For the first time climate change is expected to be a major issue in the U.S. presidential campaign. Analysts say in past campaigns candidates argued about whether global warming is actually occurring, but the results of recent scientific studies have now shifted the debate to possible solutions that could reduce the future impact of a changing climate. VOA correspondent Meredith Buel has details in this background report from Washington.

Scientists at the Academy say the most striking proof of global warming is what they call a relatively rapid and widespread increase in temperature during the past century.

They say other evidence of warming, such as increases in ocean temperatures, shrinking mountain glaciers and the decreasing polar ice cover are consistent with this trend.

David Sandalow, an energy and environment scholar at the Brookings Institution, says the evidence is clear.

"We have a good body of science on the impacts of global warming," said David Sandalow. "We have never seen greenhouse gas concentrations in this range in all of human history. They are accelerating at rates that are wildly unknown in human history. The potential threat is very serious."

The National Academy of Sciences says greenhouse gases have increased significantly since the Industrial Revolution, mostly from the burning of fossil fuels for energy, industry and transportation.

It was only in recent years, however, that most scientists and political leaders began to agree that the changes are due primarily to human behavior.

Rick Klein, a senior political reporter for ABC News, says it is now a major campaign issue for presidential candidates in the United States.

"We have seen this entire issue change over the last few years in a pretty extraordinary way," said Rick Klein. "There really is not a major debate in Washington anymore over whether climate change is happening. It has moved to [asking] what we are going to do about it."

Gregg Easterbrook is a senior editor of The New Republic Magazine and writes frequently about politics and the environment.

Easterbrook argues the next president should favor legislation putting a price on greenhouse emissions to give inventors the financial incentive to create cost-effective solutions to global warming.

"I think in the coming election everybody is going to say that they are in favor of action on climate change," said Easterbrook. "I don't see any candidate who won't say that. The question will be whether they favor a substantive reform or some kind of symbolic action."

At a recent forum at the Brookings Institution, policy advisers to some of the major presidential candidates agreed on the importance of global warming as a campaign issue, saying it plays a role in environmental considerations, economic development and national security.

Denis McDonough advises Illinois Senator and Democratic presidential candidate Barack Obama.

"So at the end of the day, if I could put one word on it, it is urgency," said McDonough. "This is a problem that is far past its prime. It now has to be confronted with the urgency it demands."

Todd Stern is a policy adviser for New York Senator Hillary Clinton, currently the frontrunner in the race for the Democratic Party's nomination.

Stern says Clinton favors a balanced approach to climate change.

"We have twin challenges here, which is oil security," he said. "We want to depend less on oil, less on foreign oil, but less on oil all together for national security and economic reasons, but also a climate challenge. We do not want to solve one, the oil security problem, while creating a bigger problem on the climate front."

On the Republican side, John Raidt, an advisor to Arizona Senator John McCain, says his candidate is a long-time supporter of nuclear energy.

All of the political advisers at the Brookings forum say their candidates support policies to create alternate energy technologies, promote fuel efficiency and reduce dependence on foreign oil.