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Global warming and forests: An overview of current knowledge

K. Andrasko

Kenneth Andrasko is senior forestry analyst for the United States Environmental Protection Agency in Washington, D.C., and a US delegate to the Intergovernmental Panel on Climate Change of the United Nations Environment Programme (UNEP) and the World Meteorological Organization (WMO).

This article summarizes what the forest and climate research and policy communities currently know and do not know - about global warming, its potential effects on forests, and possible measures within the forestry sector to mitigate global warming. It is based on a paper prepared for an FAO Expert Consultation on climate change and forests, held in Rome from 5 to 7 March 1990.

The average temperature of the earth's surface, currently 15°C, is kept relatively constant by naturally occurring radiative or "greenhouse" gases present in the atmosphere. Most of the short wavelength radiation in sunlight passes through these gases and warms the earth's surface. Long-wave thermal radiation is then emitted by the earth and heats the atmosphere. In turn, the atmosphere re-emits long-wave radiation outwards to space and downwards to further heat the earth's surface.

Greenhouse gases (abbreviated here as GHGs) include water vapour (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), nitrogen oxides NOx), stratospheric ozone (O3, the high-altitude ozone derived from fossil fuel combustion, as opposed to the tropospheric ozone near the ground that causes smog), carbon monoxide (CO), and chlorofluorocarbons (CFC).

The burning of fossil fuel is the leading source of greenhouse gases

Descriptions of greenhouse gases

Water vapour (H2O). Water vapour is the most abundant and significant infrared absorbing gas in the atmosphere, and the major constituent of clouds. Clouds play a critical though uncertain role in the general circulation models used to simulate atmospheric-surface interactions and the greenhouse effect.

Carbon dioxide (CO2) is by far the most abundant greenhouse gas other than water vapour. Over 90 percent of the carbon lost from conversion of forest to other land uses is released to the atmosphere as CO2, immediately through burning, and over time through decay of dead vegetation in the soil.

Methane (CH4). Major sources of methane are anaerobic decomposition in biological systems, including rice paddies, cattle and other livestock whose digestive systems | rely on enteric fermentation, termites I (present in huge quantities in tropical forest systems, especially on disturbed sites), biomass burning, landfills and wetlands. Forest fires emit one unit of CH4 for every 100 units of CO2.

Nitrous oxide (N2O) is a biogenic gas emitted by deforestation, biomass burning, nitrogenous fertilizers, and fossil fuel combustion. Little is known about the N2O cycle, current N2O fluxes, and processes and rates of release of N2O from soils in natural and altered ecosystems, I and from biomass burning.

Carbon monoxide (CO), while not considered a GHG, does influence the oxidizing capacity of the atmosphere, thereby contributing to increased concentrations of CH4 and N2O. Periodic burning of savannah in Africa and other continents, as a form of livestock pasture management to stimulate growth of nutritious grass shoots may be the largest single source, as significant quantities of CO are emitted during incomplete combustion and smouldering rather than hot, rapid burning.

Nitrogen oxides (NOx), sulphur dioxide (SO2), ozone (O3) and chlorofluorocarbons (e.g. CFC-11 and CFC-12) derive from non-biotic, industrial anthropogenic sources (fossil fuel burning and chemical manufacturing) and are not considered here.

Sources of greenhouse gases

Greenhouse gases are produced both by natural phenomena and through human actions. Major natural sources of greenhouse gases related to forest ecosystems include wetlands (CH4), wild ruminants and small herbivores (CH4), termites (CH4), wildfires (CO2, CH4, N2O, NOx, CO) and natural land ecosystems including forests, savannah and pastures (N2O). Natural emissions of greenhouse gases have been relatively in balance for thousands of years.

The activities of people, on the other hand, are causing significant increases in the emissions of greenhouse gases into the atmosphere. Major human-made sources of greenhouse gas emissions include the burning of fossil fuels (CO2), deforestation (:mostly by burning) to make land available for agriculture and grazing (CO2), and the burning of wood and charcoal for fuel (CO2). An estimated 5600-6000 million tonnes - 6 gigatonnes (Gt) were released into the atmosphere in 1988 in the form of CO2 from anthropogenic (human-made) sources. The industrialized countries contributed about 75 percent of this release, and the developing countries about 25 percent The measurement record for atmospheric concentration of CO2 shows observed increases of 25 percent over the historical period.

Scientists have suggested for more than 100 years, and increasingly during the past decade, that these increasing emissions may affect the atmosphere's radiative balance (i.e. the percentage of the sun's radiation allowed to pass through the earth's atmosphere directly from the sun or reflected from the earth's surface), leading to a significant and long-term increase in the earth's average temperature.

