A rigorous approach to model constraint
In an upcoming paper in the Proceedings of the National Academy of Sciences, we present the first attempt to rigorously constrain an aerosol model, then examine the implications for forcing uncertainty. This paper was based on a keynote presentation at the Sackler Colloquium in 2015. In GASSP, “constraint” means generating a huge ensemble of model simulations that sample all important uncertainties, then retaining only those simulations that are plausible when compared to measurements. Rather than finding the best model, we retain all plausible model variants, which allows us to quantify the remaining model uncertainty. A key discovery in this paper by Lee, Reddington and Carslaw is that a global aerosol microphysics model has very high equifinality. This means that, for example, similar aerosol concentrations are predicted by a wide range of model variants (i.e., different parameter settings). We show that these model variants can simulate very different aerosol forcings, so tight constraint of the aerosol model doesn’t feed through directly to forcing uncertainty.
Uncertainty in regional and decadal forcing
In two papers by PhD student Leighton Regayre et al. (GRL 2014, J Climate 2015) we have explored the causes of uncertainty in regional patterns of aerosol-cloud forcing over recent decadal periods. There are several important results for GASSP. First, we found that the causes of uncertainty are very different over decadal periods than since the pre-industrial. Natural aerosols are much less important, which suggests the uncertainty may be more easily reduced. Second, the uncertainty in regional forcing can be very high, which might affect regional climate change. Third, the causes of uncertainty in regional forcing are very different to the causes of uncertainty in global mean forcing. So significant regional uncertainty might persist even if we could reduce uncertainty in the global mean. See DOI: 10.1175/JCLI-D-15-0127.1 and doi: 10.1002/2014GL062029.
Importance of natural aerosols in forcing uncertainty
In Carslaw et al. (Nature, 2013), we showed that natural aerosols have a substantial effect on the uncertainty in pre-industrial to present-day aerosol-cloud forcing. Twenty-eight uncertain aerosol parameters were sampled in the GLOMAP model using a perturbed parameter ensemble (PPE) and model emulation. Over 40% of the forcing uncertainty is due to natural aerosol emissions, even though the emissions are assumed to be constant over the industrial period. The reason for the large contribution to uncertainty is that natural aerosols define the baseline aerosol state from which forcing is calculated, and cloud albedo is highly sensitive to aerosol concentrations in pristine environments. The importance to GASSP is that our ability to reduce uncertainty in forcing depends on the availability of aerosol measurements under pristine pre-industrial-like conditions. See doi:10.1038/nature12674.
Finding pristine aerosol environments
In a peper by PhD student Douglas Hamilton et al. (PNAS, 2014) we used a novel approach to identify regions of the planet where aerosol conditions can still be considered pristine. These are highly valuable locations for making aerosol measurements because they are representative of the natural aerosol background, which has a large effect on forcing uncertainty (Carslaw et al., 2013). GASSP has prioritised the collection of measurements in such pristine environments, but they are comparatively rare. See www.pnas.org/cgi/doi/10.1073/pnas.1415440111.
We have a list of published papers.