“Greenhouse Gases and the Carbon Cycle: Science’s Best Guess as to the Growth Path of CO2 Concentrations”

“The Carbon Cycle: Human Perturbations and Potential Management Options”

William H. Schlesinger

A paper presented at the symposium:“Global Climate Change: The Science, Economics and Policy,” The Bush School of Government and Public Service, Texas A & M University, 6 April 2001

William H. Schlesinger is James B. Duke Professor of Biogeochemistry at Duke University and the author of Biogeochemistry: An analysis of global change (Academic Press, 1997). (Email: schlesin@duke.edu)

Introduction:

A variety of gases, including water vapor (H₂O), carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), add to the radiative forcing of Earth’s atmosphere, meaning that they absorb certain wavelengths of infrared radiation (heat) that is leaving the Earth and thus raise the temperature of the atmosphere. Since glass has the same effect on the loss of heat from a greenhouse, these gases are known as “greenhouse” gases. It is fortunate that these gases are found in the atmosphere; without its natural greenhouse effect, the Earth’s temperature would be below freezing, and all waters on its surface would be frozen. However, for the past 100 years or so, the concentrations of CO₂, CH₄ and N₂O in the atmosphere have been rising as a result of human activities. An increase in the radiative forcing of Earth’s atmosphere is destined to cause large and rapid changes in climate, disrupting both human society and natural ecosystems.

Relative to a molecule of CO₂, the greenhouse warming potential of each molecule of methane and nitrous oxide added to Earth’s atmosphere is about 25 times and 200 times greater; respectively. Nonetheless, most attention has focused on CO₂, because it will contribute more than half of the increase in radiative forcing during the next 100 years, it has a long residence time in the atmosphere-ocean system on Earth, and the major cause of its increase in the atmosphere, fossil fuel combustion, is well known and potentially subject to regulation (Reilly et al. 1999).

In an attempt to understand the changing chemistry of the Earth’s surface—that is its biogeochemisty—scientists try to understand what controls the movements of gases in and out of the atmosphere and to estimate a global budget for each gas that cycles through the atmosphere. For the carbon cycle, biogeochemists assess the emissions of CO₂ to Earth’s atmosphere relative to the natural processes that add or remove CO₂ to/from that reservoir, allowing us to forecast atmospheric CO₂ concentrations and the human impact on future climate. In this, our job is far from complete: while biogeochemists have a good estimate of worldwide fossil fuel emissions, we have highly conflicting views about whether the biosphere—especially forests and soils—is now a source or sink for atmospheric CO₂.

The most recent budget for atmospheric CO₂ prepared by the Intergovernmental Panel on Climate Change (IPCC, 2000) contains an unknown sink (or fate) for CO₂ that amounts to about 30% of estimated annual emissions (Table 1). In the face of such uncertainty, policy makers will certainly demand a better accounting by biogeochemists before taking serious actions to reduce fossil fuel emissions globally. We also need to know how the terms in this equation will change in the future. What will happen, for instance, if fossil fuel combustion increases to 15 PgC/yr? Most oceanographers see a diminishing marginal uptake of CO₂ by the oceans (Archer 1995), so that ocean uptake is not likely to exceed 5 PgC/yr. If forests constitute the unknown “residual” term in the equation, then undisturbed forests now perform a great service to society, and their preservation should be ensured. Looking to the future, we need to know if forests will function more efficiently to take up CO₂ in the face of higher concentrations of CO₂ and warmer temperatures in Earth’s atmosphere. Thus, studies of forest growth are now intimately tied to questions of public policy and global biogeochemistry.

The Global Carbon Cycle:

The concentration of CO₂ is controlled by a variety of processes that add and subtract CO₂ to/from the atmosphere. Nearly all of these processes are cyclic—for example the removal of CO₂ by plant photosynthesis,

CO₂ + H₂O → CH₂O + O₂, (1)

is balanced by the return of CO₂ and the consumption of oxygen (O₂) when plant tissues burn or decompose:

CH₂O + O₂ → CO₂ + H₂O. (2)

It is important to recognize that the global carbon cycle consists of a variety of such cyclic processes operating at different rates and different timescales. The cycles are overlaid on one another, each contributing to the overall, global biogeochemical cycle of carbon.

