Keeping the global temperature increase below 1.5 °C in accord with the Paris Agreement would require prompt and substantial reductions in GHGs emissions on the global scale1. Despite considerable effort internationally, many countries are likely to miss the emission control targets proposed in the Paris Agreement, and the world is on track for more than 3°C of warming by the end of the century7,8. More severely, global warming will be irreversible if the concentration of GHGs continues to rise in the atmosphere8. To cut down GHGs emissions, numerous policies have long been proposed, including resorting to renewable energy, popularizing electric vehicles, optimizing land-use policies, as well as other GHGs cutting policies or programs1,2. Unfortunately, those policy options seem not being effectively deployed, given the continuous increase of the emissions of GHGs9,10. As the largest part of GHGs, the atmospheric CO2 has risen beyond 400 ppm in recent years, a level not experienced over the past 800,000 years3,11. The rise of CO2 levels in the atmosphere is mostly due to fossil fuel emissions and responsible for a 1.5°C increase in air temperature since the 1880s3,12. The unrelenting fluxes and pools of the terrestrial carbon cycle are widely out of equilibrium from pre-historical conditions owing to human activities3.
Though cutting fossil fuel consumption provides a direct option to reduce carbon emissions, it is hampered by the fact that the economy in many countries is still powered by fossil energy2,13. Vegetation dominates most terrestrial ecosystems (e.g., forests, grasslands, croplands, shrublands, and savannas) and absorbs substantial CO2 from the atmosphere through a biochemical process called photosynthesis14. Hence, biotic measures by improving carbon intake from vegetation provide a viable measure to counteract excessive carbon emissions5,14. The net amount of carbon captured by plants through photosynthesis over a given period is called net primary production (NPP)4,15. As a key component of energy and mass transformation in terrestrial ecosystems, NPP depends on a variety of factors, such as the supply of nutrients, water availability, soil profile characteristics, and landscape attributes (e.g., terrain and drainage)16,17. Those factors can be broadly categorized as the following three groups18: 1) climate impact (e.g., precipitation and temperature); 2) non-climatic physical environmental factors such as soil property, landforms and biomes; and 3) human-related land management practices (HUMAN).
First, vegetation will not attain the saturation of carbon sequestration capacity without appropriate climatic conditions5. Lack of rainfall will cause physiological stress and limit vegetation photosynthesis16. The dependence of vegetation carbon fixation on climatic factors is reflected by most NPP models4. For example, the climate-driven Miami model, which has been widely applied to map large-scale NPP5,19,20, highlights the importance of the climatic factors as vital drivers to carbon sequestration. Second, an ecosystem cannot maximize vegetation carbon sequestration without favorable phyical environmental conditions such as landforms17, soil properties21, and biome groups5. Lastly, carbon absorption can be updated by land management practices (LMPs)14, i.e., one LMP may result in higher or lower carbon sequestration than the other, under the same environmental contexts (the climatic and non-climatic conditions).
Land use and management has massive effect on carbon sequestration from vegetation6,20,22. An optimal land management practice (OLMP) refers to an LMP that is capable of achieving full capacity or a higher target carbon sequestration level given the current climatic and non-climatic conditions. Once LMPs are replaced with OLMPs, vegetation can be expected to sequester more carbon. Modeling the increase potential or the difference in carbon sequestration (carbon gap hereafter) with- and without- OLMPs is valuable for making improved land management policies and mitigating global climate changes. In a broad sense, OLMPs could mean the removal of negative human-related disturbances in natural vegetated areas22, or the adoption of human-related programs/practices that are useful for restoring previously degraded vegetation23,24. OLMPs pertain to local environmental and socio-economical contexts. Practically, OLMPs and the carbon gap at a particular site can be evaluated through field experiments. However, considering manifest spatial variations of the environmental conditions across the globe, there exist no universally applicable OLMPs for all locations. There is also a need for mapping carbon gaps through a globally and locally compatible approach so that both global comparison and local policy enactment are supported25. The effectiveness of implementing OLMPs for enhancing vegetation productivity has been widely documented (see Supplementary Table 1). Herein, this work addresses the following questions: how much more carbon could be further sequested from global land vegetation with OLMPs? How can the location-dependent OLMPs be decided from the local abiotic contexts at each location? Where are the most sensitive areas with carbon sequestration potentials?