The words "global warming" provoke a sharp retort from Colorado State University meteorology professor emeritus William Gray: "It's a big scam."

And the name of climate researcher Kevin Trenberth elicits a sputtered "opportunist."

At the National Center for Atmospheric Research, where Trenberth works, Gray's name prompts dismay. "Bill Gray is completely unreasonable," Trenberth says. "He has a mind block on this."


Only 55 miles separate NCAR's headquarters, nestled in the Front Range foothills, from CSU in Fort Collins. But when it comes to climate change, the gap is as big as any in the scientific community.

At Boulder-based NCAR, researchers project a world with warmer temperatures, fiercer storms and higher seas.

From CSU, Gray and Roger Pielke Sr., another climate professor emeritus, question the data used to make those projections and their application to regional climate change.

Science by its nature is disputatious - with every idea challenged, tested and retested. It's always been that way.

In the 18th century, Sir Isaac Newton and Gottfried Leibniz sparred over claims to the discovery of calculus.

About 140 years later, Charles Darwin's theory of evolution was challenged - based on the science of the day - by Harvard University professor Louis Agassiz and the British Museum's Sir Richard Owen.

Now the battle is over global warming, or more accurately over myriad details - like temperature readings and the thickness of sea ice - upon which the larger idea is based.

On one hand, the fight is a natural part of the scientific process. But it also creates dissonance and uncertainty.

"Some of this noise won't stop until some of these scientists are dead," said James Hansen, head of the Goddard Institute for Space Studies in New York City, and among the first to sound the alarm over climate change.

While science is comfortable with uncertainty, policymakers are not, and that is what has turned this scientific debate into front-page headlines.

"I think there is a debate about whether it's caused by mankind or whether it's caused naturally," President Bush said in a July interview.

To be sure, Gray and Pielke are in a scientific minority. Still, their challenges remain part of the fractious scientific process.

"Science needs skeptics," said NCAR researcher Warren Washington.

Still, a broad scientific consensus has emerged that human activity is contributing to climate change.

Findings by panels created by the National Academy of Science to resolve disputes - such as conflicting satellite and ground temperature records - have supported the trends in global climate change.

And things that the NCAR models predict - such as thinning sea ice and melting glaciers - are coming to pass, although scientists say more data are needed to verify those trends.

After more than two decades of research, scientists, even most skeptics, agree that:

Since 1750, the amount of carbon dioxide in the atmosphere, mainly from burning fossil fuels, has risen to about 380 parts per million from 286 parts per million.

It doesn't appear carbon dioxide levels have been that high in the past 650,000 years.

Carbon dioxide is continuing to build in the atmosphere by about 1.5 parts per million a year, and as a so-called greenhouse gas, it traps the sun's heat.

The Earth's average temperature has warmed about 1 degree Fahrenheit since 1880 and is now warmer than it has been in the past 400 years.

Average global temperatures are likely to rise - this is where the debate begins - somewhere between 2 and 10 degrees Fahrenheit by 2100.

The heat will cause global ocean levels to rise 3 to 39 inches this century.

In the film "An Inconvenient Truth," former Vice President Al Gore tends to fix on the upper end of the projections, while skeptics point out that the lower end may be as likely and less catastrophic.

But even small changes may have big effects. When the average temperature dropped by a little less than 1 degree Fahrenheit in about 1400, it ushered a period called the "Little Ice Age."

It was a time when advancing Swiss Alp glaciers crushed villages, England's Thames River froze and short growing seasons led to famines.

Most scientists also agree extreme weather events like Hurricane Katrina or Los Angeles' July record 119-degree Fahrenheit temperature cannot be directly attributed to global warming.

On this much there is some scientific consensus.

What the impact of rising temperatures or higher seas will be is more open to debate, according to skeptics such as Pielke, because most of the calculations are global averages.

"This tells you nothing about what's going to happen in any region," Pielke said.

While Pielke agrees carbon dioxide is forcing changes in the climate, he says, "It is not the only forcing."

Man-made changes to the land, in addition to about 30 other greenhouse gases - some man-made, some natural - may play an even a bigger role, he said.

"The public likes simple answers," Pielke said. "But there isn't any simple answer here."

Simplicity is hard to come by because Earth is a giant, complex heat-moving machine.

The sun's rays strike full force at Earth's middle and glance off the ends - making the equator hotter than the poles.

Ocean currents, winds, the jet stream and hurricanes are forces trying to balance out the Earth's heat.

Efforts to calculate what is going on in the oceans, the land and the atmosphere are an unparalleled exercise.

The task falls to mathematical models run by supercomputers like the one in NCAR's basement. These "general circulation models" attempt to keep track of a multitude of variables around the globe - such as ocean currents, air and sea temperatures, rainfall and the composition of the atmosphere.

"This is a unique exercise in science and a very difficult one," said Christopher Essex, a mathematician at the University of Western Ontario in Canada.

The models are trying to project a future world, Essex said, without a complete theoretical base on how climate works and the risk of small errors being amplified.

Another problem, Essex said, is in the inability to do controlled experiments - one of science's key tools.

"There's only one atmosphere, so you can't hold everything steady and change just one variable to see what happens," he said.

Essex offered his critique of the models at a Los Alamos National Laboratory climate conference in Santa Fe in July.

