A Tuesday Greenpeace report that studied the environmental impact of the cloud criticized firms like Apple, Microsoft, Twitter, and Amazon for lagging behind their Web counterparts.

The firms, however, took issue with that characterization.

Amazon, Apple, and Microsoft "are all rapidly expanding without adequate regard to source of electricity, and rely heavily on dirty energy to power their clouds," Greenpeace said in its report, dubbed "How Clean Is Your Cloud?"

Facebook, which has tangled with Greenpeace in the past, earned high praise from the group for committing to renewable energy, particular with its new data center in Sweden.

Greenpeace also lauded Google and Yahoo for "prioritizing access to renewable energy in their cloud expansion, and [becoming] more active in supporting policies to drive greater renewable energy investment."

Apple, Microsoft, Twitter, and Amazon, however, did not earn gold stars.

Apple: Greenpeace says Apple's huge, new data center in North Carolina, as well as one planned for Oregon "are powered by utilities that rely mostly on coal power." Apple has the available cash, Greenpeace argued, to make a difference, but its policies are putting it "behind companies such as Facebook and Google who are angling to control a bigger piece of the cloud."

"Our data center in North Carolina will draw about 20 megawatts at full capacity, and we are on track to supply more than 60 percent of that power on-site from renewable sources including a solar farm and fuel cell installation which will each be the largest of their kind in the country," Apple said in a statement. "We believe this industry-leading project will make Maiden the greenest data center ever built, and it will be joined next year by our new facility in Oregon running on 100 percent renewable energy."

Twitter: The micro-blogging site earned low marks primarily for staying mum. "While assisting in the efforts to challenge the transparency of others, the company remains quiet on its own energy use and associated environmental impacts," Greenpeace said. The group praised Twitter for moving its servers to renewable energy-friendly Sacramento, but was critical of rented space in Atlanta, which "has a high percentage of coal-fired electricity."

"The Greenpeace report raises important considerations around energy efficiency," a Twitter spokeswoman said in a statement. "We continue to strive for greater energy efficiency as we build out our infrastructure, and we look forward to sharing more on our efforts in this space in the coming months."

Microsoft: The software giant earned higher marks that Apple and Twitter, but was criticized for not having a consistent data center policy that "gives preferences to renewable energy sources." Expanding Quincy, Wash. and Dublin sites are good, Greenpeace said, but growing a site in coal- and nuclear-heavy Virginia? Not so much.

Microsoft declined to comment on the Greenpeace report, but pointed to its Software Enabled Earth blog and its Global Foundation Services blog, "which details how Microsoft is working to improve datacenter efficiency," a spokeswoman said.

Amazon: Like Twitter, Amazon was primarily criticized for its lack of transparency. "Despite its significant size and resources, AWS does not appear to have made any purchases or investments in renewable electricity for its facilities," Greenpeace said.

Amazon, however, challenged the report. Its "data and assumptions about Amazon are inaccurate," the company said. "Amazon Web Services believes that cloud computing is inherently more environmentally friendly than traditional computing. Instead of each company having their own datacenter that serves just them, AWS makes it possible for hundreds of thousands of companies to consolidate their datacenter use into a handful of datacenters in the AWS Cloud, resulting in much higher utilization rates and eliminating the waste that occurs when datacenters don't operate near their capacity. The cloud enables a combined smaller carbon footprint that significantly reduces overall consumption."