A new report from Greenpeace warns that the growth in cloud computing will be accompanied by a sharp rise in greenhouse gas emissions, and calls on big companies like Facebook, Yahoo and Google to do more to help the environment. "The cloud is growing at a time when climate change and reducing emissions from energy use is of paramount concern," Greenpeace says in the report. "With the growth of the cloud, however, comes an increasing demand for energy."

How much of an increase? Greenpeace admits that it's hard to calculate, but it estimates that the electricity consumed by the world's datacentres and telecommunications networks, which it calls "the main components of cloud-based computing", will triple between 2007 and 2020.

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The big internet companies are working hard to improve their energy efficiency, which helps reduce their operating costs. But Greenpeace argues that they will still choose to locate data centres where it makes the most business sense, even if that means using "dirty" power from coal-fired plants.

The report, titled "Make IT Green: Cloud Computing and its Contribution to Climate Change," takes as its starting point the upcoming release of Apple's iPad. Greenpeace calls the iPad "a harbinger of things to come," in the sense that its main purpose is to download video, music and books from cloud-based services.

It's not the first time Greenpeace has called attention to energy use in data centres, and the environmental group seems keen to raise awareness among consumers about where their Internet services come from.

Greenpeace recently started a campaign against Facebook, over its decision to build a datacentre that will derive its electricity mostly from coal-fired power stations. In its report, Greenpeace contrasted that decision with one made by Yahoo, which is building a datacentre that will use mainly hydroelectric power.

Facebook has responded that Oregon's climate will allow it to use a type of fresh-air cooling that removes the need for heavy cooling equipment. It also says it never "chose" to run its datacentre on coal.

"The suggestions of 'choosing coal' ignores the fact that there is no such thing as a coal-powered datacentre. Similarly, there is no such thing as a hydroelectric-powered data center. Every datacentre plugs into the grid offered by their utility or power provider," Facebook told the website Data Center Knowledge last month.

Google and Microsoft, even while they continue to build new datacentres, have done work to educate people about how to reduce their energy use. Last year Google even proposed a $3.7 trillion, 20-year plan to reduce US dependency on fossil fuels.

Still, Greenpeace wants internet companies to do more to shape the supply of renewable energy available to them, by increasing demand through choices about where they locate new data centres, or by lobbying politicians. "Ultimately, if cloud providers want to provide a truly green and renewable cloud, they must use their power and influence to not only drive investments near renewable energy sources, but also become more involved in setting the policies that will drive more rapid deployment of renewable electricity generation economy-wide, and place greater R&D into storage devices that will deliver electricity from renewable sources 24/7," Greenpeace said.