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The emissions of carbon from deforestation and degradation in the tropics: past trends and future potential Richard A Houghton
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To cite this article: Richard A Houghton (2013) The emissions of carbon from deforestation and degradation in the tropics: past trends and future potential, Carbon Management, 4:5, 539-546, DOI: 10.4155/cmt.13.41 To link to this article: http://dx.doi.org/10.4155/cmt.13.41
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Mini Focus: Sustainable Landscapes in a World of Change: Tropical Forests, Land use and Implementation of REDD+
The emissions of carbon from deforestation and degradation in the tropics: past trends and future potential
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Carbon Management (2013) 4(5), 539–546
Richard A Houghton* Land use in the tropics, including both deforestation and forest degradation, is estimated to have emitted approximately 1.4 PgC yr-1 to the atmosphere over the interval 1990–2010 (~15% of anthropogenic carbon emissions). This net emission is composed of gross emissions of at least 2.6 PgC yr-1 and gross sinks of 1.2 PgC yr-1 in forests recovering from wood harvest and in the fallows of shifting cultivation. In contrast to recent management of tropical forests, future management in the region could be used to stabilize the concentration of CO2 in the atmosphere, at least temporarily, with the following three measures: a halt to deforestation and forest degradation, protection of regrowing forests, and the re-establishment of forests on lands not intensively used now that were forests in the past. Together, these three measures have the potential to reduce emissions of carbon and increase uptake by as much as 3–5 PgC yr-1.
The primary purpose of REDD+ is to reduce emissions of carbon and help stabilize the concentration of CO2 in the atmosphere, thereby limiting the rate and amount of climatic disruption. When the predecessors of REDD+ were first proposed in the early 1990s, the emissions of GHGs from tropical deforestation were thought to account for 20–25% of global emissions [1]. That percentage has come down, largely because the annual emissions of carbon from fossil fuel combustion have increased sharply in recent years (Figure 1) [2]. The emissions of carbon from deforestation and forest degradation in the topics averaged 10–15% of anthropogenic carbon emissions in the decade 2000–2009. Today they may be less 10%, and the decline raises the question of whether REDD+ is worth the effort and expense. This paper suggests that REDD+ is indeed worth the effort, and that an all-out effort could potentially stabilize atmospheric carbon for a few decades and, thus, buy time for the development of technologies that would eventually eliminate dependence on fossil fuels. This paper has two parts. Part one provides a brief review of historic and current emissions of carbon from deforestation and degradation in the tropics. Reasons for
the variability and uncertainty of emissions estimates are discussed. Part two explores the implications of past and current land use (LU) in the tropics for future reductions in emissions and enhancement of sinks. A few definitions are in order. LU is defined here as a use of land that does not change its category, or cover. For example, harvest of wood, when followed by regrowth of the forest, is a LU; the forest remains forest. In contrast, land-cover change (LCC) is defined as the conversion of one cover type to another, for example, deforestation changes a forest cover to a largely treeless cover (a cropland or pasture use). LU and LCC together (LULCC), refers to the combined effects. Ideally, LULCC would include all types of land management, but many are not documented at a global level. It is important to stress that LULCC refers to changes that result from direct human activity. Deforestation caused by fires, storms, insects or disease is not included in LULCC, unless fires are used intentionally for LCC. Forest degradation is defined as a reduction in carbon density (mgC/ha) of either biomass or soil within a forest. Thus, it is LU, not LCC. The net effect of wood harvest lowers, at least temporarily, the carbon density of forests (although it may increase the storage
*Woods Hole Research Center, 149 Woods Hole Road, Falmouth, MA 02540, USA Tel.: +1 508 540 9900; E-mail:
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The 13 estimates of carbon emissions were calculated with two types 9 of models. Most of the analyses used Dynamic Global Vegetation 8 Models, which are based on physiological processes such as photosyn7 thesis, respiration, growth, decay, allocation, and so on. The models 6 simulate the uptake and release of carbon as controlled by environmen5 tal factors: the effects of light, water, 4 CO2, temperature and, sometimes, nitrogen on plant growth and on 3 decomposition. The fluxes of carbon vary through time as a result 2 of changes in these environmental parameters. All analyses of LULCC 1 included the conversion of lands to crops and pastures (LCC) based 0 on historic datasets [9,10]; some also included the effects of wood harvest Year (LU) [11]. The models are most often run with and without LULCC, the Figure 1. Annual emissions of carbon from combustion of fossil fuels and net emissions from difference being an approximation land use and land-cover change. Emissions from peat swamps not included. of net emissions from LULCC. A few of the analyses used what of carbon in wood products). Given enough time, a has come to be known as a bookkeeping approach. In harvested forest may attain its original carbon density this approach, all of the carbon on lands experiencing again. While growing, the forest is a net carbon sink. LULCC is redistributed when management first takes Other definitions will be given in context below. place [12]. For example, when a forest is converted to a cropland, the living biomass is assigned to the atmosphere Past & current emissions of carbon from (representing burning), to organic matter pools left on deforestation & degradation of tropical forests site to decay, and to wood product pools that decay at A recent review of 13 estimates of global carbon emis- rates simulating different wood