Rep. Steve Scalise had just won a victory against putting a tax on carbon. Then he switched from attacking taxes to questioning climate science, stepping into prickly terrain that appears to be increasingly patrolled by Democratic allies who are ready to label Republicans as deniers.

The conservative Louisianan looked back to a period of Earth's history when temperatures were higher than today's, he said, even though humans were without technology and had just started to plant crops. The comparison provides evidence that modern man isn't altering the climate with industry and vehicles, he added.

Steve Scalise

"If you look at historically, our climate has fluctuated up and down," Scalise said. "To say that man is the reason that temperatures go up or go down, I think, ignores history where 10,000 years ago you didn't have a combustion engine and the Earth's temperatures were much hotter than they were today."

The assertion, or one like it, is not new to scientists who are studying the planet's past climate profiles. There are periods when there wasn't "a snowflake" on Earth, one scientist said. Some stretches of time were so warm that crocodile fossils have been found near the North Pole.

"It might have been a hair warmer," Jeremy Shakun, a climate scientist at Boston College, said of Holocene period 10,000 years ago.

But that's not proof that man's activities don't heat the planet, he added. In fact, it serves as an underpinning for scientific evidence that carbon dioxide plays a leading role in global warming. Just because previous spurts of the gas occurred naturally doesn't diminish the impact of today's global rise of CO2, or the fact that it's coming from humans, scientists said.

"If you double the amount of CO2 in the air, is that going to cause warming? I think that's where it's like, 'Come on, seriously?'" Shakun said. "It's like holding a pen up and being like, 'If I let go, is it going to drop?'"

Can 135 Republicans be wrong?

Democrats are diving into the science debate as they defend future carbon standards for power plants and seek to define the Republican Party as being out of touch with established science. President Obama accused skeptical opponents of "sticking your head in the sand" as he rolled out his emissions plans for power plants in June.

That theme returned last week in a $2 million television and Internet ad campaign launched by the League of Conservation Voters. It targets Sen. Ron Johnson (R-Wis.) for being "a climate change denier" and accuses him of siding with fossil fuel interests. In response, Johnson released a fundraising email that accused the liberal group of "environmental jihad."

Unicorn award

The LCV campaign is also targeting three House Republicans, Reps. Dan Benishek of Michigan, Mike Coffman of Colorado and Rodney Davis of Illinois. All of them come from competitive districts. Coffman, for example, whose 6th District encompasses the suburbs of Denver, won by 2 percentage points in 2012. It's among the most politically balanced districts in the nation.

Climate advocates also introduced a mythical animal, a unicorn, to the debate last week, representing some lawmakers' assertions that man-caused climate change isn't real. Obama's former campaign apparatus, now named Organizing For America, distributed "Congressional Climate Denier" awards to 135 House Republicans, little statuettes with the imaginary one-horned creature rearing up on its hind legs.

Among those targeted were Rep. Scalise and House Speaker John Boehner (R-Ohio), whom OFA quotes as saying that it's "almost comical" that carbon dioxide is bad for the environment. "Every time we exhale, we exhale carbon dioxide," Boehner said, according to OFA's website, which encourages Obama supporters to "call these deniers out" on Twitter.

The group targets lawmakers in every state but Connecticut, Maine, Massachusetts, Nevada, Oregon and Vermont. Scalise is among those targeted, although his district along coastal Louisiana is among the most conservative, and least contested, in the nation.

His belief about past temperatures is an assertion that's frequently identified by groups that reject climate science as evidence that current warming is just a natural teetering in Earth's cooling and warming phases.


The Chicago-based Heartland Institute asserts that there's "nothing unusual, unnatural, or unprecedented about the current level of Earth's warmth."

When temperature ups and downs speed up

Most scientists disagree. A study published in the journal Science in March found that global temperatures might have slid up or down by about 1 degree Celsius over the past 11,300 years. A similar change, a rise of about 1 degree, has occurred over the last century. The authors, Shakun among them, say that shows that industrial-era greenhouse gases are accelerating the warming.

"We're going from the cold end to the warm end in 100 years. That's a little bit eye-catching," Shakun said.

These slight up-and-down changes can occur over tens of thousands of years and are caused, initially, by slow changes in Earth's orbit called wobbles.

That can change the display of sunlight on the planet, increasing temperatures in some areas while decreasing it in others. Overall, it's a wash. Heat is redistributed on Earth, but the global temperature stays about the same. One climate scientist said, "You're just rearranging sunlight."

That was true until other forces were unleashed. An example of this happened about 20,000 years ago, when humans were still dependent on roaming animals and wild plants for food. A gradual planetary wobble had occurred, and the world was emerging from its most recent ice age. More sunlight was striking the Northern Hemisphere, and it was creating a huge amount of water as it melted the frosty covering of ice and snow.

The increase in sunlight, in natural emissions of CO2 and in warming from changing ocean currents happened naturally. Not only were there no car engines, but the human species had just begun to plant crops, and that only happened in limited areas. It wasn't until about 2,000 B.C. that people in the area now known as Peru moved away from hunting and gathering in the Western Hemisphere and began burning down forests to clear land for farming. The internal combustion engine, of course, came along much later, but when it did, man made millions of them.

Alley uses a wildfire analogy to differentiate natural warming from human-caused warming. Fires can be ignited by either lightning or a match. He says climate change is similar.

"I think there are some people who hear this and say, 'OK, if nature has always changed the climate, doesn't that mean that the current change is natural?' And the answer is no," he said. "As we go back and study the history of climate ... it really is analogous to an arson investigator understanding natural fire so they can understand the human-caused ones."