RALEIGH - A climatologist's claim last month that planting trees in temperate latitudes will exacerbate global warming came as a quite a shocker, because conventional wisdom has been that trees take up the greenhouse gas CO2 and thus help cool the globe.

The assertion is particularly relevant right now, since Raleigh is re-examining its regulations that protect trees.

Ken Caldeira, a scientist at the Carnegie Institution's department of global ecology, based his claim on simulations conducted with general circulation models, also known as global climate models, which continue to become more refined and accurate. The explicit objective of this research, however, was not to realistically simulate possible future scenarios, but rather "to bracket the magnitude of temperature change that is possible in the climate system due to changes in land cover." (Gibbard et al. 2005. Geophys. Res. Lett., 32, L23705)

To accomplish this, the scientists replaced the maps of actual vegetation that occurs in broad bands around the Earth with ones containing only single vegetation types, without regard to whether specific vegetation types could realistically grow in those places.

The narrow premise is that because grass, crops, bare earth and shrubs reflect more of the sun's energy back into space than trees, planting large numbers of trees in mid latitudes will cause more heat absorption and work against our efforts to reduce global warming. In the simulated first 50 years after replacing the existing vegetation with forests, uptake of CO2 by the fast-growing trees outweighs the reduction in reflectance. But ultimately these dark forests cause the Earth to warm by reflecting less radiant energy away from it.

This result is interesting, even compelling. But it is scale dependent -- Raleigh is not even detectable at the resolution at which these climate models are applied (about 11,600 square miles at Raleigh's latitude).

Also, the urban heat-island effect has long been recognized as raising summer temperatures in and around cities. Scientists have for many years understood that trees help moderate summer temperatures, sometimes as much as 10 degrees.

At our local scale, our alternatives are not native grassland, native forest or bare ground. We choose between green tree cover and unshaded black asphalt parking lots, which absorb much more heat than trees and don't absorb any CO2. The shade of the oak tree on the south side of your house reduces your air conditioning needs, which are partially provided by burning fossil fuels, which in turn warms the planet.

The grasslands of Raleigh are not wild unkempt prairies -- they tend to be cool-season exotic grasses that require plenty of lime, fertilizer, pesticides and mowing, all made possible by burning fossil fuels.

Trees do many other things to improve our environment. Their leaves filter gaseous and particulate pollution, thus cleaning our air. They moderate flooding and soil erosion by slowing and capturing stormwater. Trees reduce noise pollution, both by absorbing noise and by masking irritating urban din with more pleasing natural sounds.

Overall, trees improve life in and around cities for all creatures, including us humans. We hope the citizens of the City of Oaks are not discouraged from planting trees and cooling our city.

When scientists become advocates for public policy, they have the responsibility to correctly apply the results of complex scientific studies to the right conditions, at the appropriate scales. Hypothetically covering broad areas of the Earth with trees, even where they cannot grow, to produce an equally fantastical warming of the globe is an interesting academic exercise that can yield important insight into how climates change. But it is undeniable that planting and conserving trees -- lots of trees -- in Raleigh will help cool the climate, locally and globally.

(Robert F. Mosher Jr. is planning program manager at the Division of Bicycle and Pedestrian Transportation at the state Department of Transportation. He is also a member of the City of Raleigh's Environmental Advisory Board and Tree Conservation Task Force. Ted Shear is an associate professor in the Department of Forestry and Environmental Studies at N.C. State University and a member of Raleigh's Tree Conservation Task Force.)