Three decades of urban densification reduced winter direct sunlight access across China's old urban cores.

Urban densification is widely promoted to improve land-use efficiency and curb sprawl, but it can reduce winter sunlight in old urban cores, an important dimension of urban livability. Although three-dimensional (3D) urban form is known to shape sunlight availability, most studies focus on present-day conditions, typical urban forms, or planning scenarios. How much sunlight has been lost through recent redevelopment, particularly in legacy neighborhoods, remains unclear. Here we quantify densification-induced winter direct-sunlight loss across the old urban cores of 77 large cities in mainland China. We map winter-solstice direct-sunlight access at 5-m resolution using 3D building data and city-specific solar geometry, comparing the observed 2020 built environment with a counterfactual (CF) circa-1990 height field for legacy buildings. The CF uses city-specific empirical height caps and is evaluated against historically documented block-scale cases. Over three decades, densification reduced sunlight across 36% of old-core areas on average, with mean duration in affected areas declining from 5.37 to 3.75 h. Around 7% of the area (603 km2) shifted from sufficient to insufficient sunlight, while 17.7% (1,525 km2) fell below the 2-h threshold by 2020. Mean losses ranged from 1.08 to 2.46 h across cities, affecting approximately 16 million first-floor residents, including around 0.61 million exposed to severe losses of 3 to 6 h. Losses were more extensive in cities with shorter baseline sunlight, greatest in second-tier cities, and steeper at higher latitudes. These findings provide a scalable basis for sunlight-sensitive urban renewal and height regulation.