In a paper published in the journal Science, Natalie Mahowald at Cornell University reports on a third effect of anthropogenic aerosols on the climate—their slow impact on biogeochemical cycles potentially affecting the fluxes of CO2 into and out of the atmosphere. The net effect of aerosols is usually considered the sum of their direct radiative effect plus the indirect effect through aerosol-cloud interactions.
Until recently, an additional climate impact of aerosols has been largely ignored: the indirect effect of aerosols on biogeochemical cycles. Similar to the indirect effect on clouds, these effects occur only to the extent that aerosols affect relevant earth system processes. For example, the indirect effect of aerosols changes cloud droplet properties once clouds form downwind of the aerosol source (cloud albedo effect) and can change the lifetime of the cloud (cloud lifetime effect). Aerosol indirect effects on biogeochemical cycles similarly affect the fluxes downstream from where they are emitted, through one of two different mechanisms: (i) changing the physical climate of the ocean or land ecosystem, and thereby changing biogeochemical fluxes; or (ii) depositing chemicals that modify the biogeochemical cycles. In the latter case, the aerosols could supply either nutrients that stimulate growth or, alternatively, toxins that suppress growth. The aerosol indirect effect on biogeochemical cycles tends to occur on a longer time scale than the aerosol indirect effect on clouds.
The identification of additional impacts by aerosols has implications for future climate policy. Emission projections suggest that globally averaged aerosol forcing will decrease as countries reduce emissions to improve their air quality and reduce public health risks. Many aerosols are created in combustion processes at the same time as CO2 is produced, so reductions in CO2 emissions could cause corresponding reductions in aerosol emissions. Aerosol effects are as diverse as the sources of aerosols, with some aerosols warming the planet (such as black carbon) and others cooling the planet (such as sulfate). The net effect of anthropogenic aerosols is to cool the planet, however, so projected changes in emissions will tend to reduce the aerosol radiative forcing. These cuts in aerosols will not only cause an increase in temperatures but will also cause a decrease in the uptake of carbon by the land and the ocean. This may preferentially affect the more aggressive carbon policies, because these also will result in the fastest decrease in aerosol emissions.
For high-carbon emission cases, changes in emissions should not be large enough to cause significant impacts due to changing aerosol production. However, for the low-emission pathways, even lower emissions will need to be achieved than previously estimated, because of the impact of the aerosol indirect effect on carbon uptake. Although there are many uncertainties in estimating future mitigation costs, it is clear that lower targets for CO2 concentrations correspond to greater costs. The relationship between cost and targets is highly nonlinear, with costs rising rapidly in a kind of “cliff” as CO2 targets decrease. Because it is generally not accounted for, the aerosol indirect effect on CO2 uptake, mediated by biogeochemical cycles as described here, tends to shift the cost cliff toward higher costs for the same CO2 level at 2100 as compared to when it is ignored. Therefore, achieving lower atmospheric CO2 concentrations may be even costlier than previously estimated. The estimates provided here suggest that more detailed studies on the effect of aerosols on biogeochemical cycles are important for understanding future climate.—Mahowald 2011
Natalie Mahowald (2011) Aerosol Indirect Effect on Biogeochemical Cycles and Climate. Science Vol. 334 no. 6057 pp. 794-796 doi: 10.1126/science.1207374