  Urban green space has been regarded as contributing to citizen happiness by
promoting physical and mental health. However, how urban green space and
happiness are related across many countries of different socioeconomic
conditions has not been explained well. By measuring urban green space score
(UGS) from high-resolution Sentinel-2 satellite imagery of 90 global cities
that in total cover 179,168 km$^2$ and include 230 million people in 60
developed countries, we reveal that the amount of urban green space and the GDP
can explain the happiness level of the country. More precisely, urban green
space and GDP are each individually associated with happiness; happiness in the
30 wealthiest countries is explained only by urban green space, whereas GDP
alone explains happiness in the 30 other countries in this study. Lastly, we
further show that the relationship between urban green space and happiness is
mediated by social support and that GDP moderates the relationship between
social support and happiness, which underlines the importance of maintaining
urban green space as a place for social cohesion in promoting people's
happiness.
