The world just agreed to a major climate deal in Paris. Now comes the hard part.

After two weeks of bleary all-nighters in Paris, diplomats from around the world have hammered out a major global agreement to address climate change. Here's the full 31-page document, which was approved by 195 countries on Saturday.

It's important to be clear on what this wad of paper actually does. The Paris climate agreement hasn't saved the planet, and it hasn't solved global warming. Not by itself. Instead, the deal is supposed to add structure and momentum to efforts that are currently underway around the world to reduce greenhouse gas emissions.

That's a worthwhile task in its own right. Since 2014, nearly every country has submitted a voluntary plan to the United Nations for tackling climate change. The US is reducing carbon dioxide from power plants. China is boosting wind and solar. And so on. But those pledges, in the aggregate, remain weak and inadequate. If you add them all up, global emissions are projected to keep rising through 2030, putting us on pace for 2.7°C (or more) of warming by century's end. That's well above the 2°C threshold that many scientists argue is unacceptably risky:

(Javier Zarracina/Vox)

What this Paris agreement does, then, is provide a set of diplomatic tools to prod countries into cutting emissions even more deeply over time. The deal's text starts with aspirational goals: The world should aim for an emissions peak "as soon as possible" and limit total warming to less than 2°C, or perhaps even to 1.5°C. (The Earth has already warmed about 1°C since pre-industrial times.) It's a signal that countries at least hope to do more than they're already doing.

The deal then adds transparency measures to verify that nations are actually restraining their emissions. Importantly, it requires that countries reconvene every five years to reconsider the ambition level of their pledges. And wealthy countries have set a goal of providing more than $100 billion per year in public and private financing by 2020 for poorer countries, to help them invest in clean energy and cope with sea-level rise, droughts, floods, and other ravages of climate change.

There are plenty of hard questions about how effective these diplomatic tools will be. Will the transparency measures work? Will that climate aid actually materialize? The basic reality, though, is that the Paris agreement can only encourage countries to step up their efforts. It can't force them to do so. That's the hard part, the part that comes next. Further action will ultimately depend on policymakers and investors and engineers and scientists and activists across the globe, not the UN.

In other words, the Paris deal is only a first step. Perhaps the easiest step. To stop global warming, every country will have to do much, much more in the years ahead to transition away from fossil fuels (which still provide 86 percent of the world's energy), move to cleaner sources, and halt deforestation. Countries will have to pursue new policies, adopt new technologies, go far beyond what they've already promised.

This @rogerpielkejr chart is a good way to visualize the scale of the climate challenge. A *lot* more work to do: pic.twitter.com/HY96FyEPgJ

— brad plumer (@bradplumer) December 12, 2015

If we want decent odds of staying below 2°C of global warming — the grandiose target laid out in the Paris deal — then most countries will have to make radical changes, and quickly. It's an extraordinarily difficult undertaking, with no guarantees we'll succeed.

What the Paris climate deal does, exactly

This crowd is riveted by Article 15. (Shutterstock)

Start with this fact: The Paris deal, negotiated through the United Nations, does not legally require any emission cuts. Negotiators tried to craft an agreement like that back in 1997, with the Kyoto Protocol, and it simply didn't work. It was infeasible to force countries to make sharp cuts they didn't want to make. (See here for more on that history.)

Instead, the Paris talks took a radically different approach. Every country started off by deciding for itself how it plans to tackle greenhouse gas emissions, taking into account its own unique domestic situation. Since 2014, every single major emitter has submitted a climate pledge to the UN. A few highlights:

• The United States is vowing to cut its greenhouse gas emissions at least 26 percent below 2005 levels by 2025, through policies like the Clean Power Plan to decarbonize power plants.
• The European Union will cut emissions 40 percent below 1990 levels by 2030.
• China has vowed that its emissions will peak around 2030 and that it will get about 20 percent of its electricity from carbon-free sources by then.
• Brazil will cut emissions 37 percent below 2005 levels by 2025, with an emphasis on curbing illegal deforestation in the Amazon.
• India will continue to reduce its carbon intensity, or CO2 output per unit of economic activity, in line with historic levels (though overall emissions will grow).

