# Evaluating Uncertainties in Marine Biogeochemical Models: Benchmarking Aerosol Precursors
## 4 Discussion  
In the present work, we have outlined the importance of marine biogeochemistry to the generation of global aerosol precursors in the lower troposphere, and further to the determination of cloud structural feedback loops. The latter influences occur partly but not exclusively through the regulation of albedo over the ocean. We underscore the importance of incorporating the relevant processes in climate and Earth system simulations. Examples include, and we focused upon, the classic CLAW hypothesis, and volatile reduced sulfur sources [4; 9]. However, we go further by noting that extensive validation must be supported if the new generation of models is to be successful. An automated framework is proposed and described based on the ILAMB package as a foundation and example [10]. The marine version is now known as IOMB, which stands for International Ocean Model Benchmarking. The present paper constitutes one of the first descriptions of IOMB; its capabilities begin with standard nutrient-carbon cycling, as we demonstrate. However, we have intentionally incorporated the option to extend to the marine aerosol and related issues involving dissolved and volatile organics [46; 56]. Benchmarking results with the global DMS database are presented and analyzed as examples of the need for further research, and here we venture to discuss the potential for future aerosol validation exercises. Since the surface ocean is a major contributor of natural aerosol to the global troposphere [3; 77], the sea necessarily acts as a point source for gases and particles, which become cloud condensation and ice nuclei. Microchemistry of the resulting aerosol distribution influences cloud reflectivity and lifetime  
Figure 5: Global and regional trends of sea-air DMS flux (\(\mu\)mol m\({}^{-2}\) day\({}^{-1}\)) in CMIP5 models from 1850 to 2100. HadGEM2-ES outputs started in 1860. There is a transition from historical simulation results to RCP 8.5 in year 2005.  
across most of the planetary area [11; 78]. Sulfate aerosols are of special interest, as high hygroscopicity and unique optical properties make them more suitable for cloud formation changes and increasing cloud lifetime. About 70% of natural sulfur emissions originate from the ocean as DMS. Our results show that approximately 30.6 Tg of sulfur get transferred into the atmosphere in a year. Tropospheric aerosol has contributed significantly to the moderation of global radiative forcing, which has been attributable to greenhouse gases since the industrial revolution [21]. By contrast, with the long-lived heat trapping gases that prevent longwave radiation from escaping the top of the atmosphere, aerosol particles have exerted a net cooling effect operating primarily from below. Accurate evaluation of the contribution of tropospheric aerosol to global radiative forcing requires a correct assessment of the natural contribution from marine sources. Although the parameterization of subgrid processes in contemporary ESMs has advanced over the decades, considerable regional biases still linger, from nutrient concentrations underlying the ecosystem to the mass of BVOCs released to the marine boundary layer. This is not unexpected, as layers of process complexity increase the uncertainty in simulated results. To reduce the degree of uncertainty associated with remote aerosols, marine biogeochemical simulations must be constrained with high quality observation datasets. All can agree that no ESM result will perfectly match observations, particularly across all temporal and spatial scales. However, well-defined and appropriately organized measurements clearly help to constrain model deviations. Biogeochemical simulation results within the accuracy bounds of their respective data serve to untangle the impacts of external forcings. Ultimately, this supports the quantification of secondary products such as the flux and concentrations of BVOCs as a class.