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The [Photosynthetic Inefficiency] of Biofuels

January 10th, 2013 by Jonathan Lewis, Senior Counsel - Climate Policy

Article: Hartmut Michel. The Nonsense of Biofuels. Angew. Chem. Int. Ed. 51: 2516-2518. (2012)

Background: Hartmut Michel, the director of the Max Planck Institute of Biophysics and the recipient of a Nobel Prize for his research on photosynthesis, considers whether biofuel production is an efficient use of land when compared to other energy options.

Summary: Michel shared the 1988 Nobel Prize in Chemistry for isolating the crystalline membrane proteins that some bacteria use to initiate photosynthesis. Michel and his colleagues also showed that the protein complex, called the photosynthetic reaction center, was similar in bacteria and plants; in doing so, says the Nobel Foundation, they enabled “a giant leap in the general understanding of the mechanism of plant photosynthesis,” and allowed “researchers to finally access and explore a raft of other important proteins housed in the cell’s living frontier.”

Drawing on this expertise, Michel begins his 2012 paper by describing the efficiency at which plants’ photosynthetic reaction centers convert sunlight to energy. Taking the average energy of the photons and the energy that plants store as a coenzyme called NADPH, Michel writes that “it is easy to calculate that only 11.8% of the energy of sunlight is stored in the form NADPH.” But because plants prevent photodamage by converting only portion of the available light energy, “4.5% is considered the upper limit of photosynthetic efficiency of C3 plants.” (C3 carbon fixation is the photosynthetic process used by many of the plants that thrive in temperate climates.) In reality, writes Michel, “values of only around 1% are observed, even for rapidly growing trees like poplars.”

Applying that analysis to data on the energy density of biofuels and the volume of biofuels produced per hectare, Michel writes:

[O]ne can easily calculate how much of the energy of the sunlight is stored in the biofuels. For German “biodiesel” which is based on rapeseed, it is less than 0.1%, for bioethanol less than 0.2%, and for biogas around 0.3%. However, these values even do not take into account that more than 50% of the energy stored in the biofuel had to be invested in order to obtain the biomass (for producing fertilizers and pesticides, for ploughing the fields, for transport) and the chemical conversion into the respective biofuel.

Michel’s conclusion is hardly a surprise, given the title of his paper. The production of biofuels, he writes, “constitutes an extremely inefficient land use.”

Because of the low photosynthetic efficiency and the competition of energy plants with food plants for agricultural land, we should not grow plants for biofuel production. The growth of such energy plants will undoubtedly lead to an increase in food prices, which will predominantly hit poorer people.

In Michel’s view, PV-powered electrification offers a far better method for converting light into useful energy. “[T]he combination photovoltaic cells/electric battery/electric engine uses the available land 600 times better than the combination biomass/biofuels/combustion engine,” he writes. Biofuels made from algae “could be better than land plants because of the absence of non photosynthetic cells and the continuous availability of water,” but “the existence of photoinhibition and a poor RuBisCO [the enzyme used in carbon fixation] will limit the advantages of microalgae together with the demands for growing and harvesting them.”

Michel also suggests that using available biomass for heat production or electricity generation would be “preferable over biofuel production.” Saving fossil fuel that would otherwise be used for heat and power and redirecting it to the transportation sector could help prevent tropical forests from being converted into biofuel plantations, he writes. “With respect to the carbon footprint, it would be even much better to reforest the land used to grow energy plants[.]”

CATF Analysis: While Michel’s assessment that “we should not grow plants for biofuel production” is based on an engineering-type analysis that draws on his specific area of expertise, his paper is also animated by more general concerns about biofuels’ net impact on global climate change and their effect on global food security. The points are related, of course: if energy crops could capture more of the energy made available by sunlight, the amount of energy yielded per acre of cultivation would improve, as would biofuels’ impact on climate change and food security. The point that unifies Michel’s photosynthesis analysis and other broader critiques is his conclusion that “the production of biofuels constitutes an extremely inefficient land use.”

Michel’s condemnation of biofuels echoes the criticisms leveled by other highly regarded scientists who are not otherwise directly engaged in the controversy around bioenergy. See, for example, Jesse Ausubel’s characterization of biofuels as an “ecological disaster,” quoted in an earlier post, or the 2007 study by Paul Crutzen (another Nobel laureate in chemistry) on the nitrous oxide emitted during energy crop cultivation.

