From the Economist

Data centres consumed 0.6% of the world’s electricity in 2000, and 1% in 2005. Globally, they are already responsible for more carbon-dioxide emissions per year than Argentina or the Netherlands, according to a recent study by McKinsey, a consultancy, and the Uptime Institute, a think-tank. If today’s trends hold, these emissions will have grown four-fold by 2020, reaching 670m tonnes. By some estimates, the carbon footprint of cloud computing will then be larger than that of aviation.