How Biofuels Can Cool Our Climate and Strengthen Our Ecosystems

By Evan H. DeLucia
Courtesy of EOS

Critics of biofuelsLiquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation.Bio fuels are liquid fuels that are produced of plant material or herbal remains. like ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. Ethanol is abbreviated as EtOH, using the common organic chemistry notation of representing the ethyl group (C2H5) with Et.Bioethanol is a readily available, clean ... argue they are an unsustainable use of land. But with careful management, next-generation grass-based biofuels can net climate savings and improve their ecosystemsA system of living organisms interacting with each other and their physical environment. The boundaries of what could be called an ecosystem are somewhat arbitrary, depending on the focus of interest or study. Thus, the extent of an ecosystem may range from very small spatial scales to, ....

Miscanthus and switchgrass, two perennial grasses that could be used for biofuels, grow in front of a corn crib. Photo: Evan DeLucia

MiscanthusElephant grass is a special cultivation of Chinese silver grass. and switchgrass, two perennial grasses that could be used for biofuels, grow in front of a corn crib. Photo: Evan DeLucia

As the world seeks strategies to reduce emissionsEmissions of greenhouse gases, greenhouse gas precursors, and aerosols associated with human activities, including the burning of fossil fuels, deforestation, land-use changes, livestock, fertilisation, etc. (IPCC) of carbon dioxideCarbon dioxide (CO2) is emitted in several ways. Naturally through the carbon cycle and through human activities like the burning of fossil fuels. These human activities have increased CO2 concentrations in the atmosphere since the beginning of the industrial revolution and these high ... (CO2) into the atmosphere, bioenergyEnergy from sustainable sources such as forests and agriculture (like wood and energy crops), but also manure and other biodegradable wastes. Includes biogas, biofuels and solid biomass. is one promising substitute for fossil fuelsEnergy from fossil sources, such as natural gas and oil. This type of energy contributes to climate change and because of its finite nature it is not a permanent resource. [Somerville et al., 2010]. Currently, the United States uses the starch component from roughly 40% of its corn harvest to produce ethanol for the transportation sector (see the National Agricultural Statistics Service website).

Cornstarch production is technologically simple—a so-called first-generation bioenergy technology. However, growing corn requires a lot of fertilizer and field preparation that ultimately depend on fossil fuels, tempering the net carbon savings.

Because of this, researchers have focused on developing fuel production using more advanced methods and second-generation bioenergy cropsA crop is any cultivated plant, fungus, or alga that is harvested for food, clothing, livestock fodder, biofuel, medicine, or other uses.. These methods make liquid fuel, primarily ethanol, from lignocellulose, which composes the structural elements of plants, leaves, stalksStalk biomass is biomass from stalky plants such as grass, straw or plant remnants from farms., and stems. Because many of these crops are perennials (they grow back year after year), they often require less fertilizer and tillage, avoiding many of the negatives associated with corn and other annual crops that need intensive management.

A challenge with second-generation energy cropsEnergy plants are plants that get cropped or felled to produce electrical or thermal energy., however, is that they yield much less energy—lignocellulose produces less than one third of the energy per unit mass compared to fossil fuels. Because of this low energy density, the United States would need to invest a considerable amount of land to meet a significant part of national demand, a land area almost the size of Wisconsin to meet the Renewable Fuel Standard mandate for 32 billion gallons of biofuelLiquid fuels and blending components produced from biomass (plant) feedstocks, used primarily for transportation.Bio fuels are liquid fuels that are produced of plant material or herbal remains. [Hudiburg et al., 2015].

The conversion of current land uses and management practices to the cultivation of bioenergy crops directly affects the climate system and is a fundamental process underlying the ecological sustainabilityIn order to survive, all life, including human life, depends either directly or indirectly on the natural environment. Sustainability is a principle where current requirements are met while the livelihoods of future generations are not threatened. of bioenergy production, as well as the ability of bioenergy crops to mitigate climate changeClimate change is a lasting change in weather patterns over long periods of time. It can be a natural phenomena and and has occurred on Earth even before people inhabited it. Quite different is a current situation that is also referred to as climate change, anthropogenic climate change, or .... Where conversion of native prairie to corn negatively affects climate by releasing CO2 and other greenhouse gasesGreenhouse gas emissions cause dangerous anthropogenic climate change. Emissions include CO2, fluoridated gases, methane which are emitted by human activity such as deforestation and burning fossil fuels, and water vapour. to the atmosphere, planting second-generation grass-based biofuel crops on marginal or degraded land can reduce our carbon footprintGreenhouse gas accounting describes the way to inventory and audit greenhouse gas (GHG) emissions. A corporate or organisational greenhouse gas (GHG) emissions assessment quantifies the total greenhouse gases produced directly and indirectly from a business or organisation’s activities. ... and provide other beneficial ecosystem servicesHumankind benefits in a multitude of ways from ecosystems. Collectively, these benefits are known as ecosystem services. Ecosystem services are regularly involved in the provisioning of clean drinking water and the decomposition of wastes. While scientists and environmentalists have discussed ....

