Guardian: To some they are a rural escape in the centre of the city, to others they are a chance to test their green fingers and design skills. Now London mayor Boris Johnson has found a new use for urban roof gardens – as a key weapon on the front line against global warming. An increase in the number of rooftop gardens to soak up rainwater across the capital is among a series of measures suggested by Johnson yesterday, as part of efforts to prepare London for the effects of climate …