I feel obliged to point out that no race capable of crossing the void is likely to lack either sophisticated replicators of the von Neumann variety or Fusion, if you've got fusion or something better, CO2 in your atmosphere is silly concern, fusion doesn't produce like fossil fuels and you could literally profitable suck it right back out of the air. So it would be a really f'd up thing to send a 'warfleet' when you could send a diagram of fusion reactor faster and basically for free.

There's also the prick factor, none of us would think genocide the proper route to deal with a culture that, say, chronically over-fished, so anyone who would go flying across light years to do so is probably a pack of SOBs simply looking for a thinly veiled excuse and 'eats meat' or 'murders and skins thinking feeling creatures to wear their hides around' are a heck of a lot more likely than a problem that ca not possibly matter, it's not like if we did colonize other worlds they'd even have fossils fuels for us to burn.