The Supreme Court just put a kink in President Obama's climate change plans. How big a kink ... well, that remains to be seen.

In a surprise 5-4 decision on Tuesday evening, the conservative justices halted implementation of the EPA's Clean Power Plan — a major regulation aimed at cutting carbon dioxide emissions from US power plants. The hold will last until legal challenges to the rule are resolved.

A little background here: The Obama administration finalized its Clean Power Plan last summer. This sweeping EPA regulation would require every state to submit a plan between 2016 and 2018 for reducing CO2 emissions from their electricity sectors by a set amount. These plans would take effect by 2022 at the latest, and states would have a lot of flexibility in deciding how to cut: more efficiency, more renewables, switching from coal to gas, carbon trading, whatever.

The Clean Power Plan is the backbone of Obama's climate agenda

This rule was a huge deal, serving as the backbone of Obama's broader plan to address global warming. At the international climate talks in Paris in December, the United States pledged to slash its greenhouse gas emissions at least 26 percent below 2005 levels by 2025. It'd be extremely tough to hit that target without the Clean Power Plan.

Yet as soon as the Clean Power Plan was finalized, it was challenged by 27 states, led by West Virginia and Texas, who called it "the most far-reaching and burdensome rule EPA has ever forced onto the states." You can read more on their legal challenges here: They dispute that the Clean Air Act gives the EPA authority to regulate CO2 in the way it did. These states also asked the courts to suspend implementation of the plan (a move known as a "stay") until there was a final ruling.

The courts initially sided with Obama on this. On January 21, a three-judge panel of the DC Circuit Court — the appeals court right below the Supreme Court — agreed to hear the legal challenges but refused to issue a stay in the meantime. Oral arguments for that case were set to start June 2. (Two of the panel's judges were appointed by Clinton and Obama, so it's plausible the DC Circuit will uphold the rule.)

Now, however, the Supreme Court has stepped in and said, no, no, let's suspend implementation of the Clean Power Plan until the courts figure out if they'll uphold it or overturn it. That means the DC Circuit Court will have to hear and rule on the legal challenges this summer, and then (presumably) the Supreme Court will take up the case and make a final decision. Only then can the EPA go forward with implementation.

It's unprecedented for the Supreme Court to step in and block a federal regulation like this, before review by an appeals court. None of the justices gave any explanation for the move. The justices voting to block the rule were Roberts, Alito, Scalia, Thomas, and Kennedy.

What happens next?

So what does today's ruling mean in practice? A few thoughts:

1) This isn't a fatal blow for Obama's climate rule, but it's not an auspicious sign. It suggests that five justices on the Supreme Court are entertaining serious doubts about the Clean Power Plan.

2) It's still unclear how this will affect state preparation. States were originally supposed to submit plans for cutting emissions by September 6, 2016 (or formally request an extension until 2018). But there's no chance that both the DC Circuit Court and the Supreme Court will resolve this case by September. As such, some states may decide to hold off worrying about their carbon emissions until the courts figure things out.

In a statement, the White House said it was confident the Clean Power Plan would be upheld and urged states to keep developing their plans in the interim: "Even while the litigation proceeds, EPA has indicated that it will work with states that choose to continue plan development and will prepare the tools the states need."

3) If the Supreme Court does eventually uphold the law, today's ruling probably won't have much practical impact in the end. Remember, the Clean Power Plan doesn't require CO2 cuts until 2020-'22, by which point all this litigation will likely be resolved.

4) Alternatively, the Supreme Court could end up overturning the Clean Power Plan entirely. In that case, the EPA would likely still keep its legal authority to regulate carbon dioxide under the Clean Air Act. So the next step would be up to whoever wins the White House in November. Maybe that will be Hillary Clinton, and she'll try to craft different CO2 rules that comply with whatever limits the Supreme Court imposes. Or maybe it will be Donald Trump, and he'll decide to do nothing.

5) It's also possible the Supreme Court could strike down just a portion of the Clean Power Plan — making it weaker than it otherwise would be — while leaving the rest largely intact. We'll have to see.

6) This is all an excellent reminder that the Supreme Court is exceedingly important for all manner of policy, and the next president will probably have a say in picking a justice or two.

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