Forests and Deforestation
Forests and Deforestation
Forests cover 30% of Earth's total land area. They are an important part of the climate system in several respects. First, wood stores carbon: all green plants obtain the carbon in their tissues by extracting CO2 from the air, breaking out the carbon, and releasing the oxygen. Trees, the largest plants, store the most carbon, sequestering it in persistent woody tissues that can keep carbon out of the atmosphere for centuries or millennia (4,000 years, in the case of certain bristlecone pines). Second, large amounts of carbon are stored in forest soils, as dead branches, leaves, needles, trunks, and other tree parts accumulate and partly decay. By keeping CO2 out of the atmosphere, forests mitigate climate change (make it less severe).
Third, trees are dark and so decrease the planet's surface albedo, especially in snowy northern regions, which tends to increase global warming. Fourth, deforestation to clear land for agriculture and to extract lumber and fuel-wood, which has been particularly severe in tropical regions for decades, releases carbon from forest trees and soils, enhancing global climate change. Throughout the 1990s, tropical deforestation was responsible for 20–30% of global anthropogenic (human-caused) greenhouse-gas emissions. Deforestation has slowed only slightly in the
early 2000s. Planting and conservation of forests is a primary goal of schemes to mitigate global climate change, such as the Kyoto Protocol.
Historical Background and Scientific Foundations
Forests have existed since shortly after the evolution of land plants about 450 million years ago. The first forests consisted of tree ferns; gymnosperms (evergreens such as pine and fir) were next to evolve, about 362 million years ago; finally, flowering plants, including most deciduous and present-day tropical tree species, first evolved about 130 million years ago. Contrary to a common misconception, trees and land plants do not supply Earth's atmosphere with significant amounts of oxygen. Land plants do emit oxygen, but the oxygen concentration in Earth's atmosphere was at approximately the modern level, thanks to emissions from single-celled green plants floating in the oceans (phytoplankton), the first multicellular plant and animal life that evolved about 600 million years ago, many millions of years before land plants existed.
Deforestation, although more rapid since the mid-twentieth century than at any earlier time, began long before the modern period. For example, about 70% of Europe was forested in AD 900, but by 1900 only 25% was forested. As much as 90% of all deforestation occurred before 1950. Yet the human relationship to forests has not always been destructive. According to ethnobotanists, much of the Amazon rainforest (the most biologically diverse forest on Earth today, though not as large in extent as the boreal pine forest of Siberia) is partly a cultural artifact produced by native peoples selectively propagating favored tree species.
Since the nineteenth century, steadily increasing population in Asia, Africa, and South America has also added pressure on forests, as tens of thousands of square miles of forest have been cleared for cropland. This has been especially intense since 1950, about which time deforestation pressure shifted from Europe and North America to the tropical regions. From 1920 to 1949, about 907,000 mi2(2,350,000 km2) of tropical forest were destroyed. From 1950, to 2001, boreal and temperate hardwood forests have declined only slightly, with growth almost keeping pace with demand for forest products, while 2,100,000 mi2(5,500,000 km2) of tropical forest have disappeared. This is having important consequences for ecosystems, soil loss, hydrologic cycles, species extinction, and climate.
State of the World's Forests
As of 2005, forests covered about 30% of total land area, just under 15,450,000 mi2(40,000,000 km2). The ten countries most rich in forest held two-thirds of global forest area: the top four, which together had about as much forest as the entire rest of the world put together, were the Russian Federation with 3,100,000 mi2(8,000,000 km2), Brazil with 1,850,000 mi2(4,780,000 km2), Canada with 1,200,000 mi2(3,100,000 km2), and the United States with 1,170,000 mi2(3,030,000 km2).
IN CONTEXT: FOREST-RELATED MITIGATION
“Forest-related mitigation activities can considerably reduce emissions from sources and increase [CO2] removals by sinks at low costs, and can be designed to create synergies with adaptation and sustainable development (high agreement, much evidence).”
“About 65% of the total mitigation potential (up to 100 US$/ tCO2-eq) is located in the tropics and about 50% of the total could be achieved by reducing emissions from deforestation.”
