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Natural aerosols such as volcanic eruptions, biological emissions from phytoplankton, and dust all contribute significantly to the Earth's albedo and energy balance

Summary​

Natural aerosols from volcanic eruptions, marine processes, terrestrial sources, and dust significantly influence Earth's climate. Marine dimethylsulfide (DMS) emissions from phytoplankton contribute 18% – 42% of global sulfate emissions. Terrestrial aerosols encompass biological particles and organic compounds such as pollen and fungi spores. Dust aerosols exhibit dual effects, cooling over dark surfaces but warming over ice. Key findings show that large reductions in phytoplankton DMS emissions could cause +3 W/m² forcing, equivalent to a 1.6°C temperature increase. Natural aerosols account for ~50% of atmospheric particles in some regions, interacting with clouds through scattering and absorption. Increases in wind speeds in the Southern Ocean since the 1980s have created -0.7 W/m² radiative forcing, offsetting anthropogenic CO₂ impacts over the same time period.

Natural aerosols form in the atmosphere from a variety of sources including volcanic eruptions, dust, sea salt, and other biological processes. Aerosols play an important role in influencing the Earth's climate and weather patterns by scattering light via the same process as anthropogenic aerosols, and by influencing the formation and properties of clouds such as their albedo and lifetime 1. Similar to other particles present in the atmosphere, natural aerosols can have negative thermal forcing on the Earth's energy balance, which is seen with sulfate aerosols released by volcanoes and dimethylsulfide (DMS) emissions from marine phytoplankton. Natural aerosols make up a large percentage of all aerosols found in the atmosphere with some estimates suggesting that as much as 50% of aerosol mass in some regions comes from natural sources 2.

Marine aerosols​

Marine aerosols come from a number of sources including sea spray and dimethylsulfide (DMS) emissions from phytoplankton. DMS is formed through the decomposition of phytoplankton, and undergoes a number of reactions in the atmosphere, first forming sulfur dioxide (SO₂), which then goes on to form sulfate aerosols. Estimates suggest that between 18% – 42% of global sulfate emissions originate from marine phytoplankton 2. The concentrations of sulfate aerosols originating from DMS vary greatly by region and are only released into the lower atmosphere where they have shorter residence times and can be easily washed out by rain. However, their influence on the global climate is still significant, with some modeling of phytoplankton DMS emissions showing that a halving of DMS emissions would result in 3 W/m² of positive forcing, equivalent to a 1.6°C increase in global temperature 3.

Marine primary aerosols include sea salt and organic material. Sea salt aerosols, which play an important role in cloud formation, are dependent on wind washing over the sea surface, as well as sea surface temperatures. Global aerosol models have shown that since the 1980s, within the latitudes of 50 and 65° S, particles for cloud nucleation increased by 22% due to climate change induced increases in wind speeds, leading to a local radiative forcing of -0.7 W/m². This is equivalent to the positive forcing caused by anthropogenic CO₂ emissions over the same time period 4. Organic aerosols such as particulate organic carbon and chlorophyll are released into the atmosphere by sea spray, but their influence on global climate is expected to be limited 2.

Terrestrial aerosols​

Terrestrial aerosols come in the form of primary biological aerosol particles (PBAP) and secondary organic aerosols (SOAs). PBAP are released into the atmosphere from natural sources such as volcanoes, forest fires, fungi spores, pollen, and bacteria. SOAs are formed through the oxidation of biogenic volatile organic compounds (BVOCs) in the atmosphere. The influence of PBAP on global climate is difficult to predict as some of their sources are sporadic and can appear out of nowhere such as with volcanoes and wildfires 2. Additionally, the composition of aerosols from PBAP such as forest fires has the potential to increase local radiative forcing via increased absorption of solar radiation, or decrease radiative forcing via scattering 2. The influence of organic PBAP such as pollen and fungi spores on the Earth's climate is poorly understood 2. The influence of BVOCs on the Earth's climate is also hard to predict as estimating BVOC emissions is challenging due to difficulties in distinguishing natural from anthropogenic sources 2.

Dust aerosols​

The influence of dust aerosols on the Earth's climate depends on the surface that it is covering, as well as its size and composition. Over open ocean, which has a very low albedo, it has a cooling effect. But over reflective surfaces with high albedo such as snow and ice, it has a warming effect 2. Global estimates of the thermal forcing of dust particles range from −0.7 – +0.5 W/m² at the top of the atmosphere and between −0.82 – −1.92 W/m² near the Earth's surface 2.

Sources​

Footnotes​

  1. Forster, P., Ramaswamy, V., Artaxo, P., Berntsen, T., Betts, R., Fahey, D. W., Haywood, J., Lean, J., Lowe, D. C., Myhre, G., Nganga, J., Prinn, R., Raga, G., Schulz, M., & Van Dorland, R. (2007). Changes in atmospheric constituents and in radiative forcing. In S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor, & H. L. Miller (Eds.), Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. ↩

  2. Carslaw, K. S., Boucher, O., Spracklen, D. V., Mann, G. W., Rae, J. G. L., Woodward, S., & Kulmala, M. (2010). A review of natural aerosol interactions and feedbacks within the Earth system. Atmospheric Chemistry and Physics, 10, 1701–1737. https://doi.org/10.5194/acp-10-1701-2010 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9

  3. Gunson, J. R., Spall, S. A., Anderson, T. R., Jones, A., Totterdell, I. J., & Woodage, M. J. (2006). Climate sensitivity to ocean dimethylsulphide emissions. Geophysical Research Letters, 33(7), L07701. https://doi.org/10.1029/2005GL024982 ↩

  4. Korhonen, H., Carslaw, K. S., Forster, P. M., Mikkonen, S., Gordon, N. D., & Kokkola, H. (2010). Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds. Geophysical Research Letters, 37(2), L02805. https://doi.org/10.1029/2009GL041320 ↩