The most basic cycle, often called the carbonate-silicate subcycle, is driven by the reaction of atmospheric CO₂ with the Earth’s crust in the process of rock weathering. Since this reaction would occur on a lifeless Earth, it is a component of the abiotic carbon cycle on Earth (Figure 1). Rock weathering transfers CO₂ to the world’s oceans, via rivers, in the form of bicarbonate (HCO₃^-). Bicarbonate is eventually removed from seawater by the deposition of calcium carbonate (limestone, or CaCO₃), which is added to Earth’s crust. When the Earth’s crust undergoes subduction and metamorphism, CO₂ is returned to the atmosphere in volcanic emanations. The presence of life on Earth has increased the rate of some of these processes (e.g., witness the deposition of marine carbonate by oysters), but this portion of the global carbon cycle appears to have turned slowly for nearly all of geologic time. Very few marine sediments are more than 150,000,000 years old (Smith and Sandwell 1997). Presumably, the carbon content of older sediments has been returned to the atmosphere.

Each year, the amount of carbon moving in the carbonate-silicate cycle is relatively small: volcanic emissions are currently estimated between 0.02 and 0.05 PgC/yr1 (Williams et al. 1992, Bickle 1994), annual riverflow of HCO₃^- is 0.40 PgC/yr (Suchet and Probst 1995), and the formation of CaCO₃ carries about 0.38 PgC/yr to ocean sediments (Milliman 1993). It would take nearly 3,000 years for rock weathering to remove the current pool of CO₂ from the atmosphere in the absence of emissions from other sources. The geologic record shows periods when volcanic emissions greatly exceeded the rate that CO₂ could react with the Earth’s crust, and high levels of CO₂ built up in the atmosphere (Owen and Rea 1985). However, for all intents and purposes, this subcycle now appears reasonably well balanced, and there is no credible evidence that the current buildup of CO₂ in Earth’s atmosphere can be attributed to recent, unusually high levels of volcanic activity or to lower rates of rock weathering.

Another component of the abiotic cycle of carbon derives from the presence of liquid water at the Earth’s surface. Any time that CO₂ rises in Earth’s atmosphere, a greater amount will dissolve in water, in the following reaction:

CO₂ + H₂O → H⁺ + HCO₃^- → H₂CO₃. (3)

The reaction is mediated by Henry’s Law, which describes the distribution of any gas, with significant solubility, between the gaseous and liquid phases in a closed system. Played out at the global level, Henry’s Law means that the oceans act to buffer changes in atmospheric CO₂ concentration. As the concentration has risen owing to industrial emissions during the past 150 years, a significant fraction of the CO₂ that might otherwise be in the atmosphere has dissolved in ocean waters. Indeed, we can document the oceanic uptake of CO₂ by comparing sequential measurements taken at the same locale during the past few decades (Quay et al. 1992; Peng et al. 1998). The total uptake of CO₂ by the oceans is determined by the downward mixing of surface waters into the deep sea, in a global pattern known as the thermohaline circulation (Broecker 1997). Marine biogeochemists are fairly confident, that as a result of rising CO₂ concentrations in Earth’s atmosphere, the net uptake of CO₂ by the world’s oceans is about 2 PgC/yr—about 20 times more than estimates of enhanced consumption of atmospheric CO₂ by rock weathering (Andrews and Schlesinger 2001). However, they are also fairly confident that the uptake of CO₂ by the oceans will not increase in proportion to the future anticipated increase of CO₂ in the atmosphere (Archer 1995).

In contrast to the abiotic cycle, the biotic carbon cycle stems directly from the presence of life on Earth (Figure 2). On land and in the sea, photosynthetic organisms remove CO₂ from the atmosphere, using it to form organic matter (Equation l). Globally, the annual production of new plant tissues is known as net primary production, which is estimated to capture 105 PgC/yr—with 54% occurring on land and the rest in the sea (Field et al. 1998).

The mean residence time for a molecule of CO₂ in Earth’s atmosphere—about 5 years—is largely determined by the uptake of carbon in photosynthesis. The well-known annual oscillations of CO₂ concentration in Earth’s atmosphere occur because a large fraction of global photosynthesis occurs in regions with seasonal climate—i.e., where plants grow only during the summer. As a result of their uptake of CO₂, marine phytoplankton maintain an undersaturated CO₂concentration in the ocean’s surface waters, which enhances the marine uptake of CO₂ from the atmosphere. However, most of the CO₂ removed from the atmosphere by photosynthesis is not captured for long, because dead organic matter decomposes rapidly in soils and seawater. The long-term accumulation of carbon in undecomposed materials in soils is about 0.4 PgC/yr (Schlesinger 1990), while the storage of carbon in marine sediments is only about 0.1 PgC/yr (Berner 1982)2.

