The Worldwide Extent of Land-Use ChangeAuthor(s): R. A. HoughtonSource: BioScience, Vol. 44, No. 5, Global Impact of Land-Cover Change (May, 1994), pp. 305-313Published by: American Institute of Biological SciencesStable URL: http://www.jstor.org/stable/1312380Accessed: 09/11/2009 08:27

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The Worldwide Extent of

Land-use Change In the last few centuries, and particularly in the last several

decades, effects of land-use change have become global

R. A. Houghton

hanges in land use reflect the history and, perhaps, the fu- ture of humankind. They are

linked with economic development, population growth, technology, and environmental change. Rates of land-use change often parallel rates of population growth, whereas they generally diminish locally with in- creased economic development.

The relationships among popula- tion growth, economic development, and land-use change are, of course, much more complex than these state- ments suggest. For example, the higher rates of change in the devel- oping world may be primarily the result of demand from developed countries. The global economy and international trade are important agents in land-use change. Land-use change also occurs in various forms, including both changes in area and changes in the intensity of use. Only changes in area will be explicitly addressed in this article.

For most of the 10,000-year his- tory of settled agriculture, the envi- ronmental effects of land-use change have been scattered in time and space. Large changes occurred long ago in Mesoamerica, Europe, and parts of Asia and Africa, some of these changes accumulating over time and others being reversed as empires declined. In the last few centuries, however, and particularly in the last several decades, the ef-

R. A. Houghton is a senior scientist at the Woods Hole Research Center, Woods Hole, MA 02543. ? 1994 Ameri- can Institute of Biological Sciences.

Deliberate land-use

change is intended to

support the human

enterprise but may instead reduce this capacity

fects of land-use change have be- come global, not only in the sense that changes in land use and their effects are present almost everywhere on the earth, but in the sense that they contribute to global changes in climate through increasing emissions of greenhouse gases. The distinction is important because the latter change affects all regions, whether or not those regions contributed to the change. There is an element of choice in what one does with one's land, but not in what happens to the earth's climate.

Land-use change can be consid- ered from two perspectives: the in- tended and the unintended effects. The purpose of deliberate land-use change is to increase the local ca- pacity of lands to support the hu- man enterprise, but many land-use practices instead reduce this capac- ity. Deleterious local and regional effects of deforestation for pastures, for example, include erosion of soils, reduced rainfall, reduced capacity of soils to hold water, increased frequency and severity of floods,

and siltation of dams. Global effects of land-use change

are also becoming apparent. They include the conversion of land that is potentially productive to land with diminished capacity to support crops, forests, and people. They also include the irreplaceable loss of spe- cies and the emission of chemically active and heat-trapping trace gases to the atmosphere.

At one extreme, a world covered with agricultural crops, interspersed with human settlements, would seem the most productive of worlds. It would probably not function in a way consistent with human well- being, however. At the other ex- treme, the largely undisturbed world of 10,000 years ago would probably have sustained hunters and gather- ers indefinitely, but at population levels considerably below current levels.

Where, between these two ex- tremes, is the balance of managed and natural systems optimal for the human enterprise? The answer is defined not only in terms of num- bers of people and quality of life but also by the role of natural and man- aged ecosystems in regulating the chemical and climatic stability of the environment. Do levels of tech- nology really matter, or do they sim- ply change the local density of people and, hence, the distribution of man- aged and unmanaged systems glo- bally? The questions are difficult, but research dealing with global change, and with climatic change in particular, has begun to address some of the environmental conse-

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305

Croplands

Forests and woodlands

Agriculture/ settled

Tundra, Ice, desert

Other lands Woods/fields

Pastures

Grass/shrub

Figure 1. The fraction of the earth's land surface in different categories of land cover according to (left) an agricultural perspective (FAO 1990) and (right) an ecological perspective. (From Olson et al. 1983.)

quences of replacing natural ecosys- tems with managed and degraded ecosystems.

