The global climate is changing because we are releasing large amounts of heat-trapping gases—carbon dioxide (CO2) in particular—into the atmosphere. These gases contribute to higher global temperatures that could increase the frequency of extreme weather events and have a profound impact on human health, sea levels, natural habitats, and agriculture.
Forests play a critical role in climate change by sequestering, or storing, large quantities of carbon (by absorbing CO2) as they grow and releasing it when they die. Though most Americans could probably name cars, trucks, and industry as major sources of CO2 emissions, the clearing and degradation of forests actually accounts for approximately 20 percent of annual CO2 emissions worldwide. This is more than the annual CO2 emissions generated in the United States by burning fossil fuels.
Nature’s Carbon Warehouse
Photosynthesis and respiration are the essential machinery by which forests store and release carbon. As a tree grows, it absorbs CO2 from the air and, through the process of photosynthesis, uses solar energy to store carbon in its roots, stems, branches, and foliage. Some carbon is released back into the atmosphere as CO2 during respiration, but a living tree acts as a carbon “sink”—storing more carbon than it releases. Trees continue to accumulate carbon until they reach maturity, at which point about half of the average tree’s dry weight will be carbon.
When trees decay and die, they become a carbon source, releasing more carbon than they can absorb. And when forests are harvested, burned, or cleared by humans, or in the event of a natural disturbance such as fire or disease, some of the carbon stored in the trees’ cells is released into the atmosphere. Stored carbon, however, can be transferred into forest products—for example, wood used for lumber, furniture, and other durable goods can hold its carbon for decades or even centuries if well maintained.
Recent estimates show that U.S. forests, grasslands, and agricultural lands form a sizable carbon sink. Even a forest that undergoes regular harvesting can act as a carbon sink as long as yearly growth exceeds the amount of carbon removed during harvest. The U.S. carbon sink absorbs 1.1 to 2.6 million metric tons of CO2 each year, which is equivalent to 20 to 46 percent of total U.S. global warming emissions.
Sadly, this sink appears to be shrinking. Carbon sequestration by forests and other lands decreased by approximately 20 percent from 1990 to 2001, a decline stemming primarily from unsustainable timber management (especially on privately owned forests) and the clearing of forests for development.
If the U.S. carbon sink were managed more effectively, it could easily be maintained and even expanded over the next 50 to 100 years before reaching a plateau. Finding ways to enhance carbon sequestration in forests is essential if we are to achieve the significant reductions in net emissions needed to stabilize the atmosphere. And, since carbon sequestration can usually be accomplished through established sustainable forest management practices, biodiversity and ecosystem health would be maintained as well.
Carbon Storage Strategies
During the past decade, ecologists, foresters, and other experts have established effective methods for increasing carbon sequestration in forests:
 | Source: IPCC, 2000. |
Forest conservation. The most significant carbon sequestration benefits can be obtained by conserving forests under imminent threat of clearing or degradation. This is especially true of older forests, which accumulate and store more carbon than younger forests. A prime example is the state of Maine, where more than 60,000 acres of forest have been cleared in recent years to make way for development. Stopping, or slowing, the rate at which such forests are cleared is essential to maintaining their carbon-storing capacity and the ecological “services” they provide: biodiversity, watershed protection, and recreation.
Afforestation and reforestation. Afforestation refers to the planting of trees on lands that have not historically supported forests. In the United States, afforestation of less-productive agricultural lands such as the lower Mississippi River alluvial valley is proving to be an effective sequestration strategy. In addition to storing up to two tons of carbon per acre each year, afforestation projects can deliver other important benefits such as improved wildlife habitat, reduced soil erosion and fertilizer runoff, and new recreational opportunities. Planting trees in cities and suburbs delivers an especially attractive climate benefit, since urban trees not only sequester carbon but also provide shade, reducing emissions associated with the energy that would otherwise have been used to cool these neighborhoods in the summertime.
Reforestation, or restoring forests that have been severely degraded, can produce similar sequestration and ecological benefits. However, to preserve local biodiversity, only native species and seed stocks should be used when implementing afforestation or reforestation projects.
Improved forest management. A variety of sustainable management approaches can improve carbon sequestration in existing forests. Allowing trees to grow for longer periods between harvests, planting longer-lived tree species (e.g., red oak, white pine, red spruce, hemlock), and setting aside wider buffer zones around streams and rivers have all been shown to increase carbon storage in forests.
Putting a Price Tag on Carbon
To make carbon sequestration economically viable, incentives will have to be offered to landowners who would be expected to forgo revenues from timber harvesting or other activities. Entire nations must be similarly encouraged to reduce CO2 emissions, and policy makers have responded with the Kyoto Protocol, an international framework for reducing global warming emissions that allows developing countries to offset their emissions by funding carbon sequestration projects.
Some important and even controversial questions about forest carbon sequestration emerged during the Kyoto negotiations: Who is responsible for the carbon loss if a forest burns down? What if a forest is preserved in one region, but timber harvesting increases somewhere else as a result? These challenges can, and must, be overcome in order to achieve a stable climate, because even the most technologically advanced solutions in the energy and industrial sectors can do nothing to stem the global loss of carbon and biodiversity associated with the loss of forests.
Michelle Manion is a senior analyst in the Global Environment Program.
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