Trees don't just add a welcome splash of green to our cities. They absorb CO2, restore biodiversity, improve our wellbeing, and even lower outdoor temperatures in the most concrete-heavy parts of town. Together with a US public broadcaster, we look at what's being done to ensure "urban forests" play a bigger part in the fight against climate change. From San Francisco's stunning Tunnel Tops, to the tiniest of gardens in suburban Tokyo, this movement takes many forms. All are equally inspiring.