Advocates of forceful action on climate change have long held a trump card. The primary source of greenhouse gas emissions in the U.S. is coal plants, and — since the Supreme Court has determined that those emissions are a pollutant — the EPA is mandated to regulate them. At some point, then, whether whatever president likes it or not, the agency had to make a rule limiting carbon dioxide emissions.

But, what the court giveth, the court can rescind in a tightly contested vote. And with a barrage of petitions raining down on a conservative-leaning Supreme Court, it's possible that the EPA's pollution-control mandate could be eradicated well before the threat of climate change is.

Among large, developed countries, only Australia emits more carbon dioxide per person than the United States. China emits the most overall, of course, but as greenhouse gas polluters, we're still in the top tier. For decades, environmentalists have pushed to cut the country's overall emissions levels, winning a significant victory in 2007 when the Supreme Court ruled in Massachusetts vs. EPA that carbon dioxide was an air pollutant. Under the Clean Air Act, that required that the EPA take action.

So far, it hasn't. When Barack Obama came into office, pledging to be the president who oversaw the moment when "the rise of the oceans began to slow," advocates hoped he'd move quickly to curb carbon dioxide emissions. In 2009, the House passed a measure to institute a market-based system to reduce emissions; it wasn't voted on in the Senate. Last January, Obama called for a similar solution, saying he'd act if the Congress wouldn't. That declaration, however belated, was interpreted as meaning he would ask the EPA to regulate coal-burning plants, even though his staff seemed unprepared to do so.

While Obama and the EPA have delayed, opponents of regulating greenhouse gas emissions — largely fossil fuel companies — have filed a number of challenges to the Court's ruling. To date, they've failed; in both July and December last year, lower courts upheld the EPA's right to regulate. But each lower court decision makes an appeal to the highest court more inevitable. Reuters indicates that a case could arrive at the Supreme Court as soon as this October.

Some of the challengers specifically ask the court to consider overturning Massachusetts v. EPA. They point out that the Clean Air Act, which passed in 1970, was not designed to tackle climate change. At least one brief, by the state of Virginia, challenges the EPA's evaluation of the climate change science that underpinned its decision to regulate greenhouse gases. Others contend the Supreme Court's holding in the 2007 ruling, which specifically addressed automobile emissions, did not give the EPA the authority to issue greenhouse gas rules that affect such a broad cross-section of the economy.

If the justices were to accept one of these broad petitions and side with challengers, they could make it impossible for the EPA to regulate greenhouse gases and could open the door to attacks on the air pollution regulations the agency has formulated for 30 years, according to Dru Stevenson, a law professor at the South Texas College of Law.

That last point is even more alarming. The EPA uses the Clean Air Act as its mandate to regulate air pollution. If the Court says that it can't use the law to regulate carbon dioxide, it will not take long for companies that emit pollution of various types to challenge other Clean Air Act regulations.

There were two dissenting opinions filed in the five-to-four Massachusetts vs. EPA, written by Chief Justice John Roberts and Justice Antonin Scalia. The make-up of the Court hasn't changed much; there are two new justices — Sotomayor and Kagan — who replace two that voted to support the majority opinion. In other words, it's not clear that the Court hearing the case means that the EPA's ability to regulate carbon dioxide emissions will be revoked.

But it's another sign of the lack of progress made in addressing climate change over the last six years. In 2007, a five-to-four vote established a critical tool that the EPA could use to tackle global warming. In 2013 — without that having happened and facing a much weaker political position — environmental advocates can only hope to keep that trump card in the deck.