Climatic change

The impact of deforestation on regional and global climate has received the most attention when environmental change is discussed in Amazonia (Bunyard 1987; Collins 1990; Dickinson 1987, 1989; Leopoldo, Franken, and Matsui 1985; Myers 1988; Prance 1986; Reis 1972; Wood 1990). Tropical deforestation is often pinpointed as a major culprit in the purported global warming trend and, since Amazonia is the largest stretch of tropical forest, its fate is thought to have an important bearing on the future of the world's climate. Amazonia was probably also heavily cleared in pre-contact times, without triggering the greenhouse effect.

Global warming

Increased atmospheric levels of carbon dioxide and other gases, such as methane, nitrous oxide, and chlorofluorocarbons (CFCs), can potentially trigger a greenhouse effect (Ravel and Ramanathan 1989). Many in the scientific community, and much of the reporting in the media, suggest that we are on a global warming course propelled by human activities such as burning forests and fossil fuels. But claims that a global warming has already begun may be premature (Byrne 1988; Flavin 1989; Schneider 1989). No firm evidence has yet emerged that the world is becoming significantly warmer (Abelson 1990a; Blinder 1992; Hansen and Lacis 1990; Ray 1993: 12; Solow and Broadus 1989; Spencer and Christy 1990). Indeed, surface temperatures over the western Arctic Ocean have become significantly cooler during the 1950-1990 period (Kahl et al. 1993). Such inconsistent reporting on "global warming" points to major defects in current models of global climate and suggests a tenuous basis for drawing policy conclusions (Bryson 1989).

Although some data suggest a recent global warming trend, no valid correlation with greenhouse gases can be made, nor can we be sure how long this trend will last (Barrett 1990). Even if such changes will soon be confirmed, it will be difficult to separate natural climatic cycles from any greenhouse effect (Mitchell, Senior, and Ingram 1989). For example, the subsurface thickness of ice around the North Pole varies markedly from year to year, and no significant trend emerged during the 1977-1990 period (Langereis, Van Hoof, and Rochette 1992). Also, the effects of clouds, volcanic dust, and oceans on any possible greenhouse effect are imperfectly understood (Abelson 1990b; Jarvis 1989; Kerr 1989; Slingo 1989).

If evapotranspiration rates are substantially reduced in Amazonia as a result of landscape changes, less latent heat may be exported from the region in water vapour (Motion 1987). Whether reductions of the moisture level in warm air circulating from tropical regions to temperate areas in the Hadley cell would affect global climate is unclear. Reduced moisture levels could result in less latent heat being released during condensation, thereby cooling climates. Such a mechanism would help to counteract any greenhouse effect. Deforestation also tends to increase the albedo affect, further reducing energy for atmospheric heating.

In the event that the greenhouse effect takes hold, tropical deforestation will be only partly at fault (Radulovich 1990). Deforestation accounts for less than 20 per cent of greenhouse gas emissions (Flavin 1989: 13). The amount of carbon in initial, undisturbed ecosystems may have been overestimated, thus exaggerating the impact of deforestation on the release of carbon dioxide to the atmosphere (Post et al. 1990).

Carbon dioxide from the burning of fossil fuels, which occurs mostly in temperate countries, is the largest component of greenhouse gases. Just seven industrialized countries produce 40 per cent of carbon dioxide emissions worldwide (Turner et al. 1990a). The industrial countries are responsible for approximately 85 per cent of carbon dioxide build-up in the atmosphere (Parikh 1992).

The release of CFCs, used to make aerosols, refrigerants, and solvents, is responsible for a larger proportion of greenhouse gases entering the atmosphere than CO2 emissions from burning forests. Industrial countries are responsible for most of the CFC emissions. The notion that developing countries must take a large share of the blame for any global warming has been ascribed to environmental colonialism (Agarwal and Narain 1991).

Deforestation in Amazonia may not be implicated in excessive emissions of nitrous oxide, a potent greenhouse gas, as previously thought. Although higher levels of nitrous oxide are released in pasture soils in the first 10 years after forest clearing, emissions of the gas subsequently decline to lower levels than those coming from tropical forest (Keller et al. 1993). This does not mean that cattle-raising is necessarily an appropriate land use for much of Amazonia from the policy standpoint; rather the ecological impacts of different land uses must be weighed according to a "basket" of criteria, and more longterm research is often needed to decipher the environmental effects of habitat change.