At the end of the presentation, CSU's Gray jumped up and demanded: "Should we base national policy on these models?"

"I'm not touching that," Essex replied.

And then Essex added: "At every stage of the history of science, there has been some element that was impossible, and we've found a way around it. I am sure we will here."

This did not assuage Bill Gray.

Gray is among the most strident critics, quick to use words like "fraud" or "gang" to describe the modelers.

Instead of model projections, Gray looks at the history and patterns of weather to find trends.

And befitting his 76 years, Gray has a long view. His first report on climate - on the return of the dust bowl - was in the early 1940s when he was in junior high school.

"We'd gone through a warming trend in the '40s, and everybody was saying we were going to win World War II but face terrible droughts," Gray said.

Soon after, temperatures went into a cooling trend and by 1975, Gray points out, there was talk of a coming ice age.

The Earth does have natural cycles of cooling and warming - during the past 740,000 years there have been eight cycles with four ice ages.

The cycles appear to be tied to slight variations in the tilt of the Earth toward the sun.

During the last ice age - which ended about 10,000 years ago - Earth was on average about 4 degrees Fahrenheit cooler, and what is now Manhattan was buried under ice.

At some point the Earth will wobble on its axis again, setting the stage for an ice age.

There are other phenomena affecting global temperatures over time, such as El Niño, a Pacific Ocean warm-water mass that appears in roughly five-year cycles and changes world weather patterns.

And there is the Atlantic thermohaline current, a conveyor belt moving heat north on the surface and then dropping it to the ocean floor and heading back to the equator - a 1,200-year trip.

Changes in the current lead to changes in temperature. Somehow the models have to account for these natural variations too.

Gray believes that the warmer temperatures in the Northern Hemisphere are linked to a natural slowing in the thermohaline current, not the carbon dioxide.

Some of the models also show the current is slowing and that, along with warming oceans, adds to hurricane risks.

This has sparked one of the biggest scientific disputes of the moment.

It is a debate in which NCAR's Trenberth and CSU's Gray are, of course, on opposite sides.

Hurricanes feed on warm water, and Trenberth says that warmer sea-surface temperatures and increased atmospheric water vapor - both of which have been measured - will contribute to more intense hurricanes.

Gray - and other hurricane specialists, including Chris Landsea, the science and operations officer at the National Hurricane Center in Miami - say that the link doesn't yet exist and that models overstate the case.

Trenberth concedes that the changes being measured are small, but he adds, "They are all going in one direction."

Gray argues that heat by itself isn't enough - that there are other variables: The air has to be cooler than the ocean, the winds have to be agreeable.

The dispute led Landsea, who is a former Gray student, to quit as a member of a working group of the United Nations' Intergovernmental Panel on Climate Change, or IPCC.

Trenberth, 61, is the lead author for that working group, whose report is due next year.

The IPCC was created to assess - through a set of working groups - scientific, technical and socioeconomic information on climate change. It does not, however, do research.

Landsea, in an open letter to the science community, said the science working group was being "motivated by pre- conceived agendas" and was "scientifically unsound."

Even with that, Landsea says: "I am concerned about the trend in global warming. It is a problem."

The IPCC science working group - with more than 100 members - has been trying to forge a consensus on the best science, Trenberth said.

"But it is a struggle to accommodate every viewpoint," he said. "I don't know why Chris Landsea acted that way."

Landsea isn't the panel's only critic.

"The IPCC has become an inbred process," Pielke said. "All the scientists I know are doing legitimate work and believe in what they are doing. ... Still, it's a narrow view."

Pielke, 59, says his doubts about the climate record began during his stint as Colorado's climatologist when he realized how inaccurate the state's thermometer network was.

Placing a thermometer close to a building or near an air- condition vent can compromise readings, Pielke said.

When the winds blow from Denver, a Front Range thermometer is influenced by urban effects, Pielke said, and by agricultural activities when it blows from the north.

Multiply that by tens of thousands of thermometers around the world and the temperature record is suspect, he contends.

The modeling groups say that what is important is the warming trend.

NCAR's Washington, 70, a pioneer in climate modeling, said that 30 years ago the climate models kept track of just the atmosphere and oceans. Today they include more than 10 measurements, including sea ice, clouds and forest growth.

This year NCAR even added human land-use impacts on climate to the modeling.

Pielke, who argues that when it comes to climate change both science and policy have focused too much on the carbon dioxide buildup and ways to contain it, said he felt "vindicated."

NCAR researcher Linda Mearns, however, said the land- use impact "doesn't obliterate or remove the greenhouse gas problem."

"Roger tries to present it as though he's the lone voice," Mearns said. "That's not true."

The models still have problems, Trenberth and the other modelers concede - particularly assessing regional impacts.

When the NCAR model tries to show Denver's weather patterns, for example, summer thunderstorms keep coming about noon.

"We all know they come in the late afternoon, so that's a problem in the model," said Trenberth, who was born in New Zealand and trained at the Massachusetts Institute of Technology in Cambridge, Mass.

Many issues still have to be resolved, said Chris Folland, a researcher at Britain's Hadley Centre for Climate Prediction, but the science continues to point in one direction.

"We've shown that the climate change is a true thing," he said. "We've done that with global averages, since that was easiest.

"The American government might not agree," Folland said. "Most American scientists do."