Forestry

Forests as a Carbon Sink

Content Cover Image

Vast Marine west coast forest expanse in Alaska. @ C.Michael Hogan

Forests as a carbon sink are often overlooked as a major source of carbon removal from the atmosphere, a process known as carbon sequestration. Conversely, deforestation, whether removal of forests by logging or burning, has been a major contribution to atmospheric carbon addition since the mid-Holocene, when the ascent of man to planetary dominance began. Carbon storage in the world's forests as of 2005 was estimated at 1036 Gigatons (as carbon dioxide), and global net loss of forests averages 0.0002% per year (Nabuurs et al. 2005). Massive deforestation at present is most prevalent in places such as the Amazon, Indonesia, Brazil and Borneo. There are two main reasons for land use conversion by deforestation; ranching and agriculture. These conversions are done in an attempt to produce short term economic benefit by forcing immediate environmental change. Cattle ranching is one of the leading causes of deforestation in numerous places including the Brazilian Amazon, that have forested area to have effects of global proportion.

Background

Some economic reasons for the removal of the valuable forest resource include currency devaluation as a result of the decrease in a country’s monetary position, control over diseases (since some forests harbour pathogens), and land regulation laws (or lack of laws) which dictate where development may occur. Conversion from forested land to agricultural land is also a major issue in subsistence farming and commercial farming. In developing countries, government land policies encourage subsistence farming, while commercial farming is increasing worldwide as a result of the demand for soybeans, corn, and other crops demanded by the expanding human population. Wood is harvested for multiple reasons related to woody biomass harvest including for uses as timber, as a source of energy, and for pulp products such as paper. Although these reasons are not the primary drivers for forest destruction, they play a significant role. Forests serve as a valuable carbon sink by removing carbon dioxide (CO2) from the atmosphere and storing the carbon in a long term reservoir. Carbon sinks are natural or manmade storage sites for carbon that regularly absorb more carbon than they release. In the decade from 1993 to 2003, 3.3 Gigatons of carbon dioxide per annum were placed into storage in terrestrial sinks.

Forests, along with their associated soils contain two to three times the amount of carbon in the atmosphere. Deforestation not only halts the positive carbon storage effects of a functioning forest ecosystem but also may cause detrimental costs to climate. Deforestation converts carbon sinks into sources which, diametrically opposite to sinks, release more carbon than they sequester. The destruction of forests also decreases the level of evapotranspiration, the process by which water re-enters the atmosphere through transpiration in plants. A decrease in evapotranspiration results in regionally decreased precipitation, increased surface temperature, and fewer clouds. This decline in average cloud cover contributes to the Earth’s albedo effect, consequently lessening the planet’s ability to reflect solar radiation. Similar to deforestation’s effects on evapotraspiration and sink conversion, a decrease in the ability to prevent solar radiation from entering the atmosphere contributes to increasing in global temperatures.

Forests role in the carbon cycle

caption Ceiba pentandra tree; individual rainforest trees
may sequester up to 100 tons of carbon
Tikal, Guatamala. Source: C.Michael Hogan

Carbon cycling is the process that transfers carbon among the earth’s systems: the biosphere, lithosphere, hydrosphere, and atmosphere. As part of the biosphere, trees and other plants are a primary mechanisms by which carbon is transferred among different systems. Forests pull CO2 out of the atmosphere as part of the process of photosynthesis. Through a series of biochemical reactions, carbon in the low energy state found in CO2 is converted into higher energy carbon in a molecule of glucose. That glucose is used to power plant cell function and biomass production. The CO2 in vegetation is conventionally released back to the atmosphere through respiration, burning, and biomass decay. In the absence of man’s interference, the carbon cycle would function as a closed cycle, fluctuating within boundaries that are ideal to sustain life. Interrupting this cycle, deforestation releases unnaturally large amounts of CO2 into the carbon cycle. This overwhelming of the carbon cycle contributes to anthropogenic climate change. Deforestation has multiple impacts on climate change since in addition to releasing CO2, deforestation also reduces our natural safeguard to adverse climate change impacts by decreasing the amount of CO2 that these forests are able to store.

