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.
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.”
The D.C. region has experienced intense warming in recent years as well. In the past two years, Washington has experienced its two hottest summers on record (2010 and 2011), second-warmest spring (2010) and third-warmest winter (2011-12) since records began in 1871.
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 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.
Environmentalists and major insurers argue that 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. In 2011, insurers paid out $35 billion to cover weather-related losses in the United States, he added.
Although extreme weather in developing countries exacts a higher human toll than in industrialized nations, the high economic cost of recent U.S. disasters is shifting more of the financial burden to taxpayers.
Cynthia McHale, who runs the insurance program at Ceres, a nonprofit network that addresses issues of environmental sustainability,said the National Flood Insurance Program now has $1.2 trillion of commercial and residential assets on its books.
“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 burden of natural catastrophes in 2011. That year now ranks as the second-most expensive on record for insured disaster claims worldwide, with $100 billion to $116 billion in claims.
The IPCC report identifies “no regrets” strategies that policymakers can pursue to 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 report.
Staff writer Jason Samenow contributed to this report.