By establishing bio-geochemistry, life on Earth has stimulated the movement of CO₂ to and from the atmosphere (Schlesinger 1997). Through geologic time, the products of photosynthesis have added a huge amount of organic matter to the Earth’s crust (≈15,600,000 PgC). Nevertheless, the current rate of carbon storage in sediments is rather small—not unlike the rates through most of geologic time (Garrels and Lerman 1981).

Past Variations in Atmospheric CO₂:

One way to gain perspective about the potential future trajectory for atmospheric CO₂ is to examine the geologic record of its concentration in the past. How high has the CO₂ concentration been in the past? How fast did it reach past high levels? Do past fluctuations offer any insight about how effective the various subcycles of the global carbon cycle are in buffering future fluctuations in atmospheric CO₂?

There is good reason to believe, and some supporting geologic evidence, that the concentration of CO₂ in Earth’s atmosphere in the distant past was much higher than today. Persistent high concentrations of CO₂ are likely to have characterized Earth’s history before the evolution of land plants, which subsequently greatly increased the consumption of CO₂ by rock weathering (Berner 1998; Moulton et al. 2000). High concentrations of CO₂ in Earth’s early history may have been instrumental in maintaining Earth’s temperature above the freezing point of water at a time when the Sun’s luminosity was significantly lower than today.

Despite such high levels of CO₂ during the Earth’s “deep” geologic past, studies of marine sediments indicate that atmospheric CO₂ has remained in a narrow range between 100 and 400 ppm3 over the past 20,000,000 years (Pearson and Palmer 2000). Bubbles of air trapped in layers of the Antarctic ice pack show concentrations in the range of 180 to 290 ppm over the past 420,000 years (Petit et al. 1999), with low values associated with glacial epochs and higher values during warmer, interglacial periods. Small variations, between 230 and 290 ppm, since the end of the last glacial epoch (10,000 years ago) suggest short-term temporal imbalances in the global carbon cycle (Indermuhle et al. 1999), with fluctuations in the amount of forest biomass partially responsible for changes in atmospheric CO₂. During the past 2000 years, concentrations of CO₂ have remained between 270 and 290 ppm, except since the Industrial Revolution (Barnola et al. 1995). The rise in CO₂ during the past 150 years appears associated with global warming (Mann et al. 1998, Crowley 2000), and the most current IPCC (2001) projections are for levels reaching 550 ppm in 2050 and > 700 ppm by 2100 (Figure 3).

Human Perturbations of the Global Carbon Cycle:

Each year, humans extract more than 6 Pg of organic carbon from the Earth’s crust (oil, coal, and natural gas) and convert it to CO₂ that is added to the atmosphere. The IPCC (2001) “business-as-usual” scenario predicts CO₂ emissions will rise to 15 PgC/yr by the year 2050, largely due to increases in fossil fuel combustion (Figure 4). Our impact on the global carbon cycle may appear small compared to some of the natural transfers, such as decomposition, that also add (or subtract) CO₂ to the atmosphere (Figure 2), but it is important to recognize that photosynthesis and decomposition are naturally-occurring, counter-balancing processes that produce no large net source or sink of atmospheric CO₂ on an annual basis. In contrast, with fossil fuel combustion, humans remove organic carbon from the Earth’s crust at a rate more than 100 times greater than the storage of organic carbon in newly-formed marine sediments. We must count on Henry’s Law and changes in the activity of the biosphere to buffer any changes in atmospheric CO₂concentration.

Forest destruction, largely deforestation in the tropics, is also thought to be a net source of atmospheric CO₂, although the exact magnitude is most uncertain. Melillo et al. (1996) estimate a release of 1.2 to 2.3 PgC/yr as CO₂ from global tropical deforestation in the 1990s; however, Houghton et al. (2000) report a net release of only 0.2 PgC/yr from the Brazilian Amazon, where tropical deforestation rates are thought to be among the highest globally. Regrowth of vegetation on deforested lands and lands abandoned from agriculture may account for the discrepancy. We can hope that the improving long-term satellite record of forest cover in the tropics will allow us to refine our estimates of deforestation rates (Skole and Tucker 1993).

