Deforestation contributes 6-17% of global anthropogenic CO 2 emissions to the atmosphere 1 . Large uncertainties in emission estimates arise from inadequate data on the carbon density of forests 2 and the regional rates of deforestation. Consequently there is an urgent need for improved data sets that characterize the global distribution of aboveground biomass, especially in the tropics. Here we use multi-sensor satellite data to estimate aboveground live woody vegetation carbon density for pan-tropical ecosystems with unprecedented accuracy and spatial resolution. Results indicate that the total amount of carbon held in tropical woody vegetation is 228.7 Pg C, which is 21% higher than the amount reported in the Global Forest Resources Assessment 2010 (ref. 3). At the national level, Brazil and Indonesia contain 35% of the total carbon stored in tropical forests and produce the largest emissions from forest loss. Combining estimates of aboveground carbon stocks with regional deforestation rates 4 we estimate the total net emission of carbon from tropical deforestation and land use to be 1.0 Pg C yr −1 over the period 2000-2010-based on the carbon bookkeeping model. These new data sets of aboveground carbon stocks will enable tropical nations to meet their emissions reporting requirements (that is, United Nations Framework Convention on Climate Change Tier 3) with greater accuracy.When forests are cleared, carbon stored above and below ground in leaves, branches, stems and roots is released to the atmosphere. As a consequence, forest clearing, especially in the tropics, is a major source of CO 2 to the atmosphere. Although the proportion of carbon stored in forests comprises 70-80% of total terrestrial carbon 5 , the spatial and temporal variability in carbon storage is substantial 6 . This variability arises from natural and anthropogenic disturbances, as well as differences in stand age, topography, soils and climate. Globally, soils hold two to three times more carbon than that stored above ground in forest vegetation, but with the exception of cultivation, peatland fires and thawing permafrost, much of the carbon in soils is physically and chemically protected and not easily oxidized 7 . In contrast, carbon stored in aboveground biomass is readily mobilized by disturbance processes such as fire, wind throw, pest outbreaks and land conversion 8 .Efforts to quantify the amount of carbon stored in aboveground biomass over large areas of the tropics have been fraught with uncertainty. For example, estimates of aboveground carbon storage in tropical African forests vary by over ref. 9). In turn, the lack of reliable estimates of forest carbon storage introduces large uncertainties into estimates of terrestrial carbon emissions 10-14 . In Amazonia, recent studies have suggested

The carbon balance of tropical ecosystems remains uncertain, with top-down atmospheric studies suggesting an overall sink and bottom-up ecological approaches indicating a modest net source. Here we use 12 years (2003 to 2014) of MODIS pantropical satellite data to quantify net annual changes in the aboveground carbon density of tropical woody live vegetation, providing direct, measurement-based evidence that the world's tropical forests are a net carbon source of 425.2 ± 92.0 teragrams of carbon per year (Tg C year). This net release of carbon consists of losses of 861.7 ± 80.2 Tg C year and gains of 436.5 ± 31.0 Tg C year Gains result from forest growth; losses result from deforestation and from reductions in carbon density within standing forests (degradation or disturbance), with the latter accounting for 68.9% of overall losses.

Mapping and monitoring carbon stocks in forested regions of the world, particularly the tropics, has attracted a great deal of attention in recent years as deforestation and forest degradation account for up to 30% of anthropogenic carbon emissions, and are now included in climate change negotiations. We review the potential for satellites to measure carbon stocks, specifically aboveground biomass (AGB), and provide an overview of a range of approaches that have been developed and used to map AGB across a diverse set of conditions and geographic areas. We provide a summary of types of remote sensing measurements relevant to mapping AGB, and assess the relative merits and limitations of each. We then provide an overview of traditional techniques of mapping AGB based on ascribing field measurements to vegetation or land cover type classes, and describe the merits and limitations of those relative to recent data mining algorithms used in the context of an approach based on direct utilization of remote sensing measurements, whether optical or lidar reflectance, or radar backscatter. We conclude that while satellite remote sensing has often been discounted as inadequate for the task, attempts to map AGB without satellite imagery are insufficient. Moreover, the direct remote sensing approach provided more coherent maps of AGB relative to traditional approaches. We demonstrate this with a case study focused on continental Africa and discuss the work in the context of reducing uncertainty for carbon monitoring and markets.

