Metrics for Regional Climate Responses to Regional Pollutant Emissions

The Absolute Global Temperature change Potential (AGTP) and Absolute Global Precipitation change Potential (AGPP) are widely used climate change indices.  They can be applied quickly and easily to estimate the global mean temperature and precipitation responses to a pulse emission of a long-lived climate pollutant at a given time horizon, making them invaluable policy-relevant metrics.  They can also be extended to short-lived climate pollutants - where a sustained emission is more useful to consider than a pulse emission - by using their time-integrated forms (iAGTP and iAGPP).

However, these metrics are only useful when taking a global-average perspective, and do not allow us to account for the regional nature of either emissions or their climate response.  Although long-lived greenhouse gases induce a relatively homogeneous radiative forcing (RF) which is not sensitive to emission location, nonetheless due to transport of heat there is not a one-to-one correspondence between the RF in a region and the local temperature response.  Moreover when considering short-lived pollutants such as aerosols, the region of emission is potentially critical because the short lifetime of such pollutants results in an inhomogeneous distribution of RF.  Therefore, for both long-lived and short-lived pollutants the AGTP/AGPP (or iAGTP/iAGPP) are not adequate when looking at climate responses on a regional scale, even though this would be the most relevant when evaluating different policy scenarios or climate change impacts.

Here, we combine the results of simulations from the Precipitation Driver Response Model Intercomparison Project (PDRMIP) where emissions (or concentrations) of multiple long- and short-lived climate pollutants were perturbed globally in nine different climate models, with the results of simulations using the HadGEM3 model where sulfate aerosol emissions are perturbed one at a time in several key geopolitical regions: the United States, Europe, India, East Asia, or the whole Northern Hemisphere Mid-Latitudes.  We use these results to adapt the (i)AGTP/(i)AGPP to the case where both the emission and the response are regional.  Data from the regional HadGEM3 simulations allow us to estimate normalised regional forcing-response relationships for aerosols, whilst the PDRMIP multi-model means and ensemble spread are used to derive estimates of radiative efficiency for both long- and short-lived pollutants and their corresponding uncertainties, as well as the regional climate sensitivities for long-lived pollutants.

Finally, using these regional temperature and precipitation change potentials, we produce a simple model in Python which allows the user to specify arbitrary combinations of different future emission scenarios for different pollutants from different regions, allowing rapid projections of the regional climate responses to diverse emissions policies.