Science 22 June 2012:
Vol. 336 no. 6088 pp. 1573-1576
DOI:10.1126/science.1217962
Report

Baseline Map of Carbon Emissions from Deforestation in Tropical Regions

Nancy L. Harris, Sandra Brown, Stephen C. Hagen, Sassan S. Saatchi, Silvia Petrova, William Salas, Matthew C. Hansen, Peter V. Potapov, Alexander Lotsch | 6 Comments

Tropical deforestation and degradation across three continents led to ~0.8 petagrams of yearly carbon emissions from 2000 to 2005.

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RED, not REDD

Removal os trees creates a scar in the landscape, which can be detected from satellites. Relatively large scars of at least 30 x 30 m were the focus of N. L. Harris et al. as they estimated gross carbon emissions in their Report “Baseline Map of Carbon Emissions from Deforestation in Tropical Regions” (22 June, p. 1573). Their new results are independent of government data and provide observations which can be tested against ground measurements. However, a caveat is needed regarding “the second D” of REDD+ (Reduced Emissions from Deforestation and forest Degradation). This caveat is important, because loss of individual trees within canopy, and the gradual growth and accumulation of biomass dominate the process of carbon transfers between forests and the atmosphere (1). Furthermore, comparisons with previously published estimates (2,3) are highly misleading because of incompatible definitions of land-cover change and accounting for different carbon pools. Harris et al. (2012) consider only changes in canopy cover whereas most other studies include forest degradation with the broader term “land-cover change”. Even though the new method is not sufficient for estimating carbon loss from degradation, or sequestration of C in regrowing forests in the tropical nations, and is therefore of limited use as a baseline for carbon assessment, it can be more directly applied to determining and locating losses of biodiversity or the deterioration of the flood protection capacity of forested landscapes. These ecosystem services relate closely to losses and gains of canopy cover, which now can be monitored with this new method. Unfortunately, change of forest density cannot yet be measured solely from satellite data. Both satellite observations and ground measurements are needed to assess REDD+, which implies that governments and/or other relevant domestic partners need to be engaged in monitoring forest dynamics and encouraged to maintain transparent monitoring systems that can at least be partially validated remotely.

Pekka E. Kauppi and Richard A. Birdsey

References 1. A. Rautiainen, I. Wernick, P.E. Waggoner, J.H. Ausubel, P.E. Kauppi, PLoS ONE 6, 19577 (2011). 2. A. Baccini et al. Nature Clim. Change 2, 182 (2012). 3. Y. Pan et al. Science 333, 988 (2011).

Submitted on Mon, 09/03/2012 - 03:36

Assessing deforestation emissions is a challenging task because deforestation rates, biomass loads, litter and soil carbon stocks, and post-deforestation land use are highly heterogeneous. By merging the best available data derived from satellite data on deforestation rates and biomass loads, Harris and colleagues have provided a new estimate of emissions from forest loss.

It is important to realize that the numbers presented in their paper exclude potential emissions from soils, from coarse woody debris, and from stems smaller than 10 cm in diameter. Soil carbon losses are assessed for the first 2.5 years in the supplementary materials, but not included in the 0.81 petagram of carbon per year estimate. Neither are they in the uncertainty estimate. Carbon losses from the burning or decay of course woody debris and stems smaller than 10 cm diameter were also excluded. This is acknowledged in the supplementary materials, but not taken into account in their best estimate. Neither of these sources are very large, but combined they can add up to a significant amount of carbon in the order of 10-30%.

Another critical point is that post deforestation land use varies between and within countries. Conversion to mechanized agriculture in the southern Amazon for soy cultivation, for example, results in more complete removal of biomass and thus higher estimates than assessed by Harris and colleagues than clearing for small-scale agriculture more typical in other regions. Because of this regional variability, it is not possible to compare their country-level estimates directly.

Whether including the abovementioned carbon sources and deforestation processes will increase (when including neglected carbon sources) or decrease (in case not all live biomass is lost) actual emissions estimates remains to be seen, but by not assessing these processes the reported uncertainty range is biased low. Due to the lack of key processes and simplifying assumptions, care should be taken with using the new data as a baseline map from which potential carbon savings are derived or as comparison with other published estimate.

Submitted on Thu, 08/09/2012 - 10:04

Biomass burning carbon emissions transitioning away from deforestation J. E. Ten Hoeve, A. L. Correia, L. A. Remer, and M. Z. Jacobson

Estimates of baseline forest carbon emissions are needed to guide international programs aimed at reducing deforestation, such as the Reducing Emissions from Deforestation and Forest Degradation (REDD+) program. Harris et al. (1) estimated global deforestation carbon emissions using remotely sensed estimates of forest carbon stocks and deforested areas. Yet, Harris et al., as well as many other studies quantifying baseline biomass burning carbon emissions (BBCE) to inform policy, neglect emissions from savanna, grassland, and agricultural burnings, which have increased substantially in the Amazon Basin during the latter 2000s and are a large component of total BBCE globally (2,3).

