Extreme hot weather and rain attributed to greenhouse gas emissions

Three-quarters of extreme hot weather and almost one in five extreme rainfall events can be attributed to greenhouse gas emissions primarily from human activity, a new study shows.

The study, published today in Nature Climate Change , analyses climate model simulations for the pre-industrial era and compares it with model simulations of present-day conditions. 

Co-author Dr Erich Fischer, of the Institute for Atmospheric and Climate Science at ETH Zurich, says in pre-industrial times hot extremes occurred once in three years (about 1000 days).

In today's world, which is 0.85°C warmer, extreme hot days occur on average about 4 - 5 times in three years.

"Therefore we attribute 3 - 4 events of the 4 - 5 events to the warming and the one that would have occurred without human influence to natural variability," he says.

Fischer says this means about 75 per cent of today's hot extremes can be attributed to current global warming, which is primarily the result of human activities.

Similarly Fischer and co-author Professor Reto Knutti found 18 per cent of extreme rainfall events today can be linked to the increase in global temperatures.

Fischer says these events are not directly caused by the warming, but are the result of the chaotic nature of weather.

"But they occur in a warmer and moister atmosphere, which favours their occurrence and increases their probability," he adds.

Predicting impact

The work helps to highlight the implications of a relatively small global temperature increase, Fischer says.

"We show at 2°C warming we would expect five times as many hot extremes than today at 0.85°C warming, and two times as many hot extremes than at 1.5°C warming. 

"In other words we demonstrate how subtle global temperature changes have large effects in terms of extremes."

Fischer says not every extreme is attributable to the warming and we cannot say that one or the other event is caused by the warming.

He uses the analogy of a loaded dice to explain: "Someone cheating with loaded dice will roll more sixes. But we don't know which of the sixes is due to the loading or not. All we know is that the loading of the dice increases the occurrence."

The researchers say the increase in extreme events will remain regardless of international efforts to reduce emissions.

"The global mean temperatures and thus the occurrence of extreme events would only reduce if we could artificially take out more greenhouse gases from the atmosphere than we emit," says Fischer. 

"This is what worries many people that the warming/change in extremes caused by our emissions are irreversible for many centuries as CO2 has a very long lifetime."Related: Storm clusters make the wet get wetter."