[In 2000, Melanie Kenderine at the U.S. Department of energy stated that: “This nation has abundant biomass resources (grasses, trees, agricultural wastes) that have the potential to provide power, fuels, chemicals and other bio-based products” (136).
That’s a good point — biofuels are the only sustainable choice after fossil fuels are gone for transportation, but they’re ALSO the only sustainable source to generate electricity, to cook and heat with, make and provide the feedstock for half a million products, the heat source for steel and cement, and so on.
But is there really enough biomass to do all of these things? Both papers below explain why biomass can’t scale up to provide more than a small fraction of energy in the future for transportation, let alone all the other needs.
Nearly all heavy-duty trucks run on diesel exclusively. Diesel engines can’t burn ethanol, diesohol, or gasoline, and most engine warranties allow zero to at most 20% biodiesel to be mixed in with petroleum-derived diesel. So why are we making ethanol? Civilization ends when trucks stop running. Why aren’t we getting cars off the road ASAP to free up fuel for trucks, locomotives, and ships? Especially since biodiesel scales up even less well than ethanol. Both probably have a negative energy return on invested (EROI) or at best are break-even, so it’s a shame that meanwhile we’re destroying topsoil, exhausting aquifers, and polluting land, air, and water with pesticides, herbicides, and fertilizers to make biofuels, when post-fossil fuels organic topsoil will be the most valuable asset we have.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer and “Crunch! Whole Grain Artisan Chips and Crackers”. Podcasts: Practical Prepping, KunstlerCast 253, KunstlerCast278, Peak Prosperity , XX2 report ]
Gomiero, Tiziano. June 30, 2015. Are Biofuels an Effective and Viable Energy Strategy for Industrialized Societies? A Reasoned Overview of Potentials and Limits. Sustainability 2015, 7, 8491-8521.
Excerpts from this 31 page article follow.
For our industrial society to rely on “sustainable biofuels” for an important fraction of its energy, most of the agricultural and non-agricultural land would need to be used for crops, and at the same time a radical cut to our pattern of energy consumption would need to be implemented, whilst also achieving a significant population reduction.
Some scholars questioned the energy efficiency of biofuels, claiming that it was an unproductive enterprise (e.g., [2–13]), a point already made in the 1970s by energy experts such as Prof. David Pimentel , and Prof. Vaclav Smil .
Biofuels, in fact, call for the adoption of those very same agricultural practices that for decades have been blamed for being highly energy inefficient and water consuming, and for contaminating the environment and threatening biodiversity and soil health [2–5,14–17].
Other works highlighted that, contrary to current belief, biofuel production may cause net CO2 emission, in particular when tropical forests and pristine land are converted to plantations and crops for biofuel production [18–20].
The interest in biofuel as a potential sustainable and renewable energy source is still high, as is attested by numerous scientific journals recently created in its name, and the number of funded research projects that focus on this topic. Private investments and public subsidies are still poured into this sector. Since the crisis, however, the focus shifted from first-generation biofuels (or the use of fuel crops) to second-generation biofuels, i.e., the use of cellulosic ethanol (crop residues, woody biomass), and then to third-generation biofuels, i.e., oil from algae.
Palm oil is also becoming of high interest for the biofuel market, and there is a risk that palm oil plantations may further increase the displacement of native forests in tropical countries (as happened for sugarcane plantations in Brazil), or replace other food crops, without providing any benefits to farmers. After the plantation is discontinued (20–25 years), the soil is then ruined and cannot easily serve for further agricultural activities.
Findings from different experts, however, diverge considerably. Some authors claim that biofuels may represent an efficient alternative to oil, some of them referring to fuel crops, while others only refer to cellulosic ethanol. Other authors claim that biofuels and biomass in general are instead an inefficient alternative to fossil fuels. So, how is it possible that highly respected scholars can reach such opposing conclusions?
