Comprehensive Evidence That Detecting Urban Signals in Large-Scale Warming Is Highly Uncertain

Increasing urbanization causes urban heat island effects that might introduce significant biases into global warming estimates. Previous studies of urban warming signals and asymmetries remain a subject of debate. Here we comprehensively assess urban-induced warmings by investigating meteorological temperatures on 2,370 stations in China during 1980-2022. There are noticeable urban-induced annual warming biases and contributions ranging from 0.016 to 0.251 degrees C decade(-1) and 0.3%-72.4%, primarily due to spatiotemporal heterogeneities and the criteria defining urban sites. Rapid urbanization tends to exacerbate diurnal and seasonal warming asymmetries, resulting in shrinking temperature differentials associated with urbanized areas and urbanization chronosequences. This study underscores that the specific definition of urban areas matters for warming biases in magnitude and complex nonlinear urbanization imprint on warming asymmetries.