Attributing the increase in atmospheric CO2 to emitters and absorbers

Climate change policies need to consider the contribution of each emitting region to the increase in atmospheric carbon dioxide. We calculate regional attributions of increased atmospheric CO2 using two different assumptions about land sinks. In the first approach, each absorber region is attributed ‘domestic sinks’ that occur within its boundaries. In the second, alternative approach, each emitter region is attributed ‘foreign sinks’ that it created indirectly through its contribution to increasing CO2. We unambiguously attribute the largest share of the historical increase in CO2 between pre-industrial times and the present-day period to developed countries. However, the excess CO2 in the atmosphere since pre-industrial times attributed to developing countries is greater than their share of cumulative CO2 emissions. This is because a greater fraction of their emissions occurred more recently. If emissions remain high over the coming decades, the share of excess CO2 attributable to developing countries will grow, and the sink service provided by forested regions—in particular those with tropical forest—to other regions will depend critically on future tropical land-use change.