Terrestrial carbon sinks grew by 30% from 1992 to 2019, with most carbon stored in nonliving pools, not forests, prompting a call to update climate models.
Recent studies have shown that carbon stocks in terrestrial ecosystems are increasing, helping to offset about 30 percent of the carbon dioxide emissions caused by human activities. The overall value of carbon sinks on Earth’s surface is relatively well understood, as it can be estimated by examining the global carbon balance, including human-caused emissions, the buildup of carbon in the atmosphere, and the absorption by ocean sinks. However, researchers still know very little about how carbon is distributed among different terrestrial pools, such as living vegetation, primarily forests, and nonliving carbon pools like soil organic matter, lake and river sediments, and wetlands.
Nonliving carbon mainly comes from the waste and decomposition of dead plants and animals, eventually serving as a food source for soil organisms. While the processes that lead to carbon accumulation in living biomass, especially photosynthesis, are well understood, changes in nonliving carbon pools remain poorly known and extremely difficult to measure.
Researchers measured fluctuations in total terrestrial carbon stocks by harmonizing a set of global estimates based on different remote sensing technologies and field data between 1992 and 2019. They combined their global estimate with the recent compilation of carbon exchanges between land, atmosphere, and oceans to distribute terrestrial carbon accumulation between living and nonliving carbon pools.
A 30% increase in terrestrial carbon sinks over the last decade
The research team, coordinated by Yinon Bar-On (California Institute of Technology), found that around 35 gigatonnes of carbon were sequestered on the Earth’s surface between 1992 and 2019. This accumulation of terrestrial carbon has increased by 30% over the last decade, from 0.5 gigatonnes per year to 1.7 gigatonnes per year. Yet, vegetation, mainly forests, accounts for only 6% of these carbon gains.
Until now, forests were considered to be the main carbon sinks, however, disturbances linked to climate change or human activities—fires, deforestation, etc.—have made them increasingly more vulnerable and now, in certain situations, they could emit almost as much carbon as they accumulate. Nevertheless, they remain important carbon stocks that need to be protected.
The main terrestrial carbon sinks are more enduring
The results show that a substantial fraction of terrestrial carbon accumulation mechanisms is linked to the burial of organic carbon in anaerobic environments, such as the bottoms of natural and artificial bodies of water. More surprisingly, the results indicate that a significant proportion of terrestrial carbon sinks could be linked to human activities such as the construction of dams or artificial ponds, or even the use of timber. A positive outcome of this study is the discovery that the majority of terrestrial carbon gains are sequestered in a more enduring manner, compared to in living vegetation.
The lack of data on carbon accumulation in soils, bodies of water, and wetlands has led current dynamic global vegetation models to greatly overestimate the role of forests in terrestrial carbon sinks. This study identifies key processes in terrestrial carbon accumulation that are not included in current models. As a result, this data could serve as a valuable resource for validating future dynamic global vegetation models in living plant biomass.
Reference: “Recent gains in global terrestrial carbon stocks are mostly stored in nonliving pools” by Yinon M. Bar-On, Xiaojun Li, Michael O’Sullivan, Jean-Pierre Wigneron, Stephen Sitch, Philippe Ciais, Christian Frankenberg and Woodward W. Fischer, 20 March 2025, Science.
DOI: 10.1126/science.adk1637
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