[Editor's note: As used in IPCC reports “a carbon dioxide equivalent (CO2-eq) is the amount of CO2 emission that would cause the same radiative forcing as an emitted amount of a well mixed greenhouse gas or a mixture of well mixed greenhouse gases, all multiplied with their respective GWPs [Global Warming Potential] to take into account the differing times they remain in the atmosphere [WGI AR4 Glossary].” tCO2 denotes tons carbon dioxide equivalents.]
“Climate change can affect the mitigation potential of the forest sector (i.e., native and planted forests) and is expected to be different for different regions and sub-regions, both in magnitude and direction.”
“Forest-related mitigation options can be designed and implemented to be compatible with adaptation, and can have substantial co-benefits in terms of employment, income generation, biodiversity and watershed conservation, renewable energy supply and poverty alleviation.”
SOURCE:Metz, B., et al, eds. Climate Change 2007: Mitigation of Climate Change: Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.
Globally, counting forest destruction, regrowth, planting, and natural increase, forest area was lost at 50,580 mi2(131,000 km2) per year for 1990–2000 and at 49,800 mi2(129,000 km2) per year for 2000–2005. However, losses were not evenly distributed worldwide; South America and Africa accounted for about 80% of forest loss from 1990 to 2005, with smaller losses in North and Central America and Oceania. Forest declined in Asia during 1990–2000 but increased in 2000–2005, the only region to show a turnaround in forest trends. Forest area increasedfrom 1990 to2005 in Europe.
Forests and Carbon
Globally, more carbon is stored in standing forest bio-mass (primarily wood and leaves), dead wood, litter (dead leaves, twigs, etc.), and soil than is present in Earth's atmosphere. Standing forest biomass alone, not counting other forms of forest storage, contains about 283 billion metric tons of carbon.
The ability of forests to sequester carbon—that is, take it out of the atmosphere and keep it there for climatically significant periods of time, i.e., decades or centuries—is the largest component of one of the two major carbon sinks or carbon-absorbing mechanisms of the planet. These two mechanisms are the terrestrial and the marine carbon sinks. The terrestrial carbon sink consists primarily of carbon uptake by forests (standing bio-mass, dead wood, litter, and soils); the marine carbon sink consists of the dissolving of CO2 in the oceans. Together, these two sinks remove a little over 50% of the CO2 being added to the atmosphere by human burning of fossil fuels each year. About half of this amount (25% of fossil-fuel emissions) is sequestered by forests. Carbon storage in forest biomass decreased in Africa, Asia, and South America in 1990–2005, but increased slightly in all other regions.
Since the late 1990s, there has been a high-stakes scientific debate about the nature and location of the terrestrial (land) carbon sink. Human activity releases about 7.1 billion metric tons of carbon to the atmosphere each year; about 2 billion tons are absorbed by the oceans, while 1.1 to 2.2 billion appear to be absorbed by the terrestrial sink, mostly forests. Efforts to discover which continent's forests are doing most of the absorbing have, however, given contradictory results.
WORDS TO KNOW
AFFORESTATION: Conversion of unforested land to forested land through planting, seeding, or other human interventions. Unforested land must have been unforested for at least 50 years for such intervention to qualify as afforestation; otherwise, it is termed reforestation.
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.
BOREAL FOREST: Type of forest covering much of northern Europe, Asia, and North America, composed mostly of coniferous evergreen trees. Although low in biodiversity, boreal forest covers more of Earth's land area than any other biome.
CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.
CARBON SINK: Any process or collection of processes that is removing more carbon from the atmosphere than it is emitting. A forest, for example, is a carbon sink if more carbon is accumulating in its soil, wood, and other biomass than is being released by fire, forestry, and decay. The opposite of a carbon sink is a carbon source.
ETHNOBOTANIST: Scientist who specializes in the study of relationships between human cultures and plants, whether wild or domesticated. Culture is intimately related to forests, herbal medicines, sacred or recreational mind-altering substances derived from various plants, crops, and the like.
