BROOMFIELD - Global climate change is "probably the most important environmental issue facing the world," the Bush administration's point man on the hot-button topic said Monday.

"We know that humans are influencing the climate. There's no question about that," said James Mahoney, director of the U.S. Climate Change Science Program.

"The real questions are: by how much, how reversible is it and what are the best means to reduce the human impacts on the climate?" Mahoney said during a meeting of about 400 atmospheric researchers at the Omni Interlocken Resort.

The Bush administration has been lambasted by climate researchers for failing to endorse the Kyoto Protocol or to otherwise play a leadership role in addressing climate change.

The planet has warmed about 1 degree Fahrenheit over the past century. Most climate scientists agree that receding mountain glaciers, declining global snow cover, thinning summer sea ice thickness in the Arctic, and rising sea levels are environmental indicators of a warming world.

The Kyoto Protocol, which calls on 35 industrialized countries to rein in emissions of carbon dioxide and other heat-trapping gases, was ratified by 140 nations and went into effect in February.

"The United States has a really deep problem with credibility right now," said David Victor of the Center for Environmental Science and Policy in Stanford, Calif.

"We have no credible emission policy at the federal level," Victor said on opening day of the International Carbon Dioxide Conference.

Other researchers agreed, despite Mahoney's assurances that the U.S. government "is doing a great deal" to combat global warming.

"It's pretty much a certainty that big changes will happen, so we should be slowing down our CO2 emissions," said Susan Trumbore, a bio-geochemist at the University of California, Irvine.

Mahoney said the U.S. government spends nearly $2 billion a year on climate change research and another $3 billion annually to promote new energy technologies.

"Not signing Kyoto doesn't mean that this government isn't doing anything," said Mahoney, who later amended his description of climate change to say it's "one of" the world's most important environmental issues.

Each year, global combustion of coal, oil, natural gas and wood emits nearly 7 billion tons of carbon, in the form of carbon dioxide, into the air.

The Earth's oceans, trees, plants and soils absorb about half of that carbon.

The rest remains in the air, contributing to the 36 percent increase in atmospheric carbon dioxide levels since pre-industrial times.

Carbon dioxide levels are higher now than at any time in the past 450,000 years, Mahoney said. If the levels of carbon dioxide and other heat-trapping gases continue to rise as projected, the planet could warm another 2.5 to 10.4 degrees Fahrenheit by 2100, according to the Intergovernmental Panel on Climate Change.

The consequences could include a greater frequency of extreme weather events, such as Katrina-intensity hurricanes, said Ken Caldeira of the Carnegie Institution.

One of the "persistent myths" about climate change is that the problem "will disappear on its own," said Jae Edmonds of the Pacific Northwest National Laboratory.

Conventional wisdom says the world's supply of easily accessible oil and natural gas will be exhausted in a couple of decades.

After that, prices for those fuels will skyrocket, and other energy sources will replace them.

It would appear the problem would be solved.

But that scenario ignores the world's abundant coal supply, Edmonds said. Coal can be burned as a solid or converted into liquid and gaseous fuels.

In addition, higher prices for petroleum-based fuels and natural gas will spur the development of new technologies to reach and extract less-accessible deposits.

So, Edmonds said, waiting for the system to run out of fossil fuels won't be a solution.