As increasing amounts of CO2 are released into the atmosphere, humans continue to exceed the carbon threshold which was stable in the early Holocene. This compounding surplus of CO2 causes feedback to occur within the carbon cycle. Feedback effects are changes in climate that may seem small but can significantly increase climate change due to their ability to force greater change. If the resulting additional change is focused in the same direction as the initial change, then the effect is considered a positive feedback. The reverse is considered negative. An example of a positive feedback loop would be decreasing potential of oceans to retain CO2 when heated. Increasing amounts of CO2 in the atmosphere raise global ocean temperatures. This increase in ocean temperature decreases the amount of CO2 that the oceans can hold, releasing additional CO2 into the atmosphere and consequently raising global temperatures, perpetuating the cycle. Other examples include the albedo effect and the melting tundra releasing methane, which is a greenhouse gas about 23 times more potent than carbon dioxide in forcing atmospheric warming. Some of these loops may seem to have negative feedback traits such as the albedo effect which releases water vapor and increases cloud cover, causing more sunlight to be reflected, thus resulting in what appears to be global cooling. However, global averages suggest that the effects of the positive feedback loops are greater than the negative feedback loops.

Specifically for deforestation, some feedback effects include impacts on El Nino, forest fires, and the release of soil carbon. The increases in temperature caused by deforestation can have an effect on El Nino and its precipitation patterns, which can in turn cause droughts in some locations and floods in others. Flammability of forests is also anticipated to increase due to higher temperatures, causing even more carbon to be released into the atmosphere. There is also the possibility of a “runaway greenhouse effect", caused by a large release of carbon from the soil because higher temperatures promote respiration of soil microbes that convert soil organic carbon to CO2. Another form of enhanced positive feedback is induced when forests are clearcut, and thence replaced by livestock grazing, which produces very large methane fluxes per hectare. 

Major carbon fluxes

caption Massive clearfelling with Durris Forest, Aberdeenshire, Scotland. @ C.Michael Hogan

It is important to quantify the major carbon fluxes, both sources and sinks, interacting with the atmosphere. There is an ongoing atmospheric contribution of carbon dioxide from fossil fuel combustion of approximately 8.7 gigatons per annum. Terrestrial vegetation including forests act as a sink for about 1.0 gigatons of carbon per annum, while oceans provide a sink for another 2.0 gigatons per annum. These summaries understate the importance of the role of forests, since they ignore the fact that prehistoric and historic deforestation of th middle and late Holocene has decimated roughly one half to three quarters of the Earth's forests. 

A alternative to carbon sequestration in living forests is biochar production, the controlled pyrolysis of forest product and agricultural wastes. This technique produces a rather stable product that can be returned to the Earth soil system and is capable of supplying plants with nutients and/or sequestering carbon for centuries. Estimates of the carbon sink value of biochar range as high as 2.0 gigatons of carbon per annum by the year 2050. While this amount of carbon flux from forest respiration, ocean respiration and biochar may not seem to be a complete match with industrial carbon production, it is a significant sink complex as well as a continuing (renewable) sink. More artificial sinks such as pipeline and well systems are not only quite expensive, but are arguably not truly renewable; in fact, the International Energy Agency estimates a maximum of 144 gigatons of total man-made well carbon storage could be brought on-line by the year 2050, given a prodigous capital investment. Furthermore, in a broader ecological context, the above analysis ignores other important natural sinks, including peat deposits and wetlands. Additionally as Archer and  Pierrehumbert (2011) point out: organic carbon in the oceans may play a major role in atmospheric carbon content, but there are presently insufficient data upon which to model the atmospheric/upper ocean/marine organic carbon fluxes.

Relationship to soil carbon

Sequestration of carbon in forests is mostly inseparable from carbon storage in soil, because the above-ground biomass in forests typically reaches an upper limit within decades. However, carbon storage in soils is worth a separate note for perspective. Many have commented on the enormous volume of carbon tied up in the Earth's soil environment, and on the fragile nature of this storage that can be disturbed by deforestation, agricultural conversion, urbanisation or a multiplicity of other drivers.

