There's more going on this month than elections. Next week a conference begins at the Hague at which one of the most important decisions ever made about the global environment will be taken.

The parties to the Convention on Climate Change will decide to what extent trees and forests can be used to offset the growing concentrations of greenhouse gases. This convention comes on the heels of a recent draft report by the Intergovernmental Panel on Climate Change indicating a substantial human role in global warming to date and projecting the highest temperature increase yet for the coming century (3 to 11 degrees). It will also play out amid compelling evidence that climate change is more imminent than previously thought.

It's well known that the polar ice is melting, but an eye-opening report last fall, based on data collected by nuclear submarines traveling under the Arctic ice cap, showed it happening faster than we knew. The declassified data reveal the polar ice sheet is 40 percent thinner than it was in the period from 1958 to 1976. Never thick to begin with, it is now only six feet on average, and thinning by four inches a year. The implications are that based on averages, the ice cap will break up within 20 years. A more recent model suggests it may take up to 50, but the result will be the same: a major change in global environment caused by warming.

Even less noticed is that glaciers in the world's tropical zones are disappearing, and on the same time scale. Whether in the Andes or on the African peaks of Mount Kenya and Kilamanjaro, glaciers present for more than 10,000 years are receding at such a rate that they will all disappear in 20 years.

The conclusion is obvious: We need to do everything we can to reduce greenhouse gas emissions right now. It has been obvious almost from the beginning that that means reducing consumption of fossil fuels through alternative energy, energy conservation and efficiency. What has been less known is what can be done with trees and forests. That is the crucial matter to be considered next week.

Forests figure substantially in the global carbon cycle: Every year, deforestation (releasing the carbon held in forests) contributes between 1 billion and 2 billion tons of carbon as CO2 to the atmosphere. Reducing deforestation, important for lots of reasons, can make a big difference. So can planting trees and reforesting--since growing trees convert C02 into the living tissue and cellulose of wood.

What is needed is a set of rules to make a carbon market work effectively--and not perversely. A carbon market is essentially one in which an emitter finds it more economical to pay for absorption or for emissions avoided by others. An old coal-fired plant could pay for reforestation, for example.

It would be foolish for such a market to encourage the destruction of existing natural forests or other important natural areas to replace them with fast-growing (carbon-absorbing) plantations, especially when plenty of already converted land is available and ready for such plantations. And there is no point in paying for protection of a natural forest because of the service it provides in carbon storage if that merely deflects deforestation to another natural forest (in that case, net deforestation doesn't change).

There are technical issues as to how much carbon is stored in different kinds of forests and the different rates of carbon uptake. Clearly, too, any plantation or forest conservation projects need to take into account the concerns of local people. All these things can be dealt with.

The key concept involves the "offsets" in a sort of carbon market that involves trades between nations by which one country's industry can avoid reducing its own emissions by paying for things that reduce emissions or absorb carbon in another place--by paying for cleaner technology, say, or using conservation to reduce emissions from forest burning. But currently the only sanctioned offsets are those based on technology. There is pretty much of a standoff among nations over the question of whether forests should be treated the same way.

European nations tend to take the attitude that if forests are included, the United States, which produces 24 percent of all emissions, will use the forest "loophole" to meet its targets by fostering forest projects in other countries--and thus avoid getting serious about cutting its own fossil fuel emissions. Some nongovernmental organizations hold a similar view--not without justification, considering that Congress has yet to ratify the Kyoto Protocol of the convention with its woefully small targets for emission reductions.

Nonetheless, carbon that is sequestered constitutes greenhouse gas not emitted. If the United States would be willing to limit the amount of credit it gets from such sources to demonstrate its seriousness about also dealing with fossil fuel emissions, it should allay such concerns.

Unfortunately, Brazil, like the Europeans, is also against the inclusion of natural forests in this regime, in part for reasons that aren't entirely clear. If it's a matter of sovereignty, the Brazilians should remember that this is a strictly voluntary mechanism; any nation that does not wish to participate with forests would not be required to do so. In the meantime Brazil's position stands in the way of many countries (including all other Latin American nations except Peru) that would like to have forests included.

The great irony is that natural forests and their biological diversity are likely to suffer terribly from climate change, with massive dieback and huge loss of species under almost any scenario. Thus inclusion of forests--with proper rules--could be the single most important achievement for biological diversity, as well as a very important step toward reducing climate change.

When the parties meet in the Hague, forests should be included in principle, and the means should be provided to work out the right rules. To do otherwise would be a heavy blow to forests, to biological diversity conservation and to the effort to deal with climate change. The two great nations of the Western hemisphere, the United States and Brazil, hold the keys. In the meantime, the Arctic ice cap continues to melt.

The writer is a Smithsonian scientist working at the World Bank.