New global carbon map for 2.5 billion ha of forests

The benchmark carbon map published in the Proceedings of the National Academy of Sciences in 2011 was produced using a combination of field-based measurements and satellite remote sensing data. The research team, led by Sassan Saatchi of NASA's Jet Propulsion Laboratory, synthesised data from 4,079 forest plot sites distributed across the tropics with satellite-derived measurements of forest structure to produce spatially explicit estimates of carbon stocks at a 1-kilometre resolution.

The satellite data used in the study included microwave radar observations from instruments that can penetrate forest canopies and provide information on vegetation structure and biomass. These were combined with LiDAR data from NASA's ICESAT satellite, which measured the height of forest canopies at thousands of locations across the tropics, providing a critical calibration dataset for the radar-based biomass estimates.

Ground-based plot data was gathered from a global network of forest inventory sites, representing diverse forest types from the Amazon basin to the Congo basin to the forests of Southeast Asia. This data provided direct measurements of tree diameter, height, and species composition that could be converted to biomass estimates using established allometric equations. Combining field data with remote sensing allowed the researchers to extend plot-level measurements across the entire 2.5 billion hectare study area.

The result was the first truly comprehensive spatial estimate of carbon stocks across the tropical forest biome, providing a baseline against which future changes in forest cover and carbon stocks could be measured. The map was made publicly available to support countries developing REDD+ programs and researchers studying forest carbon dynamics.

Tropical forests store exceptional quantities of carbon because of the combination of high biomass per tree, high stem density, and the persistence of large, old trees that accumulate carbon over long lifespans. Tropical trees often grow to massive dimensions, with individual trees containing tonnes of carbon in their trunks, branches, and roots. The warm, moist climate of tropical regions supports rapid photosynthesis and plant growth, allowing forests to absorb carbon from the atmosphere at high rates.

Below-ground carbon storage is also substantial in tropical forests. Root systems, woody debris, and soil organic matter collectively store significant quantities of carbon, adding to the above-ground biomass measured in forest inventories. In some tropical forest types — particularly peat swamp forests and montane forests with cool, moist conditions that slow organic matter decomposition — below-ground carbon can exceed above-ground carbon stocks.

The diversity of tree species in tropical forests also contributes to carbon storage. Different species have different wood densities, growth forms, and longevities, and the mixture of species in a tropical forest means that carbon is stored in many different forms and at many different rates. High-diversity forests have been found to store more carbon per hectare than lower-diversity forests, suggesting that biodiversity itself contributes to the carbon storage capacity of tropical ecosystems.

Old-growth tropical forests, which have never been logged or significantly disturbed, typically store more carbon than forests that have been selectively logged or have regenerated after disturbance. The presence of large, old trees — sometimes called "carbon giants" — is a key determinant of carbon storage. These trees, while few in number, contribute disproportionately to total forest carbon, making the protection of old-growth forests from logging a high priority for climate mitigation.

Carbon maps such as the one produced by Saatchi and colleagues in 2011 are fundamental tools for REDD+ implementation because they provide the spatial information needed to estimate how much carbon would be at risk if a given area of forest were converted or degraded. Without spatially explicit carbon data, governments and project developers cannot calculate the emission reductions that their conservation activities are achieving, which is a prerequisite for receiving results-based payments under REDD+.

For national governments, carbon maps help prioritise conservation investments by identifying where the highest carbon stocks are located relative to deforestation threats. Areas with very high carbon density that are also under significant clearing pressure are the highest priority for protection from a climate mitigation perspective. Combining carbon data with deforestation threat maps allows countries to design their REDD+ strategies to focus resources where the emission reductions achieved per dollar spent are greatest.

At the project level, carbon maps provide the baseline data needed to establish a credible reference scenario against which emission reductions are measured. Project developers must demonstrate that the forests they are protecting would have been cleared in the absence of their intervention, and that the carbon stored in those forests is accurately quantified. Carbon maps, combined with ground-based verification, provide the evidence base for these claims.

The public availability of carbon mapping data has been an important democratising force in REDD+ implementation, allowing civil society organisations, indigenous communities, and smaller countries without sophisticated monitoring capacity to access information about their forest carbon stocks. This transparency also creates accountability, as it becomes harder for governments or companies to make misleading claims about forest carbon when independent spatial data is publicly available.

The finding that Latin American forests account for approximately half of all tropical forest carbon, followed by Southeast Asia and Africa, has significant implications for international climate policy. It means that the countries of the Amazon basin — Brazil in particular, which holds nearly a quarter of tropical forest biomass — have an outsized importance in determining whether REDD+ can deliver meaningful emission reductions.

Brazil's dominant position in the tropical carbon stock makes the effectiveness of its deforestation control policies a matter of global significance. The dramatic reduction in Amazon deforestation that Brazil achieved between 2004 and 2012 is therefore one of the most important land-use conservation achievements in history from a climate perspective. Conversely, increases in Brazilian deforestation rates have global consequences that extend far beyond the country's borders.

The concentration of tropical forest carbon in a relatively small number of countries — with the top five (Brazil, Democratic Republic of Congo, Indonesia, Peru, and Colombia) accounting for more than half of the total — means that international REDD+ finance can be strategically targeted to have maximum impact. Bilateral forest deals between major donor countries and these high-carbon forest nations represent a potentially cost-effective approach to climate mitigation.

At the same time, the African Congo basin's large carbon stocks have received comparatively little international attention and finance compared to the Amazon and Indonesian forests. As pressure on Congo basin forests increases with population growth and economic development, ensuring that the forest countries of Central and West Africa have access to REDD+ finance and technical support will become increasingly important for maintaining global forest carbon stocks.

The 2011 Saatchi et al. carbon map represented a major advance in the state of forest carbon science, but the field has continued to progress rapidly since its publication. The launch of new satellite missions with improved sensors, expanded networks of ground-based monitoring plots, and the development of machine learning algorithms for interpreting remote sensing data have all contributed to more accurate and higher-resolution carbon stock estimates.

The NASA GEDI (Global Ecosystem Dynamics Investigation) mission, launched in 2018, provided the first global LiDAR data for forests, enabling direct measurement of canopy height and structure from space. LiDAR provides much more direct information on forest biomass than the radar and passive optical sensors used in earlier studies, and GEDI data has been used to produce improved global carbon maps with finer spatial resolution and reduced uncertainty.

The ESA's BIOMASS mission, designed specifically to measure forest carbon from space using P-band synthetic aperture radar, will provide further advances when its data becomes available. P-band radar can penetrate dense forest canopies that shorter-wavelength sensors struggle with, enabling more accurate measurement of biomass in the high-density forests where current estimates carry the greatest uncertainty.

On the ground, expanded networks of permanent forest inventory plots — including the Amazon Forest Inventory Network (RAINFOR), the African Tropical Rainforest Observation Network (AfriTRON), and the CTFS-ForestGEO network — have continued to accumulate long-term biomass data that provides the calibration and validation foundation for satellite-based estimates. Together, these advances are moving forest carbon science toward a level of precision that can support the robust accounting needed for results-based payments under REDD+ and Article 6 of the Paris Agreement.