Paris-consistent peak CO2 concentrations in Shared Socioeconomic Pathway scenarios

If temperatures are to remain below 1.5°C, it is likely that carbon emissions will need to reach net zero this century. This is because global mean temperature is strongly related to cumulative emissions of CO₂ while emissions are still positive. Often, this is also near the point in time where atmospheric CO₂ concentrations peak. Unlike CO₂ emissions, atmospheric CO₂ concentrations are not solely a function of societal choices, as they are dependent on carbon cycle and earth system feedbacks in addition to anthropogenic and natural sources and sinks. However, as radiative forcing is directly dependent on concentrations, they provide a strong link to temperature. It can also be argued that the parts-per-million atmospheric CO₂ concentration is an easier concept to grasp than the cumulative emissions that comprise carbon budgets.

We use emissions trajectories for CO₂ and other species from the suite of Shared Socioeconomic Pathway scenarios from six integrated assessment modeling groups. Using a 1000-member Monte Carlo ensemble we sample uncertainty in radiative forcing, climate sensitivity, ocean thermal response and the carbon cycle based on assessed ranges from the IPCC Fifth Assessment Report and CMIP5 climate models. This 1000-member ensemble is run through the FaIR simple climate model. We show that if atmospheric CO₂ concentrations remain below 525 ppm, there is a 50% chance of keeping warming below 2°C in our ensemble results. A 50% chance of limiting warming to 1.5°C requires peak CO₂ concentrations of less than 450 ppm.

There is a considerable spread in allowable peak CO₂ concentrations for each temperature limit. The peak CO₂ concentrations for 1.5°C or 2°C depends strongly on non-CO₂ (particularly aerosol) forcing and the carbon cycle. Some aspects like non-CO₂ emissions pathways are part of the scenario uncertainty, whereas many such as the climate sensitivity, carbon cycle, and relative strength of aerosol forcing, are part of the geophysical uncertainty. We attempt to quantify these components and show that the strength of the present-day aerosol forcing is a large contributor to the peak CO₂ concentrations that limit warming to 1.5°C or 2°C.