Post-script: Michel expresses doubt that the photosynthetic efficiency of plants can be improved enough to make biofuels worthwhile, in part because plants have evolved photoinhibition mechanisms to defend themselves against overexposure to sunlight. But if photodamage was not a concern, could humans use photosynthesis to make fuel? Nate Lewis is tackling that challenge at Caltech, with his fascinating effort to produce cheap, mass-producible artificial leaves.

Will Biofuels Continue to Confound Progress Toward Land Conservation?

January 4th, 2013 by Jonathan Lewis, Senior Counsel - Climate Policy

Article: Jesse H. Ausubel, et al. Peak Farmland and the Prospect for Land Sparing. Population and Development Review 38 (Supplement): 217-238 (2012)

Background: Even as the world grows more populous and more affluent, other countervailing trends like dematerialization and the intensification of land use “encourage a rational hope that humanity’s pressure will not overwhelm Nature.” Biofuel production and other “wild cards” could slow the progress toward a peak in demand for cropland, however.

Summary: Over the next fifty years, will a larger, more affluent global population be able to grow and consume enough food? According to Jesse H. Ausubel, Iddo K. Wernick, and Paul E. Waggoner, trends that have been developing over the last half-century suggest that the answer is yes. Their optimism is tempered by several big caveats, though, and of those caveats the uncertainty around future demand for biofuels looms particularly large.

Ausubel et al. use an analytic approach they call the “ImPACT identity” to organize their examination of the effect that changes in population, affluence, diet, and agricultural performance have on the use of land for crop production. The ImPACT identity determines the amount of cropland used [Im] by multiplying assumptions about future population [P] and affluence [A]; the number of food calories consumed per GDP [C1]; crop production per calorie [C2], which tracks the relationship between planting choices and the supply of food calories; and land required per unit of production [T], which tracks the application of yield-improving technologies:

Im = P · A · C1 · C2 · T

Reviewing a recent 50-year period (1961-2010) through the lens of the ImPACT identity, the authors find a set of “diverse, durable patterns,” including a deceleration in population growth, annual increases in GDP per capita that hover between one and two percent, and the decoupling of calorie consumption from GDP (to the extent that caloric intake in developed countries plateaus even as incomes continue to rise). Looking forward across the subsequent 50-year period (2010-2060), Ausubel et al. contend that these trends and a few others will produce a steady decline in the number of total hectares under cultivation:

Allowing for wild cards, we believe that projecting conservative values for population, affluence, consumers, and technology shows humanity peaking the use of farmland. Over the next 50 years, the prospect is that humanity is likely to release at least 146 MHa, one and a half times the size of Egypt, two and a half times that of France, or ten Iowas, and possibly multiples of this amount.

Notwithstanding the biofuels case, the trends of the past 15 years largely resemble those for the past 50 and 150. We see no evidence of exhaustion of the factors that allow the peaking of cropland and the subsequent restoration of Nature.

“Nothwithstanding the biofuels case ….” The authors identify a handful of “wild cards” that could slow or reverse the predicted trend toward land sparing. Shifting consumer preferences in diet (how much meat will increasingly affluent societies eat, and what kind of meat will it be?) and fabric (how much farmland will be used to grow cotton, linen, and hemp?) are two of the wild cards, along with the possibility of radical innovations in food production technology.

The most confounding wild card might be biofuels. But for “the sharp rise in C2” over the past 15 years, the authors contend the ImPACT analysis would look even more promising. (Recall that higher C2 values mean that a larger percentage of harvested biomass is being used for non-food purposes.) According to Ausubel et al., biofuels lie behind this trend:

Starting with a baseline of less than 4MHa in 1995, by 2007 according to the USDA and FAO, nearly 25MHa were devoted to crops used for fuel.* This number exceeds the additions to arable land globally from 1995 to 2010, suggesting that much of the addition to cropland over this period was used to grow fuels … The entry of biofuels as major crops in the mid-1990s helps explain the fourfold increase in C2 from 0.24 percent in the 1961-2010 period to 1.04 in the last 15 years of that period.

[*internal citation: Ronald Trostle. 2008. Global Agricultural Supply and Demand: Factors Contributing to the Recent Increase in Food Commodity Prices. WRS-0801. USDA Economic Research Service.]

CATF Analysis: The core assertion by Ausubel et al. – that humanity will use less and less cropland even as population and income increase – assumes a fairly dramatic reversal on biofuels. “As the shortcomings of biofuels become evident to governments and champions of the environment alike,” the authors write, “we conservatively project C2 as slowing to 0.4 percent annually, slightly less than half of the 1995-2010 level.”

