Modelling the climate response to anthropogenic carbon dioxide emissions: time-dependent processes, commitment, and reversibility

This thesis gives insight into key aspects of the climate system response to anthropogenic carbon dioxide (CO2) emissions. One characteristic is an approximately constant global mean surface air temperature (GMSAT) after cessation of emissions, but also changes in GMSAT to second order. Here it is shown that these second-order GMSAT changes are positive, i.e. there is a small committed warming from previous emissions, because the warming effect from declining ocean heat uptake dominates over the cooling effect from declining atmospheric CO2. The timing of zeroing emissions or the time horizon over which the warming commitment is calculated have minor effects on this warming commitment compared to the effect of the scenario prior to cessation of emissions. Another characteristic explored is the approximately constant ratio between GMSAT change and cumulative CO2 emissions (CE), referred to as Transient Climate Response to cumulative CO2 emissions (TCRE). It is shown that the TCRE diverges more strongly over time from a constant value under increasing atmospheric CO2 concentration than previously suggested. But it is approximately constant over time under constant CO2 concentration due to cancelling effects of changes in ocean heat and carbon uptake. Applying a wide range of sub-grid ocean mixing parameterizations does not change the temporal evolution of the TCRE significantly but leads to a wide range in the TCRE value. A third characteristic explored is irreversibility of sea level rise from thermal expansion (TSLR). It is shown here that TSLR under negative emissions does not return to pre-industrial levels for centuries after atmospheric CO2 has returned to pre-industrial concentrations. This result is robust against the choice of mixing parameter, although, generally an increased parameter leads to higher TSL rise and decline rates. The results presented in this thesis suggest that setting cumulative CO2 emission budgets in order to not exceed a certain warming target needs to be done with caution as the TCRE varies more strongly over time than previously shown and additional committed warming may lower allowable carbon budgets. Furthermore, TSLR is not linearly related to cumulative CO2 emissions and is slow to be reversed if net negative emissions are applied.