Consider yourself lucky. It could be much, much colder outside -- like ice age cold.

Researchers at the University of Wisconsin-Madison have used powerful computer climate models to show that, were it not for a rise in global temperatures that started thousands of years ago with the first clearing of European forests, we would be entering another ice age. Glaciers would be growing instead of melting. Your back would be even more sore from shoveling.

The findings were to be presented Wednesday at a meeting of the American Geophysical Union in San Francisco by Steve Vavrus, John Kutzbach, and Gwenaelle Philippon, all University of Wisconsin-Madison climate researchers.

"The world would be a much colder place right now,'' said Vavrus of the researchers' conclusion.

Vavrus and his colleagues used their computer models to compare a world unaffected by man's impact on the climate with a world in which humans have been pumping greenhouse gases into the atmosphere for possibly thousands of years. They also factored in what science has already told us about the regular occurrence of ice ages over the last million years.

The researchers' work involved studying and plugging into their computers data from something that sounds vaguely offensive -- fossilized air. This, Vavrus explained, is ancient air that was trapped in ice hundreds of thousands of years ago. Unlocked from ice cores taken from places such as Antarctica, the fossilized air provides very accurate data on everything from carbon dioxide to methane levels at various times through history.

The work by the UW-Madison scientists builds on earlier research on fossilized air by climatologist William F. Ruddiman of the University of Virginia. Through his studies of ancient air, Ruddiman came up with a theory known as the "early anthropogenic hypothesis.''

Ruddiman's study of ice cores led him to conclude that man's impact on the climate actually dates to a much earlier time than the industrial age of 200 years ago, the period which has received most of the blame for climate change. He linked increases in atmospheric methane and carbon dioxide in the ice cores to a time about 5,000 years ago when humans commenced large-scale rice farming in Asia and extensive deforestation of European forests. Those practices, Ruddiman said, pumped significant amounts of greenhouse gases -- methane from rice paddies and carbon dioxide from the burning forests -- into the atmosphere and set us on the course toward where we are today.

The UW-Madison researchers used their computer models to compare a world impacted by this longer period of warming with a theoretical world in which the climate remained unaffected by human activities.

In the world absent man's influence, known glacial cycles -- predictable and regular periods that have to do with the orbit of the Earth and radiation from the sun -- happened at regular 100,000-year intervals.

But Vavrus said when the researchers compared the changes caused by humans with a world absent the impact of greenhouse gases generated by man over the last 5,000 to 8,000 years, the model told them that we should be entering an ice age.

"We should have been in a relatively cool climate state by now," Vavrus said.

When they removed the human-caused greenhouse gases, Vavrus and Kutzbach said, three different models showed the same thing -- that more ice and snow should be forming in Canada, Siberia, Greenland and the Rocky Mountains, all places where glaciers first started forming during previous ice ages.

While this may seem a fortuitous finding -- especially as you survey the small glacier in your driveway -- Vavrus cautioned that our possibly having held off an ice age is not necessarily a good thing.

"It's important for people to realize," Vavrus said, "that this research suggests climate is very sensitive to greenhouse gases. And the huge amounts we're emitting through modern activities are probably going to have an even bigger impact.''