|The White House.|
The Paris Agreement, adopted at the "COP21" U.N. Conference on Climate Change, establishes a framework for reducing global greenhouse gas emissions. It takes effect once 55 countries accounting for at least 55% of global emissions formally commit to undertaking the low carbon measures it outlines.
According to a March 31 joint presidential statement on climate change, over the past 3 years, "climate change has become a pillar of the U.S.-China bilateral relationship." The statement notes domestic efforts by the U.S. and China to "build green, low-carbon and climate-resilient economies", as well as the international action culminating in the December 2015 conference decision to adopt the Paris Agreement.
The joint statement declares that U.S. and China "will sign the Paris Agreement on April 22nd and take their respective domestic steps in order to join the Agreement as early as possible this year." April 22 represents the first day that the Paris Agreement will be formally open for signature by adopting nations.
A White House blog post describes this step as a "critical milestone" because it represents a commitment by "the world's two largest polluters" who account for 40% of global emissions. According to that blog, this commitment places the 55% threshold for implementation "well within reach," "demonstrating to the international community that there is no turning back on the path towards a low carbon future."