Contribution of land use and cover change (LUCC) to the global terrestrial carbon uptake.

Terrestrial carbon uptake is critical to the removal of greenhouse gases and mitigation of global warming, which are closely related to land use and cover change (LUCC). However, understanding terrestrial carbon uptake and the LUCC contribution remains unclear because of complex interactions with other drivers (particularly climate change). By proposing an innovative approach of "trajectory analysis", this study aimed to isolate the LUCC contribution to terrestrial carbon uptake over different scales. Methodologically, global land was first divided into sub-regions of land transformations and stable land trajectories. Then, the carbon uptake change in the stable land trajectory was taken as a synthetic influence of climate change, which was used as a reference to isolate the carbon uptake alternation generated from the LUCC contribution in the land transformation trajectories. Finally, future LUCC and the terrestrial carbon uptake response were predicted under different development pathways. The results showed the global mean net ecosystem production (NEP) was 27.44 ± 36.51 g C m yr in the past two decades (2001-2019), generating 3.15 ± 0.88 Pg C yr of the total terrestrial carbon uptake. Both the NEP and total carbon uptake showed significant increasing trends. Specifically, the mean NEP increased from 17.96 g C m yr in 2001 to 37.37 g C m yr in 2019, with the trend written as y = 1.20× + 15.20 (R = 0.62, p < 0.01). Meanwhile, the total carbon uptake increased from 2.35 Pg C yr in 2001 to 4.13 Pg C yr in 2019, which could be written as y = 0.12× + 1.93 (R = 0.56, p < 0.01). Climate change acted as the dominant factor for the trends at the global scale, which contributed 21.26 g C m yr and 1.59 Pg C yr of the mean NEP and total carbon uptake changes in the stable land trajectories (94.30 million km that covered 63.29 % of the global land area), and the historical LUCC contributed -6.30 g C m yr (-40.85 %) and - 0.046 Pg C yr (-57.50 %) of the mean NEP and the total carbon uptake change in the land transformation trajectories (6.64 million km that covered 4.46 % of the global land area), respectively. The maximum LUCC contribution (-61.85 g C m yr) to the mean NEP occurred in the land transformations from evergreen needleleaf forests to woody savannas, while the maximum contribution (-0.034 Pg C y) to total carbon uptake was in the deforested regions from evergreen broadleaf forests to woody savannas. Eight SSP-RCP scenarios predictions demonstrated that future terrestrial carbon uptake would increase by an average of 0.015 Pg C yr in 2100 due to global afforestation. SSP4-3.4 and SSP5-3.4 had the greatest potential for increasing carbon uptake, which is expected to reach a maximum increase (0.045 Pg C yr) in 2100. In contrast, the minimum terrestrial carbon uptake would occur in SSP5-8.5, which had the highest CO emissions. In conclusion, although relatively limited at the global scale, LUCC (particularly forest change) exerted an unneglectable role on terrestrial carbon uptake in land transformation regions. The results of this study will help to clarify terrestrial carbon uptake dynamics and provide a basis for carbon neutral and climatic adaptation.