4.1. Comparisons with other estimates of land-use change
The revisions described in this paper increased a previous global estimate (Houghton, 1999) from 124 to 134 PgC for the period 1850–1990, most of the increase appearing before 1960 (Fig. 1). Revisions were as large as 0.3 PgC yr−1 in individual regions (Latin America during the 1980s; China during the 1950s), but were largely offsetting globally. The global annual flux declined during the 1990s but averaged 0.2 PgC yr−1 higher than in the previous decade.
Other recent analyses of the flux of carbon from land-use change give results that bound the results reported here (Table 4), although differences in the processes and regions included make comparisons somewhat misleading. The estimates of flux for the 1980s by McGuire et al. (2001), Houghton (1999) and this study, for example, are global, while the estimate by Fearnside (2000) includes only the tropics. The source estimated by McGuire et al. (2001) is low because it does not include either the harvest of wood or the clearing of forests for pastures, both of which contributed to the net global source calculated by Houghton (1999) and this study (Table 3). On the other hand, the average annual release of carbon attributed here to changes in the area of croplands (1.2 PgC yr−1 for the 1980s) is higher than the estimate found by McGuire et al. (0.8 PgC yr−1) (Fig. 2). The difference reflects uncertainties in the data used to reconstruct changes in cropland area and to define the carbon stocks of the ecosystems cleared.
Table 4. Estimates of the average annual flux of carbon in the 1980s from changes in land use (positive values indicate a release of carbon to the atmosphere)
|Flux of carbon (PgC yr−1)||Regions included||Reference|
|0.8a (0.6–1.0)||Globe||McGuire et al., 2001|
|2.4 (1.4–3.4)||Tropics||Fearnside, 2000|
|2.0 (1.2–2.8)||Globe||Houghton, 1999|
|2.0 (1.2–2.8)||Globe||This study|
Figure 2. Estimates of the annual flux of carbon from global changes in croplands. Positive values indicate a release to the atmosphere. Houghton's estimate is from this study; the four other estimates are from terrestrial biosphere models (McGuire et al., 2001).
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4.3. Implications for the terrestrial carbon sink
The flux of carbon in the 1990s calculated to have resulted from land-use change and management is larger than initially estimated by Houghton (2000) and reported in the Third Assessment Report of the IPCC (Prentice et al., 2001). Other recent changes in our understanding of the global carbon balance, as a result of accounting for the outgassing of O2 from oceans (Plattner et al., 2002), suggest that the residual terrestrial sink [the difference between the net terrestrial flux (inferred from atmospheric O2 and CO2 data) and the land-use flux] averaged 2.4 ± 1.1 PgC yr−1 during the 1980s and 2.9 ± 1.1 PgC yr−1 during the 1990s (Table 6). The difference between decades in both the net and the residual terrestrial flux is smaller than summarized by Prentice et al. (2001), although the residual sink of 2.4–2.9 PgC yr−1 is clearly significant.
Table 6. Global carbon budgets for the 1980s and 1990s (PgC yr−1)a
|Fossil fuel emissionsb||5.4 ± 0.3||6.3 ± 0.4|
|Atmospheric increaseb||3.3 ± 0.1||3.2 ± 0.2|
|Oceanic uptakec||−1.7 ± 0.6||−2.4 ± 0.7|
|Net terrestrial fluxc||−0.4 ± 0.7||−0.7 ± 0.8|
|Land-use changed||2.0 ± 0.8||2.2 ± 0.8|
|Residual ‘terrestrial’ flux||−2.4 ± 1.1||−2.9 ± 1.1|
Given the recent revisions to the estimated oceanic and terrestrial sinks (the latter is now half as large) (0.7 PgC yr−1, rather than 1.4 PgC yr−1) (Plattner et al., 2002), it might be appropriate to drop the word terrestrial from the term residual terrestrial sink and refer to it as the residual sink. Errors in the estimated oceanic uptake of carbon affect the magnitude of the residual sink as much as errors in land-use change.
To the extent that the residual sink is terrestrial, it exists in both northern mid-latitudes and the tropics. In northern mid-latitudes the sink attributable to the recovery of forests from past changes in land use (0.02 ± 0.5 PgC yr−1) is much less than the net terrestrial flux inferred from inverse calculations with atmospheric data and models (1.6–3.2 PgC yr−1) (Gurney et al., 2002) (Table 7). Part of the difference may be explained by the observation that ecosystems other than forests are significant sinks for carbon (Houghton et al., 1999; Pacala et al., 2001). It is also possible that the accumulation of carbon below ground, not directly measured in forest inventories, can account for the difference in estimates. However, the few studies that have measured the accumulation of carbon in forest soils have consistently found soils to account for only a small fraction (5–15%) of measured ecosystem sinks (Gaudinski et al., 2000; Barford et al., 2001; Schlesinger and Lichter, 2001). Thus, despite the fact that the world's soils hold two to three times more carbon than biomass, there is no evidence yet that they account for much of the residual sink.
