Do the lifetime emissions of 'three and a half Americans' kill 'one person'?

Question

On 29th July 2021, Nature published a paper titled "The Mortality Cost of Carbon" and many publications have repeated a rather sensational part of the abstract of the paper (emphasis added):

Our central estimate 2020 MCC [mortality cost of carbon] also implies that reducing (adding) 4,434 metric tons of carbon dioxide in 2020 saves one life (causes one excess death) in expectation globally between 2020 and 2100. In all, 4,434 metric tons is equivalent to the lifetime emissions of 3.5 average Americans, 146.2 Nigerians, and 12.8 average world people.

This has been repeated by The Independent, Yahoo and many others. Here's the headline from the Guardian, 29th July 2021:

Three Americans create enough carbon emissions to kill one person, study finds

I am skeptical as there is no explanation of how the calculations of emissions and the calculations of estimated excess deaths can possibly be matched together and conclusions be drawn in such a simplistic way.

COVID-19 figures have demonstrated that only a few countries have the administrative infrastructure to reliably calculate the correct statistic for excess deaths and these very countries are, inherently, not the ones with the highest populations.

But the main feature of this published statement is that no explanation is offered as to the decease of the supposed individual:

• Were they never born, because emissions caused a terminated pregnancy?
• Were they never born because their mother never reached reproductive age, due to emissions?
• Did they reach middle age?
• Old age?
• Was their life merely cut short by a few months?

And what of the "three and a half" Americans? Did they perhaps have shortened lives due to emissions? Did they lose a few years? Or many years?

To publish such a statistic from the mere matching of other (only vaguely related) statistics is highly debatable and leaves me very skeptical indeed.

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EDIT after COMMENT

@IMSoP comments and quotes the following definition from the paper:

Excess deaths are deaths attributable to climate change that occur prematurely relative to a counterfactual scenario in which the marginal emission did not occur.

I appreciate the definition and am grateful for it being quoted. But still it leaves questions as to how the statistics are calibrated to one another and as to what exactly 'premature' means.

I have worked night shifts for twenty-five years and I know that I shall live a shorter life because of it. But it is a risk I accept. I may live a few years less.

The 'death' caused by 'three and a half' Americans needs to be quantified as to when, exactly, this death will occur, (just how premature is 'premature') I would strongly suggest.

Answer 1

As so often, the claim has not been translated well into headlines, and makes a compelling soundbite without a very deep meaning.

The actual central claim is this:

adding 4,434 metric tons of carbon dioxide in 2020 causes one excess death in expectation globally between 2020 and 2100

This is compared against various things, among them the lifetime emissions of 3.5 Americans. An accompanying graph makes this comparison clearer, labelling it "Average citizen's lifetime emissions if all added in 2020".

So the claim is not that every year 10 people die for every 35 Americans; it is that if 35 Americans blinked out of existence, the reduction in carbon dioxide would save 10 people from climate-related deaths over the course of the next century.

Indeed, even that is probably overstating it, because the model used includes feedback factors, so a tonne of carbon dioxide emitted in 2020 is counted differently from one emitted in 2099 in terms of its impact in 2100. So the 10 deaths would be spread further into the future, as some would be caused by emissions later than 2020.