75% of very hot days due to man-made global warming, study claims

Man-made global warming is currrently responsible for 75% of extremely hot days across the world, say two Swiss scientists, who added that we are also causing 18% of the planet’s heavy rainfall or snowfall.

Prof. Reto Knutti and Dr. Erich Markus Fischer, both from the Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland, who published their findings in the academic journal Nature Climate Change, say theirs is the first study to put figures to the effect on the Earth’s weather extremes as a result of extra greenhouses gases such as CO2 that are being added to the atmosphere from the burning of fossil fuels.

After combining data from 25 computer models of the global climate, dating from 1901 to 2005, Knutti and Fischer calculated that three-quarters of daily heat extremes and 18% of heavy rainfall/snowfall are being caused by human activity.

Extremely hot days

Three-quarters of extremely hot days globally are caused by man-made global warming.

They also informed that human activity is responsible for an increase in average temperatures worldwide of 0.85°C over the past 100 years.

The authors believe that future record-breaking hot and/or wet days globally will probably be influenced by the extra warming caused by the addition of CO2 in the atmosphere.


Lead author Dr. Fischer said:

“Climate change doesn’t cause any single weather event in a deterministic sense. But a warmer and moister atmosphere does clearly favour more frequent hot and wet extremes.”

These latest study findings agree with earlier research on climate and weather extremes, Quirin Schiermeier wrote in Nature News.

A study published in Nature four years ago, for example, found that climate change has already doubled the risk of catastrophic floods in England and Wales, like the ones that occurred in 2000.

According to this latest study, the human influence on the ‘moderate’ extremes is set to grow with every degree celsius the temperature rises.

If Earth’s average temperature warms by 2°C above the pre-industrial level, human-induced climate change would be responsible for 40% of rain and snow extremes and 96% of heat extremes, Fischer and Knutti found.


18% of heavy rainfall is due to human influences on the climate.

The influence of higher temperatures

If the world’s average temperature rises by 2°C, the probability of a daily heat extreme is double compared to a 1.5°C increase, and five times that under present conditions.

Dr. Fischer said:

“The rarer and more extreme an event, the higher is the fraction of risk we can attribute to climate warming.”

Some critics wonder how accurate climate models are at predicting future rainfall trends.

Kevin Trenberth, a climate researcher with the US National Center for Atmospheric Research in Boulder, Colorado, said:

“All weather events are influenced by the changed environment. The global perspective the authors provide is helpful, but none of the models they use do precipitation realistically and some are quite bad.”

Michael Oppenheimer, on the other hand, a climate-policy researcher at Princeton University in New Jersey, said:

“The risk of heat-related premature deaths has already increased and it will very likely starkly increase further in the future. Clearly, governments should not only seek to slow global warming, but must also prepare societies for what warming will inevitably happen.”

Citation: Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes,” E. M. Fischer & R. Knutti. Nature Climate Change. Published 27 April, 2015. DOI: 10.1038/nclimate2617.