[antepodius]

If we're talking about intelligences that can travel between stars, there is no 'carrying capacity' aside from space on the surface for radiative cooling towers to stop the planet from boiling from the heat generated by the trillions of humans and machines. Seriously, you can probably put all the nonhuman processes of an ecosystem into a bottle that takes in air, energy, and wastewater and puts out food/crops/etc, and you can just dismantle some asteroids for the materials needed for as many fleets of solar panels as you need to put around earth, assuming they don't want to use what's powering their rockets. Really, the limiting factor is cooling the earth. You'd probably want to block all infrared from the sun for convenience.

Implying these beings would likely have any need to worry about 'decreasing' carbon emissions is ludicrous. It's like humans going to a beehive and worrying about killing some bees to stop honey wastage when the humans poke a hole in the hive to collect honey- no, they'll just completely reconstitute the hive. There's a chance it's some first-interstellar-travel thing we might do right now, but it's more likely it's from the time a civilisation is kardashev-2, given that time would be longer. Fermi paradox's still a problem, though.