# Satellite observations reveal the complex annual dynamics of tropical aboveground carbon

Yu Feng, Philippe Ciais

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## To cite this version:

Yu Feng, Philippe Ciais. Satellite observations reveal the complex annual dynamics of tropical above-ground carbon. Nature Climate Change, 2024, 10.1038/s41558-024-02119-7. hal-04720877

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Submitted on 3 Oct 2025

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# Satellite observations reveal the complex annual dynamics of tropical aboveground carbon

Tropical aboveground carbon is a crucial yet complex component of the terrestrial carbon budget. Here, remote observations reveal annual losses (from fire emissions and forest disturbances) and post-loss recovery of tropical aboveground carbon for 2010–2020, which overall resulted in tropical lands being a moderate carbon sink.

**This is a summary of:**

Feng, Y. et al. Global patterns and drivers of tropical aboveground carbon changes.

Nat. Clim. Change https://doi.org/10.1038/s41558-024-02115-x (2024).

**Publisher's note**

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Published online: 23 September 2024

## The problem

Contemporary estimates of tropical land carbon changes often rely on broad assumptions, sparse ground data and/or inaccurate estimation methods<fnref n="1" />, leading to uncertainties in identifying tropical land carbon sources and sinks. L-band vegetation optical depth (L-VOD) measurements from the Soil Moisture and Ocean Salinity (SMOS) satellite provide a proxy for the assessment of aboveground carbon (AGC) dynamics<fnref n="2" />. However, as L-VOD-based AGC measurements have a spatial resolution of 25 km, they miss losses and gains that occur within individual grid cells, which forms an obstacle to the effective management and conservation of tropical ecosystems in alignment with climate change goals.

## The discovery

We integrated passive microwave and optical satellite observations to create annual maps of AGC stocks, and the corresponding change processes across the tropics for 2010–2020. Specifically, we derived annual net changes in AGC stocks at a spatial resolution of 25 km using L-VOD data to capture the dynamics of all terrestrial vegetation, and then calculated high-resolution (30–500 m) estimates of gross AGC losses in each grid cell, separated into Landsat-based non-fire forest disturbances (deforestation and forest degradation without fires) and Moderate Resolution Imaging Spectroradiometer (MODIS)-based fire emissions (in both forests and non-forests). The difference between net AGC changes and gross AGC loss was defined as the annual AGC ‘residual change’ in each grid cell, which was generally an AGC gain representing recovery from past disturbances. We then quantified the spatial and temporal dynamics of AGC losses and gains and associated governing factors.

We found that gross annual losses in AGC from 2010–2020 amounted to –1.79 PgC yr⁻<fnref n="1" /> (Fig. 1a), with fire emissions accounting for 62% of this loss and non-fire forest degradation and deforestation accounting for 32%. Notably, at the biome scale, non-fire losses in moist broadleaf forests (–0.55 PgC yr⁻<fnref n="1" />) and fire emissions in grasslands, savannas and shrublands (–0.80 PgC yr⁻<fnref n="1" />, collectively) are the major contributors to overall losses. The positive net change in AGC (0.21 ± 0.06 PgC yr⁻<fnref n="1" />) indicates that tropical lands have acted as a moderate carbon sink of AGC during the last decade (Fig. 1b,c), mostly in African non-forested regions

and southeast Brazil (Fig. 1c). By deducting AGC losses from the AGC net changes, we derived a residual AGC gain of 2.01 ± 0.06 PgC yr⁻<fnref n="1" /> from 2010 to 2020. AGC gains had a similar pattern to AGC losses, implying that pixels characterized by greater AGC losses were also typically associated with higher AGC gains, indicating recovery from losses induced by previous disturbances. Using a boosted regression tree model, we further found that fire radiative power was the most important factor governing the spatial variability of AGC gains (representing more than 40% of the importance to model AGC gains). Change in soil moisture emerged as the principal determinant in explaining the trends in AGC gains, although this factor contributed only 10% of the importance.

## The implications

This study offers a nuanced spatial and temporal quantification of AGC changes throughout the tropics, detailing both losses and gains and associated governing factors. Such quantification is crucial in minimizing uncertainties in land carbon sinks and sources, and is paramount for better understanding regional and global carbon budgets. It is imperative to reduce ongoing losses to guarantee the longevity of carbon-rich ecosystems. Additionally, this study highlights the dominant roles of anthropogenic and hydroclimatic determinants in orchestrating tropical land carbon dynamics.

