Tiny particles suspended in the atmosphere, known as aerosols, are typically found in a distinct layer in the lower stratosphere, between about 15 and 25 kilometers above Earth’s surface. These particulates are composed primarily of sulfuric acid/water solution droplets, where the sulfur component originates from predominantly natural sources, particularly, powerful volcanic eruptions. Aerosols reflect incoming sunlight and can increase the reflectivity of clouds, which means their presence typically cools Earth’s climate. This critical role has led to controversial geoengineering proposals to manipulate the planet’s aerosol layer to help counteract a warming climate.
Following a decade of concerted scientific research, Kremser et al. provide a comprehensive overview of the advances since 2006 in our understanding of the sources, sinks, and properties of stratospheric aerosols and their potential effect on global climate. These include an estimated increase of 1.5 times the net flux of sulfur from the troposphere to the stratosphere, compared to the last comprehensive review on stratospheric aerosol in 2006, and the detection of small amounts of nonsulfate particulates (including organics and black carbon) in the aerosol composition.
According to the researchers, one of the most significant developments since 2006 is the improved agreement between in situ and space-based measurements of aerosol properties during periods of reduced volcanic activity. The improvement of these data sets, which are core inputs for climate model simulations, as well as the increased number and increased sophistication of chemistry-climate models, has greatly enhanced the representation of stratospheric aerosol processes in climate models during the last decade. Most of these models are now coupled with solar radiation and/or chemistry modules that can account for important feedbacks, allowing climate models to account for changes in hydrologic and carbon cycles as well as changes in the biosphere and cryosphere.
Despite significant progress, however, the researchers acknowledge that many crucial research challenges regarding stratospheric aerosols remain. These include quantifying the contribution of man-made sulfur dioxide emissions to the stratospheric aerosol layer and identifying the role of nonsulfur compounds, whose potential influence on chemical reactions is currently not accounted for in most stratospheric aerosol models. The climate model community is aiming to incorporate the stratospheric aerosol layer as an interactive element in global climate models, so that future simulations can assess the role of aerosol in a changing climate. Observations of stratospheric aerosol and its precursors remain necessary to test the reliability of climate model simulations in the future. (Reviews of Geophysics, doi:10.1002/2015RG000511, 2016)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2016), A decade of progress in stratospheric aerosol research, Eos, 97, doi:10.1029/2016EO050721. Published on 20 April 2016.