Just how conservative is that projection, though? It’s clear from Ausubel et al.’s analysis that even relatively low biofuel production levels can confound the trend toward peak farmland. “Absent the 3.4 percent of arable land devoted to energy crops,” they write, “absolute declines [in hectares of cropland] would have begun during the last decade.” But biofuel production in the United States and elsewhere was still fairly modest during the 1995-2010 period when “the entry of biofuels as major crops” contributed to a “fourfold increase in C2.” The United States did not begin mandating biofuel consumption until after the Renewable Fuel Standard was enacted in 2005, and by 2010 – when the RFS required Americans to use 13 billion gallons of biofuel – the policy-driven ramp-up in consumption was just getting underway. The RFS volumetric mandate is set to double by 2018 (expanding to 26 billion gallons per year) and almost triple by 2022 (to 36 billion gallons). The scheduled increase in biofuel production in the United States and other countries makes it likely that more harvested biomass will be used for non-food purposes and, for the purposes of the ImPACT identity, that C2 will rise faster in 2010-2025 than it did in 1995-2010. Taking that trajectory into account, the authors’ projection that C2 will slow to less than half what it was in 1995-2010 looks more hopeful than conservative.

Hopeful, but also doable – given that biofuel production is largely a function of policy, not economic demand. In a December 2012 interview following the release of his paper, lead author Jesse Ausubel urged environmental organizations to tackle those policies. “Conservation groups,” he told the The New York Times, “ought to attend more to the ecological disaster called biofuels.”

The Midwest U.S. land-use change predicted for 2020 biofuel targets

May 28th, 2012 by Rachel Perlman,

Article: Mehaffey, M. et al. (2012). Midwest U.S. landscape change to 2020 driven by biofuel mandates. Ecological Application 22: 8-19.

Background: As a result of current energy policy stipulated by the Energy Independence and Security Act of 2007 (EISA), the U.S. has established the target of producing 136 billion liters (36 billion gallons) of domestic ethanol by 2022. Since the U.S. Midwest has the highest overall crop production capacity, there is concern for how such a large increase in ethanol production will affect land-use change and the composition of agriculture in the region.

Summary: Mehaffey et al. developed a possible future scenario to predict how the U.S. biofuel demand for corn will alter the Midwest landscape. They combined an economic model output with a gridded land cover data to spatially determine the land use change in the 12 most-productive states between 2001 (BY, Base Year) and 2020 (BT, Biofuel Target). They adopted the Center for Agricultural and Rural Development (CARD) econometric model, which is structured around the requirements of EISA, the 2008 Farm Bill, and economic forcing factors. This economic model produced 2001 agriculture acreages by state, and yields by region necessary to meet the ethanol demand in the year 2020.  The group used these economic predictions of future acreages, crop rotation, and soil productivity to spatially allocate shifts in cropping practices in the Midwest to create their final product: a hypothetical BT 2020 land cover map, which mapped 18 classes of agriculture, 155 natural cover types, 3 urban, 1 barren, and water. In making the BT 2020 they assumed that (1) most of the initial biofuel production will be provided by planting more corn, (2) land conversion to continuous corn growing would occur first on the most fertile soils, and (3) shifting crop rotational practices on currently farmed fields would occur before farming land that is currently pasture and conservation land. The overall expected Midwest corn acreage only differed by 1% between the CARD econometric model and the BT 2020 land cover map; the two did vary in the distribution of acreage between states, with the BT 2020 land cover having greater acreage allocation in the Corn Belt states.

Mehaffey et al. predict that by 2020, several states will have watersheds that see more than a 50% increase in continuous corn planting. Iowa and Illinois will have the greatest amount of corn planting. In their scenario, they expect that a total of 40 million acres of farmland will be converted to continuous corn cultivation, with 25 million of these acres coming from land that was originally used for rotational cropping. The study highlights the fact that, already, between 2001 and 2010, the area of planted corn has increased by 13.8 million acres, which has been accompanied by an equivalent decrease in wheat, sorghum, and cotton. The group also predicts that by 2020, urbanization will result in the loss of over 7 million acres of productive farmland. The above trends have implications for high increases in fertilizer use, irrigation in areas where corn has not been traditionally grown, and loss of topsoil and organic matter from corn stover removal. Mehaffey et al. expect that the dominant problem with the Midwest’s extensive corn planting will be declining soil productivity.

