Spatialities and temporalities of metrics calculated by Integrated Assessment Models: Exceedance of ecosystem-specific Critical Loads

Integrated Assessment Models are designed to calculate a variety of metrics which describe the impacts upon ecosystems and human health resulting from specified spatial patterns of emissions of air pollutants. These metrics can provide policy makers with useful information which can assist in sustainable policy development and negotiation of international protocols. However, they can also present a very different picture of impacts depending upon the spatial resolution of the models used to describe emissions, atmospheric dispersion, deposition and air quality, and the metrics themselves may change as scientific understanding evolves over time. Using exceedance of ecosystem Critical Loads as an example of such metrics, we show how the area of critical load exceedances increases both as the spatial resolution of modelled deposition increases and as scientific advances in the representation of ecosystem specific deposition patterns are included in the models. Furthermore, advances in scientific understanding of Critical Loads, atmospheric dispersion and deposition rates can lead to apparently paradoxical quantifications of impacts when the effectiveness of policies are subsequently reviewed. Policy makers demand use of the most up-to-date science and may thus, when comparing against earlier assessments, discover an apparent increase in the ecosystem area exceeded due to scientific improvements in the models rather than reduced emissions; such results reflect the different and potentially conflicting temporalities of scientific developments and the policy making processes, as opposed to impacts in the real world. Thus, we also highlight the importance of coordinating these temporalities and using equivalent tools or models for both assessment of potential impacts of policies and any subsequent review of the effectiveness of those policies.