Anthropogenic aerosol drives uncertainty in future climate mitigation efforts

The 2016 Paris agreement set a global mean surface temperature (GMST) goal of not more than 2 degrees Celsius above preindustrial. This is an ambitious goal that will require substantial decreases in emission rates of long-lived greenhouse gasses (GHG). This work provides a mathematical framework, based on current state of the art climate models, to calculate the GHG emissions consistent with prescribed GMST pathways that meet the Paris agreement goal. The unique capability of this framework, to start from a GMST timeseries and efficiently calculate the emissions required to meet that temperature pathway, makes it a powerful resource for policymakers. Our results indicate that aerosol emissions play a large role in determining the near-term allowable greenhouse gas emissions that will limit future warming to 2 °C, however in the long term, drastic GHG emissions reductions are required under any reasonable aerosol scenario. With large future aerosol emissions, similar to present day amounts, GHG emissions need to be reduced 8% by 2040 and 74% by 2100 to limit warming to 2 °C. Under a more likely low aerosol scenario, GHG emissions need to be reduced 36% and 80% by 2040 and 2100, respectively. The Paris agreement Intended Nationally Determined Contributions are insufficient to meet this goal.