probability of a hot extreme at 2 C warming is almost double that at 1.5 C and more than five times higher than for present-day (0.85 C)," they wrote.

"This result has strong implications for the discussion of different mitigation targets in climate negotiations, where differences between targets are small in terms of global temperatures but large in terms of the probability of extremes."