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Plant a tree and help save the planet.

That's the message of a new study published Friday that finds about half of the carbon dioxide emitted by burning fossil fuels is absorbed by plants in the Northern Hemisphere. The research shows that plants play a role about equal to the oceans in balancing the so-called greenhouse effect.Pieter P. Tans of the National Oceanic and Atmospheric Administration, co-author of the study, said an analysis of thousands of air samples shows that planting trees and other plants could have a powerful effect in combatting the build-up of carbon dioxide in the atmosphere.

Tans and four other scientists analyzed carbon dioxide, or CO2, from more than 6,000 flasks that gathered air during 1992 and 1993 from throughout the Northern Hemisphere, from points on the oceans and from Australia.

The scientists measured the ratios of two different types of carbon in the air samples as a way of determining the effect of plants on the atmospheric CO2. Plants absorb CO2 and give off oxygen. Most plants prefer CO2 that is rich in carbon isotope 12, or C12. Ocean water will absorb C12 and C13 equally. Thus, by looking at the ratio of the isotopes, scientists can determine the relative effects of oceans and plants on the CO2 concentration.

Tans said earlier studies had suggested that most of the CO2 in the atmosphere was absorbed by the oceans, but the new study shows clearly that there is a concentration of C13 high enough to suggest that plants play a role equal to the ocean in absorbing the atmospheric gas.

The CO2 content of the atmosphere has been rising at the rate of about 0.4 percent per year for at least two decades. Much of this increase, it is believed, is the result of burning fuels and cutting down and burning forests.

Some scientists are convinced that this increased CO2 could cause a global temperature increase because of a phenomenon known as the greenhouse effect. Carbon dioxide in the atmosphere allows sunlight to pass through to the Earth's surface, but the gas molecules block heat from being radiated back into space, thus acting like glass panes in a greenhouse.

There is concern among some experts that this effect will make the Earth slowly warm, causing changes in the climate, sea level and rainfall.

Tans said the new studies will help scientists measure the effects of any government effort to limit the release of CO2. Some have suggested that reducing fossil fuel use and limiting deforestation may be needed to control the greenhouse effect.