There are some limitations to our study. Our dataset spans only a single decade (2010–2020), which hinders the analysis of long-term AGC dynamics. Furthermore, although the study covers most terrestrial tropical ecosystems, it fails to capture AGC dynamics in flooded regions and some carbon-rich regions such as mainland Southeast Asia due to limitations in available L-VOD observations. These limitations could result in uncertainties in the quantification of the tropical land carbon budget.

In future work, long-term data from fused SMOS and Soil Moisture Active Passive (SMAP) L-band observations³ could provide a more reliable framework for better understanding the dynamics of AGC and its response to climate change and anthropogenic activities.

Yu Feng<fnref n="1" />,<fnref n="2" /> & Philippe Ciais<fnref n="2" />

<footnote n="1">¹Eastern Institute of Technology, Ningbo, China.</footnote>

<footnote n="2">²Laboratoire des Sciences du Climat et de l'Environnement, Gif-sur-Yvette, France.</footnote>
## EXPERT OPINION

> "This study makes a significant contribution to furthering the understanding of carbon dynamics in all ecosystems of tropical regions. By combining observations from L-band passive microwave and high-resolution optical satellites, this work

> provides insights into the anthropogenic and natural factors influencing tropical aboveground carbon changes on a yearly scale from 2010 to 2020." Graciela Tejada Pinell, Geonoma, São Paulo, Brazil.

## FIGURE

<figure><img src="image_4.png" /><figcaption>**Fig. 1** | Tropical AGC changes from 2010 to 2020. a,b, Temporal patterns of AGC loss components (a) and total losses, gains and net changes (b) across all surveyed areas shown in c. The AGC losses are derived from forest disturbance and fire emissions data; AGC net changes are derived from 18 L-VOD estimates. The solid lines indicate the mean and shaded areas represent ±1 s.d. of the 18 estimates. c, Spatial patterns of AGC net changes. © 2024, Feng, Y. et al.</figcaption></figure>

## REFERENCES

1. Harris, N. L. et al. Baseline map of carbon emissions from deforestation in tropical regions. *Science* **336**, 1573–1576 (2012).
This paper reports satellite-derived estimates of gross carbon emissions due to deforestation across tropical regions.

2. Fan, L. et al. Satellite-observed pantropical carbon dynamics. *Nat. Plants* **5**, 944–951 (2019).
This paper reports aboveground biomass carbon changes derived from L-VOD.

3. Li, X. et al. The first global soil moisture and vegetation optical depth product retrieved from fused SMOS and SMAP L-band observations. *Remote Sens. Environ.* **282**, 113272 (2022).
This paper reports the potential application of fused SMOS and SMAP L-band observations for aboveground biomass monitoring.

4. Yang, H. et al. Global increase in biomass carbon stock dominated by growth of northern young forests over past decade. *Nat. Geosci.* **16**, 886–892 (2023).
This paper reports global biomass carbon stock changes derived from L-VOD.

5. Feng, Y. et al. Doubling of annual forest carbon loss over the tropics during the early twenty-first century. *Nat. Sustain.* **5**, 444–451 (2022).
This paper reports a doubling of forest carbon loss over the tropics during 2001–2019.

## BEHIND THE PAPER

The study extends our group's previous research highlighting the promising potential of L-VOD for AGC modelling²,⁴. Despite this, L-VOD-based AGC measurements fall short in separating the impacts of fire, forest degradation and deforestation on AGC loss, as well as vegetation responses to these disturbances. The earlier work³ of Y.F. used high-resolution optical satellite observations to quantify AGC loss from deforestation in tropical

regions. Building on these methods, this study incorporated additional factors, such as degradation and fire, to refine AGC loss estimates across all terrestrial ecosystems, not just forests. By subtracting gross losses from L-VOD-derived net changes, we identified a residual term that could serve as a proxy for gross gains. Ultimately, we delivered a comprehensive quantification of tropical AGC dynamics and processes.
**Y.F. & P.C.**

## FROM THE EDITOR

> "This work by Feng et al. stood out to us with its extremely detailed remote sensing data, allowing for an in-depth quantification of the complex annual tropical carbon dynamics, and highlighting the importance of various drivers — particularly fire — for management efforts." **Editorial Team, Nature Climate Change.**