Global annual emissions of major greenhouse gases: relative importance in greenhouse forcing, and contribution from forest systems


Annual emission

Radiative forcing relative to CO2

Contribution to greenhouse effect in 1980s

Carbon dioxide (CO2)



50 percent


5.6 Pg C

Biotic (natural)

0.4-2.6 Pg C

Tropical forest conversion

0.4-2.6 Pg C

Methane (CH4)



20 percent


50-100 Tg C

Biotic (natural)

20-875 Tg C

Tropical forest conversion to other land uses

140-320 Tg C

Nitrous oxide (N2O)



circa 6 percent

Note: 1 Pg (pentagram) = 109 tonnes

1 Tg (teragram) = 106 tonnes

Residence times and global warming potential of gases

GHGs vary greatly, both in their active residence time in the atmosphere before they are decomposed and also in their radiative forcing relative to CO2, the benchmark gas. In order to compare gases, the concept of relative Global Warming Potential has emerged as a means of accounting for the varying residence times and radiative forcing potency of gases (IPCC Work Group 1, 1990).

The table presents rough estimates of annual emissions of GHGs and their contribution to greenhouse forcing in the 1980s. Emphasis is given to the contribution from tropical forest conversion, which accounts for about 25-33 percent of total current CO2 emissions, 35 percent of CH4, and perhaps 25-30 percent of N2O.

Some 17 million ho of tropical forest are cleared annually, almost exclusively by burning

Greenhouse gas emissions from different forest ecosystems

Overall, forest ecosystems store 20 to 100 times more carbon per unit area than croplands and play a critical role in reducing ambient CO2 levels, by sequestering atmospheric carbon in the growth of woody biomass through the process of photosynthesis. However, uncertainties still exist with regard to carbon storage in forest cover and emissions associated with changes in or removal of forest cover for various land uses. For example, emissions of greenhouse gases from cropping practices in swidden (i.e. shifting, or slash-and-burn cycle) agriculture have not been quantified. Neither do we have reliable estimates of biomass, carbon content, and trace gas emissions in a truly representative sample of natural and disturbed tropical forests and carbon fluxes in disturbed tropical soils (which may account for one-third of carbon flux from deforestation).

Boreal forest. Recent investigations of boreal (northern largely coniferous) forest in North America suggest that the carbon content and biomass estimates commonly utilized in calculations of global carbon cycle fluxes may be seriously flawed. Botkin and Simpson (1990) estimated North American boreal forest biomass carbon content at about 9700 million tonnes only a portion of previous estimates of 13800-40000 million tonnes of carbon used routinely to balance the global carbon budget.

Temperate forest. Forests in temperate regions are now essentially in balance in terms of carbon cycling, with annual incremental growth rates roughly equal to timber harvest and deforestation for urban growth and other land uses. However, it is noteworthy that historically they have contributed heavily to global carbon emissions, as forests were cleared (and burnt) in Europe and North America to clear land for agricultural production.

Tropical forest. According to Brown (1988), tropical moist forest averages about 155160 tonnes of carbon per hectare (t C/ha) of standing biomass in Latin America and Asia and ranges up to 187 t C/ha in Africa. Dry tropical forest averages 27 t C/ha in Latin America and Asia and 63 t C/ha in Africa. Some 17 million hectares of tropical forest are estimated by FAO to be cleared annually, almost exclusively by burning.

Predictions of climate change

Most studies of possible climate-induced perturbations of forests have used standard atmospheric General Circulation Models (GCMs) of global climate. These highly complex three-dimensional models of atmospheric, oceanic and terrestrial interactions predict global climate change (variations in regional temperature and moisture) under a scenario of doubled atmospheric concentrations of CO2 (abbreviated as 2xCO2). Their generalized predictions vary widely, and range from increases in global mean temperature of 2.85.2°C increases of 7.1-15.8 percent in precipitation.

Major uncertainties of several orders of magnitude exist in modelling feedbacks in GCMs. GCMs currently only crudely model ocean circulation, clouds, snow and ice coverage, water vapour, and biogeochemical feedbacks. Furthermore, poor spatial resolution of the current GCMs severely limits our ability to forecast reliably the effects of climate change on specific forests. Nonetheless, on the basis of the GCMs, possible climate responses to large increases in greenhouse gas concentrations (doubled CO2 or its equivalent in other gases) are summarized below. The qualifiers in parentheses use the following scale: very probable = greater than 90 percent probability; probable = greater than 67 percent probability; uncertain = hypothesized effect, with little observational data or appropriate modelling.

