This study investigates the impact of urban irrigation on ambient temperatures and CO₂ exchange in major cities across the contiguous United States. The modeling results indicate that urban irrigation reduces carbon release from urban ecosystem respiration in humid climates due to evaporative cooling, but promotes it in arid/semi-arid regions due to increased soil moisture. The environmental co-benefits of heat and carbon mitigation from urban irrigation are positively correlated with urban greening fraction and can help counteract climate-carbon feedback in the built environment. Urban areas, which cover only 3% of global land but house 56% of the human population, consume over two-thirds of the world's energy and produce more than 70% of global CO₂ emissions. Urbanization, driven by anthropogenic activities, is a primary and irreversible driver of climate change, leading to challenges such as excessive heat stress, air pollution, and degraded ecosystems. Urban greening, including lawns, trees, green roofs/walls, and urban irrigation, has been widely adopted to mitigate heat stress and reduce carbon emissions. However, quantifying the comprehensive impact of urban greening on Earth's climate remains challenging due to spatiotemporal uncertainties in biogenic CO₂ exchange and limited urban observations. This study develops a modeling framework using the Weather Research and Forecasting (WRF) model and the Arizona Single-Layer Urban canopy Model (ASLUM) to investigate the impact of urban irrigation on heat and carbon dynamics. The results show that urban irrigation cools cities and their surroundings by an average of 0.26 °C, with more pronounced cooling in Salt Lake City, Dallas-Fort Worth, and Phoenix. Irrigation also reduces soil temperature by an average of 1.84 °C. The cooling and moisturizing effects influence CO₂ exchange in opposite ways, with irrigation-induced cooling potentially reducing gross primary productivity (GPP) and increasing ecosystem respiration (R_u). The net ecosystem exchange (dNEE_u) is highly dependent on the interplay between temperature, moisture, and carbon dynamics, with significant spatial variability. The study finds that urban irrigation affects NEE_u primarily through its impact on R_u, which is governed by either soil water or temperature dominant processes. Cities with reduced NEE_u experience environmental co-benefits from irrigation, while others face trade-offs. The results highlight the need for precise irrigation management to maximize carbon sequestration and minimize unintended carbon release. The study also emphasizes the importance of considering other environmental and sustainable indicators, such as water and energy efficiency, air quality, and human thermal comfort, in urban planning.This study investigates the impact of urban irrigation on ambient temperatures and CO₂ exchange in major cities across the contiguous United States. The modeling results indicate that urban irrigation reduces carbon release from urban ecosystem respiration in humid climates due to evaporative cooling, but promotes it in arid/semi-arid regions due to increased soil moisture. The environmental co-benefits of heat and carbon mitigation from urban irrigation are positively correlated with urban greening fraction and can help counteract climate-carbon feedback in the built environment. Urban areas, which cover only 3% of global land but house 56% of the human population, consume over two-thirds of the world's energy and produce more than 70% of global CO₂ emissions. Urbanization, driven by anthropogenic activities, is a primary and irreversible driver of climate change, leading to challenges such as excessive heat stress, air pollution, and degraded ecosystems. Urban greening, including lawns, trees, green roofs/walls, and urban irrigation, has been widely adopted to mitigate heat stress and reduce carbon emissions. However, quantifying the comprehensive impact of urban greening on Earth's climate remains challenging due to spatiotemporal uncertainties in biogenic CO₂ exchange and limited urban observations. This study develops a modeling framework using the Weather Research and Forecasting (WRF) model and the Arizona Single-Layer Urban canopy Model (ASLUM) to investigate the impact of urban irrigation on heat and carbon dynamics. The results show that urban irrigation cools cities and their surroundings by an average of 0.26 °C, with more pronounced cooling in Salt Lake City, Dallas-Fort Worth, and Phoenix. Irrigation also reduces soil temperature by an average of 1.84 °C. The cooling and moisturizing effects influence CO₂ exchange in opposite ways, with irrigation-induced cooling potentially reducing gross primary productivity (GPP) and increasing ecosystem respiration (R_u). The net ecosystem exchange (dNEE_u) is highly dependent on the interplay between temperature, moisture, and carbon dynamics, with significant spatial variability. The study finds that urban irrigation affects NEE_u primarily through its impact on R_u, which is governed by either soil water or temperature dominant processes. Cities with reduced NEE_u experience environmental co-benefits from irrigation, while others face trade-offs. The results highlight the need for precise irrigation management to maximize carbon sequestration and minimize unintended carbon release. The study also emphasizes the importance of considering other environmental and sustainable indicators, such as water and energy efficiency, air quality, and human thermal comfort, in urban planning.