Under a new UN deal on climate change agreed to in Lima, Peru, every single country has agreed to submit a plan next year for addressing their greenhouse-gas emissions. This will form the basis of a new global climate accord to be negotiated in 2015. That would be a first. The last climate treaty, in 1997, only targeted the emissions of wealthier nations and exempted fast-growing countries like China and India.* But there are big caveats. The plans will all be voluntary — countries can promise to cut as much or as little as they want. And there's no rigorous outside review. (The US wanted one, but this was opposed by China and India.) Experts warn this deal isn't enough to prevent significant global warming: the world is still on pace for temperature increases of 3°C (5.4°F) or more by 2100. Which means how to adapt to warming has become an equally large part of these talks — especially for poor nations.

What the new UN climate deal does (and doesn't do)

At this year's UN climate conference in Lima, Peru, representatives from 196 countries agreed to a deal that could eventually commit every nation to slow the growth of its greenhouse-gas emissions.

The US, China, and Europe have already vowed to limit emissions

Over the next six months, each nation will be required to submit a plan for how it will address future emissions. These plans will form the basis of a major new climate agreement to be negotiated in Paris at the end of 2015 and take effect by 2020.

The actual content of each country's plan, however, is entirely voluntary. In principle, countries are supposed to pledge to do more on climate than they've already been doing. But there are no rules about how emissions actually get restrained or what the timetable should be. (See items #10 and #14 here.) Some countries have already put forward pledges:

The Obama administration has pledged that US greenhouse-gas emissions will be 26 to 28 percent lower in 2025 than they were in 2005.

The European Union plans to reduce its emissions 40 percent below 1990 levels by 2030.

China also intends to stop its emissions from rising past 2030 or so — and plans to ramp up its share of renewable energy.

You'll notice that not all pledges are equal. This is by design. As part of a US-China deal struck before this conference, the United States agreed to cut its emissions immediately, whereas China's emissions wouldn't peak until 2030. The idea here was that China is poorer and should get more leeway to grow.

That principle was enshrined in this latest UN deal, which notes that national pledges should take into account "different national circumstances." A country like India — where 400 million people still lack electricity — shouldn't have to cut as quickly or as deeply as Germany. Countries are merely encouraged to explain how their pledges are both "fair" and "ambitious."

This new accord is a conceptual break from the past. The last climate treaty, the 1997 Kyoto Protocol, required only wealthy nations to cut emissions. Developing countries like China and India were exempt. There was some logic to that at the time. But nowadays, developing countries make up the majority of global carbon emissions — and excluding them doesn't make sense:

So this new deal will take a different approach. Each and every country will have to pitch in to help constrain global emissions — although it's up to them to determine how much.

The Lima deal still has a lot of question marks

There's still a lot that's very hazy about this climate agreement. For one, these national climate pledges are unlikely to prove legally binding in any way. That's something that Europe had been pushing for, but was opposed by both China and the US (it's unlikely that Congress would ever ratify a formal treaty).

China opposed a rigorous outside review of countries' climate plans

That means countries can propose whatever climate action they feel like. World leaders that submit weak plans (or fail to follow through on their pledges) won't face any sanctions or punishments. Progress will mainly depend on peer pressure between countries.

Even monitoring the plans themselves could prove difficult. During the Lima conference, the United States tried to insist on a minimum standard for what emissions pledges must look like. It also pushed for rigorous outside review of all national plans after they were submitted. But these items were strongly opposed by China, India, and others. (India was reportedly ready to scuttle the whole deal if these items were included.)

Instead, the final Lima deal simply says that countries "may include" detailed information on how and when they intend to cut emissions. (Or they may not!) There will be no formal assessment of each country's plans. All that will happen is that, in November 2015, the UN will tally up all the national pledges and estimate how they stack up to the broader goal of preventing more than 2°C of global warming. Otherwise, there's little monitoring or verification.

The Lima agreement also encourages countries to come up with ways to help poorer nations adapt to the impacts of global warming, like sea-level rise or droughts. But this, too, is vague. The US and Europe have long opposed any deals that would require wealthier nations to compensate poorer countries for "loss and damages" caused by global warming (say, low-lying islands that vanish under the rising seas). So this will continue to be a point of contention.

In the meantime, wealthier nations have pledged to provide (voluntary) climate aid. Under a separate deal, nations agreed to raise $100 billion per year from public and private sources to help poorer countries adapt and adjust to a hotter planet. It's still unclear where this money will come from, however.

The deal isn't enough to prevent significant global warming

Back in 2009, the world's leaders 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 how this target came about.)

'We're still on a course leading to tragedy'

Right now, however, the world is on pace to blow past that 2°C limit. And it seems unlikely that this Lima goal will avoid this fate. One recent analysis by MIT researchers looked at what was realistic to expect from countries in terms of short-term 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 well short of the cuts needed to stay below 2°C of global warming.

At the conference in Lima, Secretary of State John Kerry put it bluntly in a speech: "We’re still on a course leading to tragedy."

Other onlookers have been somewhat more sanguine. Even if the latest talks won't be enough to meet that 2°C goal, they note, building forward momentum on climate action is worthwhile in its own right. Over at Dot Earth, Andrew Revkin has made the case that this newer, "softer" approach to climate negotiations may prove more effective than previous approaches that tried to impose hard emissions limits on countries.

Similarly, Robert Stavins, a Harvard economist who studies global climate talks, put it this way: "What is most important is long-term action. Each agreement is no more than one step to be followed by others. And most important now for ultimate success later is a sound foundation, which is what the Lima accord provides."

In an August essay, Michael Liebreich, head of Bloomberg New Energy Finance, noted that conditions are more favorable for climate action than they have been in the past. 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. A climate deal could, at the very least, help nudge those trends forward.

Ultimately, however, slowing down global warming will require a massive shift in how the world uses energy — requiring huge changes in how we fuel our cars, power our homes, heat our buildings. At best, this deal is only a very modest step in that direction.

* Clarification: An earlier version of the second bullet point implied that China and India had never submitted greenhouse-gas goals before. They were exempt under the 1997 Kyoto Protocol but they did submit goals for slowing the growth of their emissions in the 2009 Copenhagen talks.

Further reading

How to stop global warming, in 7 steps.

Here's what the world would look like if we took global warming seriously.

Past UN climate talks have failed. Will this one be any different?

A graphical look at the deep divisions between rich and poor countries on climate change.