The area of contention is that the intermittent nature of (some) renewables means fossil fuel plants need to fire up and then down more often (cycling). When you fire up a plant it tends to emit more, at that moment, than if it was running steadily. Kind of how like starting a car uses a burst of fuel. So what is the overall impact of this?
NREL found that:
"The negative impact of cycling on overall plant emissions is relatively small. The increase in plant emissions from cycling to accommodate variable renewables are more than offset by the overall reduction in CO2, NOx, and SO2."I.E:
"Emission Impacts of Cycling Are Relatively Small Compared to Emission Reductions Due to Renewables"NREL found that moving to 1/3 renewables from solar and wind would cause carbon emissions to drop by 1/3. Toxic gasses would also decrease. This is because although there might be a small increase in some emissions associated with cycling a plant on and off these are tiny compared to the decrease in emissions due to less fossil fuels being burnt, because 1/3 of the power is now coming from renewables. The claims of some "skeptics" is like claiming you should leave your car idling for an hour instead of turning it off and then on again an hour later because turning you car on uses more fuel that idling would at that second.
Here's the impact for 1/3 renewables:
So there you have it, increasing renewables decreases carbon emissions (as well as other toxic gasses that coal plants especially produce a lot of). Common sense right?