Others have good backgrounds on the deal; the White House produced an outline of the agreement written in White House-ese. In short, it's a handshake deal for China to stop expanding its emissions by 2030 and for the United States to reduce emissions by 2025 further than the mark set by the administration several years ago. How this is done is still a bit nebulous. It includes expansions of clean energy systems and an exploratory carbon capture and storage (CCS) project in China backed by the U.S. and investors.
But the handshake is an important thing. It's the first time the premier of China has agreed to set an upper limit on emissions, after his country's emissions spiked well above the rest of the world in recent years, a side effect of its economic shifts. Combined, China and United States account for more than a third of global carbon dioxide emissions. China was already likely to peak in 2030 anyway; the U.S. has a number of climate actions already scheduled. Reductions by the two countries are not only essential to try and prevent the warming that science links to increased emissions but also set a path forward for other countries.
That point is key domestically. A staple of opposition to American action on climate change has been the economics at stake; particularly, the idea that if we acted to curb emissions, other countries would be given an economic advantage. Republicans have frequently pointed to the lack of action by major emitters like China and India as a reason for the United States not to step up -- so much so that climate activists put this video together last year.
If China won't cut emissions, the argument went, American reductions wouldn't solve the problem of climate change anyway and we'd be handing them an advantage. This week's agreement deflates that.
The agreement more subtly undermines another key argument against taking climate action: that it would hurt the economy. Analysis of the extent of that damage tends to vary. (Here's international analysis; when Obama announced planned rules to cut emissions from power plants in June, the Chamber of Commerce and the Natural Resources Defense Council had competing analyses.) The idea of China and the United States pushing more heavily into clean energy means a big, rapid expansion of a market. There's isolationism at play (China and the United States have battled over tariffs on solar power), but things like the inclusion of that CCS project, with the dangling carrot of private investment, could split opposition based on economics.
Much of the opposition to action on climate change is based less on the evidence at hand than on the emergence of climate change as a part of American political culture. That China is acting will almost certainly evolve into, well, let's see when they actually start reducing emissions and/or fine, but what are India and Indonesia doing? This agreement will at best slightly shift the American political landscape over the short term, as was made clear in a statement from Senate-Minority-Leader-for-now Mitch McConnell (R-Ky.). McConnell attacked the deal, including turning President Obama's campaign trail words against him: "The president said his policies were on the ballot, and the American people spoke up against them" in last week's elections, McConnell said.
This is an oversimplification, of course, perhaps colored by McConnell's win in coal-friendly Kentucky. National exit polls indicated that 58 percent of voters were concerned about climate change -- even in a conservative electorate. (Long-time climate change opponent and recently reelected Sen. James Inhofe (R-Okla.) has an expectedly energetic dismissal of the plan.) But political wariness extends to Obama's own party. Democrats are still balky about climate issues, thanks in part to concern about where the public stands (or will stand on Election Day). In order to bolster the shaky reelection campaign of Sen. Mary Landrieu (D-La.), Democrats are considering a vote to approve the Keystone XL pipeline, a key project opposed by the environmental movement.
Even if the agreement simply lets some wind out of opponents' sails, it could be more important internationally. UN Secretary General Ban-Ki Moon praised the deal, suggesting it could crystallize more international action at upcoming talks. Which could, itself, undermine the "we can't act alone" mantra even further.
But it's also a reminder of more concrete risks for Obama. He's got 26 more months as president, a blink in the timescale of climate action (and effects). If, say, Sen. Marco Rubio (R-Fla.) is sworn into office in two years, will he maintain his skepticism about climate action in light of international circumstances?
You may recall the last time a Republican president took over for a two-term Democrat who'd made an important international agreement on climate action. That was George W. Bush, who quickly scuttled any action on the Kyoto Protocol. The politics in 2001 were very different than they are today. The politics on January 20, 2017, could be very different, too.