EVAPOTRANSPIRATION: Transfer of water to the atmosphere from an area of land, combining water transfer from foliage (transpiration) and evaporation from non-living surfaces such as soils and bodies of water.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
HYDROLOGIC CYCLE: The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30% of the mid-latitudes' heat transport from the equatorial to polar regions.
KYOTO PROTOCOL: Extension in 1997 of the 1992 United Nations Framework Convention on Climate Change (UNFCCC), an international treaty signed by almost all countries with the goal of mitigating climate change. The United States, as of early 2008, was the only industrialized country to have not ratified the Kyoto Protocol, which is due to be replaced by an improved and updated agreement starting in 2012.
There are two basic methods for trying to trace carbon sinking by forests. One is called inversion modeling. This method is often characterized as a top-down approach because it starts not with data about forests but with data about atmospheric CO2. In inversion modeling, readings of atmospheric CO2 recorded at ocean-sampling stations from 1988 onward are examined. Although Earth's atmosphere is well-mixed, so that a CO2 measurement anywhere on the planet gives a good approximation of the value everywhere else, slight regional differences can be measured depending on whether large carbon sources or sinks are located upwind of the measuring station. Regional sourcing and sinking of carbon produces a temporary, slight increase or decrease in CO2. Measurements of these slight differences are fed into a computer model that calculates where sinks and sources of CO2 are most likely located.
The other basic method for tracing carbon sinking by forest is the inventory method. This is a bottom-up approach, whereby the acreage and standing timber volume of particular forests are counted to see how much carbon is located in which forests, and whether that amount is increasing or decreasing.
In 1998, a dramatic result based on inversion modeling was announced: North America's forests were sequestering 1.7 billion tons of CO2 per year, about as much as was being emitted on that continent by fossil-fuel burning. About half of North America's forests are in Canada and about half in the United States, while most of the continent's CO2 emissions from fossil fuels originate in the United States, which in 1998 was still the world's largest emitter of CO2. If correct, this result implied that the United States and Canada had, as it were, won the greenhouse mitigation lottery: without taking any significant national measures to abate their releases of CO2, they were already absorbing as much as they were emitting. Other countries, not the United States, the world's largest consumer of fossil fuels, were responsible for the increasing levels of atmospheric CO2 that were the primary cause of global warming.
The result was swiftly challenged by studies based on inventory methods, which found forests sequestering much smaller amounts of carbon. Some studies also pointed to non-North American forests as major sinks. For example, in 1998 a British group reported that thickening tree trunks in undisturbed South American forests accounted for 40% of the 1.7 billion tons claimed for North America's forests by the inversion modelers. Both claims could not be true: there was not enough carbon to go around. Other scientists pointed out that many of North America's forests were young and that forests sequester less and less carbon as they mature, so the great North American carbon sink, even if real, could not last for many more decades.
Over the next few years, however, a modified picture emerged. Other inverse-emissions studies agreed that there was a carbon sink of about 2 billion tons per year in the Northern Hemisphere but did not necessarily locate it in North America (Russia has more forest than the United States and Canada put together, and is another obvious candidate for carbon sinking; China has expanding forests). A 1999 analysis of changing U.S. land use concluded that at least during the 1980s, U.S. forests accumulated carbon at a rate equal to about 10–30% of U.S. fossil-fuel emissions, consistent with a Northern Hemispheric forest sink but not with the high value for the North American sink found by the 1998 study.
A 2001 study of the issue published in Science stated that the apparent conflict between inventories and inversion models could be resolved by using lower estimates from inversion modeling and from direct CO2 flow (flux) measurements in North America along with higher estimates from improved inventorying. Traditional forest inventorying does not count litter, soil carbon, wood products in landfills, or shrubs encroaching on grasslands due to suppression of wildfires. More realistic inventorying and the less extreme values from inverse modeling and flux measurements agreed on an annual uptake of CO2 by U.S. forests between .3 and .7 billion tons per year, a good deal less than U.S. annual emissions of about 1.6 billion tons per year but still equal to 20–40% of global fossil-fuel emissions.
