Abstract
Abstract. The natural environment is a major source of atmospheric aerosols, including dust, secondary organic material from terrestrial biogenic emissions, carbonaceous particles from wildfires, and sulphate from marine phytoplankton dimethyl sulphide emissions. These aerosols also have a significant effect on many components of the Earth system such as the atmospheric radiative balance and photosynthetically available radiation entering the biosphere, the supply of nutrients to the ocean, and the albedo of snow and ice. The physical and biological systems that produce these aerosols can be highly susceptible to modification due to climate change so there is the potential for important climate feedbacks. We review the impact of these natural systems on atmospheric aerosol based on observations and models, including the potential for long term changes in emissions and the feedbacks on climate. The number of drivers of change is very large and the various systems are strongly coupled. There have therefore been very few studies that integrate the various effects to estimate climate feedback factors. Nevertheless, available observations and model studies suggest that the regional radiative perturbations are potentially several Watts per square metre due to changes in these natural aerosol emissions in a future climate. Taking into account only the direct radiative effect of changes in the atmospheric burden of natural aerosols, and neglecting potentially large effects on other parts of the Earth system, a global mean radiative perturbation approaching 1 W m−2 is possible by the end of the century. The level of scientific understanding of the climate drivers, interactions and impacts is very low.
Highlights
Aerosols are important components of most parts of the Earth system
By assuming (i) a forest coverage of about one-ninth of the Earth’s surface, (ii) that the indirect effect is restricted to forests, and (iii) that the effects in the boreal forest can be extrapolated to all forests, the global mean indirect effect of changes in biogenic secondary organic aerosol (SOA) would lie between −0.07 and −0.3 W m−2
This study suggests that wildfire will be the dominant driver of the increase in organic carbon (OC) aerosol in the Western United States, causing 75%, with changes to climate and SOA being responsible for the remainder
Summary
Aerosols are important components of most parts of the Earth system. In the atmosphere, they affect the radiative balance by scattering and absorbing radiation and affecting the properties of clouds (Haywood and Boucher, 2000; Lohmann and Feichter, 2005; Forster et al, 2007). Wildfires alter the distribution of vegetation whose emissions of volatile organic compounds account for a large fraction of global sub-micron aerosol mass, and changes in dust emissions directly impact aerosol radiative forcing but may impact DMS emissions through changes in how dust fertilises the ocean with iron. These sections describe the main sources of natural aerosol, the climatic factors controlling emissions as derived from observations and process models, and the status of their treatment as components of climate and Earth system models.
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