What do you think of when you hear the words "global warming?" You might envision melting ice caps, drowning polar bears and shrinking coast lines. Or perhaps your mind turns to magazine covers, politicians and celebrity activists. Global warming has become a very divisive term, but is it real?

The short answer, according to environmental scientist David Keith, is yes.

"There is no disagreement among really anybody who is scientific in any way that the world is a lot warmer than it was 100 years ago," Keith says. "If there are interesting disagreements, the disagreements are about whether this is the warmest it's been since the ice ages 10,000 years ago."

A recipient of honors that include MIT's prize for excellence in experimental physics, Keith has spoken to governments, corporations and media outlets about climate change. As he points out, scientists use various methods to measure global warming; they produce varying answers.

"If all the scientists in the world believed there was only one answer, it would be right for all the rest of us to be skeptical," Keith said. "There's nothing in the world that one ever measures with perfect accuracy."

Those measurements include thermostatic records and satellite images that document temperature increases over the past century. Additionally, paleoclimate databases suggest the current rate of increase is substantially higher than normal.

While global warming is certainly an important aspect of climate change, the term's use in mass media may actually serve to distract people from the real issues. Keith uses the example of a human patient hooked up to a mercury drip to illustrate this point.

The hypothetical human will eventually die from mercury poisoning — it's the scientific reality of the situation. The media focus on year-by-year warming or cooling, he argues, is akin to focusing on the patient's symptoms instead of the proven underlying condition and the cause behind it. In the case of climate change, elevated carbon dioxide (CO2) levels are the deadly mercury drip.

"The core theory says if you double or triple CO2 in the atmosphere, it's going to get warmer," Keith said. "This is something we've known from pretty basic physics and proved with a lot of good science for more than 100 years. That's the reason to worry, not the warming over the last few decades."

Scientists first raised concerns over the warming effects of CO2 in the atmosphere in the 1960s, when the climate was actually cooling. While there's nothing overtly problematic about natural climate change, it's the rate of change that worries experts.

Approximately 55 million years ago, the climate was warm enough to support alligators in the high Arctic. It took 10 million years for those CO2 levels to decrease to the current level. At humanity's current rate of CO2 production, Keith said, levels will rise back up to that point in only 100 years: 100,000 times faster.

"There is no controversy among anybody, even the skeptics, that the amount of CO2 in the atmosphere is going up — unless you pick ones that are just nutballs." Keith said. "We know that without any doubt."

Global warming has become a politically charged issue, due in large part to the massive amounts of power and money tied up in industries that emit CO2.

"Some of these carbon-based industries face tremendous challenges in terms of making the shift, and quite logically there are certain pockets of resistance," Max Boykoff of the University of Colorado at Boulder said.

As David Keith pointed out, this is expected of any big issue that involves regulating individual lives and restructuring business.

"People fight over everything, including the science," Keith said. "If you look at ozone depletion, predicted air pollution, or any of the big environmental problems we've regulated on, there have always been enormous arguments about science. It's normal."

In addition to the science policy issues involved in climate change, Boykoff's research focuses on how individuals engage the issue. He believes that many climate change dissidents subconsciously adopt their world view to avoid confronting the issue.

"I would only suspect that most if not all of us wish there wasn't a problem in the first place," Boykoff said, "and on an individual psychological level there's a certain element of cognitive dissonance. We wish it to not be an issue and, if it isn't influencing us directly, then we turn our attention to other more pressing issues. So part of the politicization is fed by our own resistance to confronting a crucial issue."

For many, climate change is a difficult concept to grasp. You can't look out the window and observe it firsthand, and the average person lacks access to peer-reviewed literature on the subject. Mass media often bridges that information gap. In many cases, the information becomes sensationalized or otherwise misrepresented in the process.

"The fact is that if the science is all correct, and I think there's no reason to doubt the basic science, and if humans want to actually deal with this problem, then we're going to have to stop emitting CO2," Keith said. "And not just cut it by a few percentage points, but stop. That has huge implications for significant chunks of the world's economy. So of course there's going to be controversy. It would be stunning if there wasn't."