Changes in land use are estimated to have contributed approximately 25% to the enhanced greenhouse effect calculated on the basis of hu- man-caused emissions of greenhouse gases (Houghton 1990). Most of this contribution has been in the form of carbon dioxide released to the atmosphere as a result of defor- estation, but changes in land use also release significant quantities of other gases (methane, carbon mon- oxide, and nitrous oxide) and par- ticulates that affect the chemical and radiative properties of the atmo- sphere.

This article reviews the extent of land-use change over the surface of the earth. Four intervals of time are considered: the last several millen- nia, the last century, the last decade, and the next several decades. Dis- cussion focuses on the global extent of land-use change. The emphasis is on forests and deforestation, and particularly tropical forests, because current rates of land-use change are most dramatic there.

Changes over the last several millennia

The area of land used productively by humans today is approximately 4700 x 106 ha (FAO 1990). Approxi- mately 30% of this land is devoted to crops, including tree crops as well as annual row crops, and approxi- mately 70% is in permanent pas- tures (Figure 1, left). These lands

together comprise approximately 32% of the land surface of the earth.

The clearing of natural vegeta- tion and cultivation of soils associ- ated with croplands brings a large change to the environment. Upward of 90% of the carbon originally in the vegetation and approximately 25% of that initially held in the soils (to a depth of 1 m) is lost to the atmosphere as a result of this con- version. Less carbon is lost from soils converted to pastures because they are generally not cultivated, but the burning of tropical forests for pastures releases carbon monox- ide, methane, and nitrous oxide as well as carbon dioxide. Other emis- sions result from the subsequent use of the lands cleared from forests. Nitrogen fertilizers applied to crop- lands result in emissions of nitrous oxide, and both paddy rice and cattle ranching increase emissions of meth- ane.

Classifications by ecologists gen- erally divide the world's ecosystems into categories of vegetation rather than land use (Figure 1, right). There is some correspondence between the agriculturist's view of the earth and the ecologist's view (forests and woodlands, for example), but there are large differences, as well, that make comparisons difficult. The ecologist's grasslands, for example, are included in the category of "pas- tures" if they are grazed, but in the category of "other land" if they are not. A comparison also suggests that the ecologist's category of "woods/ fields" may appear in the agricul- turalist's "pastures" if the fields are

grazed, or in either "forests," "wood- lands," or "other land" if they are not. These differences suggest that in- compatibilities will occur as natural scientists and social scientists work together to address the human di- mensions of global change.

A more detailed comparison of the major classes of land cover in- cluded in the agricultural and eco- logical classifications suggests what fraction of the earth's land surface is suitable for the future expansion of agriculture. Approximately 4440 x 106 ha are in rock, ice, tundra, and desert (Olson et al. 1983). If these lands plus a small area of settled land (altogether approximately 31% of the land surface) are assumed unsuitable, then 63% of the land surface is either already used or un- suitable for use in agriculture.

The assumption that desert and tundra are unsuitable is not strictly valid because deserts may be irri- gated for crop production (although the long-term potential is question- able), and because pastoral and no- madic cultures persist in both tundra and arid ecosystems. Never- theless, if such lands are assumed unsuitable, the fraction of the land surface still suitable and available for agriculture (37%) is similar to the fraction (32%) already in use.

These estimates are probably an upper limit; they assume that all remaining forests, woodland, shrub- lands, and grasslands will support agriculture. If neither dry woods mosaics nor taiga (16% of global land area; Olson et al. 1983) are suitable, then only 21% of the land

BioScience Vol. 44 No. 5

Wetlands

306

Figure 2. Distribution of forests and woodlands in Africa currently and before human disturbance. (From Houghton et al. 1993.)

surface remains. In other words, the current area in agriculture already accounts for between 46% and 60% of the lands suitable for agriculture.

