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As two main drivers of vegetation dynamics, climate variability and human activities greatly influence net primary productivity (NPP) variability by altering the hydrothermal conditions and biogeochemical cycles. Therefore, studying NPP variability and its drivers is crucial to understanding the patterns and mechanisms that sustain regional ecosystem structures and functions under ongoing climate variability and human activities. In this study, three indexes, namely the potential NPP (NPPp), actual NPP (NPPa), and human-induced NPP (NPPh), and their variability from 2000 to 2020 in the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) were estimated and analyzed. Six main scenarios were generated based on change trends in the three indexes over the past 21 years, and the different relative impacts of climate variability and human activities on NPPa variability were quantitatively analyzed and identified. The results showed that the NPPp, NPPa, and NPPh had heterogeneous spatial distributions, and the average NPPp and NPPa values over the whole study area increased at rates of 3.63 and 6.94 gC·m−2·yr−1 from 2000 to 2020, respectively, while the NPPh decreased at a rate of −4.43 gC·m−2·yr−1. Climate variability and the combined effects of climate variability and human activities were the major driving factors of the NPPa increases, accounting for more than 72% of the total pixels, while the combined effects of the two factors caused the NPPa values to increase by 32–54% of the area in all cities expect Macao and across all vegetation ecosystems. Human activities often led to decreases in NPPa over more than 16% of the total pixels, and were mainly concentrated in the central cities of the GBA. The results can provide a reference for understanding NPP changes and can offer a theoretical basis for implementing ecosystem restoration, ecological construction, and conservation practices in the GBA. [ABSTRACT FROM AUTHOR]

• Inter-calibration between DMSP-OLS and NPP-VIIRS were conducted at the pixel scale. • CO 2 emissions in both lit area and unlit area were estimated. • Tradeoff between vegetation CO 2 sequestration and fossil CO 2 emission was evaluated. • Policy implications for CO 2 emissions reduction were suggested. Carbon neutrality has attracted tremendous attention. Cities contribute the most to CO 2 emissions. However, the contribution of vegetation to fossil-fuel-related CO 2 emissions in urban agglomeration is unclear. Clarifying the trade-off role of vegetation can disaggregate carbon reduction targets down to sub-units to adapt to and even mitigate global warming. In this study, the Guangdong–Hong Kong–Macao Greater Bay Area (GBA), one of the world's largest metropolitan areas, was studied using our proposed inter-calibration method. The results showed that the inter-calibration method is satisfactory and that the EANTLI model effectively decreases the blooming and saturation effects of nighttime light. In addition, fossil-fuel-related CO 2 emissions increased significantly (P < 0.0001) in the GBA during 2000–2018, while the variation in CO 2 sequestrations was far lower than that in the increase in emissions. CO 2 sequestrations by vegetation fully offset fossil-fuel-related CO 2 emissions in 2000, while the status reversed after 2001. Our findings illustrate the role of vegetation carbon sequestration in offsetting fossil-fuel-related CO 2 emissions and emphasize the importance of a CO 2 budget. Additionally, the one city, one policy strategy is a good choice for further adapting to and mitigating global warming. [ABSTRACT FROM AUTHOR]

• An assessment of conservation priorities for KBAs in highly urbanized regions was conducted. • Ecosystem conditions remain stable in most of KBAs over the past two decades. • Ecosystem conditions showed spatially significant negative correlations with anthropogenic threats. • Shenzhen Wutongshan is the highest priority KBA that needs to strengthen ecosystem conservation. The rapid and intense development in highly urbanized regions presents huge challenges to the conservation of biodiversity and ecosystem services. Progress on incorporating ecosystem conditions and anthropogenic threats into conservation assessments for key biodiversity areas (KBAs) in highly urbanized regions is still insufficient. To address this, this paper conducted an assessment of conservation priorities for key biodiversity areas in highly urbanized regions by integrating anthropogenic threats with ecosystem conditions. Taking 8 KBAs in the Guangdong - Hong Kong - Macao Greater Bay Area of China (GBA) as an example, this study found that the ecosystem condition of most of KBAs remains stable from 2000 to 2020. We found that the ecosystem conditions in most KBAs have not been negatively affected by anthropogenic threats. It is worth noting that Shenzhen Wutongshan (SZW) is the only KBA that faces a significant increase in anthropogenic threats with a significant decline in ecosystem condition during the past two decades. Overall, SZW is the highest priority KBA that needs to reduce the negative effects of anthropogenic threats and strengthen ecosystem conservation in the future. [ABSTRACT FROM AUTHOR]