Climate-Driven Temperature Changes in the Future Impair China's Terrestrial Carbon Sink Capacity

Abstract. The terrestrial carbon sink modulates the rise of atmospheric CO₂, yet its long-term persistence under climate warming remains poorly constrained, largely due to nonlinear temperature responses of ecosystem carbon fluxes. Here we integrate eddy-covariance observations, process-based simulations, and interpretable machine learning to quantify the nonlinear temperature dependence of China's net ecosystem productivity (NEP) and project changes in terrestrial carbon sinks to 2060 across shared socioeconomic pathways. Site-scale observations reveal vegetation-specific turning points in NEP–temperature relationships, yielding a broad range of NEP temperature thresholds spanning 15–25 °C. Aggregated to the national scale, NEP remains stable between 0 and 18 °C, peaks near 20 °C (18–22 °C), and declines beyond this optimum threshold, implying that sustained warming could weaken terrestrial carbon sequestration. A controlled sensitivity experiment with uniform 0.5 °C warming reduces national NEP by 0.17 PgC yr^–1, corresponding to a 31 % loss of the terrestrial carbon sink. Extending this framework to 2060, our projections show national NEP reductions of 24 %, 27 %, and 56 % under SSP1-2.6, SSP2-4.5, and SSP5-8.5, respectively. Across all three scenarios, grasslands exhibit the highest proportional vulnerability to warming, with NEP losses ranging from 44 % to 171 %, whereas forests show comparatively modest relative NEP declines (7–23%). While land-use and vegetation dynamics can partially offset these sink losses, such buffering effects remain localized and are overwhelmed by nationwide warming-driven carbon sink reduction. Our findings demonstrate that this warming-triggered NEP decline diminishes the capacity of terrestrial ecosystems to buffer atmospheric CO₂ accumulation and mitigate climate change. This study illustrates how threshold-line nonlinear NEP-temperature responses weaken terrestrial carbon sink contributions toward China’s carbon peaking and carbon neutrality goals.