This is possible because of the China Trade Agreement established between Sacramento and Beijing, following on the heels of the international agreement with China signed by US Secretary of State John Kerry on April 14 of this year:
The agreement could impact Canada and the growth of the oilsands where companies predict their expansion will triple their greenhouse gas emissions by 2020. This expansion is largely dependent on the building of pipelines such as the Keystone XL to Texas, which remains a hotly contested issue in the U.S. where it has become the symbol of the struggle for strong action on climate change. Public hearings on the pipeline open in Nebraska Thursday.
If the U.S. and China significantly ratchet up the level of climate action, “that would give the lie to the Harper government’s claims that its policies are in line with those of other developed countries,” Meyer said. “We’ll have to see how this plays out, of course. Talk is cheap, but the US-China statement, in particular, heightens expectations for something significant later this year.”
This is happening because, as a Scientific American article points out:
Speaking at a clean energy seminar in Beijing, Kerry warned that climate change is happening at a faster rate than scientists predicted 20 years ago and said the United States and China have a particular responsibility to rein in greenhouse gases.
"China and the United States represent the world's two biggest economies, we represent the world's two largest consumers of energy, and we represent the two largest emitters of global greenhouse gases. So if any two nations come to this table with an imperative for action, it is us," Kerry said.
Update: China's legal and cultural systems may prove to be problematic in these agreements. The culture is above the law.