These are crude approximations, but they suggest that there is little likelihood that the earth can feed a population twice as large as exists currently without significant in- creases in crop yields. The picture is even bleaker if one recognizes that the best lands are already in agricul- ture and that the remaining lands are likely to be marginal, with lower capacity for yields. On the other hand, more than two-thirds of the current area in agriculture is in pas- tures rather than croplands, and some of these areas might be used to produce vegetable rather than ani- mal protein, thereby increasing the yield for human consumption.

Comparisons of different global classifications are fraught with un- certainty. One way to overcome some of the limitations is to con- sider maps rather than tables or pie charts. What is the distribution of "other lands" or grass/shrublands of Figure 1, for example? Where have the changes in land use taken

place? What natural ecosystems have been converted to managed ecosys- tems?

To answer this last question, one needs to know the current distribu- tion of land cover and the distribu- tion of vegetation over the surface of the earth before human influ- ence. The latter is difficult to ascer- tain for obvious reasons, not the least of which is the effect of cli- matic variation on vegetation. Al- though changes in land use may have been relatively small before the start of settled agriculture some 10,000 years ago, until the last four or five thousand years changes in climate have had at least as large an effect as human activity. Thus, undisturbed vegetation in today's climate may never have existed, and the require- ment for a similar climate seems to limit the comparison of current and predisturbance land cover to the last few thousand years.

The geographic distribution of changes in land use over this inter- val can be estimated in at least two different ways. One method is based on a comparison of predisturbance vegetation (e.g., Matthews 1983)

with current land cover determined from satellite data. Houghton et al. (1993) used the global vegetation index (GVI) data from NOAA's TIROS-N series of satellites to clas- sify current land cover in the trop- ics. GVI is an index of greenness based on the normalized difference vegetation index (NDVI). NDVI is defined as the difference in the re- flected radiances measured in the visible (CVIS; 0.55-0.68 pm) and near-infrared (CNIR; 0.73-1.1 pm) spectral regions, normalized by the sum of the radiances:

NDVI = CNIR - CVIS CNIR + CVIS

Clouds, water, and bare ground give low NDVI values, whereas veg- etation produces a range of values related to the density and vigor of the vegetation. NDVI has been used on regional scales to monitor tem- poral variations in the areal extent of vegetation in the semiarid Sahel (Tucker et al. 1985b, 1986, 1991) and Greenland (Hansen 1991), and, on continental scales, to classify vegetation (Norwine and Greegor 1983, Tucker et al. 1985a).

Weekly images of the GVI are produced at NOAA by selecting the maximum NDVI from daily collec- tions of advanced very high resolu- tion radiometer (AVHRR) data from the satellite. This weekly composite of daily data minimizes cloud cover and maximizes the vegetation sig- nal. The 1-kilometer AVHRR data are subsequently sampled to pro- duce the 15-kilometer GVI data.

A comparison of Matthews' (1983) predisturbance map with maps prepared from the GVI data allowed identification and measure- ment of areas thought to have changed as a result of human activ- ity. Figure 2 shows a simplified ex- ample of the changes in Africa. Par- ticularly notable is the reduction in area of forests in western Africa. For all of sub-Saharan Africa, for- ests and woodlands have been re- duced by 44%, according to analy- ses based on this comparison (Houghton et al. 1993). Similar com- parisons for Latin America and for South and Southeast Asia show re- ductions of 32% and 34%, respec- tively, in the area originally in for-

May 1994 307

ests and woodlands in these regions. The loss of forest and woodland in tropical Africa, Latin America, and tropical Asia accounts for 38%, 38%, and 67% of the areas cur- rently in agriculture (crops and pas- tures) in these regions (FAO 1990). In other words, in Africa and Latin America most of the land used in agriculture was derived from grass- lands or shrublands, whereas in tropical Asia most was derived from forests and woodlands.

A second estimate of long-term changes in land use for South America (included in Latin America in the first estimate) was based on NDVI data alone (Stone 1992), with- out reference to the distribution of vegetation before human influence. In this case, the map of current land cover was based on NDVI data with a resolution of 1 km. Areas were defined as changed if they were iden- tified as agricultural, urban, or other settled areas, or if the areas were geometrically regular and embed- ded in otherwise apparently intact forest. According to this analysis, the original area of closed forest in South America has been reduced by 21%. Losses of woodlands and sa- vannas were estimated to have been 14% and 25%, respectively.

