Recent research shows that carbon stocks in terrestrial ecosystems are increasing, helping to offset about 30% of carbon dioxide emissions caused by human activities. The overall value of the Earth's surface carbon sink is relatively easy to understand because it can be estimated by studying the global carbon balance, including human-caused emissions, the accumulation of carbon in the atmosphere, and the uptake of carbon by ocean sinks. However, researchers still know little about how carbon is distributed in different terrestrial pools, such as living vegetation (mainly forests) and abiotic carbon pools (such as soil organic matter, lake and river sediments, and wetlands).

Wetlands in the Alps. Image credit: INRAE ​​– Sébastien De Danieli

Abiotic carbon mainly comes from the excrement and decomposition of dead animals and plants, which ultimately becomes a food source for soil organisms. While the processes by which carbon accumulates in biomass, especially photosynthesis, are well understood, changes in abiotic carbon pools remain poorly understood and extremely difficult to measure.

The researchers measured fluctuations in total terrestrial carbon stocks by coordinating a set of global estimates based on different remote sensing techniques and field data between 1992 and 2019. They combined global estimates with recently compiled data on carbon exchanges between land, atmosphere and ocean to allocate terrestrial carbon accumulation between biotic and abiotic carbon pools.

A research team coordinated by Caltech's Yinon Bar-On found that approximately 35 billion tons of carbon was sequestered on the Earth's surface between 1992 and 2019. Over the past decade, terrestrial carbon accumulation has increased by 30%, from 50 million tons per year to 170 million tons per year. However, vegetation (mainly forests) accounts for only 6% of these carbon increases.

Until now, forests were considered major carbon sinks; however, disturbances from climate change or human activities (fires, deforestation, etc.) have made forests increasingly vulnerable, and now, in some cases, they emit almost as much carbon as they accumulate. Nonetheless, they remain important carbon reservoirs that need to be protected.

The results show that a large part of the terrestrial carbon accumulation mechanism is related to the burial of organic carbon in anaerobic environments, such as the bottom of natural and artificial water bodies. Even more surprising, the results suggest that a large portion of the terrestrial carbon sink may be related to human activities, such as building dams or artificial ponds, or even the use of wood. One positive outcome of this study is the finding that most terrestrial carbon gains are sequestered in a more permanent way than in living vegetation.

Due to a lack of data on carbon accumulation in soils, water bodies and wetlands, current dynamic global vegetation models significantly overestimate the role of forests in terrestrial carbon sequestration. This study identifies key processes for terrestrial carbon accumulation that are not included in current models. Therefore, these data can serve as a valuable resource for validating future global vegetation models of living plant biomass dynamics.

Compiled from /ScitechDaily