CATF take-away message: The study by Mehaffey et al. contributes to an already swelling body of research that ties the expansion in corn production being driven by biofuel policies to a variety of environmental challenges (e.g., soil degradation, water pollution).  Interestingly, the US Environmental Protection Agency — the agency charged with overseeing the world’s largest corn ethanol subsidy, the Renewable Fuel Standard — is behind some of the most critical analyses of corn’s impact on the environment.  The lead author of this study, Megan Mehaffey, works at EPA’s Environmental Sciences Division; her study’s analysis of environmental impacts echoes findings that are detailed in EPA’s Biofuels and the Environment: First Triennial Report to Congress (2012).

Additionally, the litany of environmental challenges that Mehaffey et al. connect to the policy-driven expansion in corn production could — and should — include greenhouse gas emissions from indirect land use change.  The authors note that corn displaced millions of acres of wheat, sorghum, and cotton over the previous decade.  Inevitably, other farmers around the world responded to the unmet demand for displaced crops by cultivating additional farmland, resulting in the release of plant- and soil-carbon.

E85 found to be holistically worse for the environment than gasoline

May 28th, 2012 by Rachel Perlman,

Article: Yang et al. (2012). Replacing gasoline with corn ethanol results in significant environmental problem-shifting. Environmental Science & Technology.

Background: E85 is an ethanol fuel blend that is 85% ethanol and 15% convention gasoline by volume and is frequently used by flex-fuel vehicles. Corn ethanol makes up 90% of the bioethanol production in the U.S., and therefore, most of our E85 is derived from U.S.-grown corn. Most studies that use LCA to compare bioethanol and gasoline focus on quantifying the difference in energy consumption and greenhouse gas emissions between the life cycles of the two fuels.

Summary: This study, by Yang et al., is unique in that it uses LCA methods to comprehensively compare the environmental impact of gasoline and E85, taking into account 12 different environmental impacts (not just GHG emissions), as well as regional differences among 19 corn-growing states. The 12 impact categories were: global warming, human health cancer, acidification, human health respiratory, human health noncancer, ozone layer depletion, eutrophication, smog formation, ecological toxicity, fossil energy consumption, water use, and land occupation. They assumed the E85 contained corn ethanol and was produced using a dry mill powered by natural gas. Much of their data for ethanol impacts was derived from a Life Cycle Inventory database from the USDA and applies to corn ethanol produced in 2005. Their E85 impacts vary from state to state, mainly based on differences in those states’ climate, soil, topography, and transportation logistics. The gasoline calculations were developed from weighted averages of crude oil data based off the oil’s origin and its share in U.S. oil imports. The fuel lifecycles included the following steps: feedstock production, shipment of the feedstock to the refinery, refining/conversion, fuel shipment to the refueling station, and vehicle use. By normalizing and weighting their 12 categories, they also combined their results to a single environmental damage score, which they tested for sensitivity to develop one useful “weighted environmental impact” metric.

The study found that gasoline has a better environmental impact score than E85 in terms of eutrophication, water use, and land occupation, with a slight advantage for smog formation and acidification effects as well. However, E85 was seemingly better in terms of fossil fuel energy consumption and global warming impact, and had a slightly smaller ecological toxicity. For the two fuels, no clear difference was found for ozone layer depletion, cancer and noncancer human health, and respiratory effects. From a geographic standpoint, E85 from different states had variable eutrophication, water use, land occupation, and global warming impacts, since regional agricultural practices (e.g. dependence on irrigation), climate and topography were quite different. Consequently, there is much error in trying to determine the nation-wide water use or eutrophication impact that corn-ethanol has on the environment. In any case, though, E85 has a much larger need for water than gasoline, given that irrigation is sometimes used and water is needed for the ethanol conversion process. Yang et al. concludes that, overall, according to their weighted average, E85 has between a 6% to 108% (23% average) greater total environmental impact than gasoline, and that this range becomes 16%-188% (33% average) when indirect land use change data (associated with uncertainty) is incorporated.

