The Earth Science Enterprise SeriesThese articles discuss Earth’s many dynamic processes and their interactions

Tropical Deforestation

November 1998

he clearing of tropical forests across theEarth has been occuring on a large scalebasis for many centuries. This process,known as deforestation, involves the cutting

down, burning, and damaging of forests.

The loss of tropical rainforest is more profound thanmerely destruction of beautiful areas. If the current rateof deforestation continues, the world’s rain forests willvanish within 100 years—causing unknown effects onglobal climate and eliminating the majority of plant andanimal species on the planet.

Why Deforestation Happens

Deforestation occurs in many ways. Most of theclearing is done for agricultural purposes—grazingcattle, planting crops. Poor farmers chop down a smallarea (typically a few acres) and burn the tree trunks—aprocess called Slash and Burn agriculture. Intensive, ormodern, agriculture occurs on a much larger scale,sometimes deforesting several square miles at a time.Large cattle pastures often replace rain forest to growbeef for the world market.

Commercial logging is another common form ofdeforestation, cutting trees for sale as timber or pulp.Logging can occur selectively–where only the economi-cally valuable species are cut–or by clearcutting, whereall the trees are cut. Commercial logging uses heavymachinery, such as bulldozers, road graders, and log

skidders, to remove cut trees and build roads, which isjust as damaging to a forest overall as the chainsawsare to the individual trees.

The causes of deforestation are very complex. Acompetitive global economy drives the need for moneyin economically challenged tropical countries. At thenational level, governments sell logging concessions toraise money for projects, to pay international debt, or todevelop industry. For example, Brazil had an interna-tional debt of $159 billion in 1995, on which it mustmake payments each year. The logging companiesseek to harvest the forest and make profit from thesales of pulp and valuable hardwoods such as ma-hogany.

Deforestation by a peasant farmer is often done toraise crops for self-subsistence, and is driven by thebasic human need for food. Most tropical countries arevery poor by U.S. standards, and farming is a basicway of life for a large part of the population. In Brazil,for example, the average annual earnings per person isU.S. $5400, compared to $26,980 per person in theUnited States (World Bank, 1998). In Bolivia, whichholds part of the Amazon rain forest, the averageearnings per person is $800. Farmers in these coun-tries do not have the money to buy necessities andmust raise crops for food and to sell.

There are other reasons for deforestation, such as toconstruct towns or dams which flood large areas. Yet,

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these latter cases constitute only a very small part ofthe total deforestation.

The Rate of Deforestation

The actual rate of deforestation is difficult to determine.Scientists study the deforestation of tropical forests byanalyzing satellite imagery of forested areas that havebeen cleared. Figure 1 is a satellite image illustratinghow scientists classify the landscape. Contained withinthe image are patches of deforestation in a distinctive“fishbone” of deforestation along roads. Forest frag-ments are isolated areas left by deforestation, wherethe plants and animals are cut off from the larger forestarea. Regrowth–also called secondary forest–isabandoned farmland or timber cuts that are growingback to become forest. The majority of the picture isundisturbed, or “primary,” forest, with a network ofrivers draining it.

The Food and Agriculture Organization (FAO) esti-

mates that 53,000 square miles of tropicalforests (rain forest and other) were de-stroyed each year during the 1980s. Ofthis, they estimate that 21,000 square mileswere deforested annually in South America,most of this in the Amazon Basin. Basedon these estimates, an area of tropicalforest large enough to cover North Carolinais deforested each year!

The rate of deforestation varies from regionto region. Recent research results showedthat in the Brazilian Amazon, the rate ofdeforestation was around 6200 squaremiles per year from 1978-1986, but fell to4800 square miles per year from 1986-1993. By 1988, 6% of the Brazilian Amazonhad been cut down (90,000 square miles,an area the size of New England). How-ever, due to the isolation of fragments andthe increase in forest/clearing boundaries,a total of 16.5% of the forest (230,000square miles, an area nearly the size ofTexas) was affected by deforestation.Scientists are currently analyzing rates ofdeforestation for the current decade, aswell as studying how deforestationchanges from year to year.

The much smaller region of Southeast Asia(Cambodia, Indonesia, Laos, Malaysia, Myanmar,Thailand, and Vietnam) lost nearly as much forest peryear as the Brazilian Amazon from the mid-1970s tothe mid-1980s, with 4800 square miles per yearconverted to agriculture or cut for timber.