Recently, a dramatic shift in Amazon burning from forest to savanna/agriculture from 2002 to 2010 was identified (2). The forest to savanna/agricultural fire detection ratio decreased from 0.58 during 2002-2006 to 0.29 during 2007-2010. The decline in forest fires was attributed to enhanced forest law enforcement and reduced deforestation following the 2005 drought (4), and the increase in savanna/agricultural fires to drought conditions and increased agricultural and pastoral development on already-degraded land rather than newly-deforested land, a trend also shown in other studies (5). Agricultural production has increased over the same period despite large cuts in deforestation, pointing to a shift in agricultural practices. Yearly burning of savanna/agriculture increases carbon emissions since natural savanna regrowth ranges from 2-12 years (6).

Van der Werf et al. (3) estimated that savanna/woodland/agricultural burnings corresponded to 40% of total South American BBCE over 1997-2009 and 65% of total worldwide BBCE. Because carbon emissions from all land covers contribute to global warming and regional air pollution, we point out that burning of all biomes should be accounted for to achieve an effective environmental policy aimed at reducing BBCE.

References 1. N. L. Harris et al. Science 336, 1573 (2012). 2. J. E. Ten Hoeve, L. A. Remer, A. L. Correia, M. Z. Jacobson, Environ. Res. Lett. 7, (2012). 3. G. R. van der Werf et al. Atmos. Chem. Phys. 10, 11707 (2010). 4. D. Nepstad et al. Science 326, 1350 (2009). 5. M. N. Macedo et al. Proc. Natl. Acad. Sci. U.S.A. 109, 1341 (2012). 6. M. Z. Jacobson J. Climate 17, 2909 (2004)

Submitted on Thu, 08/02/2012 - 11:15

A table included in my first comment was scrambled; I provide the data here in text format to help illustrate the relative contributions of deforestation and forest degradation to carbon emissions. The main point is that the gross emissions of carbon from deforestation estimated by Harris et al. (0.81 PgC yr-1) (1) are comparable to the emissions estimated previously by Baccini et al. for the period 2000-2005 (0.96 PgC yr-1) (2). Their value is not 35% of the previous estimate (0.81 vs. 2.28 PgC yr-1), but 84% (0.81 vs. 0.96 PgC yr-1). The value of 2.28 PgC yr-1 corresponds to the gross emissions not only from deforestation (0.96 PgC yr-1) but from degradation (1.32 PgC yr-1) as well (the two ‘D’s in REDD). Note that net and gross emissions are not related in a simple way. For deforestation they are equivalent; there is no carbon uptake associated with deforestation. In contrast, the gross emissions from forest degradation (1.32 PgC yr-1) are largely offset by the uptake of carbon in re-growing forests (-1.15 PgC yr-1), for a net emission of 0.17 PgC yr-1. The gross emissions from degradation are larger than the emissions from deforestation (0.96 PgC yr-1), while the net emissions (0.170 PgC yr-1) are smaller. The activities contributing to degradation (defined as the reduction of carbon density within forests) include industrial wood harvest (0.450 and 0.004 PgC yr-1 for gross and net emissions, respectively), fuelwood harvest (0.230 and 0.084 PgC yr-1), and shifting cultivation (0.640 and 0.082 PgC yr-1). For completeness, the net annual emissions from deforestation and degradation 2000-2005 should also include the net uptake of carbon from afforestation (-0.015 PgC yr-1), for total net emissions of 1.115 PgC yr-1 (2). The values here may vary slightly from those presented in Baccini et al. (2) because they refer to different time intervals. A table is worth 367 words in this case. References 1. Harris, N.L., S. Brown, S.C. Hagen, S.S. Saatchi, S. Petrova, W. Salas, M.C. Hansen, P.V., Potapov, A. Lotsch. Science 336:1573-1576. 2. Baccini, A., S.J. Goetz, W.S. Walker, N.T. Laporte, M. Sun, D. Sulla-Menashe, J. Hackler, P.S.A. Beck, R. Dubayah, M.A. Friedl, S. Samanta, and R.A. Houghton. 2012. Nature Climate Change 2:182-185; doi:10.1038/nclimate1354.

