Climate and carbon budget implications of linked future changes in CO ₂ and non-CO ₂ forcing

The approximate proportional relationship 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, errors in the approximation increases. Using the GCAM model to produce an ensemble of ~3000 scenarios, we show that linked changes in CO₂ forcing, aerosol forcing, and non-CO₂ greenhouse gas (GHG) forcing lead to an increase in total non-CO₂ forcing over the 21st century across mitigation scenarios. This increase causes the relationship between instantaneous temperature and cumulative CO₂ emissions to become more complex than the proportional approximation often assumed, particularly for low temperature objectives such as 1.5 °C. The same linked changes in emissions also contribute to a near-term increase in aerosol forcing that effectively places a limit on how low peak temperature could be constrained through GHG mitigation alone. In particular, we find that 23% of scenarios that include CCS (but only 1% of scenarios that do not include CCS) achieve a temperature objective of 1.5 °C without temperature overshoot.