Deforestation and the Global Carbon Cycle

Deforestation increases the amount of carbon dioxide(CO

2) and other trace gases in the atmosphere. The plants

and soil of tropical forests hold 460-575 billion metric tonsof carbon worldwide with each acre of tropical forest stor-ing about 180 metric tons of carbon. When a forest is cutand burned to establish cropland and pastures, the car-bon that was stored in the tree trunks (wood is about50% carbon) joins with oxygen and is released into theatmosphere as CO

2.

The loss of forests has a profound effect on the globalcarbon cycle. From 1850 to 1990, deforestation world-wide (including the United States) released 122 billion

Deforestation

Deforestation

Regrowth

UndisturbedForest

ForestFragments

Figure 1. Satellite image of deforestation in the Amazon region, taken from theBrazilian state of Para on July 15, 1986. The dark areas are forest, the white isdeforested areas, and the gray is re-growth. The pattern of deforestation spreadingalong roads is obvious in the lower half of the image. Scattered larger clearingscan be seen near the center of the image.

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metric tons of carbon into the atmosphere, with the cur-rent rate being approximately 1.6 billion metric tons peryear. In comparison, fossil fuel burning (coal, oil, and gas)releases about 6 billion metric tons per year, so it is clearthat deforestation makes a significant contribution to theincreasing CO

2 in the atmosphere. Releasing CO

2 into

the atmosphere enhances the greenhouse effect, andcould contribute to an increase in global temperatures(see Global Warming Fact Sheet, NF-222).

Deforestation and the Hydrologic Cycle

Tropical deforestation also affects the local climate of anarea by reducing the evaporative cooling that takes placefrom both soil and plant life. As trees and plants arecleared away, the moist canopy of the tropical rainforestquickly diminishes. Recent research suggests that abouthalf of the precipitation that falls in a tropical rainforest isa result of its moist, green canopy. Evaporation andevapotranspiration processes from the trees and plantsreturn large quantities of water to the local atmosphere,promoting the formation of clouds and precipitation. Lessevaporation means that more of the Sun's energy is ableto warm the surface and, consequently, the air above,leading to a rise in temperatures.

Deforestation and Biodiversity

Worldwide, 5 to 80 millionspecies of plants and animalscomprise the “biodiversity” ofplanet Earth. Tropical rainforests–covering only 7% of thetotal dry surface of the Earth–hold over half of all thesespecies. Of the tens of millionsof species believed to be onEarth, scientists have only givennames to about 1.5 million ofthem, and even fewer of thespecies have been studied indepth.

Many of the rain forest plantsand animals can only be foundin small areas, because theyrequire a special habitat in whichto live. This makes them veryvulnerable to deforestation. Iftheir habitat is destroyed, theymay become extinct. Every day,

species are disappearing from the tropical rain forestsas they are cleared. We do not know the exact rate ofextinction, but estimates indicate that up to 137 speciesdisappear worldwide each day.

The loss of species will have a great impact on theplanet. We are losing species that might show us howto prevent cancer or help us find a cure for AIDS.Other organisms are losing species they depend upon,and thus face extinction themselves.

After Deforestation

What happens after a forest is cut is very important inthe regeneration of that forest. Different cutting tech-niques and uses of the land have diverse effects on theground and surviving organisms that make up a rainforest.

In a tropical rain forest, nearly all of the life-sustainingnutrients are found in the plants and trees, not in theground as in a northern, or temperate forest. When theplants and trees are cut down to sow the land, farmersusually burn the tree trunks to release the nutrientsnecessary for a fertile soil. When the rains come, theywash away most of the nutrients, leaving the soil much

Cerrado

Clouds

0 - 5 percent

5 - 20 percent

20 - 40 percent

40 - 60 percent

60 - 80 percent

80 - 100 percent

Deforestation

Figure 2. Deforestation in the Brazilian Amazon in 1986. The darker the area, the more forestthat is remaining.

