Whether man's burning of fossil fuel is making the Earth warmer is still an open question - one that scientists should investigate for another couple of decades, according to an expert in climatic change.

But politicians are demanding answers today, added John Michael Wallace of the University of Washington, who spoke Wednesday night at the University of Utah's Fine Arts Auditorium in the College of Science series, Frontiers of Science.While a group of scientists declared in 1996 that "the balance of evidence suggests a discernible human influence on global climate," Wallace and some others aren't so sure. He presented conflicting evidence on the issue, including long-range climate re-creations that show the Earth went through ice ages and warming periods in the past without man's intervention.

Over the past 30 years, weather patterns definitely changed, he said. Siberian winters are getting warmer while Newfoundland is becoming more frigid. "I think it's not a black and white case. I think it's still very much an unclear issue" about what is happening, he said.

A period of rapid warming occurred in the 1920s and '30s. During the 1940s and 1950s, however, Earth experienced a cooling-off. Then the temperature began to rise again.

"The rapid warming during the 1920s and '30s and the subsequent cooling after that time is difficult to explain by global warming," he said.

"It is conceivable that natural variability is responsible for the warming we've seen this century. We certainly can't rule that out."

Some opponents of the global warming theory have argued that the indicators are only an artifact. Most weather stations are in urban areas. As cities grew, the landscape in the region of these stations has become more and more urbanized. Trees were cut down, streets and buildings constructed.

As a result, the stations were surrounded by more asphalt and concrete - material that tends to retain the day's heat at night. As a result, goes this argument, the weather stations are reporting that the nights are warmer. But instead of a sign of global warming, it's an indication that the stations are in areas that are more urban than they used to be.

Wallace showed one set of charts that would seem to show the climate is confused in California: It's getting hotter fast in big cities, not so fast in medium cities and remaining level in small towns.

In his opinion, some warming really has taken place in the past 30 years in some regions. In Utah, the average temperature in urban areas has gone up by 1.14 degrees Celsius since 1967, while it has increased by 0.99 degree in rural areas over the same period. That indicates the urban effect is real, but so is a warming trend.

Comparable trends show up in studies of Oregon and Arizona, he said.

The Arctic and Antarctic seem to be warming and average barometric pressure is changing. Pack ice in the Arctic is thinner and melting sooner.

"They couldn't find any," he said.

But are these changes just the natural fluctuations that Earth goes through?

"I think the verdict is still out on greenhouse warming," he said. Greenhouse warming is the theory that man's release of "greenhouse gases" like carbon dioxide has altered the climate drastically.

Even if global warming is happening, the effects so far are much less severe than the fluctuations of natural changes such as El Nino, Wallace said. There is no reason to be alarmed for the short term, at any rate.

"I'm hopeful that in another decade or two we will have much clearer answers to whether the climate is warming and why."