The carbon balance of forests must consider changes in the temperate zone that may offset (or augment) changes that are occurring in the tropics. Using an inverse model4 of atmospheric CO₂ concentrations, Tans et al. (1990) suggested that the northern temperate latitudes were a net sink for carbon, largely as a result of the regrowth of forests on abandoned agricultural lands. Similar conclusions derive from other inverse modeling studies (Denning et al. 1995; Ciais et al. 1995), and Fan et al. (1998) estimated that the sink in North America was as large as 1.7 ± 0.5 PgC/yr between 1988 and 1992. Battle et al. (2000) postulate a net global uptake of carbon by forests at 1.4 ± 0.8 PgC/yr – i.e., the uptake in the northern latitudes more than compensated for all the losses from tropical deforestation. Their results are consistent with studies of changes in atmospheric O₂ (Keeling et al. 1996). The results of inverse modeling studies imply that the net emission from tropical deforestation has been overestimated (Ciais et al. 2000).

By themselves, inverse modeling studies would seem to identify the residual term in table 1 and to resolve the atmospheric CO₂ budget; however, estimates of actual changes in the carbon storage on land fall far short of the values predicted by such models. In a global analysis of forest greenness using satellite remote sensing, Potter (1999) found that deforestation was a source of 1.44 PgC/yr to the atmosphere, of which 0.29 PgC/yr accumulated in regrowth, for a net release of 1.15 PgC/yr from the terrestrial biosphere. Earlier, Dixon et al. (1994a) also calculated a net source of 0.9 ± 0.2 PgC/yr from an inventory of world forests. Post and Kwon (2000) concluded that the rate of soil carbon accumulation as a result of reforestation and afforestation globally (0.16 PgC/yr) also falls short of the “missing sink” of carbon on land. So as we enter this century, we have highly conflicting views about whether the world’s forests are waxing or waning in their extent and carbon storage!

Examining historical forest inventories, Houghton et al. (1999) find an accumulation of 0.037 PgC/yr in U.S. forests during the 1980s, postulating a maximal upper limit for carbon storage at 0.35 PgC/yr if a variety of other processes, including greater carbon storage in soils, are included. Alternative estimates indicate a net accumulation of 0.17 PgC/yr in eastern U.S. forests (Brown and Schroeder 1999), 0.2 PgC/yr in all U.S. forests (Birdsey et al. 1993), and <0.5 PgC/yr in all of North America’s forests (Chen et al. 2000)—similar to the North American sink determined by inverse modelling (Ciais et al. (2000). A recent workshop convened to reconcile the inverse-modeling and inventory studies agreed that there was a sink of 0.30 to 0.58 PgC/yr in the United States during the 1980s (Pacala et al. 2001).

In the face of losses of carbon from tropical forests and only a small sink in North America, we must postulate huge, recent increases in the carbon uptake and storage in Siberian forests, for which the driving mechanism is unclear. Kolchugina and Vinton (1993) estimate a net sink of 0.49 PgC/yr in forests and their soils of the former Soviet Union, but most alternative estimates are lower, and Shepashenko et al. (1998) calculate a net loss of carbon from Siberian forests in recent decades.

Prospects for the Future:

Changes in forest biomass and soil carbon storage have certainly affected atmospheric CO₂ concentrations in the past, and there is some indication that year-to-year variability in the accumulation of CO₂ in the atmosphere is affected by changes in the activity of the terrestrial biosphere (Bousquet et al. 2000; Houghton 2000). Despite the wide disparity between inverse-model and inventory estimates of forest carbon storage, there is no doubt that the growth of atmospheric CO₂ concentrations would be even greater if it were not for forest regrowth in the temperate zone. Nevertheless, while these forests grow, CO₂ concentrations continue to rise. Can we expect, or orchestrate, more uptake by terrestrial ecosystems in the future?

The carbon uptake by forests is determined by their total area, as well as factors that affect the rate of carbon accumulation per unit of area, including forest age. Total area is affected by human land-use decisions as well as increases in the spatial extent of forests, as determined by a warmer climate (Myneni et al. 1997). Changes in local carbon uptake are determined by climate, CO₂ fertilization, and the enhanced deposition of nitrogen from regional air pollution. Young forests show the most rapid carbon uptake, with the rate of carbon sequestration decreasing with time (Chapman et al. 1975, Schiffman and Johnson 1989). Separate studies using biogeochemical modeling (Schimel et al. 2000) and an analysis of historical forest inventory (Caspersen et al. 2000) agree that changes in land use dominate the current net uptake of carbon by U.S.forests.

