Reports link heat waves, deluges to climate change

Scientists are increasingly confident that the uptick in heat waves and heavier rainfall is linked to human-caused greenhouse-gas emissions, posing a heightened risk to the world’s population, according to two reports issued in the past week.

On Wednesday, the United Nations Intergovernmental Panel on Climate Change released a 594-page study suggesting that when it comes to weather observations since 1950, there has been a “change in some extremes,” which stem in part from global warming.

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 This year has been an unprecedented one for natural disasters. By the end of June, economic losses totaled $265 billion, according to German reinsurer Munich Re. That easily exceeds the total figure for 2005, which was previously the costliest year.

The report — from 220 authors in 62 countries — makes distinctions among weather phenomena. It shows there is “limited to medium evidence” that climate change has contributed to changes in flooding, for example, and there is “low confidence” that long-term hurricane trends over the past 40 years have been driven by the world’s growing carbon output.

But the IPCC team projects that there is a 90 to 100 percent probability that sea-level rise “will contribute to upward trends in extreme coastal high-water levels in the future.” Chris Field, who co-chairs the IPCC’s Working Group II and serves as director of the Carnegie Institution’s Department of Global Ecology at Stanford University, said in an interview that although many uncertainties still exist when it comes to extreme weather, “We also know the risk people face is changing as a result of climate change.”

Whether particular extreme weather events can be blamed on human-caused global warming is the wrong question to ask, since there is no method available to make such a connection, said Dim Coumou, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.

Instead, a new analysis from Coumou and a colleague, published in the journal Nature Climate Change, examines patterns of extreme weather since 2000 and asks whether the likelihood of these events was heightened by human-driven climate change.

The answer is “yes” for extreme heat waves and unusual downpours, Coumou and his colleagues found. “The evidence is solid,” he said: Human-emitted greenhouse gases trap heat in the atmosphere; warmer air, in turn, soaks up more moisture. The climate has already changed, and the sheer number of these events over the past decade reflects it, they find.

Linking hurricanes, tornadoes and other storms to climate change is much harder, because records for these events are poorer than those for temperature and rainfall.

Coumou pointed to heat waves in Western Europe in 2003 and western Russia in 2010, among others, as events made much more likely by climate change. Estimates of the toll across 16 European countries in 2003 range from 35,000 to 70,000 more deaths than normal.

The 2010 Russian summer was the hottest in 500 years of records there, causing 15,000 deaths, triggering 500 wildfires and destroying 30 percent of the country’s grain harvest, according to a study cited by Coumou. “We found very strong warming since 1970s in the Moscow region,” he said, “and this warming has dramatically increased the chances that a record summer would occur.”