These estimates of current land cover, and of change over an un- specified interval of time, are crude, but the results of the two approaches are not far apart. On the order of 20-30% of the area of tropical for- ests seems to have been converted to other uses over the thousands of years of human influence.

Changes over the last century

Beginning with the eighteenth and nineteenth centuries, a different ap- proach for determining changes in land use is possible. Data can be obtained directly from land-use sta- tistics compiled at administrative districts of varying scales. Croplands are relatively well documented in census records worldwide (Richards 1990). Areas of pasture or grazing land are more problematic, but they can be estimated from records of livestock and stocking densities.

Based on historical data and as- sumptions, approximately 28% of the forest area in Latin America was

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1850 1860 1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

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Figure 3. Changes in land cover in Latin America between 1850 and 1990. (From Houghton et al. 1991.)

lost between 1850 and 1985 (Hough- ton et al. 1991; Figure 3). The area in croplands, pastures, and fallows had grown from 357x 106 ha to 918 x 106 ha over the 135-year period. A similar study in South and South- east Asia showed a 34-38% reduc- tion in forest area over the last 140 years (Houghton and Hackler 1994, Richards and Flint 1994). Lands al- tered for human use in that area expanded by approximately 176 x 106 ha. The loss of forests and the expansion of agricultural area oc-

curred in a similar pattern in tropi- cal Africa over the last century.

For most of the tropics, rates have been increasing, with the last few decades showing the most dramatic increase (Figure 4). Before 1960, croplands were expanding more rap- idly in regions outside the tropics. In North America, Europe, the former Soviet Union, and China, the largest changes in land use occurred earlier (Houghton and Skole 1990, Williams 1990), sometimes much earlier (Darby 1956). The annual

to 0 T-

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Figure 4. Annual rates of cropland expansion over the last 130-140 years for the globe (dotted line), for temperate regions (lighter solid line), and for tropical regions (heavier line). The surprisingly high rate for the 1950s is largely the result of changes reported in the Soviet Union.

BioScience Vol. 44 No. 5

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1850 1870 1890 1910 1930 1950 1970 1990 Year

Figure 5. Reduction in the amount of carbon stored in forests of South and Southeast Asia over the period 1850 to 1990. The upper curve (solid line) shows the result obtained from consideration of deforestation alone. The lower curve (dotted line) shows the total reduction from both deforestation and logging. 1 pg = 1 x 1015 g. (From Houghton and Hackler 1994.)

rates of increase in these regions seem never to have been as high as current rates in the tropics, however (Figure 4).

Despite the fact that the period between 1850 and 1990 represents only a small fraction of the time since the development of settled ag- riculture approximately 10,000 years ago, the area of cropland worldwide more than doubled in this 140-year period. Half of the world's croplands were added in the last 90 years. In the tropics, the doubling occurred in the last 50 years. The rates resemble the his- tory of population growth.

In addition to the increased area covered by cropland, pastures, and grazing land, there are changes in land use that do not involve reallo- cation of area. One such land-use change important globally, but largely ignored in discussions of land-use change, is selective logging, the harvest of the largest and com- mercially most valuable trees. This type of logging is also called high- grading. Logging is generally not considered deforestation because, if logging is not too destructive and if logged forests are not subsequently

colonized by farmers, they generally recover.

The area in forests, either as re- ported in land-use statistics or as observed from satellite, does not change as a result of logging. Al- though most logging in the tropics is selective and the forest remains, in many portions of the tropics the average biomass of forests is declin- ing (Brown et al. 1991, Flint and Richards 1991, 1994). The best- documented example comes from peninsular Malaysia, where the loss of forest area was 18% over the period 1972 to 1982, whereas the loss of total biomass was 28% (Brown et al. 1991). Even larger reductions were determined for other regions of South and Southeast Asia (Flint and Richards 1991, 1994). The decline in biomass is proceed- ing despite the enhanced growth gen- erally thought to result from selec- tive removal of trees.

