That's.. not how this works. The miner receives a block reward which is a combination of a fixed reward (2 ETH right now) + tips from people trying to get their transaction (typically ~0.2 ETH) [0]. The rest of the transaction fees are burned by the network.
In effect this means that miners are motivated to mine (and hence use CO2) based upon the block rewards. During periods of high network usage, they aren't magically procurring more miners, and hence they aren't actually using more electricity. And since the vast majority of the reward is burnt anyways, during high periods of usage, the miner reward doesn't actually go up that much.
Regardless of all of that, Ethereum will be on proof of stake within a year (wouldn't be surprised if it's 3 or 4 months at this point), at which point the CO2 argument will go away altogether).