Like many of the farmers and ranchers across his state, Michael J. Hayes of the University of Nebraska at Lincoln hopes he never again sees the kind of record heat and extreme drought that blasted the Midwest this summer.
The severe weather left behind decimated corn fields, withered pasture land, and heavy economic losses.
Still, Mr. Hayes, an associate professor of natural resources and director of the National Drought Mitigation Center, says major heat waves do have at least one important value: They apparently can help convince people of the need for urgent action against climate change.
The key, as Mr. Hayes sees it from Lincoln, a city that had 11 days in July of triple-digit temperatures and most of the rest in the upper 90s, is to explain how hot summers and long-term climate change are related, while being careful about asserting definitive links: "We, as the proponents of climate change, could do a real disservice to the whole education of the public if we wrongly tie specific events" to the phenomenon.
And yet, after years of public- policy research, it's not clear what exactly makes some Americans accept, and others reject, the established science of man-made climate change.
It stands to reason that unusually warm weather might help convert some nonbelievers. And surveys this year show that has happened. Recent polls by Gallup, the University of Texas at Austin, and the Brookings Institution all show upticks in public acceptance of climate-change science.
But the link isn't necessarily logical. Climate often means weather on the scale of decades and centuries, and scientists emphasize that an unusual temperature swing for a few months or even a few years isn't necessarily proof of anything.
"Some people think climate change is gradually turning up the thermostat all around the world," said Barry G. Rabe, a professor of public policy and environmental policy at the Gerald R. Ford School of Public Policy at the University of Michigan at Ann Arbor. "And that's just not true."
A Fickle Public
Mr. Rabe directs the Brookings National Survey of American Public Opinion on Climate Change, a multiyear series of polls being used to study the link between belief in climate change and short-term weather conditions. The nationwide surveys began in September 2008, when 72 percent of respondents agreed there was "solid evidence" that average global temperatures had risen over the previous four decades. That number dropped to 52 percent by April 2010, rose to 58 percent by December 2010, dipped again to 55 percent in April 2011, then began rising again, reaching 65 percent this past April.
That curved trend line, according to Mr. Rabe and his co-author, Christopher P. Borick, a professor of political science at Muhlenberg College, corresponds well with weather patterns in the country over that four-year period. The winters of 2009-10 and 2010-11, driven at least in part by the periodic El Niño climate phenomenon, were unusually cold and snowy in the United States and other parts of the Northern Hemisphere.
Other researchers have found that even tests of more simple effects, such as holding surveys on hot days, have shown such unwarranted correlations.
One such study, released in July by researchers at New York and Temple Universities, found that for every three degrees above normal that local temperatures reached in the week before the day on which Americans were contacted for opinion polls, they were one percentage point more likely to agree there is "solid evidence" the earth is getting warmer.
The effects reverse as temperatures cool, "leaving no long-term impact of weather on public opinion," the authors, Patrick J. Egan, an assistant professor of politics at NYU, and Megan Mullin, an associate professor of political science at Temple, wrote in the Journal of Politics.
Another study, published last year by researchers from the University of Chicago and the University of California at Berkeley, involved placing survey participants outside in the hot sun and indoors in overheated rooms. In both cases, hotter conditions at the moment of the questionnaire increased the likelihood of survey participants endorsing global warming as a proven fact.
Many researchers accept the association with temperature, but believe other factors are even more important. They include Lyle A. Scruggs, a professor of political science at the University of Connecticut, who says economic conditions predict public acceptance of climate science better than weather does.
Mr. Scruggs has reviewed Gallup polls on climate and compared the results to the federal government's monthly unemployment data. He has found that a one-percentage-point rise in unemployment decreases support for climate change by two percentage points, while a one-degree increase in the average winter temperature decreases support by about the same amount.
The economic indicator appears to be the stronger correlation, Mr. Scruggs said, because unemployment is more "real" to people than temperature, and because the unemployment rate "has fluctuated somewhat more over the last 20 years than winter temperatures have." The linkage between a growing economy and a willingness to accept the science of climate change appears to be the result of widespread predictions—not necessarily well-founded—that tackling climate change would mean a net loss of jobs, Mr. Scruggs said.
Even those who found associations with temperature and economics agree there's at least one other major factor: politics.
"Weather matters, and it contributes to one's ultimate position on the existence of climate change," said Mr. Borick. "But if there is one factor that has the biggest effect, it would be political affiliation."
Regardless of its relative ranking, the nation's political environment clearly complicates efforts to attribute changes in beliefs to variations in weather or employment. The economy did begin improving in the early part of last year, at the same time average temperatures began rising. But that also covers a time period in which industry-financed skeptics intensified their attacks on the science of climate change, beating back attempts in Congress to limit carbon emissions.
Beyond Opinion Polls
Some researchers find a fundamental limit to even the importance of public opinion. Jon A. Krosnick, a professor of political science, communication, and psychology at Stanford University, has been conducting surveys on climate change since the 1990s, and he's concluded that the public is far more supportive of the science than most polls suggest.
Mr. Krosnick specializes in constructing opinion polls in ways that avoid posing questions that introduce a bias. By using phrasing he regards as more neutral than seen in most polls, he said his surveys have consistently found that about 85 percent of Americans accept that the climate is warming and endorse steps—even if it means higher costs—to address it.
Such high levels of agreement are "very rare" on major questions of public policy, Mr. Krosnick said. He agrees there's some evidence that short-term weather affects opinions on climate, and sees little if any evidence that the economy does. The key division he sees is between the two-thirds of the U.S. population inclined to trust scientists, whose acceptance of global warming is strong, and the one-third not inclined to trust scientists, whose assessments about climate can be affected by fluctuations in the weather.
"So weather does matter, but it matters in a very particular way," Mr. Krosnick said.
Others have identified a similar phenomenon. A team led by Teresa A. Myers, a postdoctoral researcher at George Mason University's Center for Climate Change Communication, reviewed national survey results to conclude in a study this year that weather conditions do influence attitudes among people who aren't ideologically predisposed. They look for proof of their existing beliefs.
"Among those people already highly engaged in the issue—whether alarmed or dismissive—prior beliefs exerted a strong influence on their perceived experience," Ms. Myers wrote. "While for people less engaged, the reverse was true."
Mr. Rabe and Mr. Borick reported much the same. In their April 2011 survey, following the two unusually cold winters, 43 percent of those rejecting the science of climate change cited personal observations as their most important reason. A year later, amid a warmer weather pattern, only 20 percent cited personal observations. Many more disbelievers began citing religious factors or suspicions of political manipulation, Mr. Borick said.
Whatever the causes, the country as a whole may not demand action until more crises accumulate, said James R. Angel, an adjunct instructor in atmospheric science at the University of Illinois at Urbana-Champaign, who serves as state climatologist.
"We have seen it for years in things like drought planning and response to tornado warnings," Mr. Angel said. "If we haven't had a drought in a while, everyone gets complacent about planning and responding to drought. Needless to say, a lot of people are thinking a lot more about drought these days."