Human activities have significantly impacted Earth's ecosystems over tens of thousands of years, leading to a massive decline in carbon stocks in soil and vegetation. New research from Ganzenmüller and colleagues reveals that humans have depleted global terrestrial carbon stocks. carbon by a quarter, representing a decrease of 344 petagrams of carbon (PgC). This “terrestrial carbon deficit” is a key metric for understanding human impacts on ecosystems and is essential for evaluating climate models and planning climate change mitigation measures.

The extent and impact of depletion

Global terrestrial carbon stocks were exhausted by 24 %The estimated deficit of 344 PgC is comparable to total global fossil fuel emissions from 1975 to 2023 (361 PgC), or more than 70 % of all global fossil fuel emissions since 1750. This underscores the profound human-induced degradation of global ecosystems. This deficit arises as the difference between actual and potential carbon stocks, with “potential” referring to the hypothetical amount of carbon that would exist without direct human intervention under current environmental conditions.

Main drivers of the carbon deficit

The study identifies three main causes of this widespread carbon loss:

• Pasture expansion: Responsible for 30 % of the terrestrial carbon deficit. It is the main driver of deforestation and degradation of forests and grasslands.
• Expanding arable land: Contributes to 24 % of carbon deficit. Arable land has the highest relative deficit of vegetation carbon per unit area, with the actual density of vegetation carbon in arable land being 81 % lower than the potential.
• Forestry: Represents 23 % of deficit. This includes forests with signs of logging, planted forests and agroforestry systems. Forestry has the highest contribution to the soil carbon deficit (29 %), illustrating its overall negative impact on soil carbon stocks.

In addition, urban areas account for 3 % of the terrestrial carbon deficit. Geographically, the largest carbon deficits are found in China, Brazil, Europe, and the United States. Vegetation carbon accounts for 76 % to 84 % of the total deficit, while soil carbon accounts for 16 % to 24 %.

Actual and potential carbon stocks

The authors estimate the actual global carbon stock (above- and below-ground biomass) at 420 PgC and the carbon stock in soil (at a depth of 0–30 cm) at 642 PgC. The potential carbon stock in vegetation ranges from 617 to 683 PgC and the potential carbon stock in soil from 679 to 723 PgC. This difference indicates that the actual carbon stock is 18 % to 24 % lower than the potential stock, depending on the assumed degree of human influence. Quantification of this deficit was achieved by combining semi-empirical data with machine learning methods, which allowed estimates to be obtained with a resolution of approximately 1 km².

Underestimations of global vegetation models (DGVMs)

The research also revealed serious inconsistency between estimates of soil and vegetation carbon deficit obtained by semi-empirical methods and simulations of dynamic global vegetation models (DGVMs)On average DGVM underestimates terrestrial carbon deficit by 37 %. This underestimation limits the ability of DGVM to accurately estimate carbon fluxes due to land-use change and the potential for carbon removal measures. The reasons for these discrepancies are diverse, including differences in model designs, process implementation, and external land-use change data. This has implications for current scientific and policy debates on carbon emissions and removals, as it may suggest that the remaining carbon budget to limit global warming to 2°C may be exhausted sooner than previously thought.

Consequences and solutions

The findings of this research are of great importance to policymakers and planning for climate change mitigation measures. The estimated terrestrial carbon deficit underlines the massive scale of human interventions needed to restore carbon lost from Earth's ecosystems over centuriesThis includes protecting and restoring ecosystems, making sustainable land-use decisions, and improving global vegetation models. Targets such as increasing protected areas, such as the Kunming-Montreal Global Biodiversity Framework’s target of 30 % by 2030, could be key to meeting the Paris Agreement and biodiversity goals.

Total study highlights that human impacts on terrestrial carbon stocks are as important as fossil fuel emissions for understanding the overall disruption of the global carbon cycle. Implementing improved datasets, such as those presented in this study, is crucial for more accurate estimates and more effective climate action planning. JRi

These findings, published in One Earth magazine