Forests and the mitigation of anthropogenic climate change

Forests, covering about one-third of the Earth’s ground surface, are a major carbon sink. Manipulating the amount of surface area in forests arguably offers the best chance for mitigating anthropogenic climate change.

Background

Eight thousand years before the present, before the rise of human civilization, one-half of the Earth’s surface was covered by forest. In the intervening years, 20 percent of previously forested land has lost its forest cover, resulting in the loss of a major carbon sink. One of the easiest ways for humans to prevent further carbonization of the atmosphere is to prevent further and to maintain existing forests intact.

Distribution of Forests

The forests of the world fall into three primary zones: tropical, temperate, and boreal. These zones are defined by their climate and precipitation. Trees require a minimum of 25 centimeters of rainfall per year to grow; land that, by virtue of its location, does not attract the minimum rainfall annually will not support trees. Such land will be either grassland, shrubland, or desert. Trees also require a frost-free part of the year for a growing season. Generally, the longer the growing season, the greater the annual growth.

The tropical rain forests on either side of the equator support tree growth that continues virtually all year long. About half of the world’s forests are tropical forests, and they contain the greatest biological diversity of species. Because trees through photosynthesis convert atmospheric carbon to contained carbon, the half of the world’s forests in tropical areas are of vital importance.

The temperate forest does not grow all year long. The falling temperatures reach a point at which growth is no longer supported, and the trees go into for part of the year. The temperate zone has a less varied collection of tree species than the tropics, but it contains a mix of coniferous and broad-leaved species, the latter of which generally lose their leaves during the dormant period.

The forest closest to the North Pole is the boreal forest, composed very largely of coniferous species. It grows more slowly than the other forests, because its growing season is the shortest, and it exists in dormancy for a major part of each year. While the true polar regions may have enough precipitation to support trees, their growing season is too short for trees to grow enough to support the needles that provide respiration.

Twentieth Century Changes

The conversion of formerly forested land to other land uses was greater in the twentieth century than in any previous century. This conversion was driven in large part by the growth in human populations in all parts of the world, but especially during the latter half of the century, primarily in the tropical regions. Between 1950 and 2000, the world’s population more than doubled, from 2.5 billion to 6 billion, and the developing world, largely located in the same area where tropical forests are found, contained three-fourths of that larger population. World population levels reached 8 billion in 2022. Since forests provide the largest carbon sink, the loss of many forests, especially the tropical forests, is mainly responsible for the rising proportion of carbon in the atmosphere.

The chief driver of deforestation during the twentieth century has been the demand for additional agricultural land to support the growing population. The conversion of forestland to agricultural land adds to atmospheric carbon in several ways. In many cases, the trees that are cut down to release the land for agriculture are burned, and the carbon stored in them is released into the atmosphere. Second, the carbon contained in forest soils over time is also released into the atmosphere when the trees are no longer there to prevent its release. Third, the trees that had been on the land are no longer there to capture future carbon as they grow.

Beside the need for agricultural land to grow crops to feed the growing world population, that population has experienced a change in dietary demand, particularly to include meat products. Some of the land freed up by deforestation is converted not to cropland but to pasture land. Specialized crops such as sugar or soybeans have a market price that poor populations seek to realize by converting forestland to agricultural land. In many areas, the production of specialized crops with major markets in the developed world, such as rubber, has also promoted deforestation.

Locations of Deforestation

The loss of forestland to agriculture has occurred in most parts of the world adapted to tropical forests. The Amazonian forest in Brazil and the forests in Central America have been subject to important depletion since the mid-twentieth century. The deforestation has also been substantial in Southeast Asia, notably on many of Indonesia’s islands, as well as in Malaysia and Thailand. The forest in Africa has been less affected.

The temperate forest that was heavily deforested in the nineteenth century has started to bounce back as urbanization and the mechanization of agriculture have reduced the demand for agricultural land. Thus, where the population is heavily urbanized, former agricultural land is being gradually reforested, as in the United States and Europe.

Much of the world’s sawtimber comes from the coniferous trees in the temperate forest and from the coniferous trees that cover the boreal forest. The Russian forest constitutes one-fifth of the total forestland of the world, but in recent years much cutting to supply sawtimber to the developed world has depleted some of that forest. Combined with softwood coming from Canada’s boreal forests, these two sources have supplied a major portion of the dimensional lumber used by the developed world for the construction of houses. Lumber production peaked in the United States in 1906, at 46 billion board feet, but since then lumber production supplied by U.S. forests has gone down. However, wood remains a popular commodity in world trade. A substantial proportion becomes fuelwood.

Context

At the Rio Conference in 1992, 158 countries agreed to try to prevent further deforestation. It has, however, proved difficult to accomplish partly because market forces are working against it, and partly because definition has proved elusive. The of 1999 identified “reforestation” and “afforestation” as processes that could mitigate deforestation, but compensation to those who carry out such measures has not found widespread acceptance. No way has yet been found to value existing forests such that their preservation could be financially rewarded.

Experts continued to advocation for the preservation of forests in the 2010s and 2020s. Scientists showed that large-scale forests are essential carbon sinks, and that without them, fighting global climate change might be impossible. For this reason, they advocated for the development of careful forest management practices. These included working to reduce the growing number of annual wildfires across much of the world, preventing the introduction of invasive species, and adopting climate-smart forestry practices.

Key Concepts

• afforestation: creating forests on lands not previously forested
• carbon sink: vegetation that incorporates carbon into its structure
• dormancy: the portion of the year during which no growth occurs
• growing season: the portion of the year during which occurs
• reforestation: replacing lost forests

Bibliography

"Climate Change Impacts on Forests." EPA, 22 Oct. 2024, www.epa.gov/climateimpacts/climate-change-impacts-forests#. Accessed 20 Dec. 2024.

Humphreys, David. Logjam: Deforestation and the Crisis of Global Government. London: Earthscan, 2006.

Malmesheimer, R. W., et al. “Preventing GHG Emissions Through Avoided Land-Use Change.” Journal of Forestry 106, no. 3 (April/May 2008).

Stern, Nicholas. The Economics of Climate Change: The Stern Review. New York: Cambridge University Press, 2007.

Williams, Michael. Deforesting the Earth: From Pre-history to Global Crisis. Chicago: University of Chicago Press, 2003.