products. Soil organic sions from LULCC found average emissions of 1.1 ± 0.5 carbon is lost in the years after first cultivation and recovfor the period 1990–2010 [3]. Although the estimates ers again if the cropland returns to its natural state. Abanwere of global emissions, they are thought to represent doned croplands and pastures accumulate carbon as the tropical emissions because, first, estimates for the trop- natural ecosystems that replace them recover. ics, alone are similar to global estimates [4–7] and, secIt is worth emphasizing that the net and gross emisond, because extra-tropical regions are nearly neutral sions calculated with the bookkeeping approach are only with respect to carbon emissions from LULCC. That is, those resulting from direct human activity. The sources the sources of carbon from decay of logging slash, burn- and sinks of carbon from unmanaged lands are not ing and oxidation of wood products are approximately counted. Furthermore, the gross fluxes of carbon are based balanced by the sinks of carbon in forests regrowing on changes in carbon storage, not on changes in metabofrom earlier harvests and agricultural abandonment. lism. Photosynthesis and respiration are not a part of the However, the estimate of 1.1 PgC yr-1 for the tropics accounting. What is counted are changes in the pools of includes degradation but does not include peat swamps. biomass, soils, litter and wood products as a direct conThe emissions of carbon from the draining and burning sequence of LULCC. Unlike analyses based on Dynamic of peat forests in southeastern Asia, principally Indone- Global Vegetation Models, the bookkeeping approach sia and Malaysia, are estimated to have released another does not include the effects of environmental change (e.g., 0.3 PgC yr-1 in recent years [8], and thus emissions of increasing CO2, warming temperatures, changing moiscarbon from tropical deforestation and degradation ture regimes and nitrogen deposition). Evergreen forests are likely to equal approximately 1.4 ± 0.5 PgC yr-1 in North America, for example, grow at the same rate at present. whether in 1850 or 2000 in the bookkeeping calculations. 10
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Differences among the models used to estimate the flux of carbon from LULCC account for some of the differences among estimates. Other differences result from the activities included (or not) in different analyses, but the standard deviation for 13 model estimates was only approximately ±0.25 PgC yr-1, suggesting that many errors were offsetting. The primary uncertainties result from different rates of LULCC and different estimates of biomass density. Current rates of deforestation reported by the latest Forest Resource Assessment of the FAO, for example, are based on country reports often lacking up-to-date inventories [13]. An independent satellite-based assessment [14] found lower rates of forest loss in 1990–2005 than reported by FAO [13], and an increasing trend rather than the decreasing trend. A systematic, tropics-wide monitoring of forests would reduce the uncertainties of emissions considerably [15]. The emissions of carbon from degradation are even more uncertain. Although forest degradation is widespread, it is also highly variable geographically and the estimate in Table 1, in which degradation might account for approximately 15% of net LULCC emissions, is likely an underestimate. Overall, the error for net emissions of carbon from LULCC is estimated to be approximately ±0.5 PgC yr-1 and could be higher if modeled estimates include the effects of environmental changes on LULCC emissions [3]. The results from one bookkeeping model are used here to elaborate some of the details of carbon emissions. One detail is the longterm trend in annual emissions since 1850 (Figure 2). Early on, the net emissions from LULCC were almost entirely from the mid-latitudes as agriculture expanded rapidly in North America, Australia and later in China and the Former Soviet Union. Emissions in the tropics grew significantly early in the twentieth century in Latin America, followed by tropical Asia by the mid century. The combined emissions from tropical America, Asia and Africa grew almost continuously up to the 1990s and seem to have declined a little since then. The growing emissions from the tropics, combined with the declining emissions (since the late 1950s) outside the tropics have kept global emissions from LULCC between 1 and 1.5 PgC yr-1 (if peat swamps are included) since approximately 1960. In contrast to the net emissions from LULCC, gross emissions and gross rates of uptake have generally increased (Figure 3). The gross emissions include the losses of carbon to the atmosphere from burning and decay, including the decay of soil organic matter as a result of cultivation. Gross uptake includes the accumulation of carbon in regrowing forests and in soils recovering from cultivation. Currently, gross emissions are approximately 4 PgC yr-1 globally and approximately 2.6 PgC yr-1 in the tropics. Gross rates exceed net rates
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as a result of rotational LUs; for example, the harvest of wood with subsequent forest recovery, and the repeated clearing and abandonment of shifting cultivation. The net fluxes of carbon for these rotational LUs are small relative to the offsetting gross emissions and sinks (Table 1). By comparison, the gross and net emissions from deforestation are the same; there are no sinks when forests are converted to permanent agricultural lands. Comparison of two recent studies
Key terms Management: Direct human activities; for example, clearing of forests for croplands and harvest of wood. It is synonymous with land use and landcover change in this article. In contrast to management, there are two other processes that may influence changes in terrestrial carbon storage: indirect effects (i.e., environmental factors, such as CO2, climate and nitrogen deposition) and natural effects. Distinguishing among these three processes is often difficult. Changes in carbon storage in unmanaged or natural ecosystems are not included in estimates of the land use and land-cover change flux; and the indirect and natural effects of environmental change on managed lands are also excluded to the extent possible.
