Today in our environmental science lecture, we'll discuss urban heat islands. Cities tend to be warmer than surrounding rural areas because concrete and asphalt absorb and re-emit heat. However, one surprising factor is the role of green spaces. A recent study in Melbourne found that parks can reduce local temperatures by up to 4 degrees Celsius, but only if they are larger than 2 hectares. Smaller gardens have a negligible effect. Additionally, the study showed that trees along streets are more effective at cooling than grassy areas, due to shade and evapotranspiration. This has implications for city planning, especially in densely populated districts.