We have to face the fact that data-gathering systems rely on different approaches and methodologies, involving different focuses, models, assumptions and scale of analysis. To begin with, a major problem arises with the choice of system boundaries, the “boundary dilemma” as Smil  (p. 275) put it. The choices over where to make our system end can lead to large differences in the results [12,28,32]. Borrion et al. , in their extensive review of environmental LCA of lignocellulosic ethanol conversion, conclude that results strongly depend on system boundary, functional unit, data quality and allocation methods chosen. The authors also make an important remark stating that “The lack of available data from commercial second generation ethanol plant and the uncertainties in technology performance have made the LCA study of the lignocellulosic ethanol conversion process particularly difficult and challenging.”  (p. 4648).
Assessments are scale dependent (and of course value laden, a matter which scientists often prefer not to confront). This means that before the assessment exercise takes place we have to frame properly the context in which we are operating. To put it simply, do cars pollute? It depends on how many cars we are talking about, the performance of their engines, their average speed, the quality of the fuel, etc. New “clean” engines on many new cars may cause more pollution than old dirty engines on few old cars; scale matters. But the scale has to be decided before carrying out the assessment. There is a very telling example concerning the calculation of biofuel efficiency presented by Shapouri et al.  vs. Giampietro et al. , on how to account for co-products. I quote  (p. 33)
Prof. David Pimentel was also a co-author of the paper ), as it is explained very clearly: “Shapouri et al. reported a net energy return of 67% after including the co-products, primarily dried distillers grain (DDG) used to feed cattle. These co-products are not fuel!
Giampietro et al. (1997) observed that although the by-product DDG may be considered as a positive output in the calculation of the output/input energy ratio in ethanol production, in a large-scale production of ethanol fuel, the DDG would be many times the commercial livestock feed needs each year in the U.S. (Giampietro et al. 1997).
It follows then that in a large-scale biofuel production, the DDG could become a serious waste disposal problem and increase the energy costs.” For issue of scale was also pointed out by Smil , in his assessment of the program PROALCOL, launched by the Brazilian government. Apart from a number of problems identified by Smil , (e.g., soil erosion, land conversion, productivity-related issues, economic viability), the author stressed that in order to achieve the production of ethanol from sugarcane forecast by the government, the process would have to also produce each year more than 150 million m3 of vinhoto, the residue of the process. Such a byproduct can be dried up and used as feed, but that is a highly energy-intensive process. The liquid may be used as fertilizer, but it requires logistics for concentrating it, transporting it around the country, etc. So the usual solution is dumping the fluid into the nearest water bodies, and in that context, vinhoto is a very serious pollutant.
For more examples on how the scale issue matters, I refer the reader to [12,37].
On the energy analysis of biofuels, a fierce debate surrounds the issue of providing an accurate EROI estimate for biofuels, but this really has to do with a few decimals below or above one, as the EROI for biofuels is between 0. 8 and 1.6.
This issue should not be a matter of concern, as fossil fuels, which fuel industrial societies, generate an EROI of 20–30 or more [12,27,29]. The fact that there are cases where biofuels can be produced at higher EROI does not really change the judgment over the low performance of biomass.
The power density of the energy source, that is to say the rate of energy flux per unit of area (W/m2), is a key indicator [4–7]. Concerning power density, fossil fuels perform from 300 to 3000 times better than the best biofuel.
See also Smil  (p. 265), for data about the power density of various kinds of biomass energy production.
Giampietro and colleagues [12,26] argue that developed societies, in order to sustain their level of metabolism, require an energy throughput in the energy sector ranging from 10,000 to 20,000 MJ per hour of labor. The fact that the range of values achievable with biofuel are just 250–1600 MJ per hour of labor says it all. Of course, we may argue that this is a positive outcome, as it allows the creation of more jobs and reduce unemployment. Nevertheless, if wages in those jobs have to be comparable to those in other sectors of society, the cost of energy will skyrocket
On the biophysical side, one of these indicators is energy density. The final cost of energy in economic terms is, of course, another key issue. Biofuels can be produced only thanks to subsidies. A number of qualitative indicators are also highly relevant such as: the level of contamination produced, the reliability of the supply, and the level of risk involved [5–7,12,13,29].
