PNNL

Abstract

The approximate proportional relation between cumulative carbon emissions and instantaneous global temperature rise (the carbon budget approximation) has proven to be a useful concept to translate policy-relevant temperature objectives into CO2 emissions pathways. However, when non-CO2 forcing is changing along with CO2 forcing, error in the approximation increases. Using the GCAM model to produce an ensemble of ~3,000 scenarios, we show that linked changes in CO2 forcing, aerosol forcing, and non-CO2 greenhouse gas forcing lead to a consistent increase in total non-CO2 forcing over the 21st century. This increase causes the relationship between instantaneous temperature and cumulative CO2 emissions to become more complex than the proportional approximation often assumed. The same underlying mechanisms also contribute to a consistent, decadal increase in aerosol-driven warming that effectively places a limit on how low peak temperature could be constrained through greenhouse gas mitigation alone.