Aerosols and climate - How sensitive is earth’s climate to atmospheric aerosols? [Past]

Paleoclimate records spanning the past several million years reveal large variability in the deposition of aeolian dust and other natural aerosols. Understanding this variability represents both a challenge and a useful test for Earth system models. The production, transport and deposition of natural aerosols are controlled by numerous physical and biogeochemical processes that are still not well understood. On the other hand, aerosols affect the climate via a number of physical and biogeochemical processes (see the accompanying article by Albani and Mahowald). On short time scales (several years), sulfur aerosols from volcanic eruptions play a significant role in forcing climate. On longer time scales, it is believed that climate-aerosol feedbacks amplify climate changes caused by other factors, such as changes in Earth’s orbital parameters and concentrations of greenhouse gases.

Figure 1: Modeled dust deposition under preindustrial climate conditions (left) and Last Glacial Maximum (right) based on Mahowald et al. (2006).

Variability of the dust cycle is especially significant at glacial-interglacial time scales (Fig. 1). Paleoclimate data and model simulations suggest that during the Last Glacial Maximum (ca. 21,000 years before present) dust deposition in tropics was several times higher than at present and over Antarctica and Greenland the dust deposition rates increased by more than an order of magnitude. Such large increase in atmospheric dustiness cannot be explained without invoking a large increase in dust sources during glacial times (Mahowald et al. 2006). There are a number of processes via which variations in atmospheric dust loading and deposition rates may contribute as amplifiers and modifiers of the orbitally forced glacial cycles. First, an increase in atmospheric dustiness leads to increased reflection of incoming solar radiation and thus contributes to global cooling. This effect can be additionally enhanced by the effect of natural aerosols on cloud albedo (the so-called indirect effect), but partly offset by the additional absorption of outgoing long-wave radiation by dust particles (Takemura et al. 2009). The net simulated climatic effect of dust on climate during glacial times is sensitive to the poorly known optical properties of dust and is therefore model-dependent but typically of a comparable magnitude (1-2 W/m2) to other climatic factors, such as a lowering of the atmospheric CO2 concentration and increased surface albedo due to ice sheet growth. At the same time, enhanced dust deposition over snow and ice leads to a reduction of surface albedo and thus enhances ice melt. This effect may have played a role in both preventing the ice sheets from spreading into lower latitudes (Krinner et al. 2006) and accelerating the retreat of the ice sheets during glacial terminations (Ganopolski et al. 2010).

In addition to the physical effect, enhanced deposition of dust over ocean areas where plankton growth is limited by the availability of iron can enhance biological production and thus lead to the drawdown of atmospheric CO2 (Martin et al. 1990). Recent modeling experiments suggest that the iron fertilization effect in the Southern Ocean alone can explain a significant fraction of glacial CO2 reduction (Brovkin et al. 2007). Further progress in understanding the role of dust and other natural aerosols in climate change therefore requires the incorporation of these processes into the new generation of Earth system models.

selected references

http://pastglobalchanges.org/products/newsletters/ref2012_1.pdf

Brovkin V, Ganopolski A, Archer D and Rahmstorf S (2007) Paleoceanography 22, doi: 10.1029/2006PA001380

Ganopolski A, Calov R and Claussen M (2010) Climate of the Past 6: 229-244

Krinner G, Boucher O and Balkanski Y (2006) Climate Dynamics 27(6): 613-625

Mahowald N et al. (2006) Journal of Geophysical Research 111, doi: 10.1029/2005JD006653

Takemura T et al. (2009) Atmospheric Chemistry and Physics 9: 3061-3073