Climate researchers often discuss the remaining amount of carbon dioxide humans can emit to ensure that there is a reasonable probability (say, 66%) of limiting global warming since pre-industrial times to 2 or 1.5 degrees C. The important question is how to divide remaining emissions between countries.

Since CO2 is a long-lived gas in the atmosphere, legacy effects are important. Historically, the industrial countries have been responsible for most CO2 emissions. However, today, only about one third of fossil fuel and industrial CO2 emissions are from core OECD countries, and their share is falling. When CO2 from net deforestation is also considered, ttheir share is even smaller.

Should remaining allowable emissions be simply allocated between countries on equal per capita emissions for all humans? Or, should some account be taken of existing per capita emissions--after all, a sudden large drop in allowable emissions for OECD countries would not be in the short-term interests of poor countries either. And what about climate--many countries have climates that are on average too hot or too cold, necessitating higher than average air conditioning or heating energy.

So in the past, the industrial countries accounted for both most annual and cumulative emissions. Today, they still account for most cumulative emissions, but are becoming increasingly irrelevant for current emissions. What is to be done?