Changing the Landscape Changes Our Climate

Public discussion of climate change often focuses on the atmosphere. As greenhouse gases (GHGs) like carbon dioxide, nitrous oxideOne of the six types of greenhouse gases to be curbed under the Kyoto Protocol. The main anthropogenic source of nitrous oxide is agriculture (soil and animal manure management), but important contributions also come from sewage treatment, combustion of fossil fuel, and chemical industrial ..., and methaneMethane is one of the six greenhouse gases to be mitigated under the Kyoto Protocol and is the major component of natural gas and associated with all hydrocarbon fuels, animal husbandry and agriculture. Coal-bed methane is the gas found in coal seams. (IPCC) accumulate in the atmosphere, they warm the planet by absorbing infrared radiation. But that’s not the whole picture: those greenhouse gases are also constantly cycling between the atmosphere and the land (Figure 1). Therefore, changes in land useLand use refers to the total of arrangements, activities, and inputs undertaken in a certain land cover type (a set of human actions). The term land use is also used in the sense of the social and economic purposes for which land is managed (e.g., grazing, timber extraction, and conservation). ..., vegetation, and how we manage it affect climate by altering that exchange.

Climate Regulation Value

Fig. 1. Terrestrial ecosystems affect the climate system by influencing the exchange of greenhouse gases between the land surface and the atmosphere (biogeochemical regulation) and by influencing the exchange of energy (evapotranspiration and albedoClimate feedback involving changes in the Earth’s albedo. It usually refers to changes in the cryosphere which has an albedo much larger (~0.8) than the average planetary albedo (~0.3). In a warming climate, it is anticipated that the cryosphere would shrink, the Earth’s overall ...) with the atmosphere (biophysical regulation). Biogeochemical processes affect the concentration of greenhouse gases in the atmosphere, influencing global climate on decadal time scales or longer, whereas biophysical processes cause local cooling or warming over days and months. Credit: Evan DeLucia

The metabolisms of plants and soil microbes help to regulate the exchange of GHGs with the atmosphere, as these molecules or their precursors are stored in biomassEnergy resources derived from organic matter. These include wood, agricultural waste and other living-cell material that can be burned to produce heat energy. They also include algae, sewage and other organic substances that may be used to make energy through chemical processes.Biomass, a ... and soil. Clearing a native forest, for example, releases large quantities of carbon stored in biomass and soil to the atmosphere (“storage”; Figure 2). Indeed, creating and managing farmland contribute more than 14% of the world’s GHG emissionsGreenhouse gas emissions cause dangerous anthropogenic climate change. Emissions include CO2, fluoridated gases, methane which are emitted by human activity such as deforestation and burning fossil fuels, and water vapour. (U.S. Environmental Protection Agency, global greenhouse gas emissionsGreenhouse gas emissions cause dangerous anthropogenic climate change. Emissions include CO2, fluoridated gases, methane which are emitted by human activity such as deforestation and burning fossil fuels, and water vapour. data, 2015).

Fig. 2. (a) Biogeochemical climate services reflect the greenhouse gases that would be released from land clearing and the change in ongoing exchange with the atmosphere. (b) Similarly, land clearing affects biophysical climate services, albedo, related to net radiation (Rnet), and latent heat flux from evaporation (LE), related to changes in evapotranspiration. Positive values represent a net cooling effect on the atmosphere. (c) The sum of greenhouse gases and biophysical factors is the climate regulating value (CRV) of an ecosystem. Values are normalized to the warming potential of carbon dioxide (CO2) and are expressed relative to bare ground over a 50-year time frame. Replacing an ecosystem with a high CRV with that having a low value would have a net warming effect on the atmosphere and vice versa. Reproduced from Anderson-Teixiera et al. [2012].