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ChinaIndia

Myanmar

Thailand

Andaman SeaGulf ofThailand

Cambodia

Laos

Vietnam

Indonesia Malaysia

ForestNon-ForestWater

1973

China

India

Myanmar

Thailand

Andaman SeaGulf ofThailand

Cambodia

Laos

Vietnam

Indonesia Malaysia

ForestNon-ForestWater

1985

less fertile. In as little as 3 years, the ground is nolonger capable of supporting crops.

When the fertility of the ground decreases, farmersseek other areas to clear and plant, abandoning thenutrient-deficient soil. The area previously farmed is leftto grow back to a rain forest. However, just as thecrops did not grow well because of low nutrients, theforest will grow back just as slow because of poornutrients. After the land is abandoned, the forest maytake up to 50 years to grow back.

Another type of farming practiced in rain forests iscalled shade agriculture. In this type of farming, manyof the original rain forest trees are left to provide shadefor shade-loving crops like coffee or chocolate. Whenthe farm is abandoned, the forest grows back veryquickly, because much of it was left unharmed in thefirst place. After this type of farming, forests can growback as quickly as 20 years.

Other types of farming can be more devastating forforest regrowth. Intensive agricultural systems uselarge quantities of chemicals like pesticides andfertilizers. These chemicals kill a lot of the living organ-isms in the area, seeping into the soil and washing intothe surrounding areas. On banana plantations, pesti-cides are used on the plants and in the soil to kill pestanimals. However, these pesticides also kill otheranimals as well, and weaken ecosystem health.Banana plantations also use irrigation ditches andunderground pipes for water transport, changing thewater balance of the land. After the abandonment of abanana plantation, or other intensive agriculturalsystem, it can take many centuries for a forest toregrow.

A study in Indonesia found that when only 3% of thetrees were cut, a logging operation damaged 49% ofthe trees in the forest. Yet, even with that much dam-age, the rain forest will grow back relatively quickly ifleft alone after selective logging, because there are stillmany trees to provide seeds and protect young treesfrom too much sun.

Figure 3. Deforestation in continental Southeast Asia (excludesMalaysia and Indonesia) from 1973 to 1985. The dark gray repre-sents forest, the lighter areas deforestation. The white box-like areason the 1985 map are places for which no satellite information wasavaliable. During this time period, about 50,000 square miles wasdeforested. China and India are included on the map but noassessment of their forest cover was made.

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Clearcutting is much more damaging to a tropical rainforest. When the land is commercially clearcut and allof the trees removed, the bare ground is left behindwith very little regrowth. Unlike when the farmer clearedthe land, there are almost no nutrients left behindbecause all the tree trunks were removed. A clearcutforest can require many years to regenerate–in fact,scientists do not know how long it takes for a clearcutforest to grow back.

The Future

The deforestation of tropical rain forests is a threat tolife worldwide. Deforestation may have profound effectson global climate and cause the extinction of thousandsof species annually. Stopping deforestation in thetropics has become an international movement,seeking ways to stop the loss of rain forests.

Because the loss of rain forests is driven by a complexgroup of factors, the solutions are equally complex.Simple solutions that do not address the nature ofworld economics and rain forest ecology have littlechance of succeeding. The future requires solutionsbased on solving the economic crises of countries

holding rain forests, as well as improvement of theliving conditions of the poor people often responsiblefor deforestation.

NASA Missions to Study Deforestation

NASA's Earth Science Enterprise future plans to studythe effects of deforestation include continuing analysesusing data from such instruments as the EnhancedThematic Mapper Plus (ETM+), scheduled for flight onLandsat-7, and the Advanced Spaceborne ThermalEmission and Reflection Radiometer (ASTER) and theModerate Resolution Imaging Spectroradiometer(MODIS), scheduled for flight on the EOS AM-1 satel-lite. The observational capabilities and scientific studiesplanned as part of the Earth Science Enterprise,including the Earth Observing System, will help toassess the impacts of deforestation on the globalclimate system. An overarching objective of the EarthScience Enterprise is to improve our understanding ofthe causes and effects of climatic and environmentalchange so that we may become more effective andefficient managers of our natural resources, as well asmitigate potential impacts from natural disasters.

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Acknowledgements: The following scientists from the Basic Scienceand Remote Sensing Initiative at Michigan State University contrib-uted significantly to this Fact Sheet: G. R. Urquhart, D. L. Skole,W. H. Chomentowski, and C. P. Barber