Ideally, a country’s fair share would be proportional to its capacity to act. But ”capacity to act” isn’t easy to measure. We can approximate it with national income, which is often closely correlated to historical responsibility. Importantly, we take inequality within countries into explicit account when estimating national capacity. The income of those making just enough to scrape by cannot be considered part of some kind of national capacity for climate action. The responsibility for climate action lies primarily on the rich, not the poor.
Note that, in the international climate negotiations, injustice within countries is often treated as being “off limits.” Nations are rather treated as “sovereign” entities that are not expected to explain or justify internal economic stratification to others. Be that as it may, such stratification is crucial to an ethical estimation of national capacity, for the issues here invariably revolve around one central question – how do we gauge the obligations and rights of those who have a lot, compared to those who have little? As you can see from the details of the equity benchmarks we use in this analysis (see The U.S. Fair Share – Backgrounder), this is not an afterthought.
For a detailed and very current discussion of the issues here, see Confronting Carbon Inequality , the new report from Oxfam International.
 Confronting Carbon Inequality: Putting climate justice at the heart of the COVID-19 recovery, 21 September, 2020, https://assets.oxfamamerica.org/media/documents/Confronting-Carbon-Inequality.pdf