CATF take-away message: While Yang et al. determine that overall E85 is more environmentally damaging than conventional gasoline (e.g., the study finds that E85 has a higher contribution to smog formation), they also suggest E85 has an apparent advantage over gasoline in terms of global warming impact.  It is important to note, however, that the authors analyze E85’s climate impact using highly conservative and somewhat questionable assumptions about indirect land use change emissions (ILUC).  Yang et al. note that a 2010 study by Plevin et al. “estimated that iLUC GHG emissions ranged from 10 to 340 g CO2 equiv MJ1, with a 95% confidence interval from 21 to 142 CO2  equiv MJ1.”  Yang et al. choose the extreme low end of that interval (21g CO2e MJ-1) for their own analysis, but Plevin et al. provides no basis for such a decision.  Moreover, Plevin and his co-authors pointed out that “[w]hile we chose to define the ‘plausible’ range as the central 95% interval, it is important to recognize that the further right tails of these distributions represent nonzero risk of very high ILUC emissions if fossil fuel is displaced by biofuels, and the left tail offers no such corresponding prospect of very large emissions reductions.”  In addition, Yang et al. treat the carbon released as CO2 during vehicle operation to be equal to the carbon uptake by corn grain, which is a questionable assumption (flux neutrality is complex, as discussed by Cherubini et al., 2011). In any case, this study provides further argument against pursuing U.S. corn ethanol as an environmentally-compatible liquid biofuel.

Estimation of the Total Bioenergy Potential of the U.S.

May 24th, 2012 by Rachel Perlman,

Article: Smith, K. et al. (2012). Bioenergy Potential of the United States Constrained by Satellite Observations of Existing Productivity. Environmental Science & Technology 46: 3536-44.

Background: As a result of current energy policy stipulated by the Energy Independence and Security Act of 2007 (EISA), the U.S. has established the target of producing 136 billion liters (36 billion gallons) of domestic ethanol by 2022. With the U.S. producing 40 billion liters of ethanol currently, there is concern for how such a large increase (three-fold) in ethanol production will affect land-use change and the composition of agriculture.

Summary: In order to assess the feasibility of achieving our EISA energy target, Smith et al. estimated the primary bioenergy potential (PBP) of U.S. land. They used satellite data (EOS and MODIS data specifically) to estimate net primary productivity (NPP) for every 1 km2 of the 7.2 million km2 of vegetated land in the contiguous U.S. They considered the NPP values (calculated using climate data, leaf areas and radiation indices) as an upper-limit for PBP, since agricultural productivity is usually less than the natural potential. NPP quantifies the current terrestrial biomass growth capacity for vegetated land. They estimate the PBP of the contiguous U.S. to be between 5.9 and 22.2 EJ (1018 J) per year, depending on land use. The lower estimate represents the potential if only crop residues (i.e. stalks, stubble, seed pods, etc.) were harvested, while the high estimate necessitates annual biomass harvest for a land area more than three times as larger than the current U.S. agricultural area. These results indicate that the EISA goal of 136 billion liters is hypothetically achievable, but at a cost. Such a transition would require an 80% displacement of current crop harvest, or the conversion of 60% of rangeland productivity (given current technologies). Converting such a great amount of rangeland to biofuel cropland would have detrimental impacts on biodiversity, would require large infrastructure development and use of fossil-fuel energies, and would leave the bioenergy systems in a significant initial carbon debt.

However, they found that the cellulosic-derived energy target of 79 billion liters, in particular, could be accomplished using current harvest residues, without any additional harvest land. In particular, the study identified the northcentral U.S. as the region with the largest potential for intensification, due to its high agricultural harvest and residue potential. They also concluded that the northeast U.S. has high potential for increased forest harvest, due to its currently low harvest rates.

CATF take-away message: With increasing literature showing that certain bioenergy crop systems, especially U.S. corn-based ethanol, exceed their potential CO2 offset, CATF is wary about the energetics and climate-impact of the U.S. expanding its ethanol production heavily within the next ten years. From a land area stand-point, it seems the U.S. could theoretically reach its ethanol target, but with current levels of productivity, it would require a major shift in the structure and geography of agriculture, as food and feed production land would be displaced. This would lead to a host of new problems, only one of which would be increased food prices.

Impact of biomass-derived and other aerosols on climate

March 27th, 2012 by Rachel Perlman,

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.

Understanding the impacts of aerosols on clouds and precipitation

March 27th, 2012 by Rachel Perlman,

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.

Global potential for increasing biofuel crop yields/acre

March 16th, 2012 by Rachel Perlman,

Article: Fargione, J. (2012). Boosting biofuel yield. Nature Climate Change 1: 445-446.

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.

Carbon accounting of forest bioenergy and stand thinning

March 16th, 2012 by Rachel Perlman,

Article: Hudiburg, T.W. et al. (2011). Regional carbon dioxide implications of forest bioenergy production. Nature Climate Change: 1-5.

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.

Identifying global warming potential of biofuels based on their rotation periods

March 16th, 2012 by Rachel Perlman,

Article: Cherubini, F. et al. (2011). CO2 emissions from biomass combustion for bioenergy: atmospheric decay and contribution to global warming. GCB Bioenergy.

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.