There’s three permanent intertidal rock pools teaming with life.
In the first, you pour bleach. The bleach kills everything in the pool. You deem it an environmental pollutant.
In the second, you pour crude oil. Slowly, but surely, the oil kills everything in the pool. You deem it an environmental pollutant.
In the third, you pour in fresh water. Even slower, you find that the freshening water kills the life in the pool. This fresh water you deem is not a pollutant…
Wait… What? Why?
Well, you can drink it. In fact, you need to drink it to survive.
However, you’re not a marine organism living in the pool. All three experience a change to the chemical composition of the pool which alter it so much that life, as it had existed, could no longer function in the same manner.
As far as each pool is concerned, each one, including the freshened pool, has been polluted.
In the same fashion, increasing the atmospheric concentration of CO2 leads to environments no longer functioning in the way we have experienced them throughout the Holocene. Just like the freshening pool, this is effectively polluting the atmosphere, regardless of previous times where life (species that no longer exist) thrived under different atmospheric compositions.
We are digging up long trapped carbon, which has not been part of the biosphere or atmosphere for many millions of years and converting it into CO2 emissions, measured in the gigatonnes in annual atmospheric addition.
It is a slow process, but we are already witnessing coral bleaching, die back in drying / warming forests, parasitic species getting the edge on host (due to heat stress), shifting timing of breeding, blooming and migration; all of which negatively impact on ecosystems as they currently function.
Enough of any chemical is detrimental.
We are changing the chemical composition of the atmosphere in such a way that the resulting climate is less accommodating to current ecosystems and, if it’s difficult to appreciate just how dependent upon ecosystems we really are, also our current agricultural practices. Sure, it doesn’t mean death, but it means otherwise avoidable hardship.
Even CO2 can be a pollutant.