Global - mean surface warming (very probable): Scientific consensus remains, despite recent studies and press reports stressing less projected warming (see opposing view by Windelius, p. 15), that by the middle of the next century surface warming is likely to occur in the range 1.54.5°C, for a 2xCO2 scenario.

Global - mean precipitation increase (very probable): Increased heating of the surface will lead to increased evaporation and, as a result, to greater global mean precipitation. However, some individual regions may experience decreases in rainfall.

Global - rise in mean sea-level (probable): Thermal expansion of water associated with surface and oceanic warming is predicted. Rate and extent of melting of glacial and continental ice are far less certain. Predictions of changes in mean sea-level remain uncertain and difficult.

Northern high-latitude precipitation increase (probable): High latitudes are expected to experience increased poleward penetration of warm, moist air, leading to increased annual precipitation and river runoff.

Decreases in summer soil moisture in mid-latitude interior regions are anticipated by several global warming models

Summer continental dryness/warming (probable): A significant decrease in soil moisture for some mid-latitude interior continental regions in summer is anticipated by several models. This results from earlier snow melt and earlier than normal spring and summer reductions of soil moisture.

Regional vegetation changes(uncertain): Long-term changes in surface vegetation cover are considered an inevitable outcome of the potential climate changes reviewed above. However, major uncertainties remain about the patterns, scale, rate and feedbacks (e.g. changes in surface albedo or reflectivity, and in precipitation) of vegetative changes.

Tropical storm increases (uncertain): Increased frequency and intensity of tropical storms, such as hurricanes (which play a major role in gap-phase dynamics in Caribbean forests, for example), may be associated with trends toward warmer, wetter climate, according to several studies.

It is noteworthy that little work has been done thus far on applying GCM results to the tropics. Most modellers contend that climate changes are likely to be small in the tropics. However, comparison of model results shows that temperature ranges of 2.5-6.0°C are postulated for Southeast Asia and West Africa, and significant seasonal variations in rainfall are foreseen for West Africa (Andrasko, 1990a).

Potential effects of climate change on forests

Studies on the potential impact of changes in temperature and precipitation in a scenario of doubled CO2 suggest a wide range of effects both on forests and on individual trees (e.g. Shands and Hoffman, 1987; Smith end Tirpak, 1989; Meo, 1987).

Potential effects of rapid warming on temperate forest ecosystems (North America and elsewhere)


· Southern ranges of many eastern United States forest species and boreal forest in Canada and Scandinavia may shift northward 200-1000 km.

· Northern ranges of eastern North American Species could shift up to 700 km northward, although actual migration could be as low as 100 km, owing to problems with seed dispersal and survival.

· Forest health and survival in the long term may depend on how fast climate stabilizes, and weather large scale-forest declines result from pests, stress from air pollution, fires, and drought.

· Forest composition (predominance of species) may change significantly, as species less water-dependent and in the northern-part of their range tend to become dominant.


· Productivity declines of 46-100 percent, beginning 30-80 years from the present, could result along the southern edges of species ranges, depending on levels of soil moisture.

· Boreal forests in Scandinavia may respond very favourable to heightened temperatures and moisture and improve productivity and biomass (AFOS, 1990).

· Boreal forests and hardy gods-boreal transition areas in Canada and elsewhere may, however, decline if moisture and temperature rate changes are too fast and nutrients are limited.


· Increased stand decline and tree mortality are likely from insect pest population and tree disease responses to raised temperature and moisture.

· Natural regeneration changes are uncertain and difficult to predict, hut may be enhanced for some species and provenances in Scandinavia (AFOS, 1990), and for species that use both seed and vegetative propagation methods for reproduction.

· Fire incidence and damage to forests generally will be more severe, especially in stands and in vegetative zones (e.g. semi-arid tropics) becoming drier and warmer.

This hypothesis of climate change effects on forest ecosystems has been summarized by Botkin and Nisbet (1990):

Recent research suggests that global warning will have severe and rapid effects on forest over wide areas, all other factors being equal... Projections suggest that effects might be so great that forest production and species composition will change over large regions, and forests in many regions may be no longer sustainable. Such responses would lead to major impacts on commercial forestry, timber supply, recreation and wildlife that depend on forest habitats, as well as water supply and erosion rates.

