This image provided by the National Weather Service shows temperatures in the continental United States as of 3 p.m. on Friday, July 22, 2016. The weather service outlook for the following three months shows above normal temperatures across the country. (National Weather Service via AP)

Tempers are rising in America, along with the temperatures.

Two decades ago, the issue of climate change wasn't as contentious. The leading U.S. Senate proponent of taking action on global warming was Republican John McCain. George W. Bush wasn't as zealous on the issue as his Democratic opponent for president in 2000, Al Gore, but he, too, talked of regulating carbon dioxide.

Then the Earth got even hotter , repeatedly breaking temperature records. But instead of drawing closer together, politicians polarized.

Democrats (and scientists) became more convinced that global warming was a real, man-made threat . But Republicans and Tea Party activists became more convinced that it was— to quote the repeated tweets of presidential nominee Donald Trump—a "hoax." A Republican senator tossed a snowball on the Senate floor for his proof.

When it comes to science, there's more than climate that divides America's leaders and people. The mainstream scientific establishment accepts evolution as a reality, as well as the general safety of vaccinations and genetically modified food. But some political leaders and portions of the public don't believe any of that. It's not a liberal versus conservative issue, especially when it comes to vaccinations, which are doubted by some activists on both ends of the political spectrum.

But nothing beats climate change for divisiveness.

"It's more politically polarizing than abortion," says Anthony Leiserowitz, director of the Yale Program on Climate Change Communication. "It's more politically polarizing than gay marriage."

Leiserowitz says 17 percent of Americans, the fastest-growing group, are alarmed by climate change and want action now, based on surveys by Yale and George Mason University.

Another 28 percent are concerned, thinking it's a man-made threat, but somewhat distant in time and place. Twenty-seven percent are cautious, still on the fence, and 11 percent are doubtful. An often-vocal 10 percent are dismissive, rejecting the concept of warming and the science. And about 7 percent are disengaged, not even paying attention because they've got more pressing needs.

So while the largest group is at least concerned with climate change, significant segments are not. And sometimes those segments mix in one family.

Rick and Julie Joyner of Fort Mill, South Carolina, are founders of MorningStar ministries. Most of the people they associate with reject climate change. Their 31-year-old daughter, Anna Jane, is a climate change activist.

Rick Joyner, 66, would visit New York with other evangelicals to meet with Trump and then hear a completely different world view from his daughter.

As part of a documentary a few years ago, Anna Jane introduced Rick to scientists who made the case for climate change. It did not work. He labels himself more skeptical than before.

"They're both stubborn and equally entrenched in their positions," says Julie, who is often in the middle. "It doesn't get ugly too often."

In this Monday, June 11, 2001 file photo, President George W. Bush waves as he is followed by members of his cabinet after he spoke on global warming, in the Rose Garden of the White House. Two decades ago, the leading U.S. Senate proponent of taking action on global warming was Republican John McCain. Bush wasn't as zealous on the issue as his Democratic opponent for president in 2000, Al Gore, but he, too, talked of regulating carbon dioxide. (AP Photo/Ron Edmonds)

Tribalism

Recall the 20th century, with its race to the moon, advances in medicine and information technology, and "this incredibly strong belief in the promise of science," says Matthew Nisbet, a communications professor at Northeastern University.

People in the 1960s "had faith in science, had hope in science. Most people thought science was responsible for improving their daily lives," says Marcia McNutt, president of the National Academy of Sciences. So some scientists look back at that era with nostalgia, she says.

That's because now, Nisbet says, "we see partisan polarization or ideological polarization" and the implications of science "are intuitively recognized as threatening to one side and their world view."

Yale psychology and law professor Dan Kahan argues, however, that public divides on science have existed for decades. He notes that some issues that formerly divided us no longer do, such as the dangers of cigarettes, after a public health campaign eroded the social acceptability of smoking.

The split with science is most visible and strident when it comes to climate change because the nature of the global problem requires communal joint action, and "for conservatives that's especially difficult to accept," Nisbet says.

Climate change is more about tribalism, or who we identify with politically and socially, Nisbet and other experts say. Liberals believe in global warming, conservatives don't.

Dave Woodard, a Clemson University political science professor and GOP consultant, helped South Carolina Republican Bob Inglis run for the U.S. House (successfully) and the Senate (unsuccessfully). They'd meet monthly at Inglis' home for Bible study, and were in agreement that global warming wasn't an issue and probably was not real.

"I said climate change was nonsense, Al Gore's imagination," Inglis says.

After seeing the effects of warming first-hand in Antarctica and Australia's Great Barrier Reef, Inglis changed his mind—and was overwhelmingly defeated in a GOP primary in 2010. Woodard helped run the campaign that beat him and hasn't been to his former friend's home for about a decade.

"I was seen as crossing to the other side, as helping the Al Gore tribe, and that could not be forgiven," Inglis says.

Woodward responds that the new Bob Inglis didn't fit South Carolina.

"If you want to talk climate change, you need to go up to New York and Boston to do that. You don't talk that down here," he says. "Conservatives just don't believe. They think it's like Martians."

Judy Curry, a Georgia Tech atmospheric scientist and self-described climate gadfly, has experienced ostracism from the other side. She repeatedly clashed with former colleagues after she publicly doubted the extent of global warming and criticized the way mainstream scientists operate. Now she says, no one will even look at her for other jobs in academia.

