Are Historical Emissions Harmful?

In the past, the North had much higher greenhouse gas emissions than the South. Some people deny that this fact is relevant for the just design of present day climate policy. One of their arguments is that, by themselves, historical emissions did not really create much harm. It is only the continued emitting of today's generation that turns climate change into a real problem, they claim. But: Is it true that historical emissions are not harmful?

I want to list three reasons (in three separate posts) why this question is not only an empirical question. It is genuinely unclear - additionally to what natural science tells us - whether historical emissions are harmful.

Here is the first reason:

To begin, note that the harm from emissions does not increase linearly but exponentially with the amount of emissions over time, i.e. doubling emissions causes more than doubles damage.
Let us look at an example. Assume that I come into the kitchen and I think there's no salt in the pot, and so I put a pinch of salt in it. Actually, there was already salt in there. But my pinch doesn't matter much: it just makes the meal a bit saltier (harm is of size 1). After me, my friend comes in and he, too, thinks there's no salt in the pot and puts a pinch in. Together with my salt, this now makes the meal inedible (harm is of size 200). What is the harm, then, that I caused? In some sense it is harm of size 1 because if my friend hadn't come this is the harm my action had caused. In another sense, the harm of size 200 was equally caused by me and my friend and so I could just as well be said to have caused harm of size 100. Both interpretations are reasonable.
So, if we want to ask whether historical emissions cause harm we have to specify the question more precisely: Do we mean the harm caused in the past if present and future generations wouldn't emit excessively? Or, alternatively, do we mean the harm caused by in the past given that present and future generations emit excessively? In the second case, past emissions can count as much more harmful than in the first case.