On Tuesday, the UN is holding a big climate summit in New York City as a prelude to talks on a new agreement to tackle global warming. Negotiations will go for over a year, with a meeting in Lima this December and a final meeting in Paris at the end of 2015.

THE WORLD HAS BEEN FAILING TO LIMIT CLIMATE CHANGE FOR YEARS — WHY SHOULD THIS TIME BE DIFFERENT?

Does that sound drearily familiar? It should. The world's leaders have been hammering out various climate agreements for decades now. There was the 1997 Kyoto Protocol. The 2009 Copenhagen Accord. But despite all these talks, global greenhouse-gas emissions have kept rising, putting the world on track for more warming in the years ahead.

So why should this newest round of climate diplomacy be any different? It's a fair question. Some analysts are skeptical that countries will achieve their goal of limiting global warming to no more than 2°C (or 3.6°F) above pre-industrial levels.

Other experts, however, have countered that this time really might be different: Clean energy is getting dramatically cheaper, and countries like the United States and China are slowly starting to take individual actions to tackle global warming. The talks over the next year could build on that slow progress.

Expectations are modest for the one-day summit in New York on Tuesday, which was called by UN Secretary General Ban Ki-moon. Various leaders are expected to announce a few modest climate initiatives, like a "clean energy corridor" in Africa or new pledges to slow the pace of deforestation. But the ultimate prize is an agreement with "legal force" that commits all nations to reduce their greenhouse-gas emissions. That's much, much more difficult — see here for why — and will take until at least 2015 to negotiate.

So here's a walk through the next 15 months of climate talks — how they'll work, the different possible outcomes, and how the Obama administration plans to approach them.

What are the UN climate talks?

Back in 1992, virtually every nation on Earth — including the United States — signed (and later ratified) the UN Framework Convention on Climate Change. That document was the basis for subsequent climate-change negotiations.

UNDER THE 1992 CLIMATE TREATY, COUNTRIES AGREED TO TAKE ACTION — BUT NEVER SPECIFIED WHAT, EXACTLY

Under the original treaty, the world's nations promised to work together to "stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system."

That is, every country agreed we should do something about global warming — but they didn't quite say what that entailed. Key details were left vague, like: What actually counted as "dangerous" global warming? How much do emissions need to be reduced? How do we divide up responsibility for those cuts?

In the years since, negotiators have been meeting periodically to haggle over the details. A few of the big milestones:

— Under the 1997 Kyoto Protocol, the world's wealthiest nations agreed to act first to cut their greenhouse-gas emissions. Europe mostly followed through on its promised cuts. But Canada and Russia later backed out of the agreement. And the US never ratified the treaty in the first place. What's more, the Kyoto Protocol specifically exempted developing countries like China and India — on the theory that they should be allowed to get richer before being forced to act. Now, two decades later, China is the world's biggest emitter of carbon-dioxide, so this treaty has some pretty obvious holes.

— Under the 2009 Copenhagen Accord, the world's nations agreed on how to define "dangerous" global warming. Basically, they said, we shouldn't let global average temperatures rise more than 2°C (or 3.6°F) above pre-industrial levels. Otherwise, the risks from rising temperatures, extreme weather, and sea-level rise would be too great. (Here's a more in-depth look at the 2°C target.)

The Copenhagen talks also saw all of the world's biggest economies — including the United States, China, and India — make voluntary pledges on curbing their emissions. The US, for instance, promised to cut emissions 17 percent between 2005 and 2020. Those pledges aren't legally binding, and analysts found they're insufficient to meeting the 2°C goal. But the fact that pledges were being made at all was a big change.

— More recently, negotiators have set up a new timetable. They want a new climate agreement by 2015 that covers all countries, has some sort of "legal force," and would go into effect by 2020. In theory this would require countries to commit to cuts in greenhouse gas emissions and maybe set up some way for wealthier countries to aid poorer countries in the transition. But the details are still up in the air. They'll officially get underway in Lima this December.

Have the previous climate talks failed?

They certainly haven't achieved the goal of stabilizing greenhouse-gas emissions in the atmosphere. The world is burning more fossil fuels than ever, and carbon-dioxide emissions keep rising each year:

Various nations have made voluntary promises to cut emissions in the future. And some countries (particularly the United States and the European Union) are starting to reduce their emissions and inch closer to their pledges.

THE CURRENT PLEDGES ARE INADEQUATE TO PREVENT 2°C OF WARMING

But these pledges are inadequate — an analysisfrom Ecofys found that even if every nation met its goal for emissions cuts, the world would still be on pace to warm between 3°C and 4.6°C by the end of the century (that's between 5.4°F and 8.5°F). That's well above the hoped-for 2°C goal.

A related goal of previous climate talks has been "climate finance." Back in 2009, wealthier nations pledged, as a first step, some $30 billion in aid to help poorer countries use more clean energy and adapt to the worst impacts of global warming. But that hasn't gone according to plan, either. Critics have noted that a lot of "climate financing" is just existing aid repackaged under a new name. (The UN is currently seeking another $15 billion for its "Green Climate Fund," but it's unclear whether that money will materialize.)

So has anything come out of these talks? Well, the European Union did mostly follow through with its promised cuts under the Kyoto Protocol. And the world's nations are at least getting together and continually talking about global warming. But, so far, it's hard to point to a ton of tangible progress.

Will this newest climate treaty in 2015 be any different?

