AVOID 1: Observational constraints on simple climate models

This project has found four key results:

  •  A number of recent studies have found a strong link between peak global mean warming and cumulative carbon dioxide emissions. The first part of this new study examines the relative merits of different cumulative emission metrics and compares them with considering only the emissions at 2020 as a constraint future warming. We find that considering emissions from the start of the industrial revolution to the time of peak warming provides a much better constraint on peak warming. Cumulative emissions over shorter periods or emissions in the years 2020 or 2050 are, in general, a much weaker constraint.
  • Previous studies have assumed that net emissions eventually fall to zero, in conflict with the concept of a “floor” in emissions due to sectors such as food production. Here we show that the introduction of emissions floors does not reduce the importance of cumulative emissions, but may make some peak warming targets unachievable. For scenarios that give a best-guess warming up to 3-4°C, we show that cumulative emissions from pre-industrial times to year 2200 correlate strongly with most likely resultant peak warming regardless of the type of emissions floor used.
  • The third key result is that the maximum rate of CO2-induced warming, which will affect the feasibility and cost of adapting to climate change , is not determined by cumulative emissions, and is instead tightly aligned to peak rates of emissions. We suggest, therefore, that cumulative carbon emissions and peak emissions rates could together provide part of a clear and simple framework for climate change mitigation policy.
  • The final component of this project was to compare the box-diffusion model used throughout this paper with two other models. We suggest the discrepancies between the models’ temperature responses to a similar forcing are due to their different treatments of the transient climate response. We also suggest that models’ treatment of diffusion in their ocean carbon cycles may strongly influence their projected CO2 concentrations. We recommend that constraining the ocean uptake of carbon from the atmosphere in a situation of low anthropogenic carbon emissions should be a research priority.