Although high-grading reduces the biomass of standing forests and, hence, reduces the carbon emitted to the atmosphere if the forests are subsequently cleared for agriculture, the high-grading itself releases car- bon as dead plant material and log-

ging debris oxidize during and after harvest. When only changes in area were used to calculate the net loss of carbon from land-use change in South and Southeast Asia, estimates of emissions were 30-40% lower than when degradation within for- ests was considered (Figure 5; Flint and Richards 1994, Houghton and Hackler 1994).

If some of the biomass harvested through selective logging went into long-term storage, the emissions from logging would have been smaller. The fraction of original bio- mass stored in products appears to be small, however, probably less than 20% of the original biomass. One example of the wastage is no- table: in Acre, Brazil, logs used for local construction material are cut into boards about as thick as the chain saw used in cutting.1 The wast- age, in this case, is greater than 50%.

Changes over the last decade In the last two decades, a new ap- proach for determining changes in land use has appeared. With the launch of Landsat in 1972, direct measurement of areas of different types of land cover and of changes in this cover has become possible. Landsat images of the fishbone pat- tern of deforestation in Rondonia, Brazil (Woodwell et al. 1987), show- ing clearing of small plots along many small roads perpendicular to a main highway, are by now famil- iar to readers of the scientific and popular press, but the first system- atic measurements of deforestation or other changes in land cover using satellites over large regions are just now getting under way in FAO (1993), the United States (through NASA, the US Geological Survey, and EPA; Skole and Tucker 1993), and the Brazilian and European space agencies.

Although existing estimates of the rate of tropical deforestation (FAO 1993,FAO/UNEP 1981, Myers 1991) were not based on a systematic sur- vey using satellite data, they did incorporate results from a limited

1I. F. Brown, 1993, Woods Hole Research Center, Woods Hole, MA, personal commu- nication.

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number of studies using such data. For the most part, however, these tropics-wide estimates were based largely on data obtained from ground estimates.

The first surveys (Myers 1980, FAO/UNEP 1981) reported rates for the late 1970s (Table 1). Closed forests are defined by the FAO/UNEP survey as large, dense forests that do not allow sufficient penetration of light for growth of grasses on the forest floor. Overall, estimates for closed forests seemed to agree fairly well for the entire tropics (approxi- mately 7.5 x 106 ha/yr). For indi- vidual regions, the agreement was not as good, and the two studies reported quite different estimates as to the role of shifting cultivation in land-use change. The FAO/UNEP survey (1981) reported the area of fallow to have increased between 1980 and 1985, and attributed 35%, 70%, and 50% of the deforestation of closed forests in tropical America, Africa, and Asia, respectively, to shifting cultivation. Myers, on the other hand, believed that shifting cultivation was largely being re- placed by permanently cleared land and that the area of fallow was de- creasing. He estimated that approxi- mately 10 x 106 ha of fallow forests were cleared for permanent use (Houghton et al. 1985), an annual rate of clearing greater than the rate of deforestation of closed forests. This discrepancy concerning the fate of forest fallow has not yet been resolved, although satellite data could resolve it.

In contrast to closed forests, open forests (also called woodlands or savannas) have grasses present be- tween trees or between patches of trees. Myers did not consider open forests. According the FAO/UNEP study (1981), deforestation of open tropical forests was approximately 3.8 x 106 ha. The estimate for the total rate of tropical deforestation in 1980 was thus 11.3 x 106 ha (Table 1).

In the decade since the late 1970s, estimates of the rate of deforesta- tion in the tropics increased sharply (Table 1). According to Myers (1991), the annual loss of closed forests has almost doubled from 7.34 x 106 ha in 1979 to 13.86 x 106 ha in 1989. FAO's recent estimate (1993;

Table 1. Estimates of rates of tropical deforestation (106 ha/yr).

