It's fair to say that Barack Obama has done more than any president before him to address global warming. He's taken step after elaborate step to try and nudge down US greenhouse-gas emissions in the coming years.

But it's also fair to say that those efforts are wildly inadequate in the face of the broader climate problem.

'Fifty years from now, is that going to seem like enough? i think the answer to that is going to be no.'

No need to take my word for it. John Podesta, a key adviser to the White House, said much the same thing in a recently published interview with Harper's about all of the climate policies Obama has taken: "Fifty years from now, is that going to seem like enough?" Podesta mused. "I think the answer to that is going to be no."

To Obama's defenders, this may sound overly harsh. After all, consider everything the administration has done so far:

Add it all up, and it's a far-reaching agenda — costing billions of dollars and reshaping several large industries. And the administration has done most of this despite the fact that Congress is unwilling to pass any legislation to address global warming.

But as Podesta noted in that Harper's interview, even when you put it all together, the world still isn't close to keeping global warming below the agreed-upon 2°C threshold. In part that's a reflection of the fact that climate change is an insanely large, global issue — and a smattering of regulations in the United States can't possibly suffice.

The Obama administration's current goal is to reduce overall US greenhouse-gas emissions 17 percent below 2005 levels by 2020. There's no guarantee that the country will even get there, although one State Department analysis last year suggested that we might just make it.

But now consider that the United States is responsible for just 17 percent of worldwide carbn emissions — and emissions from China, India, and the rest of Asia are growing fast:

The Obama administration's answer to this is: Well, of course. The United States can't possibly solve climate change on its own. The hope is that the US will get within range of its promised goal of cutting emissions 17 percent by 2020 — and that will spur countries like China and India to do more to cut emissions. It's a long, slow process.

That's hardly a crazy plan. There are just lots of huge unknowns here:

1) So much depends on China. At the moment, many China-watchers are predicting that the country's greenhouse-gas emissions won't peak until 2030. If so, then it could prove extremely difficult for the world to achieve its goal of keeping global warming below 2°C.

Now, there are some tentative signs that China may decide to cap its emissions in the years ahead. But no one really knows if that will pan out or not — and there are good reasons to be skeptical that any cap would make a huge difference. So far, when China has had to choose between economic growth and cutting its emissions, it usually chooses growth.

2) The hardest cuts to US emissions still lie ahead. Meanwhile, even if the world does get a new international agreement at the Paris climate talks in 2015, the hardest part is going to be figure out how to make steeper emissions cuts.

The hardest cuts in US emissions are likely out of Obama's hands

The Obama administration's 2020 goal is only supposed to be the start, after all. Over the long term, the United States has pledged to cut its emissions 83 percent below 2005 levels by 2050 — a level deemed necessary to help stabilize the amount of carbon-dioxide in the atmosphere.

But right now, there's no clear path to getting there. The EPA's sweeping new power-plant rules amount to a roughly 6 percent cut in overall US greenhouse gases from today's levels — just a small fraction of the way. (And that's assuming that the rules actually work as intended, that every state complies with the regulations, that lawsuits don't bog the whole thing down, and that a new president doesn't enter office in 2016 and dismantle or modify the rules.)

That 83 percent cut by mid-century is also likely to be a massive technological challenge. Since 2005, US emissions have fallen about 10 percent due to the recession, to a glut of cheap natural gas that's edged aside dirtier coal power, and a decline in driving. Yet many experts think that further cuts will require something else entirely: a massive, unprecedented boom in clean energy.

Back in 2013, the Clean Air Task Force summed up the scale of the task well: "Ultimately, we will need to capture the carbon from gas-fired power plants as well. Renewable energy faces challenges of scale, cost and intermittency; carbon capture and storage faces cost challenges at full scale; and current forms of nuclear power are challenged by safety, waste management, weapons proliferation and cost risks."

Ideas for advancing that technology typically involve things like an economy-wide carbon tax or vast programs to spur technological innovation in the energy sector. Most of those policies, though, require Congress — which means it will be hard to make deep cuts unless there's a critical mass of lawmakers who actually want to tackle global warming.

At best, the new EPA power-plant rules get us a bit of the way to those goals. The regulations could require electric utilities to use a smidgen more solar and wind power, helping to push those technologies forward a bit. (The US share of electricity produced by renewables is expected to rise from around 5 percent in 2012 to about 9 percent by 2030 under the rules.) And they might give a boost to global climate talks.

But the biggest changes that need to happen to avert large temperature increases — political, societal, technological — have yet to be made. And those changes are largely out of Obama's hands.

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