Most Land Carbon Locked in Nonliving Reservoirs

Challenging long-held assumptions about global terrestrial carbon storage, a new study finds that the majority of carbon dioxide (CO₂) absorbed by ecosystems has been locked away in dead plant material, soils, and sediments, rather than living biomass, researchers report. These new insights, which suggest that terrestrial carbon stocks are more resilient and stable than previously appreciated, are crucial for shaping future climate mitigation strategies and optimizing carbon sequestration efforts. Recent studies have shown that terrestrial carbon stocks are increasing, offsetting ~30% of anthropogenic carbon dioxide emissions. A primary mechanism driving this trend is the CO₂ fertilization effect, in which elevated atmospheric CO₂ levels enhance plant productivity. However, it is uncertain how much organic carbon is stored in living biomass versus nonliving organic reservoirs, such as plant detritus, soils, and sediments. Understanding this distribution is crucial because different pools have varying carbon residence times and vulnerabilities to environmental change. Nevertheless, accurately quantifying living and nonliving carbon storage pools has proven difficult. To address this challenge, Yinon Bar-On and colleagues developed a comprehensive assessment of global changes in woody vegetation carbon stocks by harmonizing diverse remote-sensing estimates with upscaled field inventory data from 1992 to 2019. This integrated approach allowed the authors to partition terrestrial carbon accumulation between living biomass and nonliving organic reservoirs providing a more complete understanding of carbon distribution across ecosystems. Bar-On et al. discovered that while terrestrial ecosystems accumulated approximately 35 ± 14 gigatons of carbon (GtC) during the study period, global living biomass stocks increased by only about 1 ± 7 GtC. The findings indicate that most of the carbon sequestered over the last 3 decades was stored as nonliving organic matter in soils, deadwood, and human-influenced reservoirs like dams and landfills. These pools persist far longer than living biomass, suggesting that terrestrial carbon storage may be more stable over time than previously assumed. "Although the study of Bar-On et al. primarily focused on woody biomass because of its large carbon stocks, the role of grass-dominated ecosystems, where soil carbon sequestration is the dominant storage mechanism, merits further investigation," writes Josep Canadell in a related Perspective. "A more regional and ecosystem-based analysis will further provide key insights to enable the design of strategies for the conservation of carbon stocks and enhancement of CO₂ sinks for climate mitigation."