It also became clear that Chinese forests were sequestering large amounts of carbon dioxide, over 20 million tons of CO2 per year by the late 1990s. This was largely the result of government policies of reforestation (restoring former forests) and afforestation (planting new forests) since the late 1970s. Planted forests, by 2001, accounted for 20% of all organic matter and 80% of sequestered carbon in China. These results tended to show that reforestation and afforestation policies could help significantly in mitigating climate change in coming decades, as well as providing traditional forest services such as wood, flood control, and habitat.
Impacts and Issues
Does Deforestation Increase Global Warming?
According to a study by G. Bala and colleagues published in the Proceedings of the National Academy of Sciences (U.S.) in 2007, total global deforestation would actually have a net cooling effect on Earth's climate. This is because deforestation has both warming and cooling effects, which vary in intensity depending on which climate region the forest being destroyed is in. Deforestation's warming effects are to release CO2 into the atmosphere, eliminate possible future uptake by trees of CO2 due to CO2 fertilization (increased plant growth because of heightened CO2 levels), and decrease evapotranspiration; its cooling effect is to decrease albedo, particularly in high latitudes (closer to the poles, especially in the Northern Hemisphere, where most high-latitude forested land is located).
The scientists stated that computer experiments in which only certain latitudes (tropics, temperate zones, high-latitude zones) are deforested show that afforestation and prevented deforestation in the tropics would definitely help mitigate global warming, but that afforestation would actually speed global warming if promoted in high-latitude regions such as the expanses of tundra now being rapidly warmed under global climate change. Temperate afforestation (e.g., in the contiguous United States), the researchers said, would “offer only marginal benefits.” However, these results seem to contradict the studies cited earlier that show a large carbon sink in the forests of North America, absorbing about 20–40% of global fossil-fuel emissions annually. The scientists cautioned that their results were based on only a single computer study and that, in any case, forests in all parts of the world remain environmentally valuable for many reasons other than mitigation of climate change.
Issues: Tropical Forests
Tropical deforestation accounts for almost a third of annual global CO2 emissions. Expansion of agricultural lands is a main driver of tropical deforestation. In the early 2000s in Brazil, about 4,000 mi2 (10,000 km2) of rainforest were being cleared yearly, mostly for cattle ranching, soy farming, and subsistence farming. This activity is often referred to as slash-and-burn agriculture, because land is cleared by cutting and burning. Burning trees immediately release their stored carbon to the air. The amounts released can be substantial, especially for mature tropical forests: each acre of old-growth Indonesian rainforest contains about 750 tons of CO2 in standing biomass alone, that is, not counting litter and soil. As of 2005, widespread slash-and-burn agriculture made Indonesia the world's third largest emitter of greenhouse gas and Brazil its fourth largest.
Because soils contain much of the carbon sequestered by forests, significant carbon releases can follow long after the cutting and burning of the trees themselves. A large-scale demonstration of this fact was given in the 1990s by an Indonesian government plan to clear and drain about 4,000 mi2 (about 10,000 km2) of peat-swamp forests. In this case, the soil of the forest region consisted of layers of peat 33–60 ft (10–20 m) thick. Peat is a dense deposit of dead plant matter too water-soaked to fully decompose—essentially young coal. Dried peat has been used as a fuel in some regions (e.g., in Scotland). Exposed, drained, and dried, vast areas of Indonesian peat caught fire in the late 1990s and again in the early 2000s, releasing an estimated 2 or more billion tons of carbon dioxide.
In 2007, the Australian and Indonesian governments announced a joint plan to preserve 270 mi2 (700 km2) of remaining Indonesian peat forests, re-flood 770 mi2 (2,000 km2) of peatland, and plant 100 million trees. The plan would, if successful, absorb or prevent the emission of more CO2 per year than is emitted by Australia.
Kyoto and Forestry
The Kyoto Protocol climate treaty, which was created in 1997 and entered into force in 2005 (with the United States and Australia not participating), called for the management of terrestrial carbon sinks, particularly afforestation and reforestation, to reduce CO2 in the atmosphere. Kyoto is particularly oriented toward the creation of young forest stands: cutting of an old-growth forest followed by replanting is not counted as a source of greenhouse emissions. In a 2000 study published in Science, however, Ernst-Detlef Schulze and colleagues suggested that preserving old-growth forests, rather than replacing them with young stands, would sequester far more carbon. Replacing old-growth with young-growth forests replaces a large pool of sequestered carbon in both biomass and soil carbon with a small pool of regrowth carbon and slowing the flow of carbon into soil.