A recent study by Harris et al. [16] reported gross emissions of carGross fluxes of carbon: Gross fluxes of bon that were only approximately carbon from land use and land-cover 35% of those reported by Baccini change (LULCC) include the emissions of carbon from fires associated with et al. [7] and another recent sumLULCC, decay of woody debris, mary of the world’s forests [17] (0.8 oxidation of wood products from decay -1 vs 2.3 PgC yr ). The low estimate of soil organic matter. For many types of by Harris et al. [16] was surprisLULCC, these gross emissions are largely offset by the uptake of carbon in ing because the analysis appeared growing forests and recovering soils. to be similar in many ways to the Gross and net emissions from analysis by Baccini et al. [7]. Both deforestation are equal. Gross and net emissions are very different for wood estimated the emissions of carbon harvest with forest recovery and for the from deforestation in the tropics, rotational aspects of shifting cultivation. and both used spatial data derived from satellites. If state-of-the-art estimates of carbon emissions from tropical deforestation vary by a factor of three, there is little hope for the implementation of REDD+. However, the term ‘deforestation’ was used differently by the two studies. Harris et al. used the term in the strict sense, consistent with the IPCC Good Practice Guidelines category of ‘forestland converted to other land,’ or LCC as used here [16]. Baccini et al. used the term to encompass the broader set of emissions from LULCC, consistent with much of the published literature, including the IPCC 4th Assessment Report [7]. In the parlance of the IPCC’s Good Practice Guidelines, Baccini et al. included emissions from activities occurring on ‘forestland remaining forestland’. To minimize confusion and misinterpretation, future analyses should make clear the definitions used and the implications of those definitions for broader discussions. Here the term LULCC is defined to include not only deforestation but forest degradation as well. The gross emissions from deforestation alone (0.81 PgC yr-1 [16]) are not the same as the gross emissions from LULCC (2.3 PgC yr-1 [7]), which include emissions from wood harvest and (rotational) shifting
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by the gross uptake of carbon in recovering forests and fallows. Because these rotational aspects of LU do not change forest cover, reductions in biomass density Type of management Gross emissions Gross uptake Net emissions represent forest degradation rather than deforestation. Change in forest area As mentioned above, the estimate that forest degradation accounts for only 14% of net LULCC emissions is Deforestation 0.960 0 0.960 probably an underestimate. Afforestation -0.015 -0.015 When the (net or gross) emissions from deforestaDegradation tion are compared, the estimate by Harris et al. is only Wood harvest (industrial) 0.450† -0.446 0.004‡ 16% lower than the estimate by Baccini et al. (0.81 vs Fuelwood harvest 0.230 -0.146 0.084§ 0.96 PgC yr-1, respectively) [7, 16]. In fact, if the emissions of Shifting cultivation cycle¶ 0.640# -0.558 0.082†† carbon from cultivated soils are omitted from the estimate Subtotal for degradation 1.320 -1.150 0.170 by Baccini et al., as they were in Harris et al., the two indeTotal 2.280 -1.165 1.115 pendent estimates are identical (0.81 PgC yr-1) (Figure 4) Negative values indicate carbon removed from the atmosphere. † Emissions from logging debris and wood products. [16,7]. Neither analysis included the emissions of carbon ‡ Emissions from logging debris and wood products, and uptake by recovering forests. from draining and burning of peatlands. Furthermore, § Both emissions and uptake by recovering forests. ¶ The first time a forest is cleared for shifting cultivation that clearing is deforestation. Subsequent the estimate of 0.81 PgC yr-1 is for deforestation alone, clearing of fallows is counted here as forest degradation. and not forest degradation. # Emissions from the reclearing of fallows. †† Net emissions from the reclearing and regrowth of fallows. However, the agreement of 0.81 PgC yr-1 for deforestation is fortuitous for at least two reasons. First, a regioncultivation, as well as from deforestation (Table 1). Defor- by-region comparison showed a reasonable agreement estation (in the strict sense) accounts for only 42% of only in Latin America (Baccini et al. 0.47 PgC yr-1; Harris gross emissions in the analysis