Or batteries and other storage methods. It looks like battery solutions could get cheap enough in the next 10-20 years to smooth things out and take care of storage for a few hours (see for instance http://news.mit.edu/2018/metal-mesh-membrane-rechargeable-ba... .. Donald Sadoway has some good talks on YouTube)
But yeah, for days that happen to have less wind/solar, I think the best thing is just keep gas power plants around. The CO2 impact for that will be minimal, the power plants are already built, and over time you could replace the fuel with synthetic gas made from renewables, which would basically be another energy storage mechanism. Cheaper renewable gas is something we need to make the world sustainable anyway.
The only unsolved problem is seasonal variations in colder climates. But those areas could import more trash and burn it (for both electricity and heat), like Sweden does. Norway has a ton of hydroelectric power, and is building more HVDC power lines to other areas of Northern Europe which will help with that area. Not sure what the solution for North America is though. But it's not nuclear. Having a nuclear power plant idle for half a year is the exact opposite of what you want. The plant is expensive and the fuel is cheap - if you build it you want it to run 24/7.