Estimate America Africa Asia All tropics

Independent estimate for the late 1970s*

FAO/UNEP (1981) 4.119 1.333 1.815 7.267

FAO estimatest 1976-1980 5.611 3.676 2.016 11.303 1981-1990 7.4 4.1 3.9 15.4 Percent increase 32 12 93 36

Myers estimatest 1979 3.71 1.31 2.58 7.60 1989 7.68 1.58 4.60 13.86 Percent increase 107 21 78 82 1989 (revised)s 4.48 10.66 Percent increase (revised)5 21 40

*Closed forests only. t From FAO/UNEP 1981, FAO 1993; closed and open forests. * Myers 1980, 1991; closed forests only. s Revised rates and percent increases are based on a rate of deforestation in Brazil of 1.8 x 106 rather than 5.0 x 106 ha/yr.

including both closed and open for- ests) is also higher (approximately a 50% increase). There are qualifica- tions, however. FAO maintains that some of the apparent increase re- sulted from underestimation of the rate of deforestation in the earlier period. They acknowledge that the rate of deforestation has generally increased in the moist tropics. How- ever, it may have declined in some Asian countries, more from running out of forests than from deliberate decisions to reduce the rate. FAO claims to have underestimated de- forestation in the late 1970s in some of the larger Asian countries, a change that brings their earlier esti- mate for Asia (FAO/UNEP 1981) more in line with Myers' early esti- mate (Table 1).

On the other hand, Myers' (1991) recent estimate for Brazil, where the largest increase was reported, now seems too high. Myers based his estimate on a study by Setzer and Pereira (1991), who used AVHRR data from the NOAA-7 satellite to determine the number of fires burned in Legal Amazonia (an area of 500 x 106 ha, including most of the Brazil- ian Amazon) during the dry season (mid-July through September 1987). Setzer and Pereira used the number of fires to calculate the area of forest burned. They determined the aver- age size of a fire and distinguished fire in forests from fires in cleared lands. They believed their estimate of deforestation (8 x 106 ha in 1987)

to be conservative. Myers (1991) reduced the esti-

mate of Setzer and Pereira (1991) to 5 x 106 ha, to reflect that some fires burned for more than one day and should not be counted twice. Never- theless, even this reduced rate seems high according to more recent stud- ies. According to Brazil's National Institute for Space Research, the average rate of deforestation of closed forests in Brazil's Legal Ama- zonia between 1978 and 1989 was 2.2 x 106 ha/yr (Fearnside 1993). Skole and Tucker (1993) recently reported an even lower average rate of 1.5 x 106 ha/yr for approximately the same interval. The actual rate probably increased between 1978 and the mid-1980s but seems to have fallen substantially since then to 1.9 x 106 ha in 1988/1989, to 1.4 in 1989/1990, and to 1.1 in 1991 (Fearnside 1993).

If these data for Brazil are more accurate than those used by Myers, the increase in the rate of deforesta- tion, tropics-wide, over the last ten years may have been approximately 40% rather than 90% (Table 1). The reduction may be reassuring, but an annual growth rate of nearly 4% in the rate of tropical deforesta- tion is still considerable. This re- vised estimate of deforestation for tropical closed forest alone is 10.66 x 106 ha/yr (equivalent to an area the size of Tennessee or Ohio). The recent estimate of the FAO (1993), including deforestation of both

BioScience Vol. 44 No. 5 310

closed and open forests, is 15.4 x 106 ha/yr.

According to these tropics-wide studies of deforestation, the major agents of deforestation are farmers clearing land for either shifting or permanent cultivation, sometimes settling in forests made accessible through logging. Thus, the main cause of deforestation could be con- sidered to be agriculture. If one con- siders the causes of land-use change from this perspective, however, one misses an important factor respon- sible for deforestation.