That's the big question. By the end of 2015, the world's nations are supposed to reach an agreement that will address greenhouse-gas emissions. The agreement should apply to all countries and take effect by 2020. But the specifics are still undetermined.

EVEN IF IT WON'T MEET THE 2°C GOAL, SOME ONLOOKERS ARE OPTIMISTIC ABOUT A DEAL

The case for pessimism: There are plenty of pessimists about a new deal. One recent analysis by MIT researchers looked at what was realistic to expect from countries in terms of emissions pledges. (This was based on "national communications, discussions with observers of conditions in various countries, and — by necessity — a good deal of guesswork.") Their conclusion? The 2015 pledges would fall short of the cuts needed to stay below 2°C of global warming.

What's more, there are still deep-seated disagreements among different countries on how to best to tackle global warming. Poorer countries argue that the US, Europe, and other rich nations are responsible for most of the extra carbon-dioxide in the atmosphere today, so they should bear most of the burden for addressing global warming. Richer countries, by contrast, say that you also have to look at future emissions when allocating blame — so fast-growing nations like China and India need to do more.

The case for optimism: Other onlookers have been somewhat more sanguine. Even if the latest talks won't be enough to meet that 2°C goal, they note, there's still a chance that they can be more productive than previous talks have been.

In a recent essay, Michael Liebreich, the head of Bloomberg New Energy Finance, noted that conditions are now more favorable for some sort of agreement than they were back in 2009. For one, many low-carbon energy sources — like wind, solar, and electric cars — are advancing faster than expected. (Others, however, like nuclear power and carbon capture for coal plants, have stalled out.) Clean-energy financing has grown to more than $250 billion per year. And countries are no longer battling a major financial crisis.

What will come out of the UN climate summit in New York this week?

The UN summit in New York was intended to be something of a prelude to these talks. It was convened by UN Secretary General Ban Ki-moon in order to spur countries to make "bold pledges" on climate. But the main action will likely be a series of smaller announcements by various countries and firms.

THE UN SUMMIT IN NEW YORK WILL BE PRETTY MODEST

For example, a variety of countries and companies are expected to announce support for an African "clean-energy corridor" that will stretch from Egypt to South Africa. And six international energy companies, including Norway's Statoil and Italy's ENI, are pledging to reduce their emissions of methane, a potent greenhouse gas.

Likewise, richer countries may contribute a bit more to a UN Green Climate Fund that aims to provide $15 billion this year to poor countries to help reduce emissions and mitigate global warming. (So far, financing has been pretty paltry, although Germany recently pledged $1 billion.)

President Obama, meanwhile, is planning to show up and tout the latest proposal by the Environmental Protection Agency to reduce greenhouse-gas emissions from US power plants by 2030. The main goal here, the administration says, is to spur other countries to take further actions and lay the groundwork for a bigger agreement down the road.

Not all leaders will be there — China, India, and Germany are sending "high-level representatives" rather than their heads of state.

What will come out of the Paris 2015 climate talks?

A few experts have noted that any climate deal that comes out of the Paris talks probably won't be some sort of "top-down" agreement that requires each country to stay within a defined carbon budget so as to meet the 2°C goal.

A MORE LIKELY MODEL IS A DEAL THAT EVOLVES OVER TIME

Instead, a more likely model is that individual countries will make their own pledges, based on what they deem realistic — those pledges are due by April 2015. The final agreement might then set up some mechanisms to nudge countries to do more over time.

Here's how Elliot Diringer described that process in a recent essay for Nature: "In the emerging model… the efforts of countries would be tracked under agreed rules. But individual emissions-reduction goals would be set by each country on its own, without negotiation. The agreement would, in essence, stitch together a mixture of self-defined contributions. To encourage ambition, countries would scrutinize each other's initial offerings."

Liebreich argues that this would represent a step forward from past climate talks. "For what it is worth, I predict a deal will be struck at [the latest climate talks] in Paris," he writes. "It will be more significant than the largely meaningless Copenhagen Accord. But it will not be enough to put the world on track to a 2°C future. That will have to wait for [future talks.]"

What is the Obama administration's strategy at these talks?

The Obama administration is currently angling for a climate deal that pushes countries to follow through on their promised emissions cuts. But that's tricky: the United States can't ratify a new formal treaty unless 67 senators approve it. And the Senate can't agree on anything these days, let alone a politically contentious issue like global warming.

OBAMA IS LOOKING FOR A 'BINDING' DEAL THAT DOESN'T REQUIRE SENATE APPROVAL

Instead, as the New York Times reported in August, the administration is trying a different approach: "President Obama's climate negotiators are devising what they call a 'politically binding' deal that would 'name and shame" countries into cutting their emissions."

Here are more details from reporter Coral Davenport: "Countries … would voluntarily pledge to specific levels of emissions cuts and to channel money to poor countries to help them adapt to climate change. Countries might then be legally obligated to report their progress toward meeting those pledges at meetings held to identify those nations that did not meet their cuts."

There's a delicate dance here. The Obama administration is trying to find a way to give any climate agreement some force without going through the formal treaty process again. This isn't unprecedented — it's similar to how a lot of trade deals happen. (See Eric Posner for a legal analysis.) But it also raises a lot of questions.

As political scientist Daniel Drezner points out, it's not clear whether other large carbon emitters will trust that the United States will carry out its own pledges in the absence of a new treaty. After all, a new president could take completely different actions. And some members of Congress are irate about the New York Times report — a reminder that the United States is hardly unified on this issue.

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