Article: Mahowald et al. 2011. Aerosol Impacts on Climate and Biogeochemistry. Annual Review of Environment and Resources 36: 45-74.
Background: Although they represent a small portion of the atmosphere by mass, aerosols have a disproportionately large impact on climate and biogeochemistry. They can change atmospheric radiation (both short- and longwave), alter cloud properties, impact public health, darken snow albedo, and modify land and ocean biogeochemistry. Since most anthropogenic aerosols tend to cool the climate, it is possible that they have partially masked warming from greenhouse gas emissions during the 20th century.
Summary: In this paper, Mahowald et al. review the impacts aerosols have on climate due to a number of complex environmental interactions, as well as discuss the large uncertainties surrounding aerosols. Since the term aerosol encompasses a very broad category of particles, they can range in size from a diameter of 1 nm to 100 mm, with the larger aerosols primarily coming from wind blown dust or salt and the finer ones from anthropogenic emissions. Sulfate aerosols have been found to be the most influential in changing the radiative forcings in the past 200 years. As a result, sulfates are the aerosols gaining the most attention for geo-engineering proposals for pro-actively cooling the atmosphere. An aerosol generally has a short residence time in the atmosphere (from less than a day to 4 weeks), but once it is deposited on the Earth’s surface, it can have effects of much longer duration. Not all aerosols are “scattering aerosols” that cool the atmosphere; in fact, very small concentrations of absorbing aerosols can severely reduce albedo (increasing the amount of radiation absorbed by the earth) if deposited onto snow and sea ice. The variable shape and optical properties determine the fraction of light an aerosol absorbs or reflects, and consequently determines its radiative forcing (net effect on the energy balance of the Earth-atmosphere). Yet, since there is still much uncertainty in the chemical composition and shape of aerosols, there is significant uncertainty in quantifying the direct radiative forcing of aerosols.
Mahowald et al. provides an extensive review of the current literature on aerosols, which includes an explanation of aerosol emissions and impacts specifically coming from the burning of biomass. The chemical composition of aerosols produced from biomass burning depends on the fuel type and combustion conditions, but are primarily forms of organic carbon and black carbon. They are thought to contain both phosphorus and soluble iron, nutrients that affect land and ocean biogeochemistry. When biomass is burned in large events (e.g., large forest fires), the aerosol lifetime and range of transport can be extended. One of the uncertainties in understanding the impact of biomass-burning aerosols is that we do not know the level of preindustrial emissions. While some sources estimate that preindustrial levels of biomass-burning emissions were only 10% of what they are today, other estimates suggest they were comparable in magnitude to present-day. Consequently, as of now, it is difficult to determine the anthropogenic radiative forcing caused by biomass-burning emissions with any accuracy.
CATF take-away message: Even assuming the growing, harvesting, and burning of bioenergy crops makes it carbon-neutral (and less of a greenhouse-gas emitting energy source compared to fossil fuel), the replacement of fossil fuels with biofuels can have other (currently uncertain) implications for climate as a result of the biofuel-burning aerosol emissions. Only a better grasp on aerosols will allow us to predict whether the burning of biofuels would be positive or negative from a climate perspective as a result of aerosols changing the radiative forcings in the atmosphere.
Article: Lee, S.S. (2011). Aerosols, clouds and climate. Nature Geoscience 4: 826-827.
Background: Aerosols are liquids or solids suspended in the atmosphere, are composed of organic and inorganic compounds, and can be either anthropogenic (black carbon from fossil fuel burning) or natural (sea salt particulates) in source. The influence of aerosols on climate is still highly uncertain, but it is believed that some aerosols (but not all) have a cooling effect on the climate, due to their optical properties that reflect radiation and that their effects on cloud formation impact precipitation patterns. (Other aerosols, such as black carbon, can have a warming effect – e.g., by reducing the reflectivity of glaciers and snow cover.) Because some aerosols are naturally emitted by vegetation, there is interest in whether commercial forests might be managed to promote the release of climate-cooling aerosols and/or reduce the release of climate-warming aerosols.
Summary: Given concern over the potential of climate change to alter precipitation patterns, it is important to know the influence aerosols have on convective clouds, the class of clouds that, on average, produce the most precipitation. A group of scientists (Li et al., 2011) conducted a long-term observational study that was published in Nature Geoscience; they regionally tested the theory shown in model simulations that aerosols stimulate convective cloud growth. Using ten-year observational data sets of aerosols and cloud and meteorological data from the Southern Great Plains in the US, they found strong evidence that supports the hypothesized relationship linking increased growth of convective clouds with aerosols. After statistically analyzing the data, they showed that low-base, mixed-phase convective clouds, cloud-top height and thickness all increase with higher aerosol concentrations. They also found that the frequency of rainfall events is positive correlated with aerosol concentration for the case of water-loaded clouds. In addition to addressing convective clouds, other atmospheric scientists are seeking to understand how aerosols affect multi-cloud systems. For example, model simulations show that aerosols induce cloud evaporative cooling, which intensifies the horizontal air flow below multi-cloud systems, leading to greater convergence, formation of stronger clouds, and more intense precipitation.
CATF take-away message: By investigating the relationships between (the diversity of) aerosols and climate, we can better predict how the emissions of aerosols affect the spatial distribution and frequency of precipitation, which is of great importance on a regional scale. Additionally, as society moves away from fossil fuels and employs other sources energy, including biofuels, the composition and quantity of aerosols in the atmosphere will likely change, which in turn could have either a warming or cooling impact on climate.