by Baccini et al., but for et al. 0.44 PgC yr-1). For tropical Asia and sub-Saharan 86% of net emissions [7]. Forest degradation, by contrast, Africa the differences were nearly a factor of two, and offaccounts for 58% of gross emissions, but only 14% of setting (Baccini et al. 0.11 PgC yr-1 and 0.23 PgC yr-1, for net emissions. The gross emissions from wood harvest Asia and Africa, respectively; Harris et al. 0.26 PgC yr-1 and shifting cultivation are large and nearly balanced and 0.11 PgC yr-1, respectively). The major reason for different estimates of emissions in tropical 1.75 Asia and sub-Saharan Africa was the Global rates of deforestation used by the two 1.50 Tropics analyses. Baccini et al. used rates of deforestation (2000–2010) reported Temperate 1.25 in the 2010 Forest Resources Assessment of the FAO [13]. The 1.00 rates were obtained from country surveys. The surveys are not necessarily based on current data, and 0.75 revisions are often reported in subsequent years. The study by Harris 0.50 et al. [16], used estimates of gross forest loss obtained from satellite [18]. 0.25 While the satellite data are more up to date than those reported in the Forest Resource Assessment, they are 0.00 not necessarily a measure of deforestation [13]. The satellite data capture -0.25 deforestation, but they also capture a number of other processes that Year would not be included in the FAO’s definition of deforestation. Natural Figure 2. Annual net emissions of carbon from land use and land-cover change in tropical disturbances and clear-cut logging, regions, temperate and boreal regions, and globally. Emissions from peat swamps not for example, may both be followed included. by forest recovery, in which case they
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Table 1. Gross and net emissions of carbon (PgC yr-1) from land-use and land-cover change activities in the tropics for the period 2000–2005.
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3.0 Non-tropics emissions
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are not deforestation as defined by FAO. The reclearing of fallows for shifting cultivation may appear from space as forest loss, but the fallows are not considered forests by the FAO until they are at least 5 years old. Aside from the offsetting differences in tropical Asia and sub-Saharan Africa, a second reason why the agreement for pan-tropical deforestation emissions may be fortuitous is that the methods used by the two groups were very different. Baccini et al. included the emissions from soils [7]; Harris et al. did not [16]. Harris et al. considered the years 2000– 2005; Baccini et al., 2000–2010. Perhaps most importantly, Harris et al. calculated ‘committed’ emissions. Committed emissions assume that all of the carbon lost as a result of deforestation is lost at the time of deforestation. The estimate is obtained by multiplying the area deforested by the carbon density of the forest lost. In contrast, Baccini et al. used a bookkeeping model that accounts for lags in emissions and uptake, thus enabling an estimate of net emissions. If the emissions today from LU activities in the past are equal to the emissions delayed to future years, then the committed emissions and the actual emissions will be the same. When rates of LU are changing from year to year, this equivalence is unlikely. Thus, despite the remarkable agreement at the pan-tropical scale, that agreement does not define the precision or accuracy available for monitoring changes in terrestrial carbon storage. Although the efforts of Harris et al. [16] and Baccini et al. [7] are both notable steps forward, neither provides emissions estimates with the level of resolution or degree of certainty needed to support performance-based mechanisms, such as REDD+. Although the resolution for accounting is national or subnational, the resolution required for measurement may be tens of meters. The limitation of coarse spatial
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Figure 3. Annual gross emissions and gross uptake of carbon from tropical regions and from temperate and boreal regions. Emissions from peat swamps not included. 2.5
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Net fluxes Baccini et al. [7]
Deforestation flux Harris et al. [16]
Figure 4. Average annual gross emissions (+) and accumulations (-) of carbon from deforestation and other land-use and land-cover change activities in the tropics. Emissions from peat swamps not included.