It should be clear, therefore, that to perform a sound and effective assessment of an energy source is far from being a simple task, and requires the adoption of a number of different indicators related to different criteria and scales. The narrative about biofuels, instead, has been and still is, dangerously simplistic.
At present, the energetic discourse on biofuels is focused on the EROI, but, as we have seen, the EROI is just part of the story. The main problem with biofuels is that they have a power density that is simply too low and this requires handling an enormous quantity of biomass, costing society a lot of working time and capital. Those characteristics make biofuels unable to supply energy to match the metabolic rate of energy consumption of developed countries [5,6,12,26,32].
For our industrial society to rely on “sustainable biofuels” for an important fraction of its energy, it would require a complete reshaping of its metabolism:
- cropping most of the agricultural and non-agricultural land, affecting food supply and food affordability, increasing the impact on natural resources (water, soil health, pollution, loss of biodiversity);
- implementing an amazing occupational shift by sending millions of people back to the fields, which will increase the cost of energy (or at least drastically reduce the wages of those working in the sector);
- cutting our pattern of energy consumption, given the reduced flow of net energy;
- a consistent reduction of population size and consumption would be required;
- dealing with a continuous risk of running out of energy due to climate extremes, pests, etc.;
- such a massive amount of biomass may not be sustainable in the long term, and in the short run, it would require increasing amounts of input.
In summary, for a society (as for any living organism) the energetic supply is a matter of vital importance. The key factors being: (1) the quality of the energy source (fossil fuels are much better than biomass as most of the work has already been done by the Earth’s ecosystems and geological forces over hundreds of millions of years); and (2) the overall efficiency of the supply process (extraction, transformation, etc.), that is to say, the net energy supplied to society at the proper rate of delivery, able to match the rate of energy demand. If the supply of energy cannot match the rate of metabolic energy consumption, society will reduce its metabolism accordingly.
Subsidies: Are They the Key for Biofuel Sustainability?
Pimentel, Smil and Youngquist, were critical towards the real efficiency of biomass as an energy source, and posed important questions concerning its economic efficiency and environmental impact (e.g., soil, water, use of agrochemicals). Youngquist claims that ethanol policy in the USA is a mere political issue, with politicians granting subsidies for inefficient ethanol production in order to secure the votes from Corn Belt electors: “The answer is that it is an example of politics overriding reason. The political block of the corn belt states holds votes crucial to elections, and companies which produce ethanol in the United States have been some of the largest contributors to political campaign funds in recent years”  (pp. 243–244).
Subsidies are still the main driving force shaping biofuel policy and trade, and ultimately they keep all this going. Even with oil at 100US$/barrel, biofuels were still not competitive and needed subsidies (and that can also be expected, as a lot of fossil fuel is required to carry out intensive agriculture) [12,44,45].
Koplow and Steenblik , estimate that in 2008, in the USA, total support towards ethanol production ranged between 9.0 and 11.0 billion US$, with subsidies between 2009 and 2012 accounting for about 50% (up to 80% in 2007) of the ethanol market price. These figures are likely an underestimate, given the many faces economic support can take (from tax exemption to price premium), rendering precise subsidy assessment a difficult task [44,45].
According to the IEA, biofuel subsidies amounted to about US$22 billion in 2010, and are projected to increase to up to US$67 billion per year in 2035 . Note that fossil fuel benefits from subsidies, too. Fossil-fuel subsidies are estimated at between US$45–75 billion a year in OECD countries and at US$409 billion in 2010 in non-OECD countries . Some authors (e.g., ) back subsidy policy of biofuels on the basis that “In any case, the size of the support of biofuels is small (the authors are referring to the figure of US$ 20 billion they present earlier), in relation to the cost of fossil fuel consumption subsidies amounted to $312 billion worldwide in 2009”. This reasoning is evidently flawed. The comparison refers to the total value, but has to be done on a per-unit basis instead. According to the BP Statistical Review of World Energy , in 2009 fossil fuel consumption amounted to about 10,000 Mt oil equivalent (3809 Mt oil, 2690 Mtoe gas, 3547 Mtoe coal), while biofuel amounted to about 52 Mt oil equivalent.
