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The special significance of green spaces in cities has been fully demonstrated. Green spaces are key elements to improve the quality of urban life. They contribute to human well-being by providing ecosystem services such as climate regulation, capture of pollutants and flood control. They also promote contact between residents and community integration, and offer a favourable place for health, relaxation and contemplation of nature. Greener environments have lower crime rates. They tend to have a positive effect on people and induce mental vitality. Green spaces can help to reduce urban heat islands and generate true cold islands in urbanized environments. Finally, urban parks provide economic value for cities, including an increase in the value of properties in their proximity. The literature on urban climate has highlighted the singular importance of urban greenery for mitigating urban heat islands (UHI) and extreme temperatures. Urban vegetation plays a fundamental role in adapting to climate change in cities. Green areas register lower temperatures than the rest of urban spaces and have a cooling effect that spreads to their surroundings to create a real “cool island” effect. The World Health Organization recommends that green spaces (of at least 0.5 hectares) should be accessible within a 300 m linear distance of residences. However, the concepts of “park” and “green area” are vague and imprecise, and there is no clear consensus about what should be considered urban greenery. Highly artificialized spaces, with high proportions of sealed soil and no vegetation, are often considered “parks” in urban planning. Likewise, forest spaces, with few artificial elements, are usually considered urban “parks”. The use of satellite images has helped to study urban vegetation. Indicators such as the normalized difference vegetation index (NDVI) and many others allow us to understand the extent and quality of greenery, and to assess its impact on day and night temperatures. In this context, the aim of this study was to examine the extent of vegetation in Barcelona Metropolitan Area (636 km2, 3,303,927 inhabitants) from various satellite sensors and indicators of greenness, to determine the thresholds from which it is possible to speak with rigor of urban green.
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