Carbon debt

Thanks to Environmental History Resources we learned of a new contribution to the fraught topic of responsibility for historic carbon emissions. "Counting carbon: historic emissions from fossil fuels, long-run measures of sustainable development and carbon debt", by Jan Kunnasa, Eoin McLaughlin, Nick Hanley, David Greasley, Les Oxley & Paul Warde, was published in the Scandinavian Economic History Review. The abstract:

This article examines how to account for the welfare effects of carbon dioxide emissions, using the historical experiences of Britain and the USA from the onset of the industrial revolution to the present. While a single country might isolate itself from the detrimental effects of global warming in the short run, in the long all countries are unable to free ride. Thus, we support the use of a single global price for carbon dioxide emissions. The calculated price should decrease as we move back in time to take into account that carbon dioxide is a stock pollutant, and that one unit added to the present large stock is likely to cause more damage than a unit emitted under the lower concentration levels in the past. We incorporate the annual costs of British and US carbon emissions into genuine savings, and calculate the accumulated costs of their carbon dioxide emissions. Enlarging the scope and calculating the cumulative cost of carbon dioxide from the four largest emitters gives new insights into the question of who is responsible for climate change.

Sir James Dyer in his lawyer's cap
(Encyclopædia Britannica)

This is an ambitious effort, combining the skills of economists and historians. But putting on my lawyer's cap, I wonder if the assumption that "the calculated price should decrease as we move back in time to take into account that carbon dioxide is a stock pollutant, and that one unit added to the present large stock is likely to cause more damage than a unit emitted under the lower concentration levels in the past" is a valid one. In varied contexts from mass torts to common property, the law tends to apportion costs according to the proportional contribution to the problem or benefit from the activity, regardless of whose contribution caused more marginal damage due to timing.

That seems right to me from a distributive justice perspective, as well as from one focused on fault: The final unit of pollution, causing the most damage, would not have caused that much damage were it not for the first unit contributing to the stock of pollution. Think of the following example: Poisoner 1 slips half a dose of poison, in this amount completely harmless, into a victim's drink. Then Poisoner 2 slips another half dose. Would we say that only Poisoner 2 is responsible because only his action resulted in any marginal social cost?