The Pace of Urbanization Regulates Global Urban Gross Primary Productivity Trends

Urban expansion replaces vegetated land with impervious surfaces, significantly reducing urban Gross Primary 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 uncertainties for estimating the long-term dynamics of urban 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 faster UERs than 5.83 km²/year, the negative impact of urban expansion surpassed the positive effects of climate change on long-term GPPu, resulting in a downward trend in GPPu (–2.48 g C/m²/year). Conversely, in cities with slower UERs (commonly observed in Europe), an opposite pattern was observed, 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 highlight that the pace of urbanization acts as a fundamental regulator of urban carbon sinks, underscoring the need for growth-rate-specific strategies to sustain regional carbon sequestration.