India heat wave: how ‘attribution science’ links climate change

Was the 2003 heat wave caused by global warming? And the storms of 2018 in France? What about the 2019 heat wave? “For many years, I replied that we should not make the link between a one-off extreme event and global warming”, admits Robert Vautard, research director at the CNRS and director of the Pierre-Simon Laplace Institute of climate (IPSL).

The reports of the IPCC are however formal, by alerting on the multiplication of extreme events due to human-induced climate change. But these weather phenomena always have multiple causes and until recently, Robert Vautard and his fellow climatologists avoided linking them individually to global warming.

Today, a discipline is changing the situation: the science of attribution. It is this science that has enabled the network of scientists from the World Weather Attribution (WWA) to affirm that the heat wave that has been falling since March in India and Pakistan has been made thirty times more likely by climate change.

Probability of occurrence of extreme events

The attribution science lays down a methodological framework for assessing the degree of influence of climate change on a meteorological event. It has only been known to the general public for a few years, but “the landmark study which laid down the concepts of the science of attribution dates from 2004 and concerned the heat wave of 2003”, recalls Robert Vautard. “It showed that we cannot say that global warming caused the occurrence of an extreme event but that we can speak of its probability with regard to climate change”, continues the scientist.

Since 2014, scientists from the WWA, a network of researchers including Robert Vautard, have specialized in the science of attribution. Studies on natural disasters are launched according to very specific criteria. “In India and Pakistan, the duration and earliness of the heat wave challenged us”, notes the director of the IPSL.

The scientists then make observations and identify trends in the evolution of the climate, then carry out simulations. By comparing the results in a pre-industrial climate and in the current human-modified climate, scientists can determine to what extent the heat wave has been made more or less likely.

In 2021, several extreme events linked to global warming

Without the effect of human activity on the climate, this heat wave would have had a one in 3,000 chance of occurring, compared to the current 1 in 100, the scientists conclude. “What we are saying is that this event could have occurred without climate change, but with a lower probability,” sums up Robert Vautard.

These research methods have also enabled the WWA to establish a link between the heat dome that hit North America in June 2021 and global warming. Just as climate change made the floods that devastated Germany and Belgium in July 2021 up to nine times more likely , according to a WWA study published last August. The particularity of the WWA is indeed that the scientists carry out their studies “in near real time”, shortly after the event studied or even when it is in progress. “It’s important because when disasters happen, that’s when there is the attention of decision-makers and the media,” says Robert Vautard.

No link with rising temperatures

The group of researchers also takes into account economic and social criteria. “We are studying the reasons, in the structure of societies, why this event took place”, specifies the CNRS scientist. This, for example, has led researchers to claim that poverty, poor infrastructure and natural weather conditions were more responsible for the famine in Madagascar than global warming.

Sometimes, analyzes conclude that an extreme event is not linked to global warming. Storms Eleanor and Friederike that hit Western Europe in January 2018, for example, were not significantly influenced by climate change, according to the WWA. On the other hand, the heat wave that crossed France in 2019 was made more likely by global warming. Scientists also agree that the link between the frequency and intensity of heat waves and global warming is clearly established, unlike other events such as cyclones where the link seems less obvious .

A sometimes significant margin of error

The science of attribution therefore does not make it possible to establish a formal link of cause and effect between a disaster and global warming. However, it goes beyond the general findings of the IPCC on the increase in natural risks in the world.

These studies always have a margin of error, which is high in the case of the India and Pakistan study due to the lack of data on such rare events. But their multiplication corroborates the warnings of the IPCC in its latest reports: the visible changes linked to global warming are not to come, they are already taking place.

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India heat wave: how ‘attribution science’ links climate change

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