According to the Production Yearbooks of FAO (1990), the an- nual net expansion of agricultural lands in the tropics was consider- ably less than the annual net reduc- tion in forest area. For the entire tropics, for example, the expansion of croplands accounted for only 27% of total deforestation. Adding the increase in pasture area accounted for an additional 18% of deforesta- tion. Fully 55% of the deforestation between 1980 and 1985 was ex- plained by an increase in "other land" (FAO 1990). Although some of this "other land" is urban land, other residential land, and roads, these uses are unlikely to have ac- counted for more than a few percent of the area deforested. Most of the "other land" seems likely to be aban- doned, degraded croplands and pas- tures, lands that no longer support crop or livestock production but that do not revert readily to forest.

Forests are not converted directly to degraded areas, of course. The transformation of land is from for- est to agriculture and, subsequently, to degraded land. The important point is that only approximately one- half of the area of tropical forest lost each year actually expands the area in productive agriculture. The other half is used to replace worn- out, abandoned lands. After a few years in production, large areas are made unproductive. The process re- sembles shifting cultivation, but on a grand scale and apparently with long fallows. If this interpretation is correct, making agriculture sustain- able may be twice as effective in halting deforestation as is increas-

ing yields. The fraction of deforestation used

to expand the area in agriculture, as

Tropical America

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Figure 6. Average annual transformation of tropical forests to agricultural lands, and agricultural lands to degraded lands over the period 1980-1985 (in 106 ha/yr; from FAO 1990). Boxes (to scale) represent the net loss of forests and woodlands, and the net expansion of agricultural (croplands and pastures) and degraded lands. Rates of loss of forests and woodlands are lower and not as current as rates reported elsewhere in this article.

opposed to replace worn-out land, varies among tropical regions (Fig- ure 6). In Africa, the expansion of croplands accounted for only ap- proximately 12% of the net area deforested. Eighty-eight percent of the decrease in forest area was matched by the expansion of "other land." In tropical Asia, only 40% of the net reduction in forests appeared as an expansion of agricultural lands. In Latin America, approximately two-thirds of the reduction in for- ests could be accounted for by the expansion of croplands and pastures. If agriculture could be made sus- tainable throughout the tropics, to- tal agricultural area could continue to grow at current rates while, at the same time, rates of deforestation could be reduced by approximately 50%.

Outside of the tropics, recent changes in land use have been small, although major changes occurred in these regions in the past (Darby 1956, Houghton and Skole 1990, Williams 1990) and may be under way again. In 1980, there were small increases in the area of forest in some countries and small decreases in others, but the changes through- out the temperate and boreal re- gions summed to zero (Houghton et al. 1987). The most significant changes in these forests may not be in area, however, but in stature, or

the amount of carbon held in the trees and soils within forests. In ar- eas adjacent to heavy industrial ac- tivity and in areas affected by air pollution, forest decline has reduced the amount of carbon held on land. In other areas, the increased deposi- tion of nitrogen and sulfur from industrial emissions may have en- hanced tree growth and the storage of carbon, at least temporarily (Kauppi et al. 1992).

Recent information also suggests that the frequency of fires over much of Canada and the United States may have increased (Stocks 1991). Fire suppression starting in the last century allowed carbon to accumu- late in both live and dead plant material, but in the last decade, the frequency of fires has increased despite continued suppression. Whether the change is related to the global warming of the 1980s and whether it will continue in the fu- ture are unknown. The changes in temperate and boreal forests as a result of logging and regrowth, fire suppression, pollution, eutrophica- tion, and increased fire frequency have probably affected the forests in many ways. The annual accumula- tion of carbon in northern forests as a result of regrowth from earlier logging may no longer balance the annual emissions of carbon from oxidation and decay of logging de-

May 1994 311

bris and wood products, as they did in 1980 (Houghton et al. 1987).