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resolution is that it fails to recognize the heterogeneity of forest biomass and fails to account for any bias in the biomass density of deforestation. That is, deforestation may not be distributed randomly over a landscape of varying carbon densities but may occur in forests with systematically lower or higher biomass density than the average. To capture such bias, co-location of data on deforestation and degradation with data on carbon densities at the spatial and temporal resolutions of deforestation and degradation would be ideal, although perhaps not achievable with satellite data. The conclusions from a comparison of these two studies are not that a ‘consensus’ exists [19], and not that the two groups are so different that uncertainties in measurement, reporting, and verification preclude the adoption of REDD+. Rather, the data and methods to monitor, record and verify carbon emissions for REDD+ in individual countries exist, but were not used in calculating the pan-tropical estimates. The community can monitor REDD+ at the country scale, but neither Baccini et al. nor Harris et al. did it with the spatial resolution that is now available. The data requirements are, first, high-resolution deforestation maps (30 m), second, high-resolution biomass maps (250 m or less) and, third, co-location of the two maps so that the biomass of the forests actually deforested can be obtained. Unfortunately the Ice, Cloud and land Elevation Satellite providing the LiDAR data used by both studies to determine biomass is no longer in orbit. Three mechanisms for helping stabilize the concentration of CO2 in the atmosphere To limit climate change and prevent further climatic disruption requires stabilization of the concentration of CO2 and other GHGs in the atmosphere. Such a stabilization will require >90% reductions in the emissions of carbon from fossil fuels, the primary source of carbon to the atmosphere. Nevertheless, management policies affecting the area and carbon density of forests can help in the short term (~50 years), while alternatives to fossil fuels are developed, and can be beneficial for a host of other reasons as well – reasons that include sustainable development, biodiversity, energy and water resources, and other ecosystem services. Stabilization of the CO2 concentration in the atmosphere immediately would require a reduction in emissions of approximately 4 PgC yr-1 (Table 2). Approximately half of the carbon emitted each year from fossil fuel combustion (7.8 PgC yr-1) and LU change (1.0 PgC yr-1) accumulates in the atmosphere (4.0 PgC yr-1). The rest is taken up by the ocean and land. The carbon sinks in the ocean and on land are responding to the concentration of CO2 in the atmosphere. If emissions were reduced by approximately
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4 PgC yr-1, the concentration in the atmosphere would remain what it had been the year before because the ocean and land sinks would still be responding to the same (air–sea or air–land) gradients. That is, the atmospheric increase would drop to zero, while the ocean and land sinks would continue as before. Their uptake would not continue at the same rate for long, however, because the gradients with the atmosphere would decline as their pools increased. Thus, emissions would have to be reduced by more than 4 PgC yr-1 in subsequent years to keep the atmosphere at a constant concentration (i.e., stabilized). Nevertheless, the CO2 concentration would be stabilized with an immediate reduction of 4 PgC yr-1 in emissions. Can such a reduction be achieved through management of tropical forests? Potentially, yes, through three mechanisms: a halt to deforestation and forest degradation; protection of regrowing forests; and the re-establishment of forests on lands formerly forested but not intensively used now. The first two of these mechanisms are quantified by the analyses reviewed in the first part of this paper. Stopping tropical deforestation and degradation would reduce emissions by 1.4 PgC yr-1. Second, allowing secondary forests and the fallows of shifting cultivation to continue growing (no further harvesting or clearing) would take another 1–3 PgC yr-1 out of the atmosphere and store it in growing forests. The third mechanism would be to re-establish forests on lands that once supported forests, but that are now without forests. An area of 500 million ha would provide a global sink of approximately 1 PgC yr-1 if the annual accumulation of carbon in trees and soil were a modest 2 MgC ha-1 yr-1. The area required would be less if rates of accumulation were higher. The area is large, approximately half the area of the USA or half the area of China. It is not clear that this area is available in the tropics. Globally, however, the land area in crops is three-times larger, and the land area in pasture and rangelands is five-times larger, so the area for new forest is not unthinkable. It is not clear, however, Table 2. Mean global carbon budget 2000–2009. Source or sink of carbon
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Emissions Fossil fuel combustion and cement production Land-use change
7.8 ± 0.4 1.0 ± 0.5
Accumulations Atmospheric growth rate Ocean sink Residual terrestrial sink Data taken from [2].
