To work out who has the cleanest cloud, Greenpeace looked at the capacity of existing and planned data centres and the sources of electricity used by each, based on the mix in the local grid and declared agreements to purchase from renewable sources - defined as wind, solar and existing hydropower.

This indicated that data centres run by Google and Facebook draw on renewables for 39.4% and 36.4% of their power respectively, compared to just 13.9% and 13.5% for those operated by Microsoft and Amazon. Apple, with 55.1% of its iCloud powered by coal and just 15.3% drawing on renewables, also scored poorly.

Our enthusiasm for cloud services means that more and bigger data centres are on the way. To lessen the impact of this growth, companies can reduce the carbon intensity of data storage. Facebook says that it has increased the energy efficiency of its data centres by 38% by redesigning the hardware - from the servers themselves, to buildings that use evaporative cooling rather than energy-hungry air conditioners. The company's blueprints are open-source so that others can use and improve on the technology.