As part of his plan to address global warming, President Obama has pledged that US greenhouse-gas emissions will be roughly 17 percent lower in 2020 than they were in 2005.

But is that actually possible? As of 2013, US greenhouse-gas emissions were just 8.5 percent below 2005 levels — and starting to rise again. We're not yet on track.

What's more, a new analysis suggests that the policies Obama has put forward so far, including the EPA's proposal to cut carbon-dioxide emissions from power plants, aren't yet sufficient to hit that 2020 target. The EPA will need to put out an extremely strong final rule, and the US will still need additional policies, like curbing methane leaks from oil and gas drilling. That's not impossible, but it won't be easy, either.

How to cut emissions 17 percent by 2020

An October 2014 analysis by the Rhodium Group's John Larsen, Kate Larsen, and Whitney Ketchum looked at what steps the US would need to hit that 2020 target. Here's the key chart:

The leftmost column shows "reference emissions." Rhodium's analysts expect US greenhouse-gas emissions to be well above the target by 2020 unless the administration takes four additional steps:

1) Strong EPA regulations on power plants. The EPA has proposed — but not yet finalized — new restrictions on carbon-dioxide emissions from existing coal-fired power plants. This is known as the Clean Power Plan, and you can read here for details.

The EPA's plan to cut emissions from power plants isn't enough on its own

The Rhodium Group analysts looked at an optimistic scenario in which the EPA puts the finishing touches on its plan without any legal delays and that all states cooperate in implementing it — a debatable assumption, given that many red states and industry groups are planning to file lawsuits against the rule. The analysts also assumed that states would comply with the regulations mainly by switching from coal to cleaner energy sources (rather than simply double-counting existing energy-efficiency measures).

In this optimistic scenario, the EPA plan could conceivably cut US greenhouse-gas emissions by between 310 to 463 million metric tons of CO2-equivalent by 2020. But even if this all came to pass, the US still wouldn't hit its goal. The EPA rule isn't enough on its own.

2) Efficiency rules and other measures to cut carbon: Lately, the Obama administration has also been putting forward other climate policies besides the EPA rule — like energy-efficiency regulations for new household appliances or curtailing emissions from federal agencies. These could nudge down US carbon-dioxide emissions from energy use a bit further.

3) Crack down on methane. This is a big one. Carbon-dioxide isn't the only gas that heats up the planet. Methane is another potent greenhouse gas that's 34 times as effective at trapping heat as CO2. Cows burp it up. Trash that decomposes in landfills releases methane. There are also methane deposits trapped underground — we call that stuff "natural gas" and we extract it for energy. But methane can also leak into the air during that process.

The US can't meet its goal without cutting methane leaks from oil and gas operations

Methane made up about 8.7 percent of all US greenhouse gases in 2012, and those emissions are expected to rise in the years ahead.

For their part, the White House and EPA have recently been working on proposals to address this, involving future regulations on new oil and gas wells that are leaking methane, proposed standards for landfills, as well as partnerships with dairy farms to speed up the adoption of methane digesters that turn cow dung into energy.

Getting these rules right will be crucial. As Conrad Schneider and David McCabe of the Clean Air Task Force have pointed out, if methane leaks from oil and gas operations can't be sealed up, the United States is very unlikely to meet its 17 percent goal. And some environmentalists have criticized the White House's current methane plan as insufficient.

4) Crack down on HFCs. The Obama administration is also planning a crackdown on hydrofluorocarbons, another potent greenhouse gas used in air conditioners and refrigerators. Some of this can be done through existing EPA regulations, but the administration is also trying to negotiate a new treaty with India and China to curtail HFCs. At the high end, these measures could cut an extra 135 million tons of CO2-equivalent, but the Rhodium Group analysts aren't positive that the US will actually cut this much.

A 17 percent cut is possible — but not yet certain

If the Obama administration followed aggressively through on all four of those policies, then the US could meet its promise of cutting greenhouse-gas emissions "in the range of" 17 percent below 2005 levels by 2020. (Note that US negotiators specifically use the phrase "in the range," giving themselves some wiggle room.)

But this isn't assured. The Rhodium Group analysts also looked at another, less optimistic scenario in which the EPA's new power-plant rules are less ambitious, the agency mostly relies on voluntary measures in the oil and gas sector to cut methane emissions, and the administration only makes modest headway in reducing HFCs. The result? The US misses its goals, cutting emissions just 12 percent below 2005 levels by 2020:

And even this scenario isn't assured. If, for instance, a Republican president took office in 2016 and decided to relax or delay some of the EPA's greenhouse-gas regulations, US emissions might fall more slowly.

Then again, it's also possible that unexpected developments could cause emissions to fall faster than expected. Perhaps states will cut emissions quicker than the EPA's new regulations urge (note that the Northeast has its own cap-and-trade program, called RGGI, to cut emissions). Perhaps solar power will continue to grow so fast that it crowds out fossil-fuel plants. Or perhaps electric cars will catch on quicker than expected. These things aren't guaranteed, but surprises are always possible.

What's more, this analysis depends a lot on economic growth rates between now and 2020. Faster-than-expected growth could lead to a faster-than-expected uptick in carbon-dioxide emissions, as people drive more and use more electricity. Conversely, a big recession could wallop emissions.

The big picture on climate policy

It's also worth reiterating that America's climate goals for 2020 are only a first step in dealing with global warming. If anything, this is actually the easiest part.

Keep in mind the broader picture here: If the world wants to meet its goal of avoiding more than 2°C of global warming (3.6°F) — a level popularly considered "dangerous" — then worldwide greenhouse-gas emissions will have to fall roughly in half by 2050. The United States will have to do a lot more. So will Europe, China, India, and so on.

New pledges from US, China, and Europe put the world on pace for 2.9°C to 3.1°C of warming

Now, many of those countries are starting to make further promises as part of these ongoing UN climate talks. The Obama administration has also formally pledged that US greenhouse-gas emissions will be 26 to 28 percent lower in 2025 than they were in 2005. This is mostly out of Obama's hands and will be determined by future presidents. But if the US can't get a 17 percent cut by 2020, then that next goal is going to be even tougher.

As part of the big recent US-China climate agreement, China also intends to stop its emissions from rising past 2030 or so — and plans to massively ramp up its share of renewable energy. Europe, meanwhile, has pledged to cut its emissions around 40 percent below 1990 levels by 2030.

Those are fairly ambitious promises. A new analysis by Climate Action Tracker found that if you add up all these pledges, the world will be on pace for roughly 2.9°C to 3.1°C of warming by 2100. That's still a lot of warming, but it's less than we were on pace for before (around 4°C or more). The trick, of course, is that all those pledges actually have to pan out. And as we've seen in the US case, that can't be taken for granted.