Near real-time monitoring of carbon effects from continuous forest change in rapidly urbanizing region of China from 2000 to 2020

Forest carbon sinks are crucial for mitigating urban climate change. Their effectiveness depends on the balance between gross carbon losses and gains. However, quantitative and continuous monitoring of forest change/disturbance carbon fluxes is still insufficient. To address this gap, we integrated an improved spatial carbon bookkeeping (SBK) model with the continuous change detection and classification (CCDC) algorithm, long-term Landsat observations, and ground measurements to track carbon emissions, uptakes, and net changes from forest cover changes in the Yangtze River Delta (YRD) of China from 2000 to 2020. The SBK model was refined by incorporating heterogeneous carbon response functions. Our results reveal that carbon emissions (−3.88 ​Tg ​C·year⁻¹) were four times greater than carbon uptakes (0.93 ​Tg ​C·year⁻¹) from forest cover changes in the YRD during 2000–2020, despite a net forest cover gain of 10.95 ​× ​10⁴ ​ha. These findings indicate that the carbon effect per hectare of forest cover loss is approximately 4.5 times that of forest cover gain. The asymmetric carbon effect suggests that forest cover change may act as a carbon source even with net-zero or net-positive forest cover change. Furthermore, carbon uptakes from forest gains in the YRD during 2000–2020 could only offset 0.28% of energy-related carbon emissions from 2000 to 2019. Urban and agricultural expansions accounted for 37% and 10% of carbon emissions, respectively, while the Grain for Green Project contributed to 45% of carbon uptakes. Our findings underscore the necessity of understanding the asymmetric carbon effects of forest cover loss and gain to accurately assess the capacity of forest carbon sinks.