Fig. 2. (a) Biogeochemical climate services reflect the greenhouse gases that would be released from land clearing and the change in ongoing exchange with the atmosphere. (b) Similarly, land clearing affects biophysical climate services, albedo, related to net radiation (Rnet), and latent heat flux from evaporation (LE), related to changes in evapotranspiration. Positive values represent a net coolingair conditioning/cooling is the process of altering the properties of air (primarily temperature and humidity) to more comfortable conditions, typically with the aim of distributing the conditioned air to an occupied space to improve thermal comfort and indoor air quality. effect on the atmosphere. (c) The sum of greenhouse gases and biophysical factors is the climate regulating value (CRV) of an ecosystemA system of living organisms interacting with each other and their physical environment. The boundaries of what could be called an ecosystem are somewhat arbitrary, depending on the focus of interest or study. Thus, the extent of an ecosystem may range from very small spatial scales to, .... Values are normalized to the warming potential of carbon dioxide (CO2Carbon dioxide (CO2) is emitted in several ways. Naturally through the carbon cycle and through human activities like the burning of fossil fuels. These human activities have increased CO2 concentrations in the atmosphere since the beginning of the industrial revolution and these high ...) and are expressed relative to bare ground over a 50-year time frame. Replacing an ecosystem with a high CRV with that having a low value would have a net warming effect on the atmosphere and vice versa. Reproduced from Anderson-Teixiera et al. [2012].

Terrestrial ecosystems also affect climate on local scales by regulating the exchange of energy between the land and atmosphere (Figure 1) [Davin et al., 2014; Zhao and Jackson, 2014]. Land covered with vegetation generally absorbs more sunlight than bare soil, contributing to local warming. Working against this, when waterClimate change is expected to exacerbate current stresses on water resources from population growth and economic and land-use change, including urbanisation. On a regional scale, mountain snow pack, glaciers and small ice caps play a crucial role in freshwater availability. Widespread mass ... evaporates and passes into the atmosphere from soil and vegetation (known collectively as evapotranspiration), it carries heat away from the land, causing local cooling. Thus, forest clearing would reduce the cooling effect of evapotranspiration but would increase the reflectivity of the surface (or its albedo), allowing it to reflect more radiation.

However, global effects are complicated because evapotranspiration means more moisture in the atmosphere, which can increase cloud cover, affecting the global radiation balance. Also, when the moisture condenses into clouds elsewhere, it releases its latent heat, which can cancel the local cooling effect [Pielke et al., 2002; Snyder et al., 2004]. Unlike GHGs that have a locally weak but globally strong effect on the atmosphere, albedo and evapotranspiration affect climate locally [Bright, 2015].

By normalizing the warming (or cooling) potential of nitrous oxide (N2O), methane, albedo, and evapotranspiration to the warming potential of CO2, the contribution of different ecosystems to climate regulation can be expressed as a single metric: the climate regulating value (CRV) [Anderson-Teixeira et al., 2012]; this value provides an integrated index of the direct effects of land clearing on the surface energy budget, where the greater the CRV is, the greater the cooling effect is (Figure 2).

Long-Term Strategic Planting

In native forests and other ecosystems with large carbon stocksThe total amount of carbon stored on a plot of land at any given time in one or more of the following carbon pools: biomass (above and below ground), dead organic matter (dead wood and litter), and soil organic matter.4 A change in carbon stock can refer to additional carbon storage within a ..., biogeochemical processes—those processes that store and exchange GHGs with the atmosphere—can play a larger role in regulating the climate than biophysical processes such as changes in evapotranspiration and albedo (Figure 2). The opposite can be true for perennial grasses that don’t store much biomass but have high evapotranspiration and albedo.

Therefore, displacing native forestsForestry is the management and care of woods, including fellings and plantation of new trees., particularly tropical forests, with annual or perennial crops for energy production will, in most cases, have a net warming effect on the atmosphere, releasing large quantities of carbon stored in biomass and soil (Figure 2). But replacing annual crops or placing high-yield bioenergy crops on marginal land (that is, land that cannot produce high-value crops) has a very different effect.

Replacing annual crops with perennial grasses such as miscanthus and switchgrass would pull carbon out of the atmosphere and return it to the ground (Figure 2). These crops allocate a large fraction of their biomass below ground in their root systems, and they can rapidly build up carbon storesAny process, activity or mechanism which removes carbon from the atmosphere. in soil, reversing losses associated with frequent tillage, particularly on degraded or heavily tilled soils [Anderson-Teixeira et al., 2009, 2013; Powlson et al., 2011].

