"Global warming is a reality," said Thomas Crowley, a geoscientist who specializes in climate modeling. "It's already here."

Crowley constructed a computer model to examine the sharp temperature increases registered at the end of the 20th century. The model provides further evidence that the increasing rate of temperature climb stems from the accumulation of greenhouse gases in the Earth's atmosphere.

Crowley's climate simulation was published in the July 14 issue of Science, the prestigious journal of the American Association for the Advancement of Science (AAAS). Scientists agree that Earth's climate over the past millenium has had its share of ups and downs, but according to this new model, it takes an unprecedented change, such as a boost in greenhouse gas emissions, to explain the past century's unusual warming trends.

Crowley's research also predicts that by the year 2100, temperatures in Earth's northern hemisphere will have risen 1.5 degrees Centigrade, for a total increase of about 2.5 degrees Centigrade -- close to 5 degrees Fahrenheit -- since the beginning of the Industrial Revolution.

"These may seem like small increases, but they have potentially big consequences," Crowley noted. Because temperature increases are expressed as mean annual temperature change, with daily temperatures averaged over the entire year, figures do not reflect the greater changes that may occur in the summer months, for example. And rising temperatures can result in shrinking glaciers, rising sea levels, and, possibly, severe weather.

"To illustrate, the Dust Bowl of the 1930s reflected a temperature change of a fraction of the amount projected by the new model," said Crowley, professor of oceanography in Texas A&M's College of Geosciences. "And both the 1930's heat wave and the 1950's drought that swept Texas resulted from only a 1 degree Centigrade change in mean annual temperature."

For decades, scientists have debated the existence of global warming and the reality of the greenhouse effect. Over the past 1,000 years, the Earth has experienced alternating cycles of relatively cooler and warmer temperatures, Crowley observed. Natural rhythms of the oceans and atmosphere contribute to these cycles, as do changes in volcanic activity and periodic fluctuations in the amount of radiation received from the sun.

"Taken together, natural causes and so-called 'forcing events' like sunspot flares and volcanic eruptions can account for about 41 to 64 percent of the Earth's temperature changes between 1000 and 1850," Crowley said. "Such factors cannot explain the unusual warming trend of the late 20th century, however."

Crowley's model, on the other hand, adds anthropogenic, or man-made, factors to the forces of nature and produces correlations explaining far more of the temperature variations over the same period.

"My basic strategy was to seek added insight into the late 20th century warming trend by interpreting it within the context of how well we understand climate change over the entire last millennium," Crowley said. His model explains more of the total temperature variance because it takes into account such a long period of time, with correspondingly more observations. Crowley focused on records from the Northern Hemisphere. He was assisted with graphics for this study by Thomas S. Lowery, an undergraduate at Texas A&M. The two also are co-authors of a paper on temperature increases during the Middle Ages, which was published by the Royal Swedish Academy of Sciences, the group that awards the Nobel Prizes.

Scientists determine past temperatures by examining natural records -- such as tree rings and ice cores -- from different sites, then correlating results. Since 1860, recorded temperature observations also have been available for the Northern Hemisphere, and scientists have been able to compare this data with that obtained from natural sources to yield a composite temperature record.

"During the last 25 years, temperatures have been far warmer than expected. This represents a very, very unusual warming trend, which is supported by the weight of recently collected evidence," Crowley said.

"Although there are still uncertainties, in a sense the glass is now 80 percent full.

The late 20th century temperature increase is best explained by the accumulation of greenhouse gases in the atmosphere," he observed. "If we remove all the forcing effects and look at what's left, the curve generated by greenhouse gas emissions agrees closely with the curve showing temperature increases."

Greenhouse gases -- primarily carbon dioxide -- are so-called because they trap re-radiation of solar heat from the Earth's surface in the atmosphere close to the Earth. As the atmosphere warms, it can hold more water vapor, which in turn traps more heat, causing a chain reaction.

Although greenhouse gases occur naturally in the atmosphere, their concentration has increased dramatically since 1850 due to burning of fossil fuels and deforestation. It is easier to determine the effects of volcanism and solar variability prior to 1850, before the man-induced changes came into play. For example, during the Little Ice Age that occurred in the 17th and 18th centuries, the Earth was colder than usual. Alpine glaciers extended farther down Swiss valleys than today, and parts of New England referred to years without summer, with snow falling in New Hampshire in July 1816.

"The amount of carbon dioxide in the atmosphere today is at its highest level in the last 400,000 years of history," Crowley observed.

"It's sobering that we have used only 5 percent of the total amount of fossil fuel available. That's the tip of the iceberg; by the year 2100, we will have still used only 30 to 35 percent of our fossil fuel reservoir," he added. "And carbon dioxide released from burning fossil fuels stays in the atmosphere for 100 years or more."

TERRADAILY.COM
Scientists Take Issue With Solar Innocence
Los Angeles - May 19, 2000 - Following an article published in the UK science magazine New Scientist, a group of solar physicists have taken issue with the article's slant that the Sun is not to blame for global warming.