Amazon's claims that its German data center is carbon neutral is a good step forward for the "dirtiest" cloud operator but show proof, says Greenpeace:

Greenpeace Senior Energy Campaigner David Pomerantz said: "While we would like to trust that Amazon's new [Frankfurt, Germany] facility will actually be powered by renewable sources, Amazon's refusal so far to disclose any data about its energy use makes it difficult for customers or the public to accept the company's claim on good faith alone."

"If, however, Amazon is buying renewable energy credits or carbon offsets while continuing to power its Frankfurt facility with coal or nuclear energy, then its latest data center will continue the company's track record as the dirtiest major cloud computing provider."

Greenpeace estimates that more than 60% of Amazon Web Services computing capacity comes from Virginia in the US, where 97% of the electricity is coal, gas, and nuclear. 

Greenpeace report: Clicking Clean: How Companies are Creating the Green Internet.