Table 7. Terrestrial sources (+) and sinks (−) of carbon (PgC yr−1) estimated by different methods
|Region||Analysis of land-use change (this study) (1990s)||Inversions based on atmospheric data and models (Gurney et al., 2002) (1992–1996)||Forest inventories (Goodale et al., 2002) (∼1990)|
|Globe||2.2 (±0.8)||−1.4 (±0.8)|| |
|Tropics||2.2 (±0.8)||1.2 (±1.2)|| |
|North||−0.02 (±0.5)||−2.4 (±0.8)||−0.65 (±0.05)|
|South||0.02 (+0.2)||−0.2 (±0.6)|| |
Forest inventories show a carbon sink in northern forests (0.6–0.7 PgC yr−1) (Goodale et al., 2002) intermediate between estimates based on inverse calculations with atmospheric data and analyses of land-use change (Table 7). Again, accounting for non-forest ecosystems might reduce the difference between the results of inventories and atmospheric analyses. With respect to the difference between forest inventories and land-use change, a regional comparison suggests that the recovery of forests from land-use change may either over- or underestimate the sinks measured in forest inventories (Table 8). In Canada and Russia, the carbon sink calculated for forests recovering from harvests (land-use change) is greater than the measured sink. The difference could be error, but it is consistent with the fact that fires and insect damage increased in these regions during the 1980s and thus converted some of the boreal forests from sinks to sources (Kurz and Apps, 1999). These sources would not be counted in the analysis of land-use change because natural disturbances were ignored. In time, recovery from these natural disturbances would be expected to increase the sink above that calculated on the basis of harvests, alone, but at present the sources from fire and insect damage exceed the net flux associated with harvest and regrowth.
Table 8. Annual net changes in the living vegetation of forests (TgC yr−1) in northern mid-latitude regions around the year 1990a
|Region||Land-Use Changeb||Forest Inventoryc||Sink from land-use change relative to inventoried sink|
|Canada||−25 ||40||65 (larger)|
|USA||−35 ||−110|| 75 (smaller)|
|Russia||−55 ||40||95 (larger)|
|China||75 ||−40|| 115 (smaller)|
|Europe||−20 ||−90|| 70 (smaller)|
In the three other regions (Table 8), changes in land use show a smaller sink in trees than measured in forest inventories. If the results are not simply a reflection of error, the failure of past changes in land use to explain the measured sink suggests that factors not considered in the analysis have enhanced the storage of carbon in forests. Such factors include past natural disturbances, more subtle forms of management than recovery from harvest and agricultural abandonment (and fire suppression in the US), and environmental changes that may have enhanced forest growth. Analysis of forest inventory data from five states in the US led Caspersen et al. (2000) to conclude that very little of the observed accumulation of carbon in trees could be attributed to enhanced growth. Instead, it was largely explained by recovery from earlier disturbance. The lack of a significant growth response is consistent with recent findings that CO2 fertilization may be short lived in forests (Oren et al., 2001, Schlesinger and Lichter, 2001).
It remains unclear whether the different estimates of flux from land-use change and inventories are real or the result of errors and omissions. The differences are small, generally less than 0.1 PgC yr−1 in any region. As discussed above, the likely errors and omissions in this analysis include rates of forest growth, natural disturbances and many types of management (Spiecker et al., 1996). These possibilities need to be addressed in future analyses.
The same uncertainties apply to the tropics, where the errors are larger. Changes in land use yield smaller sinks (or larger sources) than those inferred from inversion studies (Table 7). The difference suggests the existence of a tropical sink (unrelated to land-use change) large enough to offset at least some of the emissions from deforestation. Because of the increasing importance of human activity in the region, it seems unlikely that the sink would be caused by forests recovering from past disturbances not already included in analyses of land-use change (Table 3). There is no evidence, for example, that rates of natural disturbance are less now than in the past, so that large areas in the tropics are now recovering. However, measurement of CO2 flux by eddy covariance suggests that undisturbed tropical forests in the Amazon may be a net carbon sink (Grace et al., 1995, Malhi et al., 1998). The rates of accumulation are larger than would be expected for recovered forests and suggest that the rates may be enhanced. However, a new analysis of CO2 in rivers suggests that much of the forest uptake of carbon is offset by releases downstream, so that undisturbed forests are nearly neutral with respect to carbon (Richey et al., 2002).
Estimates of carbon exchange in the tropics vary considerably. On the one hand, analyses of land-use change consistently find reductions in forest biomass, implying carbon sources (Flint and Richards, 1994; Gaston et al., 1998; Houghton and Hackler, 1999). On the other hand, repeated measurements of forest biomass seem to show an accumulation of carbon in some undisturbed forests (Phillips et al., 1998), although the findings have been attributed to artifacts of measurement (Clark, 2002). It is possible, of course, that both increases and decreases in biomass are occurring simultaneously in different forests. The challenge is to identify the mechanisms. The distribution of people throughout most forest lands suggests that relatively little of the tropics has escaped disturbance from human activity. And, because rates of harvest and burning have generally increased over the last 50 years, the net effect of human disturbance and subsequent recovery has been to reduce carbon stocks. Clearly, there are exceptions, such as in Puerto Rico, where forests have grown back on abandoned farmlands. Overall, however, the trend is a loss of forests (Table 1) and probably a loss of carbon from within forests as well.
At present, the results from numerous independent measurements cannot distinguish between two mutually exclusive paradigms: large sources of carbon from deforestation, offset by enhanced growth (in undisturbed forests), or more moderate sources of carbon than calculated here, and natural forests close to neutral with respect to carbon. Enhanced rates of plant growth cannot be ruled out as an explanation for apparent sinks in either the tropics or mid-latitude lands, but it is possible that the current sink is entirely the result of recovery from earlier disturbances, anthropogenic and natural.