Current growth simulations indicate that by 2050, nearly 6.3 billion people out of an estimated global population of 9.1 billion will live in urban territories. Karachi is largest city of Pakistan, and highly vulnerable to extreme weather events. The aim of this study is to assess the vulnerability of the Karachi urban inhabitants to heat stress and to identify the role of ecosystem services in mitigating the future impacts of heatwaves as well as the estimation of landscape functions to urbanization. GIS/RS techniques is used to estimate the suitability of forest scenario projection models and to identify the current status of urban green spaces in the city. Long-term meteorology dataset (1989–2018) is also analysed to quantify the frequency of heat waves, along with downscaling of rise in temperature under future scenarios. The results indicate that heat waves are linked with climate warning and extreme weather events, aggravated by rapid urbanization, industrialization, deforestation, emission of CO2, degradation of Mangrove Forest, and shift of land use etc. Moreover, findings revealed that there is significant drop-off in urban green spaces and increase in build-up areas of the city during the 1984–2016. In addition, model projected that forest area around the city has ability to absorb CO2 emission up to 55.4 million-tons. This study provides initial assessment and specific policy implications in meteorological predictions to establish early warning heat waves alerts and promote smart sustainable cities plans among residents and planners.