Global warming has increased risk of record heat

Rob Jordan in Stanford News Service: Drought shriveled crops in the Midwest, massive wildfires raged in the West and East Coast cities sweltered. The summer of 2012 was a season of epic proportions, especially July, the hottest month in the history of U.S. weather record keeping. And it's likely that we'll continue to see such calamitous weather.

In the north-central and northeastern United States, extreme weather is more than four times as likely to occur than it was in the pre-industrial era, according to a new study by Noah Diffenbaugh, a Stanford associate professor of environmental Earth system science, and Martin Scherer, a research assistant in the department. Diffenbaugh and Scherer found strong evidence that the high levels of greenhouse gases now in the atmosphere have increased the likelihood of severe heat such as occurred in the United States in 2012.

The researchers focused primarily on understanding the physical processes that created the hazardous weather. They looked at how rare those conditions were over the history of available weather records, going back over the last century.

Then, using climate models, they quantified how the risk of such damaging weather has changed in the current climate of high greenhouse gas concentrations, as opposed to an era of significantly lower concentrations and no global warming. Their findings don't pinpoint global warming as the cause of particular extreme weather events, but they do reveal the increasing risk of such events as the world warms.

"Going forward, if we want to understand and manage climate risks, it's more practically relevant to understand the likelihood of the hazard than to ask whether any particular disaster was caused by global warming," said Diffenbaugh, a senior fellow at the Stanford Woods Institute for the Environment.

In 2012 alone, the United States suffered 11 extreme weather events that each caused at least $1 billion in damage. "It's clear that our greenhouse gas emissions have increased the likelihood of some kinds of extremes, and it's clear that we're not optimally adapted to that new climate," Diffenbaugh said....

Plowing in the Dust Bowl in the 1930s, shot from the US Department of Agriculture