Estimates of carbon budgets consistent with global warming of 1.5-2°C from ensembles of simulations with the MIT Earth System Model of intermediate complexity: the role of non-CO2 GHGs and SO2 emissions

We present estimates of carbon budgets for different levels of surface air temperature (SAT) increase from multiple 400-member ensembles of simulations with the MIT Earth System Model of intermediate complexity (MESM). Ensembles were carried out using distributions of climate parameters affecting climate system response to external forcing obtained by comparison of historical simulations with available observations.

First, to evaluate MESM performance, we ran two ensembles: one with MESM forced by 1% per year increase in CO₂ concentrations (and with non-CO₂ greenhouse gases (GHG) at pre-industrial level) and the other with GHG concentrations from the RCP 8.5 scenario. Distributions of climate characteristics describing model response to increasing CO₂ concentrations (e.g. TRC and TCRE) as well as values of carbon budgets of exceeding different SAT levels agree well with published estimates.

Then we ran a number of ensembles with MESM driven by emissions produced by the MIT Economic Projection and Policy Analysis (EPPA) Model. Our results show that under stringent mitigation policy concerning non-CO₂ GHGs, the SAT increase can be kept below 2°C relative to pre-industrial with 66% probability through the end of the 21^st century without negative CO₂ emissions. The SAT increase can also be restricted to 1.5°C with 50% probability if such policy is implemented immediately. If GHG emissions follow the path implied by the Paris Agreement pledges through 2030, then it would require either an unrealistically sharp drop in non-CO₂ GHG emissions or negative CO₂ emissions to stay below 1.5°C. Keeping the temperature increase below a chosen value (1.5°C or 2°C) beyond 2100 will most likely require negative CO₂ emissions in part due to difficulties in restricting agricultural methane emissions.

Further analysis shows that temperature change during the 21^st century is also significantly affected by the assumption concerning decrease of SO₂ emissions from energy-intensive industries. Implementation of technologies resulting in reduction of those emissions will decrease the probability of SAT staying below a given limit by about 7-12%. This will affect the time when negative CO₂ emissions will become necessary to prevent temperature increase.