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 20^th 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.

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.

Background: One of the main concerns with biofuel production centers on land use change; if we convert natural ecosystems or food croplands to biofuel crops, we may be worsening other environmental and food security problems. There is debate over the extent to which biofuel production directly competes with the food market and increases food prices. In order to increase global biofuel production without the agricultural expansion, biofuel yields would have to increase.

Summary: Fargione explains that Johnston et al. have calculated that an extra 112.5 billion liters of ethanol and 8.5 liters of biodiesel could be produced if global yield for crops used to make biofuels increases (with crop area staying constant). They determined the potential yield gain by comparing current production levels with a scenario in which the half of the world’s farmers having below median yield increased yields to median levels. Separate median yield values were identified for each major crop used to produce ethanol and biodiesel in each climate zone. Thus, yields from locations that have similar numbers of growing degree-days and soil moisture/types (such as Indonesia and Brazil) and grow the same crop (like sugar-cane) were grouped together. The variation in yields of a given biofuel crop creates a “yield-gap” that would ideally be reduced by adjusting management practices, inputs, equipment, or the cultivar variety. For example, if Madagascar were to have its lower-yielding half of farmers produce at median yield levels, they could double sugar-cane yields.

Despite the great potential for energy crop yield increases, the means by which this can occur are not straightforward. Especially in developing countries, yields are often lower due to lack of capital, access to equipment, and education on farm management. Solutions require some combination of monetary aid, educational outreach, and investment in infrastructure and technology. However, closing the energy crop yield gap also has concerning realities – more fertilizers and irrigation will be needed, and with global food demand growing, policy-makers have to decide what proportion of resources should go toward biofuel versus food crops.

CATF take-away message: Boosting agricultural yields is essential to meet the growing demand for food, animal feed, and other plant-based products.  However, the idea that increasing yield/acre of biofuel crops is always a “win-win” – i.e., because it would increase energy production while preventing detrimental land-use change – needs to be considered carefully. Before pouring resources into efforts to make higher yielding energy crops, we must make sure that the carbon footprint and energy balances of the biofuel production process (currently, as well as with the adjustments for higher yield) are acceptable to in the first place.

Background: Forests are valuable carbon sinks, since atmospheric carbon dioxide is taken up and stored as carbon in tree biomass (trunks, branches, foliage, and roots). However, tree thinning, which makes forests less crowded, is a strategy for preventing forest fires (which release carbon dioxide). Forests also can provide energy (as a biofuel) in the form of firewood, denser wood pellets, wood charcoal, wood-derived liquid fuel. Some strategies for reducing CO₂ emissions include substituting forest biofuels for fossil fuels, which is assumed to have zero net emissions (i.e. carbon stored in growth of new tree biomass equals carbon emitted in burning).

Summary: Hudiburg et al. used LCA and inventory data from 80 forests on the US West Coast (California, Washington, and Oregon) to do carbon accounting analyses for current management practices, as well as for the following three forest management practices: (1) fire prevention by removing fuel ladders, (2) removing fuel ladders and enough marketable wood in fire-prone areas to be economically feasible (3) thinning all forestland to support energy production while contributing to fire prevention.

They found that in 90% of the diverse forest area studied, the carbon sink cannot be matched or exceeded by replacing fossil fuels with forest bioenergy. Thinning reduced net biome production in 90% of the region’s forest area. They show that the three scenarios lead to a 2-14% (46-405 Tg C) increase in carbon emissions over a 20-yr period. Forest bioenergy production could only reduce both fire risk and carbon emissions in the near future if forests in the region naturally became weaker sinks, trapping 30-60 g C/m²/yr less (due to insect infestations or higher rates of fires). At present, only three of the 19 ecoregions studied have high enough fire emissions that thinning for fire prevention and bioenergy would likely have a net emission savings.

CATF take-away message: Forests provide a wide variety of valuable services, and it is important to consider various management options in any given forested region by doing proper carbon accounting. Hudiburg et al. wrongly assume that biomass-based energy production is “carbon neutral” (an outdated assumption that neglects how the delay in re-sequestration affects climate, improperly credits bioenergy with sequstration that may have happened anyway, and fails to account for the possibility that regrowth (and thus re-sequestration) may not occur due to development decisions, etc.).  Nevertheless, the article still suggests that it is typically not in our best interests from a carbon emissions standpoint to use temperate forests for bioenergy, unless net biome production is weakened there; generally, more carbon will be stored by minimal harvesting.