These pledges are all plausible, rooted in each country's analysis of what's politically possible and technologically feasible. But they're also insufficient to avoid serious climate change. As noted earlier, if you assume every country follows its pledge to the letter, global emissions would nonetheless keep rising through 2030, and we'll be setting ourselves up for around 3°C of global warming by century's end, well past the 2°C mark long considered bad news.

So that's where this Paris agreement comes in. The goal was to provide a support structure that will, negotiators hope, allow these national pledges to get stronger over time. Key features in the final deal include:

• An overall temperature goal. The Earth has already warmed about 1°C since pre-industrial levels, because of all the greenhouse gases we've put into the air. As part of the Paris deal, countries are aiming to overall total global warming to less than 2°C, and possibly even keep it down to 1.5°C, in recognition that many dangerous impacts are likely to occur above that level. Of course, these goals are only aspirational, and they'll be impossible to meet without policies to match.
• An overall emissions goal. Currently global emissions aren't expected to peak until 2030 or later, which would make it extremely difficult, if not impossible, to stay below 2°C. So as part of the deal, countries agreed on a vague goal of aiming for peak emissions "as soon as possible." They will also aim to achieve net zero emissions by the second half of the century, though there's no specific timeline.
• Pledges will get reviewed every five years. As noted before, the current climate pledges offered by nations don't come anywhere close to keeping us below 2°C of global warming, let alone 1.5°C. So as part of the deal, countries will be encouraged to submit new and stronger pledges every five years, starting in 2020. Again, this agreement won't require anyone to keep cutting more deeply, but the goal is to pressure countries to consider stronger action over time.
• Financing for poor countries. Poorer countries will need help in adopting clean energy and adapting to climate impacts — floods, storms, sea-level rise, and so on. So the deal requires developed countries to provide aid for this purpose, although it doesn't mandate a specific number. (Developed countries have set a non-binding goal of $100 billion per year in public and private investment by 2020, and the deal calls for an increase over time.) The accountability measures here are fairly weak. In the past, critics say, rich countries have often relabeled existing aid as "climate aid." There's no provision here to ensure that this financing is brand new.
• Loss and damage. A certain amount of global warming is already baked into the system, and some countries are going to suffer no matter what. Low-lying islands, for instance, could get consumed by sea-level rise. So many poorer countries had been pushing for compensation from richer countries, which are after all responsible for most of the emissions in the atmosphere. The US had opposed this, and the deal "does not involve or provide a basis for any liability or compensation." But it does set up committees to deal with displacement and other related issues.
• Transparency measures. The deal calls for yet-to-be-determined reporting and monitoring measures to ensure that countries are actually cutting their emissions as much as promised. Countries like China and India had opposed overly intrusive inspections, which is why the deal calls for a "transparency framework" that is "facilitative, non-intrusive, non-punitive manner, respectful of national sovereignty, and avoid placing undue burden on Parties." How this works in practice remains to be seen.
• Legal status. This deal has a fairly odd legal status. The climate pledges themselves are not binding — if a country fails to cut emissions as much as it had promised, there are no penalties or anything. But much of the supporting structure is binding: the transparency mechanisms, the promise to come back every five years, etc. The main reason for this setup is so that the treaty does not have to be ratified by the US Senate, which would never happen. (Though it also means a future US president could pretty easily abandon Obama's climate pledges.)

You'll notice that some of these provisions are ambiguously worded and will likely need to be hashed out in future UN sessions, starting with the 2016 summit in Morocco. "Many segments contain very vague language, but this kind of 'constructive ambiguity' is often the only way to get a deal done," explained Oliver Geden, who heads the EU research division at SWP, the German Institute for International and Security Affairs. "The actual meaning will only develop over time, as a result of ongoing power struggles."

The Paris deal will officially come into force when at least 55 countries accounting for 55 percent of global emissions have formally acceded. (So, for instance, you'd probably need China, the United States, Europe, India, and Russia to all come on board.)

Will this climate deal actually work? Can we stay below 2°C?

