Coupled Land‐Sea Warming Dominates the Net Land Carbon Uptake Variability in the Greater Bay Area of South China.

As Earth's warming is driven by anthropogenic activities (e.g., urbanization), land and ocean warming significantly affect the net carbon uptake (NCU) of global ecosystems. However, the driving mechanism of coupled land‐sea warming on the NCU under rapid urbanization in coastal areas remains poorly understood. Here, we used long‐term remote sensing and meteorological observation data combined with spatial analysis and multivariate statistical methods to study the impact of coupled land‐sea warming and urbanization on land net primary productivity (LNPP, a proxy of the net land carbon uptake) in the Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA), a rapidly urbanizing and climatically vulnerable area of South China. We found that the total LNPP decreased by 0.40 TgC from 1985 to 2015 and declined suddenly after 2003, driven by the reduced relative humidity and increased vapor pressure deficit (VPD). The increased VPD was caused by coupled land‐sea warming, and the indirect effect (γ = −0.51, p < 0.001) of coupled land‐sea warming on LNPP variability was much stronger than the direct effect (γ = −0.16, p < 0.05). In addition to the severe decline in the LNPP (0.20 TgC) induced by coupled land‐sea warming, urbanization caused a large loss in LNPP (0.16 TgC), which accounted for 40% of the total LNPP loss. Our findings can support future predictions of regional carbon uptake and inform future climate change mitigation strategies to implement carbon peak and neutrality goals. Plain Language Summary: Climate warming driven by rapid industrialization and urbanization has altered the processes and functions of global ecosystems. However, how the interaction of land/sea warming affects vegetation productivity and the carbon cycle in coastal areas is not clear. Here, a cascading effect of coupled land‐sea warming on the net carbon uptake (NCU) variability was detected in the Guangdong‐Hong Kong‐Macao Greater Bay Area (GBA), the area with the most rapid urbanization and climate warming in South China. Climate warming and urban sprawl, driven by urbanization, caused a consistent reduction in the net carbon uptake from 1985 to 2015 across all types of land cover in the GBA. A turning point of a sudden decline in net carbon uptake was detected when the vapor pressure deficit increased and the land/sea surface temperature sharply increased after 2003. An increased vapor pressure deficit, associated with coupled land‐sea warming, was the primary driver of reduced net carbon uptake. Our findings can support future predictions of regional carbon sinks and inform future climate change mitigation strategies from a coupled land‐sea warming perspective. Key Points: The cascading effect of coupled land‐sea warming on the net carbon uptake (NCU) variability was detectedThe increased vapor pressure deficit directly drove the NCU decrease after 2003, which caused more than 50% of the total lossUrbanization‐induced NCU loss aggravated the decline in the carbon sequestration function, accounting for 40% of the total loss [ABSTRACT FROM AUTHOR]