The world’s scientists are now more than 90% sure that human-caused global warming will increase the length, frequency and intensity of extreme weather events such as heat waves, the most authoritative scientific report yet on the subject says.
The Inter-governmental Panel on Climate Change (IPCC) report into extreme weather events found with “high confidence” that climate change has already made heat waves more severe in areas including southern Europe and the Mediterranean.
By 2100, the financial cost of extreme weather conditions could be some €15 trillion, mostly borne by what is currently the developing world.
The worst affected will be women, and people who are poor, elderly, uneducated, disabled, or in poor health.
“The information is all on the table,” Thomas Stocker, one of the report’s lead authors told EURACTIV. “If you have high emissions of anthropogenic greenhouse gas emissions, then you will increase the incidence of ‘hottest days’ by a factor of 10.”
“If you choose a mitigation scenario you may contain that same extreme event,” he said.
The report was compiled by 220 authors from 62 countries, and received more than 18,000 expert and government review comments in three rounds of its review process.
“It is probably the most thoroughly reviewed report concerning extreme events so far,” Stocker said.
New uncertainty language
It also used a “new uncertainty language” which cautiously ranks the likelihood of predictions and confidence in attributions according to a scale ranging from ‘virtually certain’ (99-100% sure) to ‘exceptionally unlikely’ (a 0-1% probability).
So when the report says that it is “likely” that heavy rainfalls will increase across the globe this century, it is reporting a probability of between 66%-90%, according to the best science available.
To gain this degree of certainty, the report was compiled according to a ‘consensus’-based model which posed some problems for evaluating the contribution of human-caused climate change to phenomena such as tropical cyclones, where data gaps appear before the middle of the last century.
Issues were also raised by contributing scientists over the quantity of rainfall that technically constitutes a drought – and whether droughts should be measured by soil moisture or consecutive dry days. In the end both metrics were used.
Such a sure-footed approach is aimed at policymakers and may provide an indisputable barrier to climate sceptics who have questioned past IPCC reports, and trumpeted any slip-ups.
But other scientific studies based on a ‘perspective model’ suggest that the evidence linking climate change to extreme weather events could be even more confidently stated.
Earlier this week an inquiry by the Potsdam Institute for Climate Research concluded that it was very likely that extreme weather events had increased over the past decade because of global warming.
“We just focused on record-breaking meteorological events in the last decade,” one of the report’s authors Dim Coumou told EURACTIV.
“So we’ve seen a large number of unprecedented extremes – heat waves like the one in Russia in 2010, and the European one in 2003, but also extreme rainfalls that happened in Pakistan 2010, and Australia in 2011,” he said
The peer-reviewed study, which was published in the journal Nature Climate Change, found that the length of European heat waves had nearly doubled in the last decade, and that the frequency of hot days had almost tripled between 1880-2005.
Extremely hot summers
Extremely hot summers were now being measured in about 10% of the world compared with 0.1%-0.2% in the period 1951-1980, and nearly twice as many hot as cold days were being observed in the United States and Australia.
Coumou said that the findings of the two reports were “not fundamentally different,” despite their different approaches. Stocker disagreed.
“That’s like comparing a bean with a huge crate of oranges,” he said, noting that the IPCC report had considered more than a thousand different studies, and a vast array of experts.
However, both reports were virtually certain that the 21st century would see a warming trend, with extremely cold days becoming rarer.
“The key message is that there is now strong scientific evidence linking human-caused global warming and extreme rainfall and heatwaves,” Coumou said.