The idea that Amazonian countries should arrest forest clearing to save the world's climate while North Americans and Europeans continue to drive their increasingly more powerful cars and burn natural gas and coal does not rest well in Brasilia, Bogota, or Lima (Nisbet 1988). Developed countries will need to do more to reduce their own carbon dioxide emissions if they expect third world countries to tackle the issue (Caccia 1991). Some scientists have urged policy makers to separate "survival emissions," such as resource-poor farmers practising slash-and-burn agriculture, from "luxury emissions," particularly gas-guzzling cars plying the streets of major cities, particularly in the industrial countries (Agarwal and Narain 1991). Some industrial countries, such as Germany, the Netherlands, and Japan, have adopted carbon dioxide stabilization or reduction targets (Miller 1991), but many others are apparently waiting for a global consensus to emerge on appropriate action.

A cautious approach to formulating environmental and economic policy to address global warming has been adopted by several governments. Three main factors account for this wait-and-see attitude (Riebsame 1990). First, climate change predictions are too uncertain, particularly at the regional level. Second, current systems are thought to be capable of absorbing climate change without major disruption, at least for the next few decades. Third, technologies can be deployed to mitigate or compensate for some of the changes wrought by global climate change.

Sceptics about global warming can also point to the fact that C crops, such as rice and wheat, would likely produce higher yields with a doubling of atmospheric carbon dioxide levels if sufficient water and nutrients were available. The C crops, such as sugar cane and maize, are unlikely to be affected by increased carbon dioxide levels, at least for the foreseeable future. Also, some regions, such as the drier tropics, might benefit from a global warming since they could receive more rain.

The ability of countries to respond to global warming will depend in part on the strength of their agricultural research and extension systems to deliver new technologies. Areas with a shift to wetter climates will probably need crop varieties more resistant to fungal and bacterial diseases. Unfortunately, as the need for research institutions to be primed and ready to confront new challenges increases, their scientific capacity is at a low ebb.

In the 1970s and early 1980s, Brazil had one of the strongest agricultural research programmes among the developing countries, with an annual budget of close to US$200 million. Since the mid-1980s, however, high inflation and severe financial constraints have hampered EMBRAPA's (Empresa Brasileira de Pesquisa Agropecuária) ability to raise and sustain agricultural productivity (Ruttan 1991). The agricultural research systems of other countries with territories in Amazonia have also weakened over the past decade or so.

Another constraint on the deployment of technologies to meet the challenge of a possibly warmer world is the loss of biodiversity. As discussed in more detail later, the loss of habitats, particularly in the humid tropics, could have grave consequences for agriculture. The loss of genetic resources for crop improvement, and the disappearance of new crop candidates, could "tie the hands" of plant breeders trying to develop crops adapted to changing environments.

In spite of uncertainties about global warming trends and hazards, pressure is mounting for governments in both industrial and developing countries to take concrete steps to halt the build-up of greenhouse gases in the atmosphere. Tree planting is seen as one way to counteract the greenhouse effect, by providing a carbon sink (Myers and Goreau 1991). Although this popular notion empowers people to do something about a widely perceived problem, the impact of tree planting on the build-up of carbon dioxide in the atmosphere pales compared with what could be accomplished by the more efficient use of fossil fuels. At least 100 million ha would have to be planted to fast-growing trees to sequester a little over 10 per cent of the current annual build-up of carbon in the atmosphere (Myers and Goreau 1991). Once the trees reached maturity, they would no longer act as carbon sinks. That is an area equivalent to Britain, France, and Germany that would then cease to serve as a trap for carbon. The conversion of old-growth forests to fastgrowing plantations would release carbon to the atmosphere, in spite of the greater net photosynthesis of younger trees (Harmon, Ferrell, and Franklin 1990).

Although rehabilitating degraded areas with trees would be desirable, simply filling the landscape with "greenhouse" trees without regard to cultural and economic needs could be counter-productive. Cultural landscapes could not easily accommodate hundreds of millions of hectares of tree planting without disrupting food production and other economic activities, even if the "greenhouse gas" trees formed integral parts of agro-forestry systems. Cleared lands are often fully occupied, and the logistical and managerial implications of massive tree planting would need careful study (Churchill and Saunders 1991).

Reduction of methane emissions, an often overlooked contributor to the greenhouse effect, could help mitigate any global warming. Methane is a far more potent greenhouse gas than carbon dioxide and landfills in the developed world are a major source of methane emissions (Hogan, Hoffman, and Thompson 1991). Recovery of methane from landfills not only would reduce the greenhouse effect, but could supply gas to generate electricity. Improved coal-mining and oil-production techniques would also reduce methane emissions. Ruminant livestock and rice cultivation are also significant sources of methane. Deforestation and burning also increase methane concentrations in the atmosphere, but, again, a large share of the onus for reducing this greenhouse gas rests with the industrial countries.

