Urban Vegetation Carbon Sink Response to Urbanization Depends on Urban Expansion Rate

Urban expansion reduces vegetation cover, thereby diminishing vegetation gross ecosystem productivity (GPPu). Conversely, rising CO2 levels and warming associated with climate change can enhance GPPu. The interplay between these opposing effects on GPPu remains poorly understood, resulting in considerable uncertainty regarding the long-term dynamics of urban vegetation carbon storage. Using an improved Carnegie-Ames-Stanford approach (CASA) model that accounts for urban CO2, we evaluated the long-term trends and driving factors of GPPu in 2126 cities worldwide, spanning both historical and future periods. The study revealed that the average urban expansion rate (UER) of global cities from 1982 to 2024 was 21.3 km² per year. Rapid urban expansion was predominantly observed in cities in China and North America, whereas cities in Europe exhibited relatively slower expansion rates. The response of GPPu to urban expansion during this period was found to be dependent on the UER. In cities with a faster UER, the negative impact of urban expansion on long-term GPPu surpassed the positive effects of climate change, resulting in a downward trend in GPPu (–2.48 g C/m²/year). Conversely, in cities with slower UER (commonly observed in Europe), the negative impact of urban expansion on long-term GPPu was smaller than the positive impact of climate change, leading to an upward trend in GPPu (1.21 g C/m²/year). The projected future response and driving factors of GPPu to urbanization were expected to resemble those observed in the historical period. These findings underscored the importance of implementing tailored strategies for cities with varying urban expansion rates to enhance regional carbon sequestration capacity in the context of intensified urbanization and climate change.