Forests and Climate Change

Forests play a major role in Earth’s carbon cycle. Trees convert atmospheric carbon from CO2 into organic woody biomass as part of a respiratory process called photosynthesis. Trees then store the carbon until the woody biomass is destroyed. This carbon storage is called sequestration.

When forests are cut down, not only does photosynthesis– and thus carbon absorption– cease, but also the carbon stored in the wood of the trees is released into the atmosphere as CO2 if the wood is burned or decays.

Overall, forest ecosystems store between 20 and 100 times more carbon per unit area than do croplands.

When forests burn, stored carbon dioxide is released to the atmosphere. Photo courtesy National Oceanic and Atmospheric Administration.

Until the early 1900s, deforestation was the dominant source of increased CO2 in the atmosphere. Since that time, the burning of fossil fuels has surpassed deforestation, but deforestation alone is estimated to be responsible for up to 35 percent of the greenhouse effect.

Tropical vs. temperate deforestation:

Mass deforestation first occurred in temperate zones as forest land was converted for agricultural use. In the last 50 years, however, the fastest rates of deforestation have taken place mostly in tropical regions as impoverished nations have converted their land for farming and grazing. Thus as important as temperate forests are in the carbon cycle, most of the change worldwide currently is taking place in tropical regions.

Rates of tropical deforestation increased between 50 and 90 percent in the 1980s, while the area of temperate forests– albeit encompassing smaller second- or third-growth trees– has remained constant or increased in the last 50 years. (This statistic does not take into account the loss of biodiversity found in many temperate "plantation" forests. In other words, the replacement of an acre of older or larger trees with an acre of seedlings almost always represents a major loss of habitat values, as well as aesthetics.)

In temperate regions, forests primarily are cut for timber or paper. While this has a devastating effect on forest-dependent species, with regard to the carbon cycle most of the carbon still is stored in the wood, regardless of the wood’s use. After a few decades, however, most of this stored carbon is released when the wood or paper is burned or decays

Rainforest is often burned to clear it for agriculture. Photo courtesy University of Leeds Dept. of Geography/RAINFOR
http://www.geog.leeds.ac.uk/research/earth/tfe.html

In tropical regions, however, conversion is occurring at a rapidly increasing rate. A 1991 study shows 90 percent of deforested tropical regions becoming farms or ranchland, while only 10 percent is replanted for future timber harvests.

Scientists estimate that the largest loss of carbon from both biomass and soil occurs with a change of land use from tropical forest to permanent agriculture. Conversion of forest land to pasture causes similar carbon loss, and the effective greenhouse gas emissions are significantly higher when methane emissions from cattle grazing are included.

Forests are often cleared for cattle grazing. Photo courtesy the Bureau of Land Management.

Forests as Carbon Sinks

One point of contention in the negotiation of the Kyoto Protcol has been whether to take the carbon-absorbing capacities of forest lands into account in measuring a nation's carbon dioxide emissions.

There are still unresolved questions about how to account for a gain or loss of carbon-absorbing forest "sinks"—for example, should harvesting timber count as part of a nation’s emissions because of the resulting loss in carbon-absorbing forest? And should re-planting deforested areas count as emissions "credits"?

While a forest can act as a carbon repository, the way that a forest is managed can increase or decrease its capacity. Clearcutting not only removes carbon-absorbing trees, but also allows the CO2 stored in the soil to escape. According to scientists at the University of Alberta, letting trees grow longer before harvesting them allows the soil to store more carbon. Thinning and fertilizing forests may also increase their ability to store carbon.

Although the practice of giving a nation emissions credits for net increases in forest stocks and taking away these credits for reduction of these stocks might appear to be fair, the actual effect of forestry activities on carbon dioxide emissions is extremely difficult to monitor. There is significant concern that emissions credit trading could result in a net gain in CO2 emissions.

Sequestration of carbon by biomass is not a long-term solution. Undisturbed forests reach an equilibrium after about 100 years, with the net absorption of carbon roughly equaling the net emission.

A paper on the Pew Center for Global Climate Change website, "Land Use and Global Climate Change: Forests, Land Management, and the Kyoto Protocol," points out that "increasing carbon storage cannot by itself solve the problem of increasing atmospheric CO2, but it can help, especially in the short term."

Forests Forever is opposed to emissions credit trading because we believe it is unlikely to either reduce net carbon dioxide emissions or promote sustainable forestry. Current approaches to carbon credit trading raise significant concerns regarding environmental justice, monitoring and enforceability, and the potential for abuse when applied to major hydro projects. Emissions credit trading could shift the onus for reducing atmospheric carbon onto those countries least responsible for creating the problem and least equipped to effect a solution.

Global Climate Change

Impacts of Climate Change on California Forests

Selected Resources