Primary Source Connection
Tropical deforestation, the process where tropical forests are cut and usually burned, has been one of the greatest anthropogenic sources of greenhouse gases over the last several decades. In addition to releasing large quantities of carbon dioxide (CO2) into the atmosphere, tropical deforestation further contributes to a global increase in greenhouse gases due to the decreased ability of tropical forests to remove CO2 from the air. This article, from the journal Science, examines possible policy that could be implemented to reduce tropical deforestation. The reduction of tropical deforestation is one of the most cost-effective and easiest to implement CO2 reduction plans.
TROPICAL FORESTS AND CLIMATE POLICY: NEW SCIENCE UNDERSCORES THE VALUE OF CLIMATE POLICY INITIATIVE TO REDUCE EMISSIONS FROM TROPICAL DEFORESTATION
Tropical deforestation released ~1.5 billion metric tons of carbon (GtC) to the atmosphere annually throughout the 1990s, accounting for almost 20% of anthropogenic greenhouse gas emissions. Without implementation of effective policies and measures to slow deforestation, clearing of tropical forests will likely release an additional 87 to 130 GtC by 2100, corresponding to the carbon release of more than a decade of global fossil fuel combustion at current rates. Drought-induced tree mortality, logging, and fire may double these emissions, and loss of carbon uptake (i.e., sink capacity) as forest area decreases may further amplify atmospheric CO2 levels.
A combination of sovereignty and methodological concerns led climate policy-makers to exclude “avoided deforestation” projects from the 2008–12 first commitment period of the Kyoto Protocol's Clean Development Mechanism (CDM). The United Nations Framework Convention on Climate Change (UNFCCC) recently launched a 2-year initiative to assess technical and scientific issues and new “policy approaches and positive incentives” for Reducing Emissions from Deforestation (RED) in developing countries. This process was initiated at the request of several forest-rich developing nations, an indication of willingness to explore approaches to reduce deforestation that do not intrude upon national sovereignty. Recent technical progress in estimating and monitoring carbon emissions from deforestation and diverse climate policy and financing proposals to help developing countries reduce their deforestation emissions are currently being reviewed by the UNFCCC Subsidiary Body on Scientific and Technical Advice.
Whether a successful RED policy process can make an important contribution to global efforts to avoid dangerous climate change depends on two issues. First, are the potential carbon savings from slowing tropical deforestation sufficient to contribute substantially to overall emissions reductions? Second, is it likely that tropical forests (and the forest carbon) protected from deforestation will persist over coming decades and centuries in the face of some unavoidable climate change? The available evidence indicates that the answer to both questions is yes, especially in a future with aggressive efforts to limit atmospheric CO2….
Reducing deforestation rates 50% by 2050 and then maintaining them at this level until 2100 would avoid the direct release of up to 50 GtC this century (equivalent to nearly 6 years of recent annual fossil fuel emissions, and up to 12% of the total reductions that must be achieved from all sources through 2100 to be consistent with stabilizing atmospheric concentrations of CO2 at 450 ppm. Emissions reductions from reduced deforestation may be among the least-expensive mitigation options available. The IPCC estimates that reductions equal to or greater than the scale suggested here could be achieved at £U.S.$20 per ton CO2.
Reducing deforestation not only avoids the release of the carbon stored in the conserved forests, but by reducing atmospheric carbon, it also helps to reduce the impacts of climate change on remaining forests. The experience of the 1997–98 El Niño Southern Oscillation Event (ENSO) demonstrates how climate change can interact with land-use change to put large areas of tropical forests and their carbon at risk. The extended dry conditions triggered by the ENSO across much of the Amazon and Southeast Asia increased tree mortality and forest flammability, particularly in logged or fragmented forests. Globally, increased forest fires during the 1997–98 ENSO released an extra 2.1 ± 0.8 GtC to the atmosphere.