Sedjo and Solomon (1989), building on the work by Emanuel et al. (1985) using the Holdridge Life Zone classification, forecast a forest life zone area decrease of 6 percent, a loss of 444 million ha of forest. Boreal forest declined 37 percent in area and lost 60.4 Gt in biomass, while tropical forests increased 28 percent in area and 57.8 Gt in biomass. Total global forest and non-forest were expected to decline 14.1 Gt in carbon overall under 2xCO2.

Potential effects on forms range and composition

Boreal forest. Growth may be enhanced for boreal forests in Scandinavia (AFOS, 1990), although significant shifts in northern and southern ranges and in species composition may occur. However, Solomon, at IIASA's Biosphere Project, suggests that up to 40 percent of current boreal forest would no longer support boreal species under 2xCO2. Very rapid warming might produce conditions changing too rapidly for replacement species to invade from the south and boreal species to migrate northwards. Seedlings capable of survival in the conditions at the time of planting might not be able to reach maturity before conditions changed, awl seedlings capable of survival later might not survive today.

Wheaton et al. (1987) concluded that the southern boundary of the Canadian boreal forest zone is likely to move north 250-900 km, while the northern boundary would shift only 80-700 km, indicating a loss of area of boreal forest. Harrington (1987) suggests that, since pioneer species advance only 100-150 km a century et the northern edge of the boreal forest, the southern margin is likely to be replaced by grassland (steppe).

Temperate forests. An overview of potential effects of doubled CO2 on selected forest systems and species in temperate North America and elsewhere is presented below in the box.

Tropical and subtropical forests (humid and dry). Apparently no effects studies have been performed for tropical forests. Solomon and Leemans (in IIASA, 1989) have extended the Holdridge Life Zone vegetation mapping globally, though They conclude that areas in the tropics where existing vegetation is unlikely to survive in a warmed world include northern and central Amazonia, northwestern South America West Africa' dry forest in Ethiopia and Somalia, southern Philippines, Indonesia, Sarawak, Papua New Guinea and northern Australia.

An increase of 12 percent in total forested area in tropical ecosystems under 2xCO2 has been forecast by Emanuel et al. (1985) largely as a result of increases in precipitation in marginal sites, with increases in area of tropical wet, moist and especially tropical dry forest, and declines in area of subtropical wet and especially subtropical moist.

Panich (1989) offers an overview of potential impacts on forestry in Southeast Asia, focusing on Thailand, by reviewing general GCM results. He notes Salati's finding that indicates that about 50 percent of the rainfall in tropical forest in Amazonia results from the forest's own transpiration; thus, declines in precipitation under climate change might greatly alter precipitation and hydrologic regimes in tropical forests. This might increase fire danger and inhibit recovery of forests after logging or natural disturbance. The impacts of altered seasonal distribution patterns of rainfall (even if the total amounts change little) might include delay of the growing season, increased flooding in the rainy season, an elongated dry season, changes in cyclone numbers and intensity, and increased vulnerability of coastal forests.

Speculation about the effects of climate change on 4.5 million km2 of mesic savannah vegetation in semi-arid tropical areas is offered by Mabbutt (1989). Latitudinal shifts of about 200 km in current boundaries of climate zones are suggested. Two competing scenarios are explored: enhanced precipitation in the tropical latitudes, but reduced rainfall in the semiarid tropics, due to diminished soil-moisture availability, versus a contrasting scenario of 10-20 percent increases in semi-arid area rainfall.

Potential effects of climate change on tree growth and yield

The implications of CO2 enrichment on tree growth and yield remain unclear. Harrington (1987) summarizes current knowledge by quoting the conclusions of Occhel and Strain with respect to perennial plants, that "increases may be accumulated from year to year and carbon may be sequestered in larger plants. However, in ecologically balanced ecosystems with animals feeding on plants, disease organisms operating, and plants competing for light, water, and nutrients, it is uncertain whether ecosystem production will increase".

Laboratory studies of the effects of elevated CO2 levels on plants have documented increased rates of photosynthesis, lowered plant water use requirements, increased carbon sequestering and increased soil microbial activity fixing nitrogen for fertilizer, thereby stimulating growth (Hardy and Havelka, 1975; Drake et al., 1988). Therefore, CO2 increases could theoretically provide significant benefits for plants and trees undergoing water stress in drier climates.

However, very little work has been done in situ on forest or other natural communities over extended time frames (Drake e! al., 1988). The net effect of CO2 enrichment combined with forest decline from climate change and air pollution remains uncertain. Sedjo and Solomon (1989) conclude that " the phenomenon of CO2 fertilization has not been detected in trees, despite extensive searches for it in the field... and in growth chambers".

