Carbon dioxide is the main culprit behind man-made climate change, and power plants are the main culprit behind our CO2 surplus. This week, after decades of delay (and a timely episode of "Cosmos"), the U.S. took one of its biggest steps yet to address this climate-altering elephant in the room.

The "Clean Power Plan" would create the country's first rules on how much CO2 can escape from existing power plants. It's designed to curb U.S. carbon pollution 30 percent from 2005 levels by 2030, using President Obama's executive power to bypass Congress and regulate CO2 via the Clean Air Act. This strategy arose after Congress nixed a 2009 bill that would have set limits on CO2 emissions.

The full EPA proposal spans a dry 645 pages, so here are five of the top takeaways:

1. It's kind of a big deal.

The rule would only apply to about 600 power plants nationwide, but it's widely seen as the most significant single move the U.S. has ever made to fight climate change. That's because those 600 plants are responsible for 38 percent of U.S. carbon dioxide emissions, making electricity production the No. 1 source of CO2 emissions in a country that ranks No. 2 globally for emitting CO2.

Although U.S. power plants face rules on pollutants like arsenic and mercury, no national limits exist for CO2. The Obama administration is already using a different route to curb automobiles' carbon output, setting mileage standards that will nearly double the average fuel efficiency of U.S. cars and trucks by 2025. But by directly limiting CO2 from coal-fired power plants, the EPA rule could both speed up domestic decarbonization and set a better example to boost U.S. influence at U.N. climate talks — potentially helping coax CO2 cuts from other top emitters like China and India. (In fact, on Tuesday a top Chinese adviser suggested Beijing may soon follow suit with its own CO2 cuts.)

2. It's a compromise.

The plan has two main elements: state-specific CO2 targets, which are based on past emissions rates, and guidelines for meeting them. States would choose from a mix of policy options to customize their compliance, including tactics like expanding energy-efficiency programs, investing in renewable power and joining regional cap-and-trade networks. This mimics CO2-cutting proposals from the Natural Resources Defense Council, which were designed to minimize spikes in energy prices.

The average U.S. coal plant is 42 years old, and CO2 limits could accelerate the retirement of some older, less efficient plants. But the EPA plan includes efforts to soften the blow for coal-reliant industries, such as the use of 2005 instead of 2012 as a base year for emissions goals. Environmental advocates had sought the latter, since a drop in U.S. emissions after 2005 makes it a more lenient base for calculating cuts. The proposal also specifies "coal and natural gas would remain the two leading sources of electricity generation in the U.S." in 2030, with each providing more than 30 percent of the total. Today, 37 percent of U.S. electricity comes from coal and 30 percent from natural gas.

"From first impressions, it appears EPA is trying to strike the right balance – a rule that's strong enough to drive real action, yet flexible enough for states and utilities to meet it at a reasonable cost," says Eileen Claussen, president of the Center for Climate and Energy Solutions, in a statement.

3. It saves more than it costs.

By 2030, compliance with the rule is estimated to cost either $7.3 billion or $8.8 billion, depending whether a regional or state-specific approach is used. That's a lot of money, but it's still dwarfed by the savings. The projected climate-related benefits alone — which reflect "global impacts from CO2 emission changes" but not other greenhouse gases — are valued at about $30 billion. Improved efficiency is also expected to shrink consumers' electricity bills by roughly 8 percent in 2030.

On top of CO2, the Clean Power Plan would help reduce emissions of soot, sulfur dioxide and nitrous oxide, offering a windfall of public-health perks. Based on epidemiology research, the EPA projects co-benefits from the cleaner air will save anywhere from $24 billion to $62 billion. That means a net monetized benefit up to 10 times higher than the estimated cost of compliance. It also means avoiding 2,700 premature deaths and 140,000 asthma attacks per year, according to the EPA, which says the soot and smog reductions alone will return $7 in health benefits for every dollar invested.

4. It's not set in stone.

Despite its attempts at flexibility and balance, the plan has disappointed some supporters of regulating CO2 and angered many critics, including a few in Congress who have vowed to fight it. Obama can veto such efforts while in office, but states might be able to delay submitting their plans until 2017 or 2018, the Associated Press points out, well into the term of a different president.

There is no legal deadline to finalize the rule, but Obama has directed the EPA to do so in June 2015 so there's time to implement the plan while he's still in office. The draft will be open to public comment for the next 120 days, including four public hearings this summer. Held the week of July 28, the hearings will take place in Atlanta, Denver, Pittsburgh and Washington. Some environmental groups plan to push for more ambitious emissions targets during the four-month public comment period, while industry advocates will likely try to secure more of an economic cushion.

5. It's a giant baby step.

Despite its leading role in America's domestic carbon drama, CO2 from U.S. power plants still only accounts for 6 percent of global CO2 emissions. The EPA rule may thus be more about precedent than direct impact, paving the way for similar and stronger measures to rein in carbon pollution.

The move "shows that the United States is taking climate change seriously," says European Commissioner for Climate Action Connie Hedegaard. That's a good step before next year's major U.N. climate summit in Paris, but maybe not good enough. Scientists widely say deeper emissions cuts are needed to prevent catastrophic climate change, and to meet the international goal of keeping Earth's average temperature rise below 2 degrees Celsius, or 3.6 Fahrenheit.

"This of course sends a positive signal ahead of the Paris conference to finalize a new global climate agreement next year," Hedegaard says in a statement about the Clean Power Plan. "But for Paris to deliver what is needed to stay below a 2°C increase in global temperature, all countries, including the United States, must do even more than what this reduction trajectory indicates."


For more about the EPA's carbon-cutting proposal, check out the agency's explainer video below:

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