This is a big deal. US President Barack Obama and Chinese President Xi Jinping just announced a major agreement to curtail greenhouse-gas emissions and tackle global warming over the next few decades:

By itself, this agreement isn't enough to solve climate change. And these are only aspirational pledges, not legally binding goals. But politically, this marks a sharp break from the two countries' long stalemate on the issue. And the agreement could help nudge along the ongoing UN climate talks, which hope to culminate in a global deal on emissions in Paris by December 2015.

Here's a breakdown of the deal, which was hashed out over nine months of negotiations:

For the first time ever, China has set a hard deadline for its emissions to peak

The US pledge: As part of the bargain, the US government has pledged to reduce emissions 26 to 28 percent below 2005 levels by 2025. This is a new and significant extension of the Obama administration's existing goal to reduce emissions 17 percent by 2020.

The biggest question here is whether US policymakers will actually follow through on this pledge. The country's carbon-dioxide emissions are currently 10 percent below 2005 levels, but they've started to rise again of late. The US Environmental Protection Agency has proposed new rules to curb emissions from existing power plants, but that's unlikely to be enough to achieve a 28 percent cut. So where will additional policies come from? Note that Congress is deadlocked on climate, with many Republicans furious about this new deal.

The China pledge: For the first time ever, China has set a goal of having emissions stop growing by "around" 2030 — and possibly earlier. China will also aim to get 20 percent of its energy from non-fossil sources by 2030. (China isn't reducing its emissions as quickly as the US; the logic is that this is fair since China is still poorer.)

Critics will argue that China's target is vague (it leaves out the crucial question of what level emissions will peak at) and not ambitious enough. And, yes, a few experts had been predicting China's emissions would peak around 2030 anyway. On the other hand, a 2030 peak was never a sure bet. And China had long refused to set a deadline here. This is a real shift in stance, even if it's not legally binding.

On top of that, China's pledge to get 20 percent of its energy from clean sources by 2030 is genuinely audacious. "It will require China to deploy an additional 800-1,000 gigawatts of nuclear, wind, solar and other zero emission generation capacity by 2030 — more than all the coal-fired power plants that exist in China today and close to total current electricity generation capacity in the United States." That's staggering — and it remains to be seen if China can actually do all that.

Other parts of the deal: The two countries will also continue to cooperate on clean energy R&D. They'll launch a pilot project in China for capturing coal-plant emissions and sequestering the carbon underground. And they'll continue to cooperate on phasing out hydrofluorocarbons, a potent greenhouse gas. You can see more details in this fact sheet.

Why is this deal important?

For starters, these are the two biggest carbon-dioxide emitters in the world. It's impossible to address global warming without action from both China and the United States:

Granted, China and the US are far the only two emitters out there. But the hope is that if these two countries pledge drastic action to reduce their emissions, other nations will follow. (The EU has promised to cut emissions 41 percent below 2005 levels by 2030.)

Both the US and China have long pointed to each other as an excuse for inaction

This deal is also a step away from the long-standing deadlock between the two nations on climate. Many US politicians have long argued against cutting greenhouse-gas emission on the grounds that China would never act — so what was the point? And China, for its part, has long insisted that rich countries should cut their own emissions and give developing countries like China time to grow.

With this deal, the two countries are beginning to cooperate rather than use each other as an excuse for inaction.

Is this deal enough to solve climate change?

No. For starters, both the United States and China actually need to achieve their stated emissions goals. That won't be easy for either country, but it could prove especially difficult for the US, which needs to make deep cuts in the next decade. And the current GOP-dominated Congress is opposed to any sort of action on greenhouse-gas emissions.

It's also debatable whether either pledge is sufficient to avoid drastic levels of global warming — particularly if China lets its emissions keep rising until 2030. Some analyses have suggested that China's emissions would need to peak around 2025 to meet its goal of preventing more than 2°C (3.6°F) of global warming. (The White House said it thinks China can peak earlier, particularly if it meets that ambitious clean-energy target. But that's far from certain.)

More crucially, this deal only includes two nations. As climate modeler Chris Hope points out, this deal in isolation still puts the world on course for a likely 3.8°C (6.8°F) rise in temperatures by 2100. "These pledges are only the first step on a very long road," he concludes.

For the time being, however, this is a significant shift in climate politics — and possibly a first step toward a broader global climate agreement.

Further reading

Philip Bump has a nice post on the politics of the US-China climate deal.

Michael Levi takes a closer look at the math behind some of the emissions pledges

A look at the next year of climate talks, which are expected to culminate in a deal at the end of 2015.

7 charts that show why UN climate talks keep breaking down

A more detailed look at what various nations would need to do to avoid drastic global warming. It entails a lot more more than what got discussed today.