In terms of climate effects, this is doubly helpful. In addition to displacing fossil fuels by providing a renewable biofuel, replacing low-CRV annual crops with high-CRV perennial grasses would have a net cooling effect on the atmosphere because of the changes in biogeochemical and biophysical properties. In particular, the increased albedo from perennial grass and the heat carried away by evapotranspiration can amount to a considerable cooling effect compared to annual row crops [Georgescu et al., 2011].

The U.S. Midwest: From Carbon SourceA carbon source emits carbon into the atmosphere. Note a carbon sink can turn into a carbon source, like a forest that can absorb carbon as it grows but release it when it burns. to Sink

Of the approximately 40 million hectares in the United States that are planted with corn, mostly in the Midwest, only 8% of them directly feeds humans; most of the rest (73%) is for feeding livestock and producing ethanol (National Agricultural Statistics Service website, 2015). Displacing corn currently grown for ethanol with high-yielding perennial grasses would have enormous environmental benefits, without displacing land used for food production. Davis et al. [2012] predict that replacing ethanol-bound corn with perennial grasses would reduce emissions of GHGs to the atmosphere while increasing soil carbon. The emissions of N2O in particular would be reduced because perennial grasses require so much less nitrogen than corn [Smith et al., 2013]. Over the entire region, this transition would convert soils in the Midwest from a net source to a net sink for GHGs while simultaneously increasing fuel production and reducing the contamination of groundwater by fertilizer-derived nitrate.

Prime corn land is expensive. Restricting most bioenergy grasses to more affordable marginal land would also drive a reduction in U.S. GHGGreenhouse gas emissions cause dangerous anthropogenic climate change. Emissions include CO2, fluoridated gases, methane which are emitted by human activity such as deforestation and burning fossil fuels, and water vapour. emissions, albeit a smaller one than replacing corn ethanol, and would still meet the Renewal Fuel Standard’s mandate for 32 billion gallons of renewable biofuel, with negligible effects on food crop production [Hudiburg et al., 2015].

The biophysical processes will also help to regulate climate. Evapotranspiration would increase slightly—less than 10% [VanLoocke et al., 2010]—but combined with an increased albedo, that’s enough to provide an additional local cooling effect.

The Promise and Challenges of a Bioenergy Landscape

In addition to displacing CO2 emitted from fossil fuels, the expansion of perennial bioenergy crops in the U.S. Midwest will likely have positive effects on the climate system. This, however, is not necessarily the case elsewhere. In much of the United States west of the 100th meridian (a line that roughly bisects the Dakotas and Texas), the ability of the atmosphere to remove water (potential evapotranspiration) exceeds precipitationNo reegle definitive available.. There, the irrigation necessary for energy crops would pose severe environmental challenges.

In Southeast Asia, displacing native forest with palm oil plantations, in part for biodieselA biodegradable transportation fuel for use in diesel engines that is produced through the transesterification of organically- derived oils or fats. It may be used either as a replacement for or as a component of diesel fuel.Biodiesel is a biosynthetic fuel with similar properties as mineral diesel., has increased the amount of atmospheric GHGs and hurt biodiversityThe total diversity of all organisms and ecosystems at various spatial scales (from genes to entire biomes). (IPCC) [Danielsen et al., 2009]. Furthermore, by increasing grain prices, displacing food crops with bioenergy crops may encourage deforestation in the tropics for expanding agricultureCultivation of the ground and harvesting of crops and handling of livestock, the primary function is the provision of food and feed. [Searchinger et al., 2008].

We must take care to avoid these unintended negative consequences of expanding lignocellulosic bioenergy production. But with appropriate financial incentives [Dwivedi et al., 2015], there are many strategies to use land sustainably to contribute to the U.S. demand for transportation fuel: the use of marginal or underproductive lands [Gelfand et al., 2013] or replacing intensively managed corn for ethanol with high-yielding, low-input perennials. When annual crops that require intensive management are replaced with high-yielding perennial plants, bioenergy crops can simultaneously reduce the emissionEmissions of greenhouse gases, greenhouse gas precursors, and aerosols associated with human activities, including the burning of fossil fuels, deforestation, land-use changes, livestock, fertilisation, etc. (IPCC) of GHGs to the atmosphere and improve the health of agricultural landscapes [Werling et al., 2014].

Evan H. DeLucia, Department of Plant Biology, Institute for Sustainability, Energy, and Environment, Energy Biosciences Institute and Carl R. Woese Institute for Genomic Biology, University of Illinois at Urbana–Champaign; email: [email protected]

References

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