'U' researchers debate cause of global warming

As concern for global warming continues to rise, most scientists agree that the Earth's climate is changing.

But the causes of this problem are not as clear. While many experts believe that global warming is a result of man-made greenhouse gases in the Earth's atmosphere, others maintain that such changes are a natural occurrence.

The global warming debate only will heat up after yesterday's announcement that 1997 was the Earth's warmest year on record. Researchers with the National Oceanic and Atmospheric Administration said the Earth's average temperature last year was 3/4 of a degree Fahrenheit above normal.

University researchers have formed several different theories explaining the phenomenon.

Joyce Penner, University professor of atmospheric, oceanic and space sciences, is one of the few researchers arguing that global warming might be a natural occurrence. Her research indicates that aerosols created by sulfur emissions from the burning of fossil fuels - substances such as soot and sulfuric acid - actually help cool the planet.

"These particles can reflect solar radiation," Penner said. "Not as much sunlight gets to the surface of the Earth to warm it up, which has a cooling effect."

Aerosol particles also become cloud nuclei, forming water droplets that increase cloud coverage. This further reduces the amount of sunlight that reaches the Earth.

Penner's study showed that these clouds and free-floating particles cool the Earth more than previously thought, at a rate almost twice that of which greenhouse gases warm the planet. So if the global climate is getting warmer, as studies show, it might be because of natural reasons, Penner said.

"There's natural variability in the warming system," Penner said. "If the two effects were nearly balanced, then the hundred-year trend that we've seen might be due to natural causes."

Although Penner's results are preliminary, she believes they are conclusive enough to cast doubt on conventional theories about global warming during the past hundred years. She also said the results confuse the issue of global warming in our immediate future.

"It adds a complication, which may mean that we would not anticipate a warming as soon as we would otherwise expect," she said.

But because aerosol emissions are being cut, global warming could well increase. Europe and the United States have cut aerosol emissions because they cause acid rain. But greenhouse gases, which Penner believes cause global warming, continue to increase.

"Aerosols have a very short lifetime; CO2 has a very long lifetime," Penner said. "If we continue on the same emissions trends that we're on, CO2 concentrations will rise faster than aerosols will."

Henry Pollack, a University professor of geological sciences, agrees with Penner on one thing - the Earth is getting warmer, and it will continue to do so. But the cause of global warming is different, Pollack said.

"The temperature increase is real, there isn't any real quarrel about that," Pollack said. "The causes are more debatable."

Pollack examined the temperature readings from 300 sites around the world. These sites were used to measure temperature changes over a 500-year period. This gives information about climate changes before 1900, when meteorological data began to be recorded globally.

"We have looked at a five-century interval, which lets us look at the pre-industrial era as well as the industrial era," Pollack said. "Our results show that the Earth's temperature did warm between 1500 and 1750." But Pollack adds that "about 80 percent (of the warming) occurred after 1750, and 20 percent occurred before."

Pollack's data confirmed that the average global temperature has increased about one degree Celsius during the past 500 years. Alarmingly, fully one half of the total warming occurred in the past 100 years, indicating that the increase in greenhouse gases was a key factor in the warming process.

"What we found is that the 20th Century is a very anomalous century compared to previous ones," Pollack said. "The 20th Century is the time when greenhouse gases have increased most dramatically; temperature has also increased most dramatically. The correlation between those two observations makes us suspicious that there is a human signature on this global warming."

In light of this data, Pollack concluded that natural climate changes may have played a part in the warming trend, but only a small one.

"What we know about natural fluctuations suggests while that it plays a role, it's unlikely to account for more than 20 percent of the temperature increase that we've seen," Pollack said.

Pollack's views are shared by many other scientists and researchers, including Mary Ann Carol, a University professor of chemistry and atmospheric, oceanic and space sciences.

"There are certainly natural phenomena that lead to warming and cooling of the planet," Carol said. "But the rate at which we've increased greenhouse gases ... would indicate an artificially induced or enhanced temperature change. That level of change over a short period of time is something that would not be typical."

Although there is debate over the true causes of global warming, most researchers share the same outlook on the future: if left unchecked, greenhouse gas emissions will continue to grow, and global warming will only get worse.

"The concern is that the human impact can only grow, while the natural signature is at its upper limit," Pollack said. "If greenhouse gases continue to increase, then one can expect significant global warming in the 21st Century."

01-09-98