Subsidies turn out to be 3.1 million US$ per Mt oil eq. in the case of fossil fuels (US$ 3/t), and 423 million US$ per Mt oil eq. in the case of biofuels (US$423/t), 136 times more. We may well wonder what are we doing with biofuels!
Who benefits most from these subsidies? In the USA, federal and state subsidies for ethanol production, that total more than US$7 per bushel of corn, have been always mainly paid to large corporations [9,45,49]. It thus seems that those who will gain from subsidies are large corporations that sell the fossil-fuel-derived inputs, and the losers are the farmers, the consumers and the tax payers! And the environment, of course.
The USA population, 310 million in 2009, will reach 440 million by 2050 (US Census Bureau, 2009). According to Nowak and Walton , the rate of rural land lost to development in the 1990s was about 0.4 million ha per year and the authors warn that if this rate continues until 2050, USA will have lost an additional 44 million ha of rural countryside. Such areas will be lost mostly at the expense of agriculture or conservative land programs. Brown  points out that the USA, with its 214 million motor vehicles, paved an estimated 16 million ha of land (in comparison to the 20 million ha that US farmers plant in wheat). About 13% of U.S. land area is currently dedicated to highways and urbanization, so adding other 150 million people will dramatically affect both the demand for food, as well as the demand for space (e.g., urbanization and highways).
Promoting the extensive cultivation of species suitable for biofuel production would increase two of the major causes of biodiversity loss on the planet, namely the clearing and conversion of yet more natural areas for monocultures, and the invasion by non-native species.
“Carbon Debt”: Biofuels and Increasing Carbon Emissions
The belief that burning biomass is carbon neutral has been questioned. Such an idea is founded upon the rather simplistic reasoning that CO2 released in the burning is picked up again by plants, giving a net release of zero. There are a number of reasons why this is not so. Displacing tropical ecosystems in favor of plantations causes the loss of aboveground biomass, and also the release of a huge amount of carbon stored in the soil (about 50% of the total carbon in tropical forests is stored in the soil). Plantations will never store as much biomass as native ecosystems, and that leads to net carbon emissions. Converting grasslands into fuel crops will cause the net emission of the carbon stored in the native ecosystem.
Estimates concerning the “carbon debt” (the carbon that is lost in land use change) have been already published (e.g., [18,19]:
- the conversion of rainforests, peatlands, savannas. Brazil and Southeast Asia may create a “biofuel carbon debt” by releasing 17 to 420 times more CO2 than the annual GHGs reductions that these biofuels would provide by displacing fossil fuels;
- in the USA, corn-based ethanol will nearly double GHG emissions over 30 years, while cropping grasslands to produce biofuels (e.g., with switchgrass), will increase GHG emissions by 50%. Some USA public institutions concluded that much worse problems may be caused by fuel crops than by fossil fuels, due to corn ethanol and biodiesel made from soybean oil causing a large amount of land conversion to create a high “carbon debt” [88,89];
- in a meta-analysis carried out by Piñeiro et al.  on 142 soil studies, the authors conclude that soil C sequestered by setting aside former agricultural land was greater than the C credits generated by planting corn for ethanol on the same land for 40 years, and that C releases from the soil after planting corn for ethanol may, in some cases, completely offset C gains attributed to biofuel generation for at least 50 years.
It has been suggested that agricultural intensification may help reduce the expansion of plantations into pristine ecosystems. However, recent analysis found that using high-yielding oil palm crops to intensify productivity and then preserving the remaining biodiversity may not work either. Carrasco et al. , for example, argue that using high-yielding oil palm crops could actually lead to further tropical deforestation. That is because palm oil will become cheaper on the global food markets and will outcompete biofuels grown in temperate regions. That in turn will increase the planting of oil palm in tropical regions. In fact, paradoxically, while developed countries are claiming to import biofuels from tropical regions in order to reduce their CO2 emission, they are actually contributing to an amplification of the problem, and concurring to fuel the process of tropical deforestation [18,19,44,96,97]. Houghton  warns that, between 1990 and 2010, forest degradation and deforestation accounted for 15% of anthropogenic carbon emissions and argues that we have to work to stop this trend. The author is rather critical about the international biofuel trade, which, he claims, is driven by distortions generated by the high subsidies in place in the USA and the EU, and is not going to work towards halting deforestation.
