The summer of 2012 was a season of epic proportions, especially July, the hottest month in the history of US 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.
The study is published in the Bulletin of the American Meteorological Society.