The implementation of green roofs as a nature-based strategy to mitigate climate change is urgently being applied in many cities, as in addition to providing important and numerous benefits to the quality of public spaces, it offers a reduction in the energy consumption of buildings, especially on hot or summer days. Valparaiso is a city located in the central zone of Chile, with a coastal Mediterranean climate. The thermal behaviour and the presence of an atmospheric and surface heat island have been evaluated in summer and winter using urban transects and remote sensing. The atmospheric heat island has been recorded up to 5.2 degrees C at 8 pm, on a sunny winter day, and up to 8.8 degrees C on a sunny summer day. Regarding the surface heat island, values up to 4 degrees C in winter and up to 6 degrees C in summer have been recorded for sunny days at 2 p.m. In this paper, and as a way to mitigate these behaviours, local scenarios and strategies have been simulated through the implementation of green roofs on buildings in the city centre, using the ENVI-met software. The results of these simulations are presented in order to mitigate both surface and air temperatures in the city. The strategy of implementing green roofs is able to reduce the surface temperatures of building roofs, but more important is the reduction of air temperature at pedestrian level, which favours urban life.