Regrowing natural forests is a prominent natural climate solution, but accurate assessments of its potential are limited by uncertainty and variability around carbon accumulation rates. To assess why and where rates differ, we compiled 13,112 georeferenced measurements of carbon accumulation. Climate explained variation in rates better than land use history, so we combined field data with 66 environmental covariate layers to create a global, 1-km resolution map of potential aboveground carbon accumulation rates for the first 30 years of forest regrowth. Our results indicate that on average default forest regrowth rates from the Intergovernmental Panel on Climate Change are underestimated by 32% and miss 8-fold variation within ecozones.Conversely, we conclude that previously reported maximum climate mitigation potential from natural forest regrowth is overestimated by 11% due to the use of overly high rates. Our results therefore provide a much needed and globally consistent method for assessing natural forest regrowth as a climate mitigation strategy. BackgroundTo constrain global warming, we must reduce emissions and capture excess carbon dioxide (CO2) in the atmosphere 1,2 . Restoring forest cover, defined here as the transition from < 25% tree cover to > 25% tree cover where forests historically occurred, is a promising option for additional carbon capture 3 and has been prioritized in many national and international goals 4,5 . It is deployable, scalable, and provides important biodiversity and ecosystem services 6 . Yet the magnitude and distribution of climate mitigation opportunity from restoring forest cover is poorly described, with large confidence intervals around estimates 2,3 . To evaluate the appropriateness of forest cover restoration for climate mitigation, compared to the multitude of other potential climate mitigation actions, countries, corporations, and multilateral entities need more accurate assessments of its potential 7 .Mitigation potential from restoring forest cover (reported here in terms of MgCO2 yr -1 ) is determined by the potential extent and location of new forest ("area of opportunity") and the rate at which those forests remove atmospheric CO2 (reported here in terms of MgC ha -1 yr -1 ). While there are now multiple estimates of area of opportunity based on diverse and often heavily debated criteria (e.g., references 3,8-11 ), we lack spatially explicit and globally comprehensive estimates of accumulation rates. This is especially true for natural forest regrowth, defined here as the recovery of forest cover on deforested lands through spontaneous regrowth after cessation of prior disturbance or land use. Many countries do not have nationally specific forest carbon accumulation rates and instead rely on default rates from the Intergovernmental Panel on Climate Change (IPCC) 12,13 . Although these rates were recently updated 8,12 , they nonetheless represent coarse estimates based on continent and ecological zone, and do not account for finer scale variation in rates due to mor...

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Halving carbon emissions from tropical deforestation by 2020 could help bring the international community closer to the agreed goal of <2 degree increase in global average temperature change and is consistent with a target set last year by the governments, corporations, indigenous peoples' organizations and non‐governmental organizations that signed the New York Declaration on Forests (NYDF). We assemble and refine a robust dataset to establish a 2001–2013 benchmark for average annual carbon emissions from gross tropical deforestation at 2.270 Gt CO 2 yr−1. Brazil did not sign the NYDF, yet from 2001 to 2013, Brazil ranks first for both carbon emissions from gross tropical deforestation and reductions in those emissions – its share of the total declined from a peak of 69% in 2003 to a low of 20% in 2012. Indonesia, an NYDF signatory, is the second highest emitter, peaking in 2012 at 0.362 Gt CO 2 yr−1 before declining to 0.205 Gt CO 2 yr−1 in 2013. The other 14 NYDF tropical country signatories were responsible for a combined average of 0.317 Gt CO 2 yr−1, while the other 86 tropical country non‐signatories were responsible for a combined average of 0.688 Gt CO 2 yr−1. We outline two scenarios for achieving the 50% emission reduction target by 2020, both emphasizing the critical role of Brazil and the need to reverse the trends of increasing carbon emissions from gross tropical deforestation in many other tropical countries that, from 2001 to 2013, have largely offset Brazil's reductions. Achieving the target will therefore be challenging, even though it is in the self‐interest of the international community. Conserving rather than cutting down tropical forests requires shifting economic development away from a dependence on natural resource depletion toward recognition of the dependence of human societies on the natural capital that tropical forests represent and the goods and services they provide.

Better land stewardship is needed to achieve the Paris Agreement's temperature goal, particularly in the tropics, where greenhouse gas emissions from the destruction of ecosystems are largest, and where the potential for additional land carbon storage is greatest. As countries enhance their nationally determined contributions (NDCs) to the Paris Agreement, confusion persists about the potential contribution of better land stewardship to meeting the Agreement's goal to hold global warming below 2°C. We assess cost-effective tropical country-level potential of natural climate solutions (NCS)—protection, improved management and restoration of ecosystems—to deliver climate mitigation linked with sustainable development goals (SDGs). We identify groups of countries with distinctive NCS portfolios, and we explore factors (governance, financial capacity) influencing the feasibility of unlocking national NCS potential. Cost-effective tropical NCS offers globally significant climate mitigation in the coming decades (6.56 Pg CO 2 e yr −1 at less than 100 US$ per Mg CO 2 e). In half of the tropical countries, cost-effective NCS could mitigate over half of national emissions. In more than a quarter of tropical countries, cost-effective NCS potential is greater than national emissions. We identify countries where, with international financing and political will, NCS can cost-effectively deliver the majority of enhanced NDCs while transforming national economies and contributing to SDGs. This article is part of the theme issue ‘Climate change and ecosystems: threats, opportunities and solutions’.