The intensification of urban heat due to climate change poses risks to both human populations and biodiversity. Urban green spaces (UGS), such as parks and gardens, are typically cooler than surrounding areas, providing respite for residents and habitat for many species. However, whether UGS will maintain sufficient cooling under future warming remains uncertain. This study investigated the microclimate conditions of 15 UGS across three Swiss cities (Zurich, Geneva and Lugano) and evaluated the effectiveness of different adaptation scenarios to maintain their cooling benefits. The Urban Tethys-Chloris (UT&C) model was applied to simulate the universal thermal climate index (UTCI) and surface temperature under historical and future climate conditions. Adaptation scenarios were designed based on the four most influential vegetation parameters on UTCI identified by a sensitivity analysis. Driven by rising air temperatures and a decline in latent heat flux due to reduced precipitation, UTCI was projected to rise by 1.4–4.0 °C and surface temperatures by 1.8–3.2 °C by the 2080s if vegetation remains unchanged. Vegetation-based adaptation cannot fully offset this warming. Trees deliver the largest per-unit cooling and should be prioritized where additional planting is feasible. Where tree expansion is constrained, increasing ground vegetation cover offers a complementary cooling pathway, reducing UTCI by up to 1.4 °C, while denser and taller ground vegetation reduces surface temperature by up to 1.7 °C. We recommend planting dense and tall ground vegetation where tree expansion is limited, to support both human thermal comfort and ground-dwelling species under a warming climate. Practical implications • Green spaces are cooler than built-up areas, providing recreational areas for city residents and serving as refugia for many species. Yet it is unclear whether they will stay cool as Switzerland warms under climate change with longer dry spells (MeteoSwiss and ETH Zurich, 2025). By the 2080s, UTCI in urban green spaces is projected to rise during summer by 4.0 °C in Zurich, 2.8 °C in Geneva, and 1.4 °C in Lugano. This study evaluated the effectiveness of different adaptation measures for green spaces to maintain their cooling benefits. • Vegetation-based adaptation, such as increasing the vegetation coverage and canopy density, can reduce this warming by up to 1.4 °C, but cannot fully counteract future warming. Cities should therefore combine greening strategies with complementary measures such as built-environment changes (cool surfaces, reduced built fractions), blue-green infrastructure (water bodies, permeable pavements), and active water management. • Trees cool more per unit area than ground vegetation and should be prioritized where space allows. In settings where tree planting is constrained by infrastructure, historical preservation, or the need to maintain open public space, increasing ground vegetation coverage is a robust alternative to reduce human heat stress. • When ground vegetation is replaced or expanded, plants that produce a denser canopy (larger leaves, taller stems, higher structural complexity) should be prioritized. Denser, taller vegetation lowers ground surface temperature, which protects ground-dwelling species and biodiversity. Plants with deeper roots should also be planted when possible, to increase proximity to groundwater and limit water stress. Planners should maximize vegetation coverage and canopy density (leaf area, height) enabling faster and simpler implementation. • The cooling achieved by a given vegetation intervention will differ across green spaces, depending on local climate, urban morphology, and green space characteristics. Site-specific assessment is therefore essential when planning adaptation measures.