Even in non-ENSO years, global warming may be putting tropical forest regions at risk of more frequent and severe droughts. Over the last 5 years, a number of Amazon Basin and Southeast Asian droughts have been uncoupled from ENSO events but have coincided with some of the warmest global average temperatures on record.
In recent decades, carbon losses from tropical deforestation have been partly or largely offset by a tropical sink. Forest sinks are, however, unlikely to continue indefinitely, and continued warming will likely diminish and potentially even override any fertilization effects of increasing CO2. Climate change might also adversely impact tropical forests by reducing precipitation and evapotranspiration, making them drier, more susceptible to fires, and more prone to replacement by shrublands, grasslands, or savanna ecosystems, which store much less carbon. In the Amazon Basin, continued deforestation may disrupt forest water cycling, amplifying the negative impacts of climate change. A new generation of coupled climate-carbon models is being used to explore the prospects for the persistence of tropical forests in a changing climate. A widely discussed early study projected that business-as-usual increases in CO2 and temperature could lead to dramatic dieback and carbon release from Amazon forests, raising concerns that high sensitivity of tropical forests to climate change might compromise the long-term value of reduced deforestation, with dieback releasing much of the carbon originally conserved. However, of 11 coupled climate-carbon cycle models using the IPCC's mid-to-high range A2 emissions scenario, 10 project that tropical forests continue to act as carbon sinks, albeit declining sinks, throughout the century. The moderate sensitivity indicated by the new results suggests that reducing deforestation can result in longterm carbon storage, even with substantial climate change. Aggressive efforts to reduce industrial and deforestation emissions would likely further reduce the rate of decline and risk of reversal of the tropical sink.
While no single climate policy approach is likely to address the diverse national circumstances faced by forest-rich developing countries seeking to reduce their emissions, there are promising examples of countries with adequate resources and political will that have been able to reduce forest clearing. In some countries, it may be possible at relatively low cost to reduce emissions from deforestation and forest degradation that provide little or no benefit to local and regional economies. For example, reducing accidental fire and eliminating forest clearing on lands that are inappropriate for agriculture are two promising lowcost options for reducing greenhouse gas emissions in Brazil and Indonesia.
Other measures are unlikely to be implemented at large scales without financial incentives that may be feasible only within the framework of comprehensive environmental service payments, such as through carbon-market financing. In forests slated for timber production, for example, moderate carbon prices could support widespread adoption of sustainable forestry practices that both directly reduce emissions and reduce the vulnerability of logged forests to further emissions from fire and drought exacerbated by global warming. On forested lands threatened by agricultural expansion, financing could provide significant incentives for forest retention and enable, for example, more effective implementation of land-use regulations on private property and protected area networks.
Parties to the UNFCCC should consider adopting a range of options, from capacity building supported by traditional development assistance to carbon-market financing to help developing countries meet voluntary national commitments for reductions in forest-sector emissions below historic baselines. Voluntary commitments, which were put forward by several tropical forest nations, would substantially address a concern associated with the project-based approach of the CDM that emissions reductions from a site-specific project might simply be offset by increased deforestation elsewhere.
Key requirements for effective carbon-market approaches to reduce tropical deforestation include strengthened technical and institutional capacity in many developing countries, agreement on a robust system for measuring and monitoring emissions reductions, and commitments to deeper reductions by industrialized countries to create demand for RED carbon credits and to ensure that these reductions are not simply traded off against less emission reductions from fossil fuels.
Beyond protecting the climate, reducing tropical deforestation has the potential to eliminate many negative impacts that may compromise the ability of tropical countries to develop sustainably, including reduction in rainfall, loss of biodiversity, degraded human health from biomass burning pollution, and the unintentional loss of productive forests. Providing economic incentives for the maintenance of forest cover can help tropical countries avoid these negative impacts and meet development goals, while also complementing aggressive efforts to reduce fossil fuel emissions. Industrialized and developing countries urgently need to support the RED policy process and develop effective and equitable compensation schemes to help tropical countries protect their forests, reducing the risk of dangerous climate change and protecting the many other goods and services that these forests contribute to sustainable development.
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