Climate change may severely compromise urban forest ecosystem services across North America and Europe

Urban forests provide essential ecosystem services, including cooling, carbon sequestration, air purification, and support for human health that sustain cities worldwide. With ~70% of humanity projected to live in cities by 2050, these benefits are threatened by rising temperatures and shifting precipitation patterns stressing urban forests and reducing their functional performance. We assessed climate risks to 1,724 woody species across 387 North American and European cities. These urban forests, representing ~11.25 million inventoried trees and shrubs, currently store ~2.13 million metric tons of carbon and sequester ~146 thousand metric tons annually, while providing hydrological and air quality services valued at ~USD 14 billion per year. Using climatic safety margins and i-Tree Eco modelling, we project 61-82% of species exceed tolerance limits by 2050 (SSP3-7.0) across cities, driving 45% average declines across 15 ecosystem services. Some high-latitude cities face >90% losses. Warmer baseline climates, wetter warm seasons, northern latitudes, and lower tree density drive the greatest losses. Even slight exceedances create climate-service debt—gradual functional decline preceding severe failure—while large exceedances cause immediate service collapse. Our continental-scale translation of species risk to functional losses reveals severe urban forest vulnerability, closing critical gaps between climate exposure and ecosystem service outcomes. Immediate climate-adaptive planting is essential to sustain vital green infrastructure amid rapid urban expansion and warming.