In this Aug. 7, 2015 file photo, scientist Oliver Grah measures the velocity of a stream of melt from Sholes Glacier on one of the slopes on Mount Baker in Washington. Glaciers on Mount Baker and other mountains in the North Cascades are thinning and retreating. Dozens of scientific measurements show Earth is warming. Since 1997, the world has warmed by 0.44 degrees (0.25 degrees Celsius) and 51 monthly or annual global heat records were broken, according to the National Oceanic and Atmospheric Administration. (AP Photo/Manuel Valdes)

"What's wrong with disagreement? People disagree. You listen or you don't," Curry says. "This polarization comes down to being intolerant on disagreement."

What Changed

In 1997, then-Vice President Gore helped broker an international treaty, the Kyoto Protocol, to reduce heat-trapping gases from the burning of coal, oil and gas. The U.S. later withdrew from the treaty.

"And at that moment" says Leiserowitz, "the two parties begin to divide. They begin to split and go farther and farther and farther apart until we reach today's environment where climate change is now one of the most polarized issues in America."

The election of Barack Obama and the Tea Party revolt made the schism even bigger, he says.

Stanford University's Jon Krosnick agrees that things changed around 1997, but he thinks Americans are fairly united—it's just they don't realize it. Krosnick's surveys show that nearly 90 percent of Democrats, 80 percent of independents and 70 percent of Republicans believe the increase in world's temperature over the past century was mostly or partly caused by humans.

His studies show fairly consistent numbers, except for a drop in Republicans to 50 percent in 2011 that since has returned to 70 percent.

A bigger split in Stanford surveys indicates that while about 90 percent of Democrats and 80 percent of independents believe global warming will be a serious or very serious problem for the United States, barely half of Republicans feel that way.

To illustrate how the issue plays out in all sorts of ways, let's take lobster scientist Diane Cowan in Friendship, Maine, who expresses dismay.

"I am definitely bearing witness to climate change," Cowan says. "I read about climate change. I knew sea level was rising but I saw it and, until it impacted me directly, I didn't feel it the same way."

Republican Jodi Crosson, a 55-year-old single mother and production and sales manager in Bexley, Ohio, thinks global warming is a serious problem because she's felt the wrath of extreme weather and rising heat. But to her, it's not quite as big an issue as the economy.

And then there's Ken Martig Jr. An engineer and business owner in Allyn, Washington, he paid little attention to global warming until he learned that one proposed solution involved regulations and taxes. Now he doesn't think climate change is man-made or a major worry.

"If you put it down to one word today, it's a trust issue," the 73-year-old Martig says. "Do you really know for a fact that it's burning of the (fossil) fuels that are creating these greenhouse gases" that are causing the world to warm?

Scott Tiller, a 59-year-old underground coal miner in West Virginia, has seen mine after mine close, and he agrees with Martig.

In this July 11, 2012 file photo, a farmer holds a piece of his drought- and heat-stricken corn while chopping it down for feed in Nashville, Ill. Scientists have connected man-made climate change to extreme weather, including deadly heat waves, droughts and flood-inducing downpours. (AP Photo/Seth Perlman, File)

"I think we've been treated unfairly and kind of looked down upon as polluters," Tiller says. "They say the climate is changing, but are we doing it? Or is it just a natural thing that the Earth does?"

Bridging Differences

Overwhelmingly, scientists who study the issue say it is man-made and a real problem. Using basic physics and chemistry and computer simulations, scientists have repeatedly calculated how much extra warming is coming from natural forces and how much comes from humans. The scientists and their peer-reviewed research blame human activity, for the most part.

Dozens of scientific measurements show Earth is warming. Since 1997, the world has warmed by 0.44 degrees (0.25 degrees Celsius) and 51 monthly or annual global heat records were broken, according to the National Oceanic and Atmospheric Administration.

Arctic sea ice, ice sheets and glaciers are melting faster. The seas have risen and hot water has been killing coral in record numbers. Scientists have connected man-made climate change to extreme weather, including deadly heat waves, droughts and flood-inducing downpours. Allergies, asthma and pest-borne diseases are worsening public health problems, with experts blaming climate change.

Scientists keep acting as if they just do a better job showing data or teaching, then people can understand that climate change is a problem—and that's just not the way people work, says Yale's Kahan.

He points to polling showing that if you ask people if scientists are sure global warming is real, man-made and a threat, they'll say yes.

"They know that scientists say we're screwed," Kahan says. "But it's not what activates them."

Twice in the last seven years, scientific societies sent group letters to Congress explaining that warming is real, man-made threat.

"I honestly believe that low science literacy allows people to fall for things that make no sense," says University of Georgia meteorology professor Marshall Shepherd. "For example, when it is cold or a snowy day, I may get a comment like 'There is 20 inches of global warming in my yard.' While that is a cute, snarky comment, it really illustrates a lack of understanding of weather versus climate."

Kahan says the most ardent doubters of climate change are also among the best-educated groups on the science—along with the most ardent believers. They are driven by ideology, he says.

So instead of spouting statistics, some climate activists and even scientists try to build bridges to communities that might doubt that the Earth is warming but are not utterly dismissive.

The more people connect on a human level, the more people can "overcome these tribal attitudes," Anna Jane Joyner says. "We really do have a lot more in common than we think."

Disagreement is OK, says her father, Rick.

"True unity is not a unity in conformity, but a unity in diversity," he says. "We look at differences as an opportunity to learn, not to divide."

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