Fossil fuels don't just emit CO2, they produce waste heat. The microwave beam would produce the same waste heat, without the CO2. The waste heat from fossil fuels is well under 1% of their greenhouse effect, so that's a pretty good deal.

If we got all our energy from solar panels, the albedo change from all those block panels would also cause the planet to absorb more energy (and of course that'd be a relatively minor effect, too).