Keeling (1993) notes that the increasing amplitude of the annual oscillation of atmospheric CO₂ must mean that some process has stimulated the biosphere—presumably via increased rates of photosynthesis. However, there are several indications that the stimulation of photosynthesis by CO₂ fertilization, while widely observed in short-term experiments (Curtis and Wang 1998), does not result in large increases in plant mass, when the exposure is long-term and plants can acclimate to the higher CO₂ levels (Hattenschwiler et al. 1997; Idso et al. 1999). The initial 25% growth response in a young (15-year-old) stand of loblolly pine in the Duke Forest Free-Air CO₂ Enrichment (FACE) experiment dropped below statistical significance during the 4^th year of exposure, apparently owing to nutrient deficiencies in the soil (DeLucia et al. 1999, Oren et al. 2001). Large increases in the rate of root respiration and decomposition minimize changes in the pool of carbon in soil organic matter, despite greater inputs of dead plant materials to the soil (Schlesinger and Lichter 2001).

Increased deposition of nitrogen from the atmosphere should also stimulate the growth and carbon content of forests (Holland et al. 1997). However, the growth enhancement from nitrogen deposition may simply allow forests to attain maximum biomass more rapidly, rather than at higher final values. Excessive nitrogen deposition is often a cause of acid rain, leading to soil acidifications that can reduce forest growth. Simultaneous exposure to other air pollutants, such as ozone, may also explain the relatively low growth enhancements in forests of the eastern U.S. (Caspersen et al. 2000).

Estimates of the N-derived sink also need to be discounted to the extent that emitted nitrogen falls on non-forested lands (Townsend et al. 1996, Asner et al. 1997). Furthermore, only a fraction of the added inputs of nitrogen accumulates in vegetation, where C/N ratios are high and carbon storage is most efficient (Nadelhoffer et al. 1999, Schlesinger and Andrews 2000). Abiotic processes can add nitrogen to soil organic matter, lowering its C/N ratio without adding significantly to soil carbon storage (Johnson et al. 2000). Accounting for many of these effects, Townsend et al. (1996) estimate the N-derived sink at 0.44 to 0.74 PgC/yr.

Without explicit management to enhance carbon storage on land, reforestation of abandoned agricultural land is the most plausible cause of a carbon sink in the terrestrial biosphere, both now and in the foreseeable future. A large amount of land in the eastern U.S. has reverted to forest since agricultural abandonment in the past century (Hart 1968; Delcourt and Harris 1980). These lands now support growing forests, which are accumulating carbon dioxide from the atmosphere. While reforestation of these lands may be helpful in mediating the rise of atmospheric CO₂ concentrations, it offers no long-term solution to the greenhouse-warming problem. It would require reforestation of all the once-forested land on Earth, including that now used for agriculture or covered by urban areas, to store 6 PgC/yr—the amount emitted each year from fossil fuel combustion (Vitousek 1991).

The IPCC (2000) panel on Land Use, Land-Use Change, and Forestry evaluated the potential for direct human intervention to enhance the storage of carbon in forests and soils, concluding that a significant potential exists to mediate the rise of CO₂ in Earth’s atmosphere. However, many of the recommended management procedures, including afforestation and intensification of agricultural management need careful scrutiny to ensure that the costs associated with the practice do not exceed the benefits or credits received for incremental carbon storage. The afforestation of marginal lands is likely to require especially large inputs of energy in planting, irrigation, and fertilization of young trees (Dixon et al. 1994b). Turhollow and Perlack (1991) calculate an energy ratio (i.e., energy in biomass grown/energy input) of 16 for hybrid poplar grown for biomass energy in Tennessee. Amortizing the initial cost to establish forestry plantations over a 50-year rotation, the cost of carbon sequestration ranges from $1 to $69 per metric ton, with a median value of $13 (Dixon et al. 1994b). However, the rate of carbon storage in forests declines as they mature, so “the only way by which reforestation programs can continue to sequester carbon over the long term is if they transition into programs that produce commercial biomass fuels” (Edmonds and Sands, this volume)—that is, we must replace fossil fuel with biomass energy.