Background: Bioenergy from biomass combustion is often assumed to have a net zero impact on GHG emissions in LCA and national GHG inventories. It is presumed that keeping the harvested area forested offsets the carbon released into the atmosphere. However, these assumptions ignore the climate impact of CO₂ from temporary change; CO₂ is emitted rapidly when biomass is burned at one point in time, but then is sequestered over a longer period of time that depends on the growth rate of the biofuel crop.

Summary: Global warming potential (GWP) of bioenergy can be represented on a scale of 0 to 1; it is a metric that allows scientists to aggregate emissions of different gases to a common unit (kg CO₂ equivalent). The assumption that the net flux balances to zero leads some to assign a GWP of zero to direct CO₂ emissions. Others assign the GWP equal to 1 (equivalent to impact of fossil fuels), believing that the emissions from combustion must be offset by an equal amount of sequestration (credit) somewhere else. Cherubini et al. challenges both of these assumptions, in order to improve the methodology for quantifying CO₂ emissions from bioenergy combustion with a unit-based indicator that can be used in future carbon accounting studies. The group ultimately concluded that the GWP should be some fraction between 0 and 1 and should be a function of the biomass rotation period (although it should be noted that a minority of model scenarios imply a GWP above 1, higher than that for fossil fuels).

Cherubini et al. modeled the atmospheric decay of CO₂ emissions to determine the fraction of emissions that still remain in the atmosphere, as limited by the equilibrium response of the ocean-atmosphere system. Their model was simplified with the following assumptions: biomass combustion creates a pulse of emissions, the biomass is clear-cut, the land is immediately re-vegetated with the same species, and the whole process is carbon flux neutral. They ran their model of CO₂ over time with three variations of impulse response functions or IRF (mathematical ways of representing CO₂ atmospheric decay), which differ from each other in the assumed extent of ocean vs. terrestrial removal. The IRF that considered the full carbon cycle with ocean and terrestrial sinks was found to be most reliable and accurate. The three different IRF were compared for different rotation period lengths (between 1 – 100 years) and for different time horizons (20, 100, 500 years); for all of the IRF, the mean residence time of CO₂ in the atmosphere was longer for longer biomass rotation periods. This means that even though both are “flux neutral,” short rotation biomass has less climate impact than long rotation biomass per unit of CO₂ emitted from the combustion of the biomass. However, this does not mean that one rotation length is better than the other, since other properties of the biomass growth/management and all climate forcing agents need to be considered as well. Cherubini et al. also concludes, “bioenergy is a climate change mitigation strategy particularly effective for long-term targets,” or longer time horizons.

CATF take-away message: The assumptions made in the models allow the results to be applied to CO₂ emissions from combustion of a variety of biomass species ranging from annual crops to slower growing trees. The study does not necessarily endorse fast-growing over slow-growing biocrop species, but does highlight the relationship between GWP, rotation period, and time horizon.  Studies like this are instrumental for understanding that carbon flux neutral systems can still contribute to climate change.

Background: Burning sugar cane fields prior to harvest is a common practice employed to remove dried leaves, considered “trash.” The burning decreases labor and equipment needs for harvest, but emits quantities of air pollutants that may be of concern.

Summary: Tsao’s research group used a life-cycle approach to quantify the air-pollutant emissions associated with producing sugar-cane ethanol in Brazil. To inventory emissions in Brazil, they used agriculture survey data, emissions factors and life-cycle assessment tools. Based on changes seen between 2000-2008, they found that regional emissions of pollutants are continuing to increase as sugar-cane cropland expands. This group’s estimates are (up to four-times) higher than pre-existing estimates obtained using satellite-based approaches, which tend to underestimate burned area.  Satellites can underestimate burned area, since the burning occurs at a small spatial scale of individual fires.

