Responsibility for past and future global warming: time horizon and non-linearities in the climate system

The Brazilian proposal for sharing the burden of emissions reductions among Annex-I Parties is based on the relative effect of a country's emissions on the global-mean surface-air temperature. This paper presents calculations of these relative effects, analysing the influence of the time horizon of emissions and of including non-linearities in the global carbon cycle. The analysis shows that an early start date for historical emissions increases the Annex-I contributions to global warming. Choosing an end date of emissions relatively late in time increases non-Annex-I contributions, giving more weight to their larger share in 21st century emissions. Delayed effects of global warming can be taken into account, if contributions are calculated some time after the emission end date. A calculation date long after the emission end date reduces non-Annex-I contributions, mainly because of their relative large share of relatively short-lived methane in total emissions. Our proposal for a new ('non-linear'), transparent approach for attributing CO2 concentrations generally increases non-Annex-I contributions. The impact is larger than that of including non-linearity in radiative forcing ('saturation effect'). The latter effect increases in time, until the two effects almost cancel out near the end of the 21st century.The analyses were performed for several aggregations of parties in the climate convention (Annex-I/non-Annex-I, 4 IPCC SRES regions, or 17 smaller RIVM IMAGE-regions). We found considerable heterogeneity within aggregated IPCC groups, so that general conclusions drawn for groups as a whole often do not apply to the individual regions within the groups