"It is a glimmer of good news in what would otherwise be a cloud of bad news," Monson, of the University of Colorado in Boulder, said in an interview.

Increasing levels of CO2, generated mainly through the burning of fossil fuels, contribute to global warming by trapping the sun's heat.

Trees soak up CO2 as they grow but release it when they die and rot. Monson and his team showed that declining snow linked to warming temperatures slows the release of CO2 from microbes in the soil which are very sensitive to temperature change during the winter.

Monson and researchers from San Diego State University and the National Center for Atmospheric Research in Colorado uncovered the impact of snow on CO2 using five 100-foot towers equipped with climate instruments at the Niwot Ridge Long Term Ecological Research site west of Boulder, Colorado.

They took millions of CO2 readings between 1998 and 2004 and noticed the deeper the snowpack was the more CO2 they noticed leaving the forest.

"In years with a deep snowpack it insulates the soil and you have a fairly warm temperature and micro-organisms release a lot of CO2 into the atmosphere," said Monson.

"But with a shallow snow pack the soil is colder and they don't emit as much."

Less snow is not generally considered good news for forests because it makes trees more susceptible to fire and disease and limits how much CO2 they take up in the summer.

"While winter CO2 emissions from forest soils have slowed, the lack of winter moisture is stressing the trees during the spring and summer and inhibiting the much larger amount of CO2 they absorb during their growing season," Monson, who reported the findings in the journal Nature, explained.

But he added that if the thinner snowpack is coupled with summer rains more CO2 could potentially be soaked up than is released.

Snowpack in areas of the Rocky Mountains and other ranges in the western United States and Europe has dropped by 50 to 75 percent in recent decades, according to the scientists.

Levels of CO2 in the Earth's atmosphere, which were about 250 parts per million for thousands of years, have risen steadily since the Industrial Revolution and are nearing 380 parts per million.