General circulation and earth system models (ESMs) that are used to make global climate projections are computationally expensive to run. Furthermore, as full-complexity models take time to develop and test, and intercomparisons take several years to coordinate, simpler models have become an important tool in assessing climate responses to a broad pathway of emissions. A recent example is the IPCC Special Report on Global Warming of 1.5°C, where results needed to be produced quickly and timely engagement with stakeholders is critical.
We have developed a simple open-source earth system model emulator, FaIR, which projects global mean temperature responses as a function of input emissions and radiative forcing. FaIR contains an interactive carbon cycle which replicates the behaviour of complex ESMs, and can be tuned to simulate a particular model, whereas emissions and forcing relationships are based on the IPCC Fifth Assessment Report and more recent peer-reviewed literature. The advantage of being a simple model is that large probabilistic ensembles can be run, where input parameters such as aerosol radiative forcing are varied within the assessed ranges of uncertainty, to produce a range of plausible temperature projections for a given emissions pathway.