Implementation of reduced and conservation tillage practices in agriculture appears to offer a consistent net benefit by enhancing soil carbon storage (Kern and Johnson 1993, Robertson et al. 2000, West and Marland, in review); however, greater use of nitrogen fertilizer often does not (Schlesinger 2000). The release of CO₂ by pumping irrigation water also greatly exceeds the enhanced carbon storage found in irrigated agricultural soils (Schlesinger 2000). Wildly positive forecasts (e.g., 0.4 – 0.8 PgC/yr) have been made for the potential to increase carbon storage in agricultural soils (Lal 2001), but reality is not nearly so sanguine. Pacala et al. (2001) estimate the carbon storage in cropland soils of the U.S. was only 0 to 0.04 Pg/yr during the 1980s. Kern and Johnson (1993) estimated that immediate implementation of conservation tillage on all U.S. farmland with this potential would provide a sink (<0.015 PgC/yr) accounting for only about 1% of the fossil fuel emissions in the U.S. at today’s levels. Substantial areas are already in conservation tillage regimes (Uri 1999), for which the net carbon sequestration potential is estimated at 0.0003 PgC/yr (Uri 2000). Moreover, similar to the pattern of carbon storage during forest regrowth, storage in soils is finite, and the rate will diminish with time (Schlesinger 1990).

Warming:

If the Earth's temperature rises due to the greenhouse effect, we can expect soils to be warmer, especially at high latitudes. Except in some deserts, the rate of decomposition in soils increases with increasing temperature—as seen both in compilations of literature values (Raich and Schlesinger 1992) and nearly all studies that have imposed experimental warming (Rustad et al. 2001). The increase in soil respiration5 doubles with a 10^o C rise in temperature—that is, the Q₁₀ of the relationship is about 2.0 (Kirschbaum 1995, Palmer-Winkler et al. 1996, Kätterer et al. 1998). The greatest response is found in samples of surface detritus and in soils from cold climates (Lloyd and Taylor 1994). Nearly all models of global climate change predict a loss of carbon from soils as a result of global warming (Schimel et al. 1994, McGuire et al. 1995).

As a result of cold, water-logged conditions, organic matter accumulates in boreal and tundra soils (Harden et al. 1997, Trumbore and Harden 1997). Radiocarbon measurements indicate limited turnover, but nearly all the organic matter is found in labile fractions that will be easily decomposed should the climate warm (Chapman and Thurlow 1998, Lindroth et al. 1998). In the tundra, melting of permafrost and concomitant lowering of the water table may lead to a large increase in decomposition (Billings et al. 1983, Moore and Knowles 1989). Indeed, Oechel et al. (1993, 1995) found evidence of a large loss of soil organic matter in tundra habitats as a result of recent climatic warming in Alaska, and Goulden et al. (1998) found a significant loss of carbon from soils during several warm years that caused an early spring thaw in a boreal forest of Manitoba. Recent measurements of European forests show greater respiration, and lower net uptake, by forests at high latitudes, perhaps as a result of climatic warming during the past several decades (Valentini et al. 2000). In response to global warming, large losses of CO₂ from boreal forest and tundra soils could reinforce the greenhouse-warming of Earth’s atmosphere (Woodwell 1995).

Conclusions:

The IPCC (2001) offers a number of scenarios that predict the future course of atmospheric CO₂ concentrations (Figure 3). The Business-as-Usual scenario shows emissions rising to 15 PgC/yr and atmospheric concentrations rising to 550 ppm by the year 2050. Even the most rigorous abatement scenarios show concentrations of >500 ppm in the year 2100, nearly all scenarios show emissions >10 PgC/yr in the year 2050 (Figure 4), dwarfing even the most optimistic scenarios for enhanced carbon storage in the terrestrial biosphere. Thus, if we are serious about preventing climate change, I see no alternative but to cut emissions, substantially and immediately. Alternative suggestions simply divert our attention from this problem, and precious time is lost in our attempt to control the emissions of this gas, which will otherwise take centuries for natural processes to remove from Earth’s atmosphere.

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Table 1. Atmospheric Budget for Carbon Dioxide, 1989-1998, in units of PgC/yr (IPCC 2000).

Fossil Fuel Deforestation Atmospheric Ocean Residual Emissions Increase Uptake

6.3 + 1.6 = 3.3 + 2.3 + 2.3

Figure 1. Abiotic processes contributing to the global carbon cycle of the present-day Earth (Modified from Schlesinger 1997)

Figure 2. Biotic and anthropogenic processes contributing to the global carbon cycle of the present-day Earth (Modified from Schlesinger 1997)

Figure 3. CO₂ emissions projected from fossil fuel combustion, showing high, low and Business-as-Usual (BAU) scenarios (IPCC 2001).

Figure 4. Atmospheric CO₂ concentrations resulting from emissions scenarios outlined in Figure 3 (IPCC 2001).