On Friday, most of the world’s governments are set to sign the most sweeping climate agreement in history. Their signatures will codify promises they made in Paris last December to reduce emissions of greenhouse gases.

The two largest sources of those gases are the U.S. and China. Whether they keep their promises will in large part determine whether the Paris deal succeeds. And it is by no means clear that they’ll be able to keep their promises.

The Paris climate meeting was the culmination of years of failure by the world’s nations to cut a deal on curbing climate change.

Source: Texas Public Radio