Secondary effects

One of the few confident predictions about the effect of global warming is that wildland fire activity will significantly respond to regional climate change and that the magnitude of the response will be roughly proportional (within a factor of 2) to the magnitude of the change'' (Main, 1987).

Raised temperatures may produce more generations a year of some insects, since they have high reproductivity potential, making them likely to adapt and evolve at least an order of magnitude more quickly than the c. 30- 100-year life cycle of forest tree species serving as their hosts (Hedden, 1987). In the semi-arid tropics, changes in rainfall and the location of the savannah zone could alter locust invasion patterns in West and East Africa and the semi-arid zone in the Indian subcontinent, with deleterious effects on vegetation (Mabbutt, 1989).

Forestry sector mitigation measures to reduce greenhouse gases

Mitigation measures or technical control options involving forestry have been widely proposed to sequester carbon through the growth of woody plants and the slow loss of forested areas (especially in the tropics) and to reduce anthropogenic production of GHGs from forest and agricultural land use practices. The implementation of forestry response options alone is, in general, not likely to stabilize atmospheric concentrations of greenhouse gases or balance countries' total CO2 emissions budgets (AFOS, 1990). Nonetheless, all options for reduction of atmospheric CO2, including those involving the forestry sector, should be given full consideration.

Major forestry sector mitigation strategies


· Substitute sustainable agricultural technologies for slash-and-burn deforestation.

· Reduce the frequency and amount of forest and savannah consumed by biomass burning to create or maintain pasture and grassland.

· Decrease consumption of forest for cash crops and development projects, through environmental planning and management.

· Improve the efficiency of biomass (fuelwood) combustion in cookstoves and industrial uses.

· Decrease the production of disposable forest products by substituting durable wood or other goods, and by recycling wood products.


· Conserve standing primary and old-growth forests as stocks of biomass

· Introduce natural forest management systems utilizing sustainable harvesting methods to replace destructive logging.

· Substitute extractive reserves producing timber and non-timber products sustainably, through integrated resource management and development schemes

· Increase harvest efficiency in forests, by harvesting more species with methods that damage fewer standing trees and utilize a higher percentage of total biomass.

· Expand fuelwood plantations to provide energy and reduce pressure on natural forests.


· Improve forest productivity on existing forests, through management and biotechnology.

· Establish plantations on surplus cropland in industrialized temperate zones and abandoned lands in the tropics.

· Restore degraded forest and savannah ecosystems through natural regeneration and reforestation.

· Increase soil carbon storage by leaving slash after harvest and expanding agroforestry.

Source: Andrasko, 1990a, b.

Forestry sector mitigation strategies can be grouped into three classes: those that reduce sources of greenhouse gases; those that maintain existing sinks of greenhouse gases; and those that expand sinks of greenhouse gases (Andrasko, 1990b).

Regardless of which of the three groups they fall into, all strategies should to the extent feasible be ecologically sustainable overtime; capable of addressing the direct and indirect causes of forest loss by providing an equivalent spectrum of forest products and jobs comparable to current forest use patterns; economically viable, preferably with low start-up costs; socially integrative, building on local needs and traditions; technologically simple; and relatively adaptable to changing economic, political, social, ecological and climatic conditions. Before any strategy or combination of strategies is adopted on a wide scale, however, to accurately determine their relative benefits true net GHG balance analyses will be necessary, incorporating consideration of all fluxes of multiple gases associated with all phases of growth, harvest, and final disposition of biomass and carbon.

A number of potential options to address climate change in the forestry sector are set out in the box and in the accompanying short article by Kyrklund. More detailed descriptions appear in Andrasko (1990a).


The concept of the greenhouse effect has been widely if not universally accepted. However, because of the still-crude capabilities of current global circulation models to model complex terrestrial-ocean-atmospheric interactions, debate remains about how much future climates are likely to warm, and when.

International political interest is beginning to focus on the need to identify appropriate options for forest sector policy response to potential climate change. The final report of the forestry subgroup of the Intergovernmental Panel on Climate Change (IPCC) of UNEP and WMO, a science assessment and policy process begun in 1988, has recommended early negotiations on a Global Forestry Protocol to address climate change, along with a broader energy protocol (AFOS, 1990). Policy and programmatic responses to climate change are already under way by FAO and other local, national and international development organizations.

To date, most of the considerable research into potential effects of and responses to climate change on forests has taken place in North America, Europe and Australia. Investigations into potential effects on tropical forests are only now beginning.

The threat of climate change confronts the forestry sector in all regions and at all levels with both biophysical and policy challenges of a far larger scale, longer time-frame, and shorter policy decision-making horizon than other recent forest sector concerns.


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