More Than Half of Hot Extremes Caused by Climate Change

It seems that extreme weather including torrential rains and blazing heat is becoming the new norm, and now new research indicates that more than half of hot extremes are caused by climate change.

Extreme weather is part of the chaotic nature of weather and occurs due to a complex interplay of many factors. Some climate change critics may argue that since weather extremes existed in the past, warming temperatures cannot be blamed for such events. However, researchers at ETH Zurich in Switzerland say that is not so.

According to the study, published in the journal Nature Climate Change, a good amount of today's extreme high-temperature and heavy rainfall events can be attributed to the observed warming. Though the same cannot definitively be said of other kinds of extreme weather, such as hail storms and tornadoes, despite the fact that they are becoming more frequent.

Extremes are rare by definition, so a localized change in their frequency is statistically difficult to prove. However, combined measuring stations around the world suggest there has been a global trend towards more frequent and intense hot extremes since the 1950s, as well as more heavy precipitation events.

For example, Europe experienced a summer-long heatwave back in 2003, and since then scientists have determined that heatwaves will be 10 times as likely in the future due to climate change. Currently, summers with temperatures 1.6 degrees Celsius (2.9 degrees F) greater than the historical average now happen every five years, rather than every 52 years like before.

To determine what proportion of all extreme heat and precipitation events occurring worldwide are due to global warming, the ETH Zurich team used new climate models, simulations, and observations of extremes from the last several decades.

They concluded that more than half of the hot extremes worldwide and nearly a fifth of precipitation extremes can be attributed to global warming.  (Scroll to read on...)

It should be noted that not one of these events is solely the direct result of warming, but rather warming increases their frequency. And the less common and more extreme the hot extreme or heavy rainfall event, the more this can be blamed on human activity, such as the burning of fossil fuels.

"We show that at the present-day warming of 0.85 °C about 18% of the moderate daily precipitation extremes over land are attributable to the observed temperature increase since pre-industrial times, which in turn primarily results from human influence," the research team said.

However, this is just talking about global warming at present. With each increment of warming, the frequency of hot extremes and heavy precipitation events worldwide rises dramatically. If temperatures rise globally by 2 C (3.6 F) - a goal that has been deemed utterly inadequate - we expect twice as many extreme heat events worldwide than we would with a 1.5 C increase.

Now, this research shows that climate goals greatly influence the frequency of extremes, and this should be considered when preparing for future climate change. Since a heat or precipitation event does not have the same socio-economic impact everywhere in the world, it is necessary to combine the new study approach with regional information on exposure and vulnerability in order to carry out a comprehensive risk assessment. This type of risk assessment could serve as an important scientific basis for decisions on warming targets or even for global questions of liability.

"Combined with information on vulnerability and exposure, it serves as a scientific basis for assessment of global risk from extreme weather, the discussion of mitigation targets, and liability considerations," the researchers concluded.

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