WASHINGTON — 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 U.N. 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.
The report — the product of a collaboration of 220 authors from 62 countries — makes distinctions among different 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 percent 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 the 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.”
Asking whether a particular extreme weather event can be blamed on human-caused global warming is the wrong question to ask, said Dim Coumou, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.
There’s no way to determine whether a single event was triggered by climate change, he said.
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.
For extreme heat waves and unusual downpours, the answer, Coumou and his colleagues found, is yes.
“The evidence is solid,” he said: Extra heat in the atmosphere from human-caused greenhouse gases has made these two types of events much more likely.
The climate has already changed, and the sheer number of these events over the past decade reflects it, they find.