4.0 ± 0.1 2.4 ± 0.5 2.4 ± 0.8
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where these lands are, or whether they would support forests again, even if they supported forests in the past. Many of the available lands may now be impoverished or infertile as a result of overuse. Even if they did not immediately support native forests, however, the lands might support plantations of certain species. In terms of carbon, reduced emissions or increased uptake are equivalent. If implemented all at once, the reduced emissions from these three measures (3–5 PgC yr-1) are enough to stabilize the concentration of CO2 in the atmosphere. Each of these measures has costs. The financial costs are beyond the scope of this calculation, but they would be large, as would the benefits. Allowing all fallows within the shifting cultivation cycle to regrow would require that shifting cultivation become permanent. Allowing all secondary forests to regrow would effectively preclude wood harvest for either timber or fuel wood. It might be better, in terms of carbon emissions, to use land for energy production (biofuels) other than for carbon storage [20–22]. Furthermore, the carbon sink in growing forests would diminish as forests aged. The sink might continue for 40–50 years before gradually diminishing. It offers a window during which the world might make the transition out of fossil fuels. Finding as many as 500 million ha of degraded lands in the tropics, or globally for that matter, will be a challenge, not only because the area is large, but, more importantly, because those same lands will be sought after by those seeking to increase agricultural production for a growing
number of people. Indeed, the competition for land for food, fiber, feed, fuel, carbon storage, biodiversity and other ecosystem services is going to intensify even in the absence of climatic change. Nevertheless, informed decisions about how much land to use, for what purpose, and where, are fundamental to sustainable uses of resources. Future perspective Understanding how past LULCC activities affect today’s sources and sinks of carbon on land provides perspective and knowledge for planning how lands might be used henceforth to reduce carbon emissions and increase carbon sinks. The obvious conclusion is that current tropicswide rates of deforestation and forest degradation have to be reduced, and the good news is that those rates are declining in some cases, locally and nationally. Outside the tropics, the trends were reversed decades ago, and in Brazil rates of deforestation have been declining steadily over the last 9 years [101]. Forests are accumulating carbon nearly everywhere [17], and large areas are suitable for reforestation [23]. The challenge is to provide incentives to make such carbon accumulation continue. REDD+ offers that incentive. The technical capacity for measurement, reporting, and verification will continue to improve, but it is already adequate for REDD+. The adoption of REDD+ is largely a question of political will and economics at this point. Acknowledgements The author thanks S Goetz and two anonymous reviewers for helpful comments on an early draft.
Executive summary Past sources & sinks of carbon from land use & land-cover change in the tropics 13 recent estimates of carbon emissions from land use and land-cover change (LULCC) indicate mean net emissions of 1.1 PgC yr-1 for the period 1990–2010 (~15% of total anthropogenic carbon emissions). Most of these emissions are from tropical regions, although none of the studies included emissions from tropical peat swamps. Net emissions from the tropics are likely to equal approximately 1.4 PgC yr-1 when the draining and burning of southeast Asian peatlands are included. Deforestation accounts for most of these annual net emissions. Forest degradation is estimated to account for 15–35% of LULCC emissions, but the uncertainty is large. The large difference in emissions reported by two recent studies was mostly the result of different definitions and measurement of deforestation. With similar definitions the two studies agreed within ±0.15 PgC yr-1 in all regions. A combination of field measurements and satellite data is capable right now of meeting measurement, reporting and verification requirements for REDD+. Improvements will come from co-locating rates of deforestation and degradation with aboveground carbon densities at temporal and spatial resolutions similar to those of LULCC activities (tens of meters). Future sources & sinks of carbon from LULCC Although combustion of fossil fuels accounts for approximately 90% of current carbon emissions worldwide, the concentration of carbon dioxide in the atmosphere could be stabilized immediately through massive efforts of forest management in the tropics. A halt to deforestation and forest degradation would reduce emissions by 1.4 PgC yr-1. Allowing secondary forests to regrow could remove 1–3 PgC yr-1 from the atmosphere. Reforesting 500 million ha of forest on lands that were once forested and not intensively used for food production could remove another 1 PgC yr-1. Together, these management efforts could reduce current emissions by 3–5 PgC yr-1 and stabilize the carbon content of the atmosphere long enough (~50 years) for low-carbon energy technologies to replace fossil fuels.
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Financial & competing interests disclosure Support for the preparation of this paper was generously provided by the Woods Hole Research Center (MA, USA). The author has no other relevant affiliations or financial involvement with any organization or
entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.
References Papers of special note have been highlighted as: of interest of considerable interest
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