Actually, we can argue that whether anyone signs up or doesn't to the Paris Accord isn't a matter of importance. For here's some talking about the effects of the US not remaining:

Existing climate efforts expected to keep US goals on track

More specifically:

The momentum of climate change efforts and the affordability of cleaner fuels will keep the United States moving toward its goals of cutting emissions despite the Trump administration's withdrawal from the Paris global accord, business and government leaders in a growing alliance said.

In detail:

The momentum of existing climate-change efforts and the availability natural gas, wind and solar power mean those loyal to the Paris accord in the U.S. will have an easier time, with emissions expected to fall overall for years, said Robert Perciasepe with the Center for Climate and Energy Solutions, who worked with Bloomberg's group on the climate pledge.

Some studies suggest the United States will cut emissions as much as 19 percent by 2025 if it simply moves forward as is, he said. That's not far from former President Barack Obama's goals for a reduction of 25 to 28 percent as part of the Paris accord, Perciasepe said.

If it doesn't matter much whether the US is in or out of Paris then Paris itself doesn't matter very much, does it?

Which underlines a point we continually make. The world has already done rather a lot to create the possibility of reduced emissions. Absolutely all of the predictions of future emissions - such as this one in The Guardian - resolutely ignore what has already been done. Those charts you see going off into infinity are ignoring the manner in which we're already closing or have closed coal stations, we've brought solar power down in price by an order of magnitude, people are already buying electric cars and so on and on.

Leave aside the basic contentions for a moment and consider this important detail. The predictions of total disaster are all based upon our doing nothing at all. Yet we've done a lot, we are already quite obviously not on the higher emissions pathways. Thus the predictions of total doom are simply wrong for we've already done quite a lot about it.

Hey, maybe we need to do more, maybe we don't, but we should at least start from the current position, not the predictions based upon our having done nothing for 20 years, for we've not done nothing in those decades.