The spectre of parched deserts

In addition to temperature changes, deforestation has the potential of adversely affecting rainfall regimes. Half of the rain that falls in Amazonia is thought to come from evapotranspiration (Molion 1975: 101; Salati 1987; Salati and Vose 1984; Salati, Marques, and Molion 1978). Accordingly, it has been assumed that continued deforestation might lead to a drier regional climate (Hecht and Cockburn 1989: 43). A linkage between the loss of forests and reduced rainfall has been surmised for centuries, and was much discussed in India and parts of the Caribbean in the nineteenth century (Glacken 1967; Grove 1990). Cattle-ranching and the destruction of Latin America's tropical forests have been blamed for reduced rainfall and increased droughts (Salati1992; Shane 1986: 23). Some computer models predict a sharp drop in rainfall with continued, large-scale deforestation in Amazonia, thus raising the spectre of dust-bowls and desertification (Anderson 1972; Barros 1990: 20; FAO 1991b: 3; Goodland and Irwin 1975; Modenar 1972; Paula 1972; Roddick 1991: 206; Sioli 1987). Cattleranching is sometimes singled out as most likely to provoke desertification (Wesche 1974). But such rainfall models assume that Amazonia will be turned into a barren landscape (EMBRAPA 1989: 6).

It is highly unlikely that substantial areas of Amazonia will be converted to asphalt or a desert. Second growth soon begins the regeneration path to forest in all but the sandiest soils (Moran 1993a). The widespread struggle to keep pastures and crops free of weeds is a testament to the striking speed of secondary succession. Weeds, not deserts, are a major headache for farmers, ranchers, and plantation owners in Amazonia. How soon mature forest returns depends primarily on the texture and fertility of the soil and the proximity of seed sources. Also, more rainfall may be derived from the flux of water vapour from the Atlantic than has previously been supposed (Paegle 1987).

How much forest can be removed without affecting rainfall is not known. Realistic predictions of climatic change as a result of landscape changes in the humid tropics are fraught with difficulties (Henderson-Sellers 1987). Current models of forest and climate interactions in Amazonia are too imprecise to predict with any degree of certainty the impacts of deforestation on rainfall (Salati 1992). Evapotranspiration from groves of perennial crops and silvicultural plantations may be close to that of forest. Even pastures release substantial quantities of water to the atmosphere during the rainy season. During the dry season, though, pastures transpire less water than forest and experience significantly warmer mean surface temperatures (Nepstad, Uhl, and Serrao 1991; Nobre, Sellers, and Shakla 1991).

No evidence is available to prove that deforestation in Amazonia has led to reduced rainfall. The dry season around Manaus was accentuated in 1976, and again in 1979, when no rain fell for 73 days (Fearnside 1986: 50). The "summer" (verão) of 1992 was also severe, with many farmers experiencing reduced yields or the loss of seedlings of perennial crops. But it is hard to separate little-understood climatic cycles from long-term trends. To keep matters in perspective: after two particularly heavy burning seasons in Amazonia in 1987 and 1988, 1989 was a very wet year. So much rain fell in eastern Amazonia in 1989 that the dry season virtually disappeared.

Rainfall patterns are highly variable in Amazonia. In 1774, a drought assailed the Rio Negro watershed when deforestation rates were much lower than at present (Hemming 1987). River levels in Amazonia were unusually low in 1860 owing to poor rainfall (Chandless 1866). In 1958, 64 days passed without any rain in the Bragantina zone east of Belém (Penteado 1968: 138). The Amazon River was particularly low again in 1963. The big "push" to develop and open up the Amazon started only in the late 1960s.

Unusual weather patterns also prevailed in the southern United States in the 1980s. Record heat and drought seared the southern and eastern parts of the United States during 1987 and 1988, thereby provoking widespread concern about global warming. Summer 1989 and spring 1992 in the eastern United States, however, were wetter and cooler than usual, and talk of global warming in the media subsided. In Western Europe, several hurricane-force gales during the winters of 1988 and 1989 led to speculation among some politicians that global warming was under way (Maddox 1990). Some in the environmental movement may be concerned that a return to more "normal" weather will cool the ardour of politicians to tackle the issue of global warming.

Predictions about how dry the Amazon might become with continued deforestation are fraught with shaky assumptions. With satellite imagery, it should be possible to document the area of forest cleared each year. Through its remote sensing agency (INPE - Instituto Nacional de Pesquisas Espaciais), the Brazilian government monitors forest burning in Amazonia. But what happens to the land after it is cleared is crucial to the question of climatic drying. Landsat and Spot imagery can separate second growth, cropland, grassland, and forest, but it cannot readily differentiate between secondgrowth stages or types of crops. Aerial photography from aircraft would be too expensive on the scale needed to document annual vegetation changes in the region as a whole. More sophisticated satellites in the future may be able to help. Also, more information is needed about evapotranspiration rates in various vegetation communities, including croplands.