As early as 1954, Hutchinson pointed out that a principal cause of global warming is the loss of forests and the concomitant addition of carbon dioxide to the atmosphere. These conclusions were re-inforced by seminal work of Houghton et al (1983) who considered all terrestrial biota as well as soil carbon as the carbon source of deforestation. Most recently, Archer and  Pierrehumbert (2011) performed more detailed calculations to demonstrate Hutchinson's hypothesis is essentially correct and must be a major part of the explanation for global carbon dioxide rise through the 20th century and early  21st century.

Lack of regulation

A large percentage of forested land is privately owned, some of which is not protected by law. These forests may lack sustainable management;  therefore, landowners can execute unsustainable cuts and personally release large amounts of carbon into the atmosphere. This is especially a problem within the United States. According to the American Forest Foundation, more than half of the forest land in the USA is privately owned. This situation may make the USA vulnerable to the effects of deforestation, since these private land owners can undertake certain discretionary actions which are not subject to national, state or local regulations.In contrast, approximately 90 percent of all forests in all highly industrialized nations are under official or unofficial forest preservation; as a result the UN estimates that from current time until 2050 the industrialized nations will add between 60 to 230 million hectares per annum to the world forest stock. (Millennium Ecosystem Assessment. 2005)

Due to lax enforcement of regulations in some other countries fail to prevent unsustainable acts such as clear-cutting and diameter limit cutting. In many developing countries, farmers are encouraged to move into forest lands by government land policies and then burn the forests in hopes to make a profit. Illegal logging is also a prevalent issue in many countries. This illegal logging is closely linked to road-building, which gives access to rainforest and the exploitation of its resources.

Deforestation in developing countries

In continents containing primarily developing nations the amount of carbon stock in living biomass is steadily decreasing. In South America 4.3 million ha/yr were lost from 2000-2005 and in Africa 4.0 million ha/yr were lost in the same time-frame, in comparison to developing countries which are losing biomass at slower rate or gaining total biomass.

Developing countries often oppose restrictions on their carbon emissions because some environmental preservation strategies may not be initially cost effective given the developing countries’ current standing. Some developing countries, such as China and India, feel they have a right to their emissions and therefore suggest a carbon emission standard related to population. This is met with opposition from developed countries due to the difference in practices of developed and developing countries.

Presently deforestation is more likely to occur in developing countries than in developed ones, since (a) much of the damage is already done in Europe and North America, and (2) there is a fairly advanced degree of forest proection in place for the developed countries. Developing countries are more inclined to deforest considering the potential short-run economic income that the practice presents. The Millennium Ecosystem Assessment (2005) estimates forest area in the developing regions will decrease by about 200 to 490 million hectares per annum. Agricultural conversion is a more significant path to short term income for developing nations, because it supplies essential short term human needs while producing economic benefits.

Reducing deforestation as a mitigation tool

Reducing deforestation can be a beneficial mitigation tool to enhance carbon storage. Halting deforestation altogether can reduce the rate of carbon emissions of one billion tons of carbon per year by 2054, which is equivalent to halving the projected fuel required in automobiles from now until that date. A reduction in deforestation practices results in fewer carbon emissions which would otherwise be released during the act of deforestation and allows the positive effects of the forest ecosystem to remain in function. As a mitigation tool, reduced deforestation is beneficial because the trees continue to sequester the carbon as they grow, and the sequestered carbon remains in the soil.

Reducing deforestation would lessen the effects on climate change and would reduce the associated effects such as an increase in temperature, rise in sea level, biodiversity loss and nutrient loss, and adverse effects on human health. If implemented as a mitigation tool, reduced deforestation could aid decision makers in better understanding the weight of the ecological side of the economic benefits versus ecological loss argument. Reducing deforestation in comparison to other mitigation strategies is cheaper, less intrusive to industrial development and easier to implement as the strategy utilizes naturally occurring processes. There is no need for new infrastructure or any new technology, only for the implementation of policies and enforcement of existing laws. It is also more effective than many other strategies in keeping carbon out of the atmosphere. Other strategies such as afforestation or reforestation sequester less carbon because as newer forests grow they store far less carbon per hectare than the mature stands potentially being deforested.

