In fact, in places it will likely have increased the CO2 emissions, again rounding error figures, due to the electricity generation mix and associated losses with generating that electricity (30-60% efficiency), plus transport losses (0.5 - 10%), plus charging loses, plus the efficiency of the motor itself.
Petrol cars can do a lot better than 20%
There is a study, link below, showing what the subsidy should be for electric cars and in a lot of places in the US the subsidy should be negative.
Granted things might've changed since then, but probably not to overturn the gist of it, which is the point I'm making. In places, EV cars are not the solution.
Quite happy to be proven wrong, at the of the day I've only got a passing interest on the topic so I'm not exactly 100% up to speed with any new studies, etc ...
https://www.citylab.com/environment/2015/06/where-electric-v...
"[I]t’s important to realize that the situation reflected in this data represents the past, not the present. States across the country are rapidly cleaning up their grids, thanks to cap-and-trade programs, investments in renewable energy, tightened air quality standards, and more coal plants put to sleep." [2]
"Here’s the cool thing about electric vehicles in the U.S. right now: In many areas, the longer you own one, the cleaner it gets." [1]
[1] https://www.citylab.com/life/2017/06/electric-vehicles-are-c...
[2] https://www.citylab.com/transportation/2016/12/the-surprisin...
[1] https://greentransportation.info/energy-transportation/evs-n...
[2] https://electrek.co/2017/06/05/electric-cars-cleaner-than-ga...
[3] https://cleantechnica.com/2017/05/16/coal-use-china-india-li...
In the short term, yes, folks will need to rely on existing fossil fuel technologies to build factories, transport materials, and power the grid. That is because there is no other economically viable option. To expect the entire clean energy industry to bootstrap itself only using clean energy is a fallacy. In 20-30 years when the grid is more green and clean transportation is widely available, then arguments like these would be valid.