Answer: All of them stand in the way of Donald Trump, who seems bound and determined to make the United States the biggest source of greenhouse gases, especially if (as rumored) he is determined to pull the U.S. out of the Paris climate accord next week.
What's that you say, especially if you're a Trump supporter? China and India produce more greenhouse gases than we do. Perhaps that's currently the case, but not for much longer. Of course, you're entitled to view a story in the New York Times as "fake news," but you're not entitled to be justified in holding that view. Especially when, in the process of accelerating their development of alternative energy sources, China and India (as pointed out in the story) are helping to make these sources more affordable for everyone.
And "everyone" includes at least some of us in the U.S., in states such as California, where Trump's misplaced nostalgia for dirty fuels and the economy they formerly powered has been soundly rejected. Trump may very well succeed in slamming the brakes on clean energy development on a national scale. But that may not matter if California, home to one-tenth of the nation's population, continues to focus on that development at the state level. Indeed, and again according to the Times, California appears ready to force that development not only at the national level, but even at the international one.
In fact, California appears ready to fight Trump on a number of fronts, including immigration.
I've said this many times before, and I'll say it again. California, the state that launched the tax revolt in the 1970s (and the so-called Reagan Revolution with it), may very well be the state that drags the rest of the country into the 21st century, and beyond. Let's hope so. We don't have a lot of other sources of hope out there.