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 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.
Linking hurricanes, tornadoes and other storms to climate change is much harder, as records for these events are poorer than temperature and rainfall records.
He pointed to the 2010 heat wave in Moscow and western Russia as an example of an extreme event made much more likely by climate change. The hottest summer in 500 years of temperature records caused 15,000 deaths, shaved billions off of Russia’s economic output, triggered 500 wildfires and destroyed 30 percent of the country’s grain harvest.
“We found very strong warming since 1970s in the Moscow region,” said Coumou, “and this warming has dramatically increased the chances that a record summer would occur.”
Across the United States this year so far, warm weather records have outnumbered cold records by a factor of 12, according to the National Oceanic and Atmospheric Administration’s National Climatic Data Center.
Coumou used a “loaded dice” analogy that’s become popular with climate scientists. Rolling one six is not evidence of a loaded die. Rolling 10 in a row? Now you’re suspicious. Human-induced climate change has loaded the dice toward certain extreme events, Coumou said.
Both environmentalists and several major insurers argue policymakers must move quickly to cut carbon emissions and devise strategies to adapt to climate impacts.
“The IPCC report is yet another reminder of the pressing need to tackle climate risk in both the near and long term,” said Mark Way, head of sustainability for the Americas at Swiss Re. “Last year in the United States, even with the absence of major hurricane impacts, the insurance industry paid out approximately $35 billion in losses due to weather-related events. Severe weather will continue to impact the economy, and society in general, until we take the necessary measures to increase our resilience.”
Although extreme weather in developed countries exacts a higher human toll than in industrialized nations, the high economic cost associated with recent U.S. disasters is shifting more of the financial burden on taxpayers.
Cynthia McHale, who runs the insurance program at the nonprofit Ceres, said the National Flood Insurance Program now has $1.2 trillion of commercial and residential assets on its books. Beyond flood risk, the risk pools backed by state guarantees known as “residual markets” grew from $55 billion in 1990 to $758 billion in 2010.
“If we continue on this path, extreme weather is certain to cause more homes and businesses to be uninsurable in the private insurance market, leaving the costs to taxpayers or individuals,” McHale said.
On Wednesday, Lloyd’s of London posted its first pretax loss in six years, citing the costs it incurred related to natural catastrophes last year. That year now ranks as the second-most expensive on record in terms of insured disaster claims, with industry estimates ranging between $100 billion and $116 billion.
The IPCC report identifies “no regrets” strategies policymakers can pursue that reduce the risk of disasters while promoting sustainable development and climate adaptation, including early-warning systems for hurricanes and better building design and regulation to lower the impact of flash floods .
“There are lots of opportunities which pay off,” said Field, who co-edited the new report.