It's hardly a rounding error, but I'm sympathetic to the fact that there are a lot more important concerns with respect to climate change than PoW, however, there is still a valid ecological critique of PoW due to it's inherent wastefulness relative to every other technology.

The amount of waste necessary to support the network must always grow since any new efficiencies are immediately obviated by the incentive to bring on more miners, the total utility provided by the network (i.e. the rate of transactions it securely processes) has no relationship to the amount of energy that the network burns. With every other technology, new effeincies make the technology able to do more useful work while burning less energy, in this way anything based on PoW is fundamentally flawed. You could hook up a fusion reactor of the future to the bitcoin network and it would not provide any more utility, yet the network would eventually consume all the energy produced by the reactor given enough time to increase mining capacity.

I know the typical response to this is PoS, and I think a switch to PoS would be great since its impact on the environment is within the realm of normal software. Whether or not PoS can actually work for a large network is a different discussion.