Roughly half of sugar-cane croplands are still burned, and most of the burning in Brazil occurs in the north-central region of the Sao Paulo state. Their comparison of life-cycle emissions per unit energy produced from sugar-cane ethanol versus from conventional gasoline and diesel showed that: (1) greenhouse gas (CO2 equivalent) emissions for sugar-cane ethanol were lower (2) the emissions of volatile organic compounds, CO, NO_x, SO_x, and particulates were all significantly higher for sugar-cane ethanol. In the life-cycle of sugar-cane ethanol, the field burning phase is the primary source of all of these species except SO_x.

CATF take-away message: The life-cycle emissions of a biofuel crop can vary greatly depending on the methods used in harvesting. The LCA of sugar-cane ethanol may show that it “beats” fossil fuels in terms of greenhouse gases, but could be worse for air pollutants that directly affect human health if the fields are burned. Pre-harvest burning of residue from sugar-cane grown for ethanol production is a practice that must be questioned, given its release of air pollutants (VOC, CO, CO₂, NO_x, SO_x, and particulates).

The Valdez-Vasquez et al (2010) article is a “first step towards estimating the bioenergy production capacity from crop residues.” From there, “it will be necessary to propose the best technology [direct combustion, anaerobic fermentation, etc.] for their [crop residue] use at the local level.”

Mexico has the third largest cropland area in the Latin-American Caribbean region which constitutes 13% or the worlds cropland production.  For these reasons, Mexico is expected to have high bioenergy potential from primary and secondary crop residues.  The objective of this study is to evaluate crop residue types, quantities and locations (broken down by municipality).

Valdez-Vasquez et al (2010) estimated the crop residue index (CRI) as the ratio of dry weight crop residue to total crop production.  Both primary and secondary crop residues were evaluated.  ARCGIS was then used to represent the biomass potential of the region in thematic maps.

The results of the study found an estimated 60.13 million tons of dry matter from primary crop residue and 16.5 million tons of dry matter frm secondary crop residue.  However, not all of the estimated dry matter is available for bioenergy production as crop residues constitute an estimated 23.6% of animal feed in the region.  Typically the animal feed comes from primary crop residues.

Corn by-products followed by Sorghum by-products were found to have the highest biomass potential of the primary crop residues while sugarcane, coffee, and maguey bagasse were found to be the highest of the secondary crop residues.

Overall, Baja California, Campeche, Chiapas, Chihuahua, Guanajuato,Hidalgo, Jalisco, Oaxaca, Quintana Roo, San Luis Potosi, Sinaloa, Sonora, Tobasco, Tomaulipas, and Veracruz municipalities were found to have the highest estimated bioenergy potential in Mexico.

The full article can be downloaded here.

Sara Gonzalez-Garcia, M. Teresa Moreira, and Gumersindo Feijoo of the Uiversity of Santiago do Compostela find that, “using ethanol derived from lignocellulosic feedstocks as liquid fuel would reduce fossil fuel dependence and greenhouse gas emissions but would increase acidification, eutrophication and photochemical smog, compared to using gasoline as liquid fuel.”

Gonzalez et al (2010) compare 5 lignocellulosic feedstocks ( alfalfa stems, poplar, Ethiopian mustard, flax stives, and hemp hurds) as substitutes for conventional motor vehicle gasoline at E10 and E85 concentrations.  The study was conducted using standard life cycle analysis methods to evaluate which feedstock had the largest environmental benefit when considering global warming, photochemical oxidant formation, eutrophication, and acidification.

The results of the analysis demonstrated reductions in GHGs for all feedstocks, however for some of the feedstocks the carbon sequestered did not make up for the methane and nitrous oxide emissions from the agricultural practices at the E10 concentration.  Ethiopian mustard offered reductions at both E10 and E85 concentrations.  E10 concentrations were found to be more beneficial than E85 with respect to photochemical smog, acidification and eutrophication due to emissions from upstream processes such as agricultural practices and machinery.  Ethopian mustard was found to offer a 10% reduction in fossil fuel extraction at the E10 concentration and a 63% reduction for E85.

