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Scientists debate whether we’re changing the climate

The global warming debate isn't likely to cool off anytime soon, with experts disagreeing about its causes, and even its existence.

What you hear about global warming can vary as much as, well, the weather.

One day, California Gov. Arnold Schwarzenegger is quoted saying: "We simply must do everything we can in our power to slow down global warming before it is too late. The science is clear. The global warming debate is over."

Another day, former Pennsylvania Sen. Rick Santorum says this: "Americans are coming to understand that global temperatures have actually cooled over the last 10 years and are predicted to continue cooling over the next 10."

On another day, dozens of papers proclaim that the sun is at fault, since the Max Planck Institute for Solar System Research demonstrated that the sun has burned brighter in the last 60 years than any time in the last 1,100 years, or 8,000 years, depending on the news source.

Most climatologists maintain that global warming is real and human activity is likely the major contributor. They believe that as we release more CO2 into the atmosphere, the planet will get hotter.

The contradictory statements tend to cluster around a couple of sticking points: the heating trend over the last few years, the influence of the sun, and the lessons from the distant past.

Where there's good agreement is laid out by MIT meteorology professor Richard Lindzen, a critic of Al Gore and others he considers alarmists. In an editorial for the Wall Street Journal several years ago called "Climate of Fear," he wrote that he agrees that global temperature has risen about a degree (Celsius) since the late 19th century.

He also agrees with most other climatologists that the level of CO2 in the atmosphere has increased by about 30 percent over the same period, and that CO2 should contribute to future warming, thanks to basic atmospheric physics.

Lindzen, however, said in a recent interview that the carbon dioxide buildup may not be causing the current warming trend, and it therefore may not play a significant role in the future climate.

And the trend may already have turned around, he said. "If you look from 1995 you don't see any change that could be regarded as statistically significant . . . . For the last 13 or 14 years nothing has been happening."

Pennsylvania State University climatologist Richard Alley said the recent cooling claim is everywhere. If you search for the phrase "global warming stopped in 1998," you get thousands of hits, and he's most recently heard this argument from U.S. senators.

He called it "cheating" to start in 1998 and then pick some subsequent cooler year to argue for cooling. It's a little like taking your first measurements in July and your last ones in January, he said.

To understand the climate, you have to look at longer-scale trends, he said, smoothing out those little ripples known as weather.

Over the long term, he said, it's getting hotter. "If you take the last 30 years, it's completely evident that it's going up," he said of the global temperature.

But could it be the work of the sun? Solar physicists say the sun has been temperamental in the past, and flare-ups and calm periods have rocked the climate.

One clue to the sun's variability comes from the Little Ice Age, during the 17th and 18th centuries, when Europe and North America were much colder and snowier than they are today.

During that time, astronomers noted a nearly complete absence of sunspots, said atmospheric physicist Peter Pilewskie from the University of Colorado, Boulder. Called the Maunder Minimum, he said, the coincidence of this spotless period on the sun with the notable chill of the ice age suggests a sun-climate connection.

Could the sun be getting warmer? If that were so, Pilewskie said it would be unlikely to account for more than a fraction of the observed warming trend. With satellites, scientists have been able to record the total radiation emitted by the sun, and found it has changed very little in the last 30 years.

But those records only go back so far. Several years ago, a team from Germany's Max Planck institute published a paper showing that carbon isotopes trapped in tree rings reveal unprecedented solar activity during the 20th century.

Carbon 14, a heavy form of carbon, is formed when cosmic rays from outer space impact carbon on the Earth. The greater the solar activity, the more the sun's magnetic field deflects the cosmic rays and the less carbon 14 should be stored in tree rings.

According to the Max Planck team, we've been experiencing more solar activity in the last 100 years than we did for the last 8,000 years.

Judith Lean, a solar physicist at the Naval Research Laboratory, calls that conclusion erroneous. She said the isotopes also vary much more with changes in the Earth's own magnetic field and processes on the Earth, thus complicating the analysis.

She said the consensus among solar physicists now is that the sun is in a relatively warm phase, but nothing unprecedented and not hot enough to have raised the temperature a degree in the last century.

The other major point of contention surrounds the Earth's erratic climate history, with its many ice ages and long steamy periods. Lindzen argues that the climate is ultimately unstable, and so we should expect constant change regardless of human-generated carbon dioxide.

Many factors can influence climate on time scales long and short. Slight fluctuations in the Earth's distance from the sun have periodically conspired with the angle of the planet's tilt to create ice ages over the course of the last five million years.

More ancient hot periods that lasted millions of years are raising bigger questions. About 95 million years ago, during the age of the dinosaurs, for example, it was so warm that alligators lived in what's now Canada and tropical trees grew as far north as Greenland. Dinosaurs roamed where penguins and polar bears live today.

It looks as if it would take about eight times today's atmospheric carbon dioxide concentration to make things that hot, said Brian Huber, who studies past climate for the Smithsonian Institution in Washington. "This is a challenge for climate modelers - we don't fully understand it," he said.

But it could be that CO2 has a greater warming power than previously realized, in which case we will be in for a more severe global warming than has been predicted, Huber said.

To Lindzen, the inability to fully account for past warm spells makes it hard to trust predictions of our near-term future. "If we knew the answer to things like this, one would have more confidence."

Lindzen takes the view that the climate doesn't need human intervention to change over the course of decades or centuries. "The whole notion that something outside has to cause a change is absurd," he said. "The system is never in equilibrium; it's always wobbly."

But Penn State's Alley said CO2 from volcanoes looks to be the prime suspect for the warm spells of the distant past, and fossil-fuel burning is by far the most likely driver of current trends. "We've looked at the sun. Volcanoes are not doing anything weird. The oceans can dump heat into the atmosphere, but they're getting warmer too," he said.

Perhaps some of the disagreement boils down to a philosophical difference. Some fret over the pace of humanity's impact on the planet, and consider the cautious approach to be to hold off on further changing our atmosphere.

Others are comfortable with the notion of human beings altering the global atmosphere, and argue the cautious approach would be to avoid changing our way of life until we know more.