The next year is critical for addressing climate change. The United Nations Climate Summit next Tuesday will bring together more than 100 heads of state in New York City. President Barack Obama will be at the summit; China and India will send special envoys in their country leaders’ absence. There will be speeches and announcements related to investments in clean energy, and ahead of the summit an estimated 100,000 climate activists will march for action. All this is to secure the next big step in the fight against climate change—an international agreement on greenhouse gas reductions. Obama hoped the U.S. could shape and then sign such an accord at the United Nations Climate Change Conference at the end of 2015, when world leaders are scheduled to meet in Paris.

It would take a two-thirds vote in the U.S. Senate to ratify an official treaty. That is virtually impossible to get when Republicans hold half the seats—and many of them don’t think climate change is real. Presumably, that is why the Administration is thinking about a different approach, as Coral Davenport of The New York Times reported last month. The idea is to secure an international pact, rather than a formal treaty. Obama could agree to this all on his own, without Senate approval.

Still, what could such a pact actually accomplish? No less important, what couldn’t it accomplish? The answers to those questions seem to be “quite a lot” and “still not enough.” Here’s a breakdown.

What’s the overall goal of these upcoming international negotiations?

You know the basics. The planet is warming. In order to slow down that process enough to avoid the most catastrophic effects, the world needs to reduce the emission of carbon and other greenhouse gases.

World leaders informally set reduction targets in 2009, when they met in Copenhagen. Some countries are struggling to meet those goals (as an example, just look at Australia’s reversal of its carbon tax). There’s no promise that future agreements will be any stronger.

But we’re seeing progress, too. China just pledged to create a national market that allows companies to trade permits for excess carbon emissions. It’s supposed to be in place by 2016. Here in the U.S., the Environmental Protection Agency has proposed to cap carbon emissions from coal-fired power plants.

When world leaders meet in Paris next year, the hope is that they will publicly commit themselves to new, more aggressive targets. For the Chinese, that would likely mean identifying a date by which emissions will peak. In order to contain global warming, analysts argue that date would need to be as soon as possible—like 2025. For the U.S., it could mean something like the goals Obama has floated previously—a reduction in emissions by 80 percent before 2050, for instance.

More broadly, analysts have said they hope emissions of greenhouse gases worldwide—i.e,. not just a few countries, but all of them—will peak by or shortly after 2015.

Does Obama actually have authority to make an agreement supporting such goals?

Obama’s critics certainly don’t think so. They’re already calling it a power grab. But presidents sign informal pacts all the time. Eric Posner, writing at Slate, says it’s a perfectly legitimate exercise of the executive branch’s power. In addition, Obama and his advisers have indicated they see future agreements as part of efforts to comply with the 1992 U.N. Framework Convention on Climate Change. That’s an important distinction, because the Rio framework, as the 1992 agreement is known, is a treaty. The Senate ratified it, effectively committing the U.S. to pursuing its goals.

So how would this informal agreement actually function?

It’s impossible to be certain. After the Times story outlining this ostensible approach appeared, State Department officials were quick to point out they were still deliberating over what, exactly, to do.

Still, environmental advocates say the Administration is likely thinking about what they call a “name and shame” scheme. Here’s how it’d work. Once countries set their targets for emission reductions, they’d agree to disclose publicly the progress they are making—and to answer a series of questions written by a panel of other countries. There would be no formal penalty for failing to provide that information, but countries that failed to live up to their obligations would risk embarrassment and various forms of economic retaliation.

Could such an agreement actually make a difference?

Environmental advocates seem to think so. The big test will be whether both China and the U.S. sign on. If the countries most responsible for current emissions commit to real reductions, the rest of the world is likely to take notice—and to take the agreement seriously.

These advocates also see an informal agreement as an important political step here in the U.S.—something that could encourage future congressional action, perhaps when there are more members who believe climate change is real and worth fighting. “It is important not just to judge these agreements on what the piece of paper says,” says Jake Schmidt, international climate director at the Natural Resources Defense Council. “It’s about the follow through that to me is the most important thing to judge the system on. We should not build a system that locks us in to a Congress that denies climate change in public opinion forever.”

But an accord would still be weaker than a treaty, right?

Right. A formal treaty would likely impose penalties upon countries that violate it or fail to live up to its terms. A less formal accord probably wouldn’t do that. Except for shame, there’d be nothing to keep China or Canada or, yes, the U.S. from pulling out of the agreement altogether. It also relies on a lot of good faith that countries will put forward ambitious targets, voluntarily. Smaller countries worry the most about this, since their citizens are most vulnerable to the effects of climate change (poor people tend to live in more vulnerable places and have fewer resources for coping with climate change).

That’s why the Paris meetings are not the end of climate change negotiations, but another step in what will have to be a long journey. An August MIT report, "Expectations for a New Climate Agreement," found that the targets under discussion are still too small to avoid nearly 4 degrees Fahrenheit of warming, which scientists say will impose dire, even catastrophic costs on the world a century from now.

“If we are building a system for the next 20 years we need to build it for the near term realities and the long term,” Schmidt said. “What the U.S. is trying to do is find some place within this sweet spot.”