Excessive greenhouse gases emissions, particularly carbon dioxide, have caused continuous rise in atmospheric carbon dioxide concentration which was reported to be over 400 parts per million in 2020 as compared to ~280 parts per million in the pre-industrial era. At the same time, global climate changes, especially global warming, have attracted critical concerns in both academia and government. Numerous studies have shown that there exists a close relationship between the global warming and the increased atmospheric concentration of carbon dioxide as a result of unbalanced carbon emission and sequestration. More severely, the continuous rise in temperature is likely to become irreversible if no immediate global efforts are made to cut down carbon emissions and/or increase the sequestration. In 2015, 195 countries have assigned the Paris Agreement and declared national obligations to release less greenhouse gases as the way to curb the trend of global warming. However, some obstacles to reduce the greenhouse gases emissions are hard to overcome for many nations.

In fact, scientists have long been looking for various ways to reduce the carbon dioxide in the atmosphere. Those ways can be categorized into two groups. One is to cut down carbon emissions by utilizing new or clean energy instead of depending on the traditional charcoals or gases, and the other is to pump more carbon out of the atmosphere. However, massively utilizing clean energy is difficult either because of unavailable technology or high cost involved, especially in developing countries. Terrestrial vegetation such as forestry, grasslands and croplands that can sequester carbon dioxide through photosynthesis, a natural process that requires nothing but water or rain, energy from the sun, carbon dioxide in the air, and nutrients from the soil. The net amount of carbon absorbed by green vegetation is called net primary production (NPP). Every year global terrestrial vegetation sequesters 67-112 PgC (1PgC=10¹⁵g carbon). Terrestrial vegetation covers different biomes that show varied capacity in terms of NPP. For example, forestry generally produce higher NPP than croplands. Although it is helpful to sequester more carbon by planting more trees, the potential is limited because we cannot replace all croplands with forestry through land cover and land use conversion, as enough food supply must be guaranteed. In the present work, instead of converting land cover and land use, we emphasize that vegetation carbon sequestration can be improved substantially by simply optimizing land management practices. Optimal land management practices are site-specific, meaning they must adapt to environmental-and-social contexts and can differ from place to place. To decide the optimal land management practices for a place, we resort to existing land management practices implemented at other places with similar environmental conditions such as soil, landforms and vegetation type because they have historically been proved effective in sequestering more carbon, rather than come up with new ones which may not be applicable in reality.

Numerous studies have investigated the effect of land management practices on improving vegetation productivity and carbon sequestration. In some areas, for example, if tillage for croplands is found to improve vegetation growth, it is regarded as an optimal land management practice which can be transferred to other locations having the same land use mode and environmental conditions. Similarly, reseeding or/and protective grazing on grasslands can be implemented in vegetation degraded areas to restore vegetation productivity and taken as optimal land management (Fig. 1).

Our study answers two questions, 1) how much more carbon can global terrestrial vegetation sequester through implementation of optimal land management, and 2) where the most potential areas are located. To get the answers, we use multiple datasets, including satellite remote sensing imageries, and spatial analysis. We consider three groups of impact that differentiate the capacity of vegetation carbon sequestration, including climate impact, non-climatic environmental/natural impact, and human-related land management practices. First, the difference of vegetation carbon sequestration from climate variations is isolated to derive a climate-rectified NPP index (NPP_CR). The result of this isolation will leave NPP_CR to be determined only by the non-climatic environmental factors, such as different soil properties and landforms, and human- related land management practices. To further level off the differed impact from the varied environmental factors, we divide landforms (L), vegetation (V), and soil (S) type into homogeneous and areal patches, a process called LVS segmentation. We then use zonal analysis to analyze the variations of NPP_CR in the patches labeled by the same soil, landforms and vegetation type. The logic behind the LVS segmentation is that every locations in the segmented patches labeled by the same soil, landforms and vegetation type should have comparable NPP_CR if an identical land management practice is applied across all the locations in the LVS labeled patches. If the variations of NPP_CR is indeed observed, it suggests that land management practices are actually not uniform and that locations with optimal land management practices will have higher NPP_CR than those otherwise. The variations of NPP_CR across the locations in the same LVS labeled patches imply that a lower NPP_CR under non-optimal land management could be improved to as much as the high NPP_CR level from optimal land management. The difference from the lower NPP_CR to the high NPP_CR is referred to as carbon gap, which is equivalent to the amount of carbon that can be additionally sequestered. To minimize the impact of the possible noises in the datasets, we define the 90th percentile, instead of the maximum, of NPP_CR values as the high level standard within the patches labeled by the same LVS. The carbon gap is processed location-wise and the candidates of the optimal land management practices are identified within a 20km neighborhood only, making them feasible to be transferred from one place to the other.

Our study shows the carbon gap density varies considerably in the vegetated lands among the 12 continents/regions (including Europe, Africa, Australia, East Asia, North Asia, South Asia, South-east Asia, South-west Asia, Central Asia, North America, Central America, and South America) and different terrestrial biomes. We find that 15% of the vegetated lands accounts for about 50% of the total carbon gap which is 13.74 (±0.78) PgC per year. After deducted from soil respiration, the total carbon gap is estimated to be able to counterpart about one third of the direct human-made carbon emissions, which was ~10PgC in 2019. To achieve a cost-effective implementation of the optimal land management practices, we recommend the priority be focused in areas showing high carbon gap density, for example the 15% of the vegetated lands that account for half of the total carbon gap. The study concludes that international collaboration to implementing optimal land management is a promising way to reduce carbon in the atmosphere, which also contribute to fulfil the nations’ Nationally Determined Contributions, or the declared obligations in the Paris Agreement to reduce national emissions.