Different feedstocks performed better in different categories.  For example, poplar had the least impact on photochemical oxidation formation where flax shives were best for acidification.  Ethopian mustard was found to offer the greatest greenhouse gas reduction as well as more significant reductions in fossil fuel extraction rates.  Overall, lignocellulosic ethanol is expected to reduce greenhouse gas emissions while increasing acidification, eutrophication and photochemical oxidation.

The Gonzalez-Garcia et. al (2010) paper concludes that in order for lignocellulosic ethanol to become a more economically viable transportation fuel alternative, the biorefinery process must become more efficient and production must occur in much larger quantities.

The full article can be downloaded here.

Synopsis: Shelie A. Miller of Clemson University developed a system of ranking biofuel feedstocks based on land use and nitrogen intensity in order to determine which feedstocks were the most environmentally favorable with respect to these factors.  Miller’s results suggest that, “current energy policies either undervalue or do not consider nitrogen and land use impacts.”   

 This paper explains that widely practiced lifecycle analysis methods tend to focus almost entirely on greenhouse gas emissions, despite the evidence that many environmental systems are negatively impacted by bioenergy production.  In particular, most biofuels are known to be water, nutrient and land intensive.  This study ranked feedstocks according to nitrogen and land use intensity as these two factors are almost entirely limited to feedstock cultivation stages, whereas water and energy are factors throughout cultivation, manufacturing and “end-of-life” stages.   

 In terms of the ranking system, 14 biofuel feedstocks were evaluated to determine the “minimum nitrogen and land required to produce 1000GJ of unprocessed energy.”  The calculations represent ideal values for maximum energy yield (Maximum Energy Yield = harvestable yield x high heating value) and maximum nitrogen requirements (Maximum nitrogen requirements = harvestable yield x percent nitrogen composition) of each system averaged for a range of conditions and regions for each feedstock but do not account for thermodynamic losses.  Land use and nitrogen intensity were weighted equally and the feedstocks were then ranked relative to one another.  

 The results of the Miller article indicate that sugar crops and algae are the least land intensive when accounting for nitrogen and land intensity, with sugarcane ranking as the most favorable feedstock overall.  Food crops, soy and rapeseed in particular, ranked consistently low. 

 Environmental damages are not limited to greenhouse gas emissions; water, land and nutrient cycling are all vital ecosystem services that may be impacted by biofuel production.  Going forward, this research suggests that efforts taken to determine the ecological viability of a particular fuel should include a wider scope of environmental impacts and should not be limited to greenhouse gas emissions.

The full article can be downloaded here.

Synopsis: The paper, “Sustainability considerations for electricity generation from biomass,” synthesizes available research relating to the feasibility of electricity generated from biofuel feedstocks.  The article explains that while electricity generated from biofuels only accounts for approximately 2% of total global generation, it may offer a sustainable alternative to existing generation methods going forward.

The Evans et al. article includes information on residue feedstocks as well as dedicated feedstocks and the trade-offs between the two.  Generally, residues from agriculture and forestry practices have lower fuel densities and are therefore less competitive once transportation costs have been taken into consideration.  Evans et al suggest that dedicated feedstocks, or feedstocks grown specifically for energy, will be essential for biofuel electricity generation on a larger scale, however, there are environmental and social implications to this type of agriculture.  The authors explain that ideal feedstocks will not be chemical, energy or water intensive, will not compete with food crops, will have high yields, and will have shorter rotation periods.  Corn and wheat are deemed inappropriate for a variety of these listed reasons.  Favored crops include willow, poplar, and non-woody perennial grasses.

Evans et al evaluate the crops based on available research on price of production, investment costs, efficiency and greenhouse gas emissions.  The results were highly variable across these subject areas.  On the subject of greenhouse gas emissions, the authors find that most available research suggests there are low net emissions associated with biofuel generated electricity, however, even “the highest emission is less than one third of the lowest natural gas and one-fifth of the lowest coal fired power station emissions proven at present.”

Evans et al conclude that relative to traditional energy sources, the generation of electricity from biofuels “appears favorable” with respect to “electricity price, efficiency and greenhouse gas emissions.”  The authors do note that there are still significant barriers to overcome by way of land and water intensity as well as social implications including food competition, biodiversity and labor.

The complete article can be downloaded here .