And in many cases, that may be a lot harder than simply getting the agreement ratified or otherwise approved at home.

Take the U.S. It pledged, as part of the Paris process, to cut its emissions 26-to-28 percent below the level where they were in the year 2005 by the year 2025. And it outlined a bevy of policies, most prominently the contested Clean Power Plan, in order to get there.

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Since then, there have been questions about how achievable this U.S. goal is — and now, a new study in Nature Climate Change appears to raise concerns to a new pitch. The paper, by Jeffery Greenblatt and Max Wei of the Lawrence Berkeley National Laboratory, does the math on current and proposed future U.S. climate policies and basically finds that it will be difficult (although certainly not impossible) for the country to hit its embraced target, without doing even more than is being contemplated right now.

“They said we’re going to make a 26-to-28 percent reduction, and here are the different ways we’re going to do that,” said Greenblatt in an interview. “We’re going to pass the Clean Power Plan, improve the efficiency of heavy duty trucks…We just looked at each of those policies, and did the best we could to look at what the impact of any of them would be.”

Greenblatt and Wei first sought to calculate what 2005 emissions actually were, and found that we don’t really know — or at least, not to a high level of precision. They estimate that we emitted between 6.323 and 7.403 billion tons of carbon dioxide equivalent emissions in that year. That’s right: The high end and low end estimate diverge by over a billion tons.

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Based on these numbers, Greenblatt and Wei calculate that the U.S. needs to reduce its annual emissions, in 2025, to between 4.553 billion and 5.478 billion tons in order to meet its goal. However, the study finds that based on three sets of expected policies — already finalized laws or regulations (like the Clean Power Plan); suggested laws, regulations, or executive actions; and future potential actions that have already been “announced” — the U.S. is still on course to fall short.

Specifically, the study finds that in 2025, with all three categories of policies, the U.S. might fall within the target range by as much as 356 million tons if we’re lucky — so success is possible — but emissions could also still exceed the range by as much as 924 million tons. “Further reductions will be necessary to close this gap with confidence,” the authors write.

So why are we so off? Here’s where the new research partly merges with one of the most contentious matters in current climate policy — the question of how much of the fast-acting greenhouse gas methane we’re emitting, and where it is coming from.

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Earlier this year, the U.S. EPA increased its estimate for how much methane is being emitted by the oil and gas sector, and by the U.S. overall, in recent years. The new study has more or less done something similar.

“We made some corrections to the 2005 and 2025 estimates for methane,” says Greenblatt. In particular, he said, in 2005 these changes added 400 million additional tons of carbon dioxide equivalents emitted as methane.

Greenblatt emphasized that assumptions of higher methane emissions aren’t the only reason that the U.S. could miss its goals, but that it’s a significant one. “An increasing amount of methane emissions is part of the story,” he said.

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Another reason for lingering uncertainty about whether the U.S. will meet its goals involves trees: There’s a large range in estimates of how much carbon they’re likely to absorb in coming years. And although this rarely gets mentioned, the U.S.’s overall policy goal relies not only on emitting less greenhouse gases, but also on storing large amounts in forests.

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Problems such as these extend far beyond the U.S., of course. Similar questions are sure to be asked about other countries pledges, too. Moreover, multiple analyses have found that even if you hold countries at their word and they do achieve their pledges, it still won’t be enough to keep the warming of the planet below 2 degrees Celsius, which is an explicit target of the Paris agreement. So no matter what, global ambition is going to have to be raised.

“There is certainly need for further policy action,” said Greenblatt in an interview. But he added, “I think the U.S. should be complimented. They set their own target and they set out a path to meet it as best they could. I think if they need to work a little harder, that’s not an unexpected outcome.”

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