Multiple Heat Waves Cap Planet’s Warming Trend

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Climatewire: This time, the heat is really on. From Boston to Washington, D.C., temperatures have soared to 100 degrees or more in recent days, stressing electrical grids, scrambling rail transportation and prompting the swift creation of cooling centers for those who lack air conditioning. Central Canada, portions of the Middle East and China are also coping with searing heat. Overall, according to the National Oceanic and Atmospheric Administration, the combined global land and ocean temperatures from January to May were warmer than in the same period in any year on record — a comparison that reaches back to the 1880s. But how much of the heat can be blamed on climate change? “We can’t say that one individual or even two heat waves are due to global warming,” said David Easterling, a climatologist with NOAA’s National Climatic Data Center. “But what we can say is that warming temperatures do increase the probability of a heat wave.” Scientists have documented a pronounced warming trend in the United States and around the world over the last several decades. In its 2007 report, the Intergovernmental Panel on Climate Change concluded with 90 percent certainty that human-caused greenhouse gas emissions have been the primary factor in Earth’s overall temperature rise since 1950. Meanwhile, the World Meteorological Organization said late last year that the 2000-2009 decade appears to be the warmest since record-keeping began in the 1850s. Easterling said studies also show an increase in the occurrence of heat waves in the United States since 1960. One recent analysis he published with colleagues at the National Center for Atmospheric Research, Climate Central and the Weather Channel concluded that climate change is skewing the proportion of record high temperatures to record low temperatures in the continental United States.

Record highs outpace record lows

Record highs outnumbered record lows 2-to-1 over the last decade, and the study — published in Geophysical Research Letters last year — predicted that disparity could balloon to 20-to-1 by the end of this century without sharp curbs on greenhouse gas emissions. While it’s impossible to pin blame for one heat wave on climate change, since naturally occurring weather patterns like El Niño can magnify or counteract human-caused warming over short periods, experts said an emerging crop of studies suggests that heat waves will become more frequent and intense without strong cuts in greenhouse gas emissions. “Model simulations do suggest that we can expect more and longer heat waves in the future … [but] that really does kind of depend on the trajectory of greenhouse gas emissions through the 21st century,” Easterling said. Without a dramatic decrease in heat-trapping emissions, he said, “what we currently consider a heat wave or an unusually hot day are very likely to become more then norm. The current spate of heat waves could be a harbinger of things to come.” One new study by researchers at Stanford University finds that extreme high temperatures could become far more common even if the world manages to limit warming to 2 degrees Celsius — the climate “guardrail” the European Union, the Group of Eight industrialized nations and many scientists believe will stave off severe climate change. A 1-degree-Celsius temperature rise between now and 2039, enough to bring the total temperature rise since the preindustrial era up to the edge of the 2-degree guardrail, would prompt as many as five intense heat waves between 2020 and 2029 in the western and central United States.

Western U.S. set for frequent frying

The 2030s, the study concludes, could see seven to eight intense heat waves in Western states, with at least four heat waves in other regions. Researchers based their findings on comparisons among a dozen climate models. “I was definitely surprised to see such large changes occur within the 2-degree-Celsius envelope, and to see that those changes are as robust as they are across a large suite of climate model simulations,” said lead author Noah Diffenbaugh, a Stanford climate scientist and a fellow at the Woods Institute for the Environment. “To me, that’s a suggestion that 2 degrees Celsius is not sufficient to avoid damage from climate change.” Another study that grabbed headlines recently suggested that if greenhouse gas output continues growing at its current rate, rising temperatures could make large areas of the globe uninhabitable for humans in coming centuries. Within 200 to 300 years, concluded the study lead by Purdue University scientist Matthew Huber, the global average temperature could increase by 10 to 15 degrees Fahrenheit, pushing heat and humidity in many areas beyond the point at which humans can adapt by sweating or giving off heat to cool themselves. “We’re not claiming this is a central projection for the future,” Huber said. “It represents a potential future, and the interesting thing about it is, we’ve already committed to at least 2 degrees [Fahrenheit] of warming. The kind of warming we’re talking about here, at least 10 degrees Fahrenheit, maybe 15, is something we still can decide to avoid.”

Lauren Morello, E&E reporter

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