I never get invited to any of these fancy conferences. (Shutterstock)

There's ample room for skepticism about this agreement. Countries are offering up entirely voluntary climate pledges that are, so far, awfully flimsy. (India, for instance, has said that its emissions will keep rising indefinitely as it burns more coal to climb out of poverty.) The parties have only agreed to vague feel-good goals at Paris — limit global warming to 1.5°C, have emissions peak "as soon as possible" — without a well-defined plan for how to actually achieve those targets.

So, yes, there's a chance that the Paris deal, and the processes it sets in motion, could prove ineffective. That's the risk with any treaty based on voluntary actions. It's why you see critics like James Hansen, a prominent climate scientist, calling these talks "worthless."

Again, though, the rationale behind the Paris approach was that no one could ever get China, India, or the United States to agree to a legally binding UN deal that mandated deep cuts or global carbon taxes from the get-go. So instead, negotiators decided to start with voluntary pledges, ensure universal participation, and then try to iterate from there, through cooperation and political persuasion.

As political scientist David Victor told me: "The encouraging precedent here is in trade — where you get a bicycle theory of cooperation." This is the idea that once the process of trade liberalization gets underway, it keeps gathering momentum in subsequent talks: "You build credibility and trust over time and then move to bigger issues."

Or, if you like metaphors: Think of the countries making climate pledges as a bunch of out-of-shape slobs trying (and failing miserably) to qualify for a relay event. The Paris agreement can't force these people to train harder. But it can put their names up on a whiteboard, track their progress, work out gym subsidies for those who can't afford it, and facilitate peer pressure. Obviously the exercise is the crucial part, and that ultimately depends on each individual. But that other stuff can help.

So it's worth keeping these talks in perspective. The Paris agreement can support ongoing efforts to reduce fossil fuel emissions and curb deforestation. But whether Earth warms 2°C or 2.5°C or 3°C simply won't be decided by this deal alone. That will depend on what future policies get enacted by individual countries, on how quickly we switch over to alternative energy sources, on how technology evolves.

And the bottom line is that current policies will have to undergo a serious revision if we want to avoid drastic temperature increases. A recent report from MIT and Climate Interactive looked at how national pledges would have to be ratcheted up between now and 2030 to stay below 2°C of warming:

(Sterman et al, 2015)

Those are wrenching changes. China would have to do much more to ramp up low-carbon energy and shift away from coal so that emissions peak by 2025 instead of 2030. The United States would have to double or triple its current climate policies. Coming even close to 2°C would involve far-reaching efforts to decarbonize homes, vehicles, power plants, and factories. Clean tech would have to proliferate far more rapidly. Plus, we'd likely need to develop technology to suck massive amounts of CO2 out of the air by the latter half of the century. No one knows if that's even feasible.

But that would be true with or without this agreement. Tackling climate change is a massive, herculean task, the work of generations. That was true before the Paris deal, and it's just as true now.

What else should I read about the Paris deal?

• Some of the best, most detailed analysis of the Paris climate agreement is happening over at Carbon Brief. Check out its rundown of the final deal here.
• Here is my history of the 2°C global warming target, and here's a look at how the math of meeting it looks increasingly brutal.
• Robert McSweeney and Roz Pidcock looked at why negotiators adopted the even more stringent goal of 1.5°C in the final deal. Many of the risks of climate change, such as damage to coral reefs, are thought to happen even before we hit 2°C. The problem is that it's extremely unlikely, at this point, that we can cut emissions enough to stay at 1.5°C.
• Michael Levi has a smart analysis of the final Paris climate deal, noting that many of the divisions and acrimony that plagued past climate talks could still resurface in the future. He also notes that public perception of the deal could matter a lot in determining its success.

Coral Davenport of the New York Times argues that the Paris deal will be a success if it provides a signal to markets and investors that clean energy is the future. We'll see if that happens, but that could end up being the deal's most concrete impact.

Finally, one thing the Paris deal didn't address was international shipping and aviation, which account for 4 percent of global emissions. That will be left for future talks. Read Julian Spector's piece for a look at why this is such a tricky issue.

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