The risk of record heat events like those of the summer of 2012 is as much as four times more likely now than in pre-industrial America, researchers say. Extreme weather is more than four times as likely n the north-central and northeastern United States than it was in the pre-industrial era, when there was much less carbon dioxide in the atmosphere, a study by Stanford University environmental science Professor Noah Diffenbaugh found. The summer of 2012 was a season of epic heat, especially in July, which ended up the hottest month in the history of U.S. weather record keeping, the researchers said. Diffenbaugh and research assistant Martin Scherer found strong evidence the high levels of greenhouse gases in the atmosphere have increased the likelihood of more episodes of severe heat. \"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. Using climate models, Diffenbaugh and Scherer 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. While Diffenbaugh cautioned against trying to determine whether global warming caused any individual extreme event, he said the observed warming clearly appears to have affected the likelihood of record heat. \"Knowing how much our emissions have changed the likelihood of this kind of severe heat event can help us to minimize the impacts of the next heat wave, and to determine the value of avoiding further changes in climate,\" he said.