If the Clean Power Plan is upheld, it will reduce carbon pollution from utilities by 30%. That would require a severe decrease in reliance on coal for electricity. The scheme will raise the cost for utilities to produce electricity, but not the price that consumers pay.

The Clean Power Plan is central, but not essential, to the United States' role in global climate action. Obama's success in climate climate diplomacy is notable; whether he earned the Nobel Peace Prize when he won it or not, he probably has now. His leadership abroad was by all accounts integral to the international passage of the Paris Agreement, and in 2016 he threw the country's weight squarely behind bilateral deals that brought China and India towards joining as well.

The Paris Agreement is a framework for moving ahead global action on climate change. Once it is ratified by an international quorum, all 191 signatories to the Agreement become party to its rules, which include mandated emissions cuts by each state. It's up to the country to say how it will achieve those emissions reductions, and the Clean Power Plan is Obama's signature policy behind America's intended contribution.

Necessary context: The Clean Power Plan is important, but it's not enough on its own to reach America's target.

Despite the hold put on the Clean Power Plan, Obama has been able to reduce climate pollution through other domestic instruments. That includes expanding regulation to cover trucks and planes, using existing rules to require states to reduce unhealthy pollution that is cogenerated with carbon dioxide at coal plants, and withholding funding from states that don't comply.

The stay also didn't prevent the EPA from helping some states form carbon cap-and-trade style markets and defining a social cost of carbon that guides how government projects are evaluated.

The legal challenge to the Clean Power Plan essentially contends that the executive branch doesn't have the power to intepret laws passed by Congress in new ways based on new information. It's a sort of specific application of legal originalism that Justice Scalia, may he rest in peace, invoked in joining four other justices in staying the Clean Power Plan last year. 

Environmentalists had hoped a swift resolution to Scalia's death would strengthen the Clean Power Plan against a court challenge, but his ghost continues to haunt Obama: yesterday a judge invoked a Scalia precedent that suggests Congress, not regulators, must be the source of decisions with "economy-wide" impacts.