Environmental groups called the Environmental Protection Agency's proposed rule for curbing power plant carbon emissions a good step, but many were hoping the Obama administration would be more aggressive.

At issue is the year EPA chose to use as a benchmark for emissions reductions. The agency wants to get carbon emissions 30 percent below 2005 levels by 2030, and figured out state-by-state emissions reductions needed based on 2012 levels to arrive at that mark.

How far back states can go to use certain policies — such as renewable electricity mandates or energy efficiency program — to meet the standard varies, EPA spokeswoman Liz Purchia said. She noted each state estimate is based on recent historical data, but that no single year was relied upon to set state goals.

Using a more recent year as a benchmark, though, would have required states to make more drastic cuts, not least because power sector emissions have fallen since 2005. In fact, the sector already is more than halfway toward the EPA's goal, as emissions declined 16 percent between 2005 and 2030.

Environmental groups took note of that arithmetic, and thus will seek through the proposed rule's comment period a more recent benchmark year against which to measure emissions reductions. "[National Wildlife Federation] will continue to work throughout the comment period to get the maximum reductions possible, but we understand that the President has to balance a desire for greater reductions with legality, viability and flexibility. And this rule really does a good job of hitting all of those," Josh Saks, the group's legislative director, told the Washington Examiner in an email.

To that point, an oft-cited Natural Resources Defense Council policy study that is considered a model for the proposed rule said a 31 percent reduction below 2012 levels was possible by 2020 — the EPA, instead, is shooting for a 25 percent cut below 2005 levels by 2020.

"We will be pushing for the strongest possible standards. We will push, for example, for a faster ramp-up of energy efficiency and renewables. That enables a stronger standard," David Doniger, director of the climate and clean air program with the NRDC, told the Examiner in an email.

The proposed rule would reduce 730 million metric tons of carbon pollution by 2030 for 1,000 of the nation's power plants, which is equivalent to taking two-thirds of United States cars and trucks off the road. The EPA expects to finalize it in June 2015, with states submitting implementation plans one year later.

Most environmental groups issued glowing statements on the proposed rule. That the EPA was taking its most significant step ever to restrain emissions that drive climate change was certainly nothing to frown upon, they said, especially considering the gridlock on climate policy on Capitol Hill since a sweeping cap-and-trade bill foundered in 2010.

"The American people support these commonsense safeguards and are sick of the lie that pollution has to be the fuel of our economic engine. The desperate and dirty opponents of these safeguards are using a failed, outdated playbook to protect their profits," League of Conservation Voters President Gene Karpinski said in a statement.

Still, some hinted that the proposal wasn't as stringent as they'd like.

"Today, the president made good on his promise to American families that his administration would tackle the climate crisis, and clean up and modernize the way we power our country," said Sierra Club executive Michael Brune, later adding, "In the month ahead, we will work to ensure a strong and just standard for cutting carbon pollution."

Using 2005 as the benchmark year pleased electric utilities, though the industry's response was mixed on some other elements of the proposal.

The EPA said that it wanted to respect early actors, meaning those states who have enacted policies that range from renewable electricity mandates to energy efficiency programs.

"EPA is setting state goals based on an assessment of the potential for each state to implement a set of proven measures to reduce emissions, including improvements in efficiency at existing coal-fired power plants, increased utilization of existing natural gas fired power plants, and additional low- and zero-emitting energy capacity and increased energy efficiency," agency spokeswoman Liz Purchia said.

This story was originally posted at 2:57 p.m. and has since been updated.