Apple made the pledge. So did Google and Facebook. But Amazon stayed silent.

Over the past few years, Apple, Google, and Facebook pledged to run their online empires on renewable energy, and considering how large these empires have become—how many data centers and machines are now required to keep them going—this was a vital thing. But despite pressure from the likes of Greenpeace, the environmental activism organization, the other big internet name, Amazon, didn't budge.

That all changed on Wednesday. With a post on its website, Amazon's cloud computing division—Amazon Web Services—said it has a "long-term commitment to achieve 100 percent renewable energy usage for our global infrastructure footprint."

Amazon is the largest cloud computing company in the world, providing services where developers and business can rent computing power. Many popular websites and services, such as Netflix, Spotify and Pinterest, all use the Amazon cloud. If Amazon cuts its emissions, it could have a significant impact on the tech industry's carbon footprint.

But don't celebrate too much. Like other companies committing to switch to 100 percent renewable power, Amazon will likely take many years to complete the transition. Facebook, for example, estimated in 2012 that it would actually increase its use of non-renewable energy over the next year, even as it adds more renewable power to the mix. The company estimated that only 25 percent of its power would come from renewable sources by 2015.

In a statement sent to WIRED on Wednesday, Greenpeace IT analyst Gary Cook pointed out that, unlike its peers, Amazon hasn't published a roadmap outlining its plans to wean itself off dirty energy. That makes it harder to assess how serious the company really is.

None the less, with his statement, Cooks says that Amazon's commitment is still a welcome sign of progress. "With the world's largest public cloud apparently joining Apple, Google, Facebook and others in committing to power with 100 percent renewable energy, the race to build a green internet may be gaining a crucial new competitor," the statement reads.

Greenpeace has often criticized Amazon for not following in the footsteps of rivals like Google. Amazon ranked among the worst companies in the organization's Clicking Clean report last April.

Amazon runs a large number of data centers in Virginia, where Cooks says the vast majority of electricity comes from nuclear power, natural gas, and coal. Amazon has, however, added locations in other regions that it claims are 100 percent powered by renewable energy, starting with its Oregon-based data centers, built in 2011.

One of the biggest problems, Cook says, is that Amazon hasn't been forthcoming about its energy use. For Amazon, more transparency will be the next step in proving that its future is green.