Trees remove carbon dioxide, the dominant greenhouse gas, from the atmosphere. The more we have, the better. But deforestation -- the current trend -- liberates additional carbon and makes global warming worse.
The role of "sinks"
Trees and other green plants, using only sunlight for energy, take carbon dioxide out of the atmosphere, releasing oxygen and storing carbon in a safe and useful way. (Any skeptic of the potential of solar power needs only to look at the near-miracle of photosynthesis.) Forests, which provide all kinds of undervalued benefits for mankind, can be major allies in the battle against climate change and global warming if only humans start planting them and stop cutting them down.
The unglamorous term "sinks" is the term used by climatologists for vast swaths of trees and other green vegetation which "drain away" the most dominant greenhouse gas.
Forests are a major ally in combating global warming.
Deforestation, which is occurring all over the world, has a doubly damaging effect: it reduces the number of trees that can recover the carbon dioxide produced by human activities, and it releases into the atmosphere the carbon contained in the trees that are cut down.
The world at large currently doesn't "pay" much for the positive effects of forests. The value of trees as lumber and as firewood, and the value of the land they occupy for housing or farming, tend to be short-term and specific. In fact, these benefits may be a matter of survival in some regions. The value of forests for preventing global warming and preserving the earth's biodiversity, by contrast, are long-term and their rewards apply to everyone generally. A way has to be found to make the expansion and nurturing of forests appealing and cost-effective to the local populations that usually decide their fate.
In terms of efforts to reduce global warming, a forest in one place is as good as a forest in another. That can give rise to certain practical arrangements and efficiencies. Under the Kyoto Protocol, once it takes effect, industrialized countries which lack space or cost-effective options for expanding forests on their own territories may partially compensate for their greenhouse-gas emissions by paying for the establishment and maintenance of forests in other countries.
Changing agricultural methods.
Carbon stored in agricultural soils often can be preserved or enhanced by switching to "no-tillage" or "low-tillage" techniques, which slow the rate at which organic soil matter decomposes.
In rice fields, emissions of methane, a powerful greenhouse gas, can be suppressed to some extent through tillage practices, water management, and crop rotation.
Using nitrogen fertilizers more efficiently can reduce emissions of nitrous oxide, another potent greenhouse gas.