The agreement comes after another similar announcement recently by the European Union. EU members committed to reduce their collective emissions by 40 percent below their 1990 level. They are promising to do that by 2030.
With Europe, China and the United States all pledging to take action, the world's three largest sources of emissions are for the first time working to significantly prevent the warming of the planet. "If China, the U.S., and the E.U .… got in a room, that's about 60 percent of global emissions right there," said Princeton climate scientist Michael Oppenheimer.
Despite the burst of activity, however, climate experts warn that even these ambitious efforts are unlikely -- without concerted further action -- to solve the world's climate challenges. On Wednesday, for example, a grim report released by the International Energy Agency suggested that the world is on track to pump so much carbon dioxide into the atmosphere by 2040 that it will cause irreparable harm to the planet's ecosystems.
The IEA says that humans can only emit about 1,000 additional gigatonnes of carbon dioxide (one gigatonne is equal to a billion metric tons) into the atmosphere before passing a dangerous threshold.
Climate scientists warn that pumping more carbon dioxide than that into the atmosphere would likely cause a more-than-2 degree Celsius rise in global temperatures over levels that existed before the rise of industry in the 19th century.
Such a rise could cause dramatic sea-level rise due to the melting of the planet's ice sheets, threats to agriculture, and more severe weather.
Experts were scrambling on Wednesday to determine how the new U.S.-China agreement affects the 2 degree target. "The answer we’ve come to so far is that it depends," said Taryn Fransen, a senior associate at the World Resources Institute, an environmental think tank. "I think the big question in terms of total emissions is going to be when and at what level China peaks" its emissions.
According to projections from the U.S. Energy Information Administration, the world is churning out more than 30 gigatonnes of carbon dioxide annually from energy use. The United States accounts for a little over five gigatonnes, Europe produces about four gigatonnes, and China emits about 9.5 gigatonnes.
But global emissions are also still on a rise, meaning that in coming years the global annual number is projected to exceed 40 gigatonnes. The International Energy Agency says under current its projections -- which do not account for the brand new U.S.-China agreement -- the world will produce the threshold-breaking 1,000 gigatonnes of carbon dioxide by 2040.
Forecasting changes to emission patterns is extremely difficult and imprecise, and it's not clear just how significant an impact the new agreement would have on these projections. But climate experts say that it's unlikely the new agreement -- without further action by both developing and developed countries -- would be enough to avert some of the worst outcomes.
"Obviously, if only the U.S. and China were doing something, then we’d fall short," explains David Hawkins, director of climate change programs at the Natural Resources Defense Council. "And it’s also true that if we don’t do anything more than this deal, we would bust [the] budget."
Nonetheless, the experts underscore that this deal has a symbolic value that goes far beyond the literal emissions cuts (or caps) that have now been pledged, precisely because the world's top two greenhouse gas emitters have now both come to the table.
If the agreement lays the groundwork for a broader global agreement -- one that encompasses other major emitters like India, Japan, and Russia -- then that is the real payoff. That agreement could happen in Paris in late 2015, when the nations of the world gather to try to achieve a global agreement to cut greenhouse gas emissions.
"I take what happened last night as really one of the most important developments that I’ve seen in the international negotiations over the last 5 to ten years," says Robert Stavins, an environmental economist at the Harvard Kennedy School. That is not because of the literal emissions reductions that China and the United States have pledged, but rather because of the fact that they lay a foundation for more movement in reducing the emissions of other developing nations (besides China).
The Earth system, after all, doesn't care about political victories -- only raw physics. "The planet doesn’t give A’s for effort," says Nathaniel Keohane, who heads the international climate program of the Environmental Defense Fund. "Deeper reductions will be needed but this is a very good first step."