Policies

There are solutions that could be implemented to reduce deforestation and some are already in use or development. One such policy deals with the REDD- “reducing emissions from deforestation and degradation” concept in which global banks, financial experts, policy makers, and environmentalists pay nations and their inhabitants not to cut down large portions of the rainforests. Billions of dollars a year would be donated through REDD or other avoided deforestation projects. This strategy allows poorer countries to capitalize on their natural assets without destroying them. This strategy also promotes ecotourism by having more vibrant and robust forest resources to attract visitors; moreover, such countries as Belize, Romania, Panama and Botswana are capitalizing on the rewards of enhanced ecotourism resulting from forest preservation.

Another proposal to mitigate climate change is the use of carbon credits for avoiding deforestation, however there is still much controversy over this strategy. Some critics argue that countries should not be given credit for the temporary carbon storage that forests provide because it could potentially be released in the atmosphere again. Others say that forests are valuable for being able to store carbon even if only temporarily, as any storage of carbon can aid in slowing down the rate of climate change. There are multiple other policies being considered that require a reduction in carbon dioxide emissions, and some suggest that reducing deforestation is a logical way to meet these standards, including those implemented by the Kyoto Protocol. Opposition to these standards. led by Brazil and some other powerful nations, is formidable. Brazil is a significant international player, since many of the mitigation options focus on Brazilian Amazonia. Brazil fears a potential loss of control over the region, and has expressed concern over international pressures.

See also

Further reading

  • David Archer & Ray Pierrehumbert. 2011. The Warming Papers. John Wiley and Sons. 432 pages
  • Erik Eriksson and Pierre Welander. 1956. On a Mathematical Model of the Carbon Cycle in Nature. Tellus 8: 155-75.
  • R.A.Houghton et al. 1983. Changes in the Carbon Content of Terrestrial Biota and Soils between 1860 and 1980. A Net Release of Carbon Dioxide to the Atmosphere. Ecological Monographs 53: 235-62.
  • G.E.Hutchinson. 1954. in The Earth as a Planet. G.Kuiper ed. University of Chicago Press. Chapter 8.
  • International Energy Agency. 2009. IEA Technology Roadmap Carbon Capture and Storage.publisher OECD/IEA
  • Millennium Ecosystem Assessment (MEA). 2005. Millennium Ecosystem Assessment. Ecosystems and Human Well-being: Synthesis. Island Press, Washington, D.C., 137 pp.
  • Nabuurs, G.J., O.Masera, K.Andrasko, P.Benitez-Ponce, R.Boer, M.Dutschke, E.Elsiddig, J.Ford-Robertson, P.Frumhoff, T.Karjalainen, O.Krankina, W.A.Kurz, M.Matsumoto, W.Oyhantcabal, N.H.Ravindranath, M.J.Sanz Sanchez, X.Zhang. 2007. Forestry. In: Metz B, Davidson OR, Bosch PR, Dave R, Meyer LA, eds. Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 541-584
  • Ruddiman, W.F. 2008. Earth's Climate: Past and Future. Second Edition. W.H. Freeman and Company, New York.
  • Schmidt, G. 2004. Methane: A Scientific Journey from Obscurity to Climate Super-Stardom. NASA Goddard Space Center.
  • Shakhova, N., I. Semiletov, A. Salyuk, D. Kosmach, and N. Bel’cheva (2007), Methane release on the Arctic East Siberian shelf, Geophysical Research Abstracts, 9, 01071.
  • Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Eds.). 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Cambridge University Press, Cambridge, United Kingdom.
  • Takahashi, J. and B.A. Young (Editors). 2002. Greenhouse Gases and Animal Agriculture. Proceedings of the 1st International Conference on Greenhouse Gases and Animal Agriculture, Obihiro, Japan, 7-11 November, 2001. Elsevier Sciences, Amsterdam, The Netherlands.
Glossary

Citation

Hogan, C., Papinchak, C., & Young, M. (2013). Forests as a Carbon Sink. Retrieved from http://www.eoearth.org/view/article/169930

0 Comments

To add a comment, please Log In.