Amazon Web Services says that one day it will rely solely on renewable power. But Greenpeace says a ramp-up in data-center construction in Virginia, where power comes mostly from coal and nuclear plants, makes that goal elusive.

Share story’s cloud-computing unit says that one day it will rely solely on renewable power. But Greenpeace says a ramp-up in data-center construction in Virginia, where electricity comes mostly from coal and nuclear plants, makes that goal elusive.

In a report released Tuesday, the environmental group says Amazon has taken many welcome steps toward diminishing the carbon impact of cloud computing, including throwing its weight behind renewable-energy policy, committing to use only clean electricity in the long term, and building solar and wind farms.

But Greenpeace claims the renewable and hydroelectric power mix available in the areas where Amazon’s data centers are, adjusted for various factors, has shrunk to 17 percent from 23 percent in 2015.

Part of the reason is that the “near doubling of data center capacity in Virginia,” where a large chunk of the nation’s cloud infrastructure lies, “far outstripped the addition of renewable energy supply,” the report says.

Greenpeace’s comments come as clean-energy advocates look more closely at cloud computing, a fast-growing source of demand amid otherwise flat consumption of electricity.

Amazon’s cloud-computing unit, Amazon Web Services (AWS), is by far the biggest cloud provider.

To be sure, the concentration of computing power and storage in data centers owned by cloud giants such as Amazon, Microsoft and Google is more efficient than having companies run their own facilities.

A report last year by the Berkeley Lab concluded that cloud computing and other efficiency gains have allowed energy usage by U.S. data centers to remain “nearly constant” over the past decade, a big slowdown from the rapid rise seen after the turn of the century.

But Greenpeace argues that the cloud’s explosive growth, driven by the popularity of videostreaming and other data-rich services, could offset some of these efficiency gains.

In any case, the large companies that are building out the cloud’s infrastructure are going to be key influencers of decisions on what power plants to build, and where; hence Greenpeace’s call for them to make renewables a priority.

AWS, like other tech firms, has put the environment on its agenda. Last year the company, along with other tech giants, filed an amicus brief backing the U.S. Environmental Protection Agency’s Clean Power Plan, which seeks to curb power-plant emissions.

It also has joined the American Council on Renewable Energy and other advocacy forums for public policy related to the environment.

AWS responded to a request for comment about the Greenpeace report by highlighting its environmental achievements. In 2016, some 45 percent of its energy came from renewables, the company said, and that should grow to 50 percent by the end of this year.

Amazon has commissioned 10 renewable-energy projects, including solar and wind farms, to deliver enough electricity to feed a city the size of Portland into the grid supplying its data centers in Ohio and northern Virginia. Four of these projects are already online.

“That said, we are nowhere near done,” Amazon said. “We will continue to make progress toward our 100 percent goal and have many exciting initiatives planned.”

But Greenpeace says these projects are not offsetting the requirements of the Amazon’s huge expansion in Virginia.

The 14 AWS facilities Greenpeace counted in Virginia are in a grid that is about 3 percent “clean” (which is how the organization refers to solar, wind and existing hydroelectric projects). Coal, nuclear and natural gas accounted each for about a third of the power supply available to those centers.

AWS data centers in Oregon were in much greener grids, with 85 percent of power generated by renewables or hydroelectric power.

According to Greenpeace, AWS’ energy needs are met on average by utilities that rely on coal for 30 percent of their power, on nuclear generation for 26 percent, and on natural gas for 24 percent.

The group estimates that 17 percent of AWS needs are met with “clean energy.” That’s below 32 percent for Microsoft and 56 percent for Alphabet’s Google.

(Google announced last month that in 2017 both its data centers and its offices would run 100 percent on renewable energy.)

Greenpeace acknowledged Amazon’s increased advocacy on energy and climate. But the environmental group called Amazon ”the least transparent” when it comes to revealing its energy footprint.

In fact, the group called Amazon “one of the single biggest obstacles to sector transparency,” because it didn’t disclose what Greenpeace considered “basic details” about its energy footprint.

Microsoft, which used to draw similar criticism from the advocacy group, has started disclosing more details on the energy mix powering its own network of data centers.

The Redmond company “has begun to lift the veil,” Greenpeace says, but still lags behind industry leaders Apple and Facebook in its disclosures.

If the technology sector is going to lead a transition toward renewable power, “what we need to happen is to have more transparency, especially from Amazon. This is critical,” Greenpeace’s information technology sector analyst Gary Cook said in an interview.