One other change in land use, increasingly important in recent de- cades, is the growth of urban areas. Although the fraction of the earth covered by urban areas is trivial (less than 1% of the land surface), the sprawl of suburban areas is dis- placing both agricultural and natu- ral ecosystems. Furthermore, emis- sions of greenhouse gases from transportation are large and con- centrated in urban and suburban areas. When one considers that the greatest increases in population will be in tropical urban centers, and that these relatively small areas draw on much larger areas for food, fuel, and fiber, urban areas will be in- creasingly important in the future.

The past as predictor of the future

The major change in land use, his- torically, has been the worldwide increase in agricultural land. Ten thousand years ago, terrestrial eco- systems were largely undisturbed by humans. Today, approximately a third of the land surface is devoted to either croplands or pastures. The area amounts to approximately half of the world area suitable for agri- culture. Although the expansion of agricultural lands was slow over most of this 10,000-year interval, the rate of expansion has been in- creasing. For croplands alone, half of the long-term increase occurred in the last 90 years. In the tropics, this doubling occurred in the last 50 years. At current rates of deforesta- tion, clearing for both crops and pastures will have eliminated tropi- cal forests in approximately 100 years. If the rates of deforestation continue to accelerate, most tropi- cal forests will be cleared in ap- proximately half that time.

The point here is not to predict the end of tropical forests, but to emphasize the need for a change in the way ecosystems are managed. From the perspective of natural sci- ence, the change must be toward a sustainable management of land. As discussed previously, fully half of the forests cleared for agriculture each year are degraded after a short period of use. Without changes in

management, this rate of impover- ishment may be expected to increase in the future, in part because of increased numbers of people, in part because of increased need of rev- enue, and in part because the best lands are already in use.

On the other hand, application of existing techniques for sustainable cropping and for restoration of de- graded lands could help slow or reverse current trends. Agroforestry, for example, offers the opportunity for sustainable food production and, at the same time, for the accumula- tion of carbon in woody biomass and soils. Successful examples of sustainable management exist in Europe and the United States as well as in the tropics. The natural regen- eration of forests over much of the eastern United States in the last 100 years shows that deforestation can be reversed. Knowledge from the natural sciences alone cannot be expected to reverse current trends. Ultimately, improved management of the landscape will require politi- cal, social, and economic changes consistent with long-term sustain- ability.

The global perspective of this dis- cussion does not allow an analysis of the social mechanisms respon- sible locally for determining changes in land use, mechanisms such as land tenure. On the other hand, the very large changes in the global land- scape described here suggest that the underlying causes are not local but global. The global forces in- clude economic development, inter- national trade, population growth, and environmental degradation. In the future, climatic change may also be important.

Population growth is important because the major change in land use, globally, has been the expan- sion of agricultural land, at least in part for food. A large fraction of agricultural production is not for local consumption, however, but for export, to generate revenue for re- payment of large national debts. To some extent, the underlying cause of land-use change in the tropics is related to the distribution of wealth between developed and developing countries and between different seg- ments of society within countries. National debt and international

trade are now major factors in de- termining agricultural expansion. These same factors could be designed to reverse current trends.

The irony of this discussion is that poverty, with its concomitant effect on environment, is in many ways less destructive than affluence. Developed countries are the major source of greenhouse gases, acid rain, and industrial poisons of all kinds. They consume the greatest share of natural resources. If the level of de- velopment exemplified by the United States were to be achieved by the rest of the world's nations, the ef- fects on global climate might be far worse than the current effects of land-use change.

To the extent that current land- use policies in less-developed coun- tries are determined by past and current patterns of global trade and foreign debt, the developed nations bear some responsibility for envi- ronmental deterioration in develop- ing countries. The greatest need is for a new definition of development, one that includes sustainable use of resources. It needs to be one that both the overdeveloped and under- developed nations of the world can aspire to, and one that both can realize.

Acknowledgments The author thanks three anonymous reviewers for helpful suggestions. The research was supported by the Carbon Dioxide Research Program, Environmental Sciences Division, US Department of Energy; by the US Environmental Protection Agency, Corvallis; and by the National Aero- nautics and Space Administration's Office of Mission to Planet Earth.

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May 1994

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today for all the world

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