Submitted on Wed, 08/01/2012 - 08:28

That the Harris et al. estimate of gross emissions was approximately 30% of the Baccini et al. estimate (2) seems due to definitional issues: the term “deforestation” is used differently by the two studies. Harris et al. (1) use the term in the strict sense, consistent with the IPCC Good-Practice-Guidelines category of “forestland converted to other land.” Baccini et al (2) use the term to encompass the broader set of emissions from land use and land-cover change (LULCC), consistent with much of the published literature, including the IPCC 4th assessment report. In the parlance of the IPCC’s Good Practice Guidelines, Baccini et al. include emissions from activities occurring on “forestland remaining forestland”. Both definitions are valid and accepted in the published literature. To minimize confusion and misinterpretation, future analyses should take pains to present both the definitions used and the implications of those definitions for broader discussions. Far from supporting the notion that the previous estimate of carbon emissions was overestimated by a factor of 3, the study by Harris et al. suggests that the two independent estimates (1 and 2) are remarkably consistent. Reliable benchmarks of emissions from tropical deforestation are central to the integrity of “pay for performance” REDD+ mechanisms, and, as Zarin (3) indicates, the science is getting close to providing unbiased assessments of emissions reductions. However, more work is needed. Although the efforts of Harris et al. and Baccini et al. are both notable steps forward, neither provides emissions estimates with the level of resolution or degree of certainty needed to support performance based mechanisms. Co-location of data on deforestation and disturbance with carbon densities at the spatial and temporal resolutions of disturbance seems an essential next step. References 1. Harris, N.L., S. Brown, S.C. Hagen, S.S. Saatchi, S. Petrova, W. Salas, M.C. Hansen, P.V., Potapov, A. Lotsch. Baseline map of carbon emissions from deforestation in tropical regions. Science 336:1573-1576. 2. Baccini, A., S.J. Goetz, W.S. Walker, N.T. Laporte, M. Sun, D. Sulla-Menashe, J. Hackler, P.S.A. Beck, R. Dubayah, M.A. Friedl, S. Samanta, and R.A. Houghton. 2012. Estimated carbon dioxide emissions from tropical deforestation improved by carbon-density maps. Nature Climate Change 2:182-185; doi:10.1038/nclimate1354. 3. Zarin, D.L. 2012. Carbon from tropical deforestation. Science 336:1518-1519.

Submitted on Fri, 07/27/2012 - 15:03

The recent paper by Harris et al. (1) is similar in many ways to the analysis by Baccini et al. (2). Both estimated the emissions of carbon from deforestation in the tropics, and both used spatial data derived from satellites. It is, therefore, surprising that the Harris et al. estimate of gross emissions was approximately 30% of the earlier estimate (2). The explanation for the difference lies in the ‘apples to oranges’ comparison made by Harris et al. (1). Their gross emissions from deforestation (0.81 PgC yr-1) (1) are not comparable to the gross emissions from land use and land-cover change (LULCC) (2.2 PgC yr-1) (2), which include emissions from wood harvest and shifting cultivation as well as from deforestation. If gross emissions from deforestation, alone, are compared, the new estimate (1) is only 16% lower than the previous estimate (2) (0.81 vs. 0.96 PgC yr-1, respectively) (Table 1). The emissions from wood harvest and shifting cultivation represent forest degradation (the second ‘D’ in REDD) rather than deforestation because logged and fallow forests are, nevertheless, forests. Deforestation (in the strict sense) accounts for 86% of net emissions, by these estimates, but for only 42% of gross emissions.

Table 1. Gross and net emissions of carbon (PgC yr-1) from LULCC activities in the tropics for the period 2000-2005. Values may vary slightly from those cited in (2) because of different dates. Gross emissions Net emissions Deforestation 0.960 0.960 Afforestation -0.015 Wood harvest (industrial) 0.4501. 0.0042. Fuelwood harvest 0.230 0.0843. Shifting cultivation 0.6404. 0.0825. Sub-total for degradation 1.320 0.170 Total 2.280 1.115

1. Emissions from logging debris and wood products 2. Emissions from logging debris and wood products, and uptake by recovering forests 3. Both emissions and uptake by recovering forests 4. Emissions from the re-clearing of fallows 5. Net emissions from the re-clearing and regrowth of fallows

References 1. Harris, N.L., S. Brown, S.C. Hagen, S.S. Saatchi, S. Petrova, W. Salas, M.C. Hansen, P.V., Potapov, A. Lotsch. Science 336:1573-1576. 2. Baccini, A., S.J. Goetz, W.S. Walker, N.T. Laporte, M. Sun, D. Sulla-Menashe, J. Hackler, P.S.A. Beck, R. Dubayah, M.A. Friedl, S. Samanta, and R.A. Houghton. 2012. Nature Climate Change 2:182-185; doi:10.1038/nclimate1354.

Submitted on Fri, 07/27/2012 - 14:34