Ah, no, not quite
"The article says about the "cost of polution" and puts a currency value to it. What this value is, and how it is worked out is beyond the article, and I'm OK with that, but for there to be any cost, someone must have worked out where that cost needs to be spent to offset the polution (e.g. the cost to plant enough trees to counter the 1 tonne of CO2)."
No, we're not estimating the cost of the pollution at the cost of cleaning it up. We're estimating it by the damage it does. Entirely different approach.
Imagine that there's some pollution that "costs" a million quid. That's around the statistical value of a life (right ballpark anyway). Sticking, for example, the radioactive cobalt from a hospital scanner into landfill will kill one person (this is a deliberately absurd example BTW). The cost of not having that pollution is say £35. The cost of sending the cobalt off to be reprocessed at the radioactive metals reprocessing plant.
Another deliberately silly example, not putting some slightly harmful chemical into landfill will cost us tens of billions and putting the same chemical into landfill will kill one person. So we're spending tens of billions to avoid 1 million in costs (and we do in fact have regulations like this, which cost tens, even hundreds, of billions for each statistical life saved).
You can see that there are two entirely different concepts of "cost" here then? What does it cost if we do pollute and what does it cost not to pollute?
What we want is some method of getting people to stop doing the polluting when not polluting is cheaper than polluting. We also want people to carry on polluting when polluting is cheaper than not polluting. (Third entirely absurd example: we could stop climate change simply by killing everyone on Thursday lunchtime. This most would think is a higher cost than the costs of carrying on polluting). So, what do we do? We impose a tax of the costs caused by polluting. This is not the cost of cleaning up the pollution, it is the cost that is caused by the pollution.
People will then naturally stop doing the polluting we want to get rid of, the polluting which is more expensive than not polluting, because it is cheaper for them to stop polluting.
Just as an example of how mainstream this idea is (no, really, it is mainstream) it's the basis of a lot of the Stern Review. It's the idea behind petrol taxes, APD, landfill tax and a whole series of others. As the piece above points out, it gets distorted in the political process, but it is the intellectual justification for almost all "green taxes".