The Bush administration has received enormous worldwide criticism for its refusal to support the Kyoto Treaty on Global Warming, but new scientific evidence suggests that one of the treaty’s key strategies may be dangerously flawed.

The protocol gives governments the option to plant trees to soak up carbon dioxide, rather than cutting emissions of greenhouse gas. But this provision is not realistic, warns Richard Betts of Britain?s Meteorological Office. He says it does not take into account other ways that new forests can affect climate.

Betts presented detailed calculations showing that planting trees across the snow-covered swathes of Siberia and North America will heat the planet rather than cool it. And even away from the tundra, the cooling potential of forests is much less than previously supposed, he says.

Green forest canopies reflect much less solar radiation than most other land surfaces. They also absorb more, heating the Earth?s surface. This effect is greatest where forests replace snowy tundra, which normally reflects large amounts of solar radiation.

Betts calculates that at northern latitudes, warming as a result of planting forests will overwhelm any cooling effect due to the trees soaking up CO2.

Both Canada and Russia want to plant forests in their empty tundras to help meet their Kyoto commitments, because a hectare of immature forest can absorb more than 100 tons of carbon each year, despite growing slowly. But Betts calculates that the net warming effect of heat-absorbent forests in both regions is equivalent to an annual emission of 75 tons of carbon per hectare.

?Even in places where the cooling effect is still dominant,? says Betts, ?the cooling influence is generally much smaller than expected when considering carbon sequestration alone.? So should some countries be destroying forests instead? ?I am not suggesting that we deforest,? he says. ?But afforestation is not always an effective alternative to cutting fossil fuel emissions.?

Certain varieties of trees emit air pollutants, creating ozone and fine particulate matter that make it hard to breathe. Planting a peach or an avocado tree can reduce air pollution, but the California Air Resources Board says that not all trees provide cleaner air. Planting a California sycamore, for instance, can add to air pollution.

Research by the California Environmental Protection Agency?s Air Resources Board shows that it takes informed selection and planting of trees to reduce urban air pollution. Studies of over 1,400 tree species were compiled and rated for their various pollution impacts and pollen production.

Air Resources Board Chairman Alan Lloyd says, ?People plant trees for many reasons: to beautify their home, provide shade, and reduce energy bills. But they may not be aware that certain trees can decrease air pollution and pollen counts. Necessary information is now available to choose the most effective smog and pollen reducing species.?

The emissions of trees are classified as low, moderate, or high, based on the sum of the hourly emission rates of the chemicals isoprene and monoterpenes. These compounds are very reactive and play a role in tropospheric ozone formation and aerosol production, according to the Washington State University Laboratory for Atmospheric Research. To reduce ozone concentrations in urban areas, it is particularly important to use low emitting species.

The emissions from deciduous trees such as oak, poplar, aspens and willows are mainly isoprene, while coniferous trees such as pines, cedars, redwood and firs emit mostly monoterpene. There are several species which emit both isoprene and monoterpenes, such as spruce and eucalyptus.

The differences in emission rates from one species to the next can vary significantly. Some plant species can release as much as 10,000 times more volatile organic compounds than the more atmospherically friendly low emitters. Low-emitters include the Chinese Hackberry, Avocado, Peach, Ash, Sawleaf Zelkova and the Eastern Redbud.

A few of the high emitters include the London Plane, California Sycamore, Liquidamber, Chinese Sweet Gum, Goldenrain Tree, and the Scarlet, Red and Willow Oaks.

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