The greater availability of crop residues and weed seeds translates to increased food supplies both for invertebrates and vertebrates, which play important ecological functions in agro-ecosystems, influencing, among other things: soil structure, nutrients cycling and water content, and the resistance and resilience against environmental stress and disturbance [57,115–120].
When compared to corn grain, it takes 2 to 5 times more cellulosic biomass to obtain the same amount of starch and sugars. This means that 2 to 5 times more biomass has to be produced and handled in order to obtain the same starches as for corn grain .
Tilman et al.  suggest that all 235 million hectares of grassland available in the USA, plus crop residues, can be converted into cellulosic ethanol, recommending that crop residues, like corn stover, can be harvested and utilized as a fuel source. I have already mentioned residues; as for the use of grassland, this cannot be considered an empty space. There are tens of millions of livestock (cattle, sheep, and horses) grazing on that land, as well as all the wild fauna and flora living in those ecosystems ;
Some energy analysts consider the biofuel “solution” so completely unrealistic that it should not even be worth any attention (e.g., [4,6,10,12]). Pimentel in his edited book on renewable energies , closes the work with chapter 20, on algae, consisting of two pages, summary and references included  (pp. 499–500). Pimentel claims that properly accounting for all the costs and assuming a realistic energy production level would lead to an estimated algal oil barrel cost of 800 US$.
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Excerpts from this 44 page report:
What is the role of bioenergy in a sustainable food future? The answer must recognize the intense global competition for land, and that any dedicated use of land for bioenergy inherently comes at the cost of not using that land for food, feed, or sustained carbon storage.
The world needs to close a 70% gap between the crop calories that were available in 2006 and the calorie needs anticipated in 2050. During the same period, demand for meat and dairy is projected to grow by more than 80%, and demand for commercial timber and pulp is likely to increase by roughly the same percentage.
Yet three-quarters of the world’s land area capable of supporting vegetation is already managed or harvested to meet human food and fiber needs. Much of the rest contains the world’s remaining natural ecosystems, which need to be conserved and restored to store carbon and combat climate change, to protect freshwater resources, and to preserve the planet’s biological diversity.
A growing quest for bioenergy exacerbates this competition for land. In the past decade, governments have pushed to increase the use of bioenergy—the use of recently living plants for energy (biodiesel, ethanol, cellulosic fuels)—by using crops for transportation biofuels and increasingly by harvesting trees for power generation. Although increasing energy supplies has provided one motivation, the belief that bioenergy use will help combat climate change has been another. However, bioenergy that entails the dedicated use of land to grow the energy feedstock will undercut efforts to combat climate change and to achieve a sustainable food future.
Would cellulosic biofuels avoid this competition for food? Cellulosic biofuels (sometimes referred to as “second generation”) may use crop residues or other wastes, but most plans for these biofuels rely on planting and harvesting fast-growing trees or grasses. At least some direct competition with food is still likely because such trees and grasses grow best and are most easily harvested on relatively flat, fertile lands—the type of land already dedicated to crops.
The push for bioenergy is extending beyond transportation biofuels to the harvest of trees and other sources of biomass for electricity and heat generation.
Some organizations have advocated for a bioenergy target of meeting 20% of the world’s total energy demand by the year 2050, which would require around 225 exajoules of energy in biomass per year. That amount, however, is roughly equivalent to the total amount of biomass people harvest today—all the crops, plant residues, and trees harvested by people for food, timber, and other uses, plus all the grass consumed by livestock around the world.
The world will still need food for people, fodder for livestock, residues for replenishing agricultural soils, wood pulp for paper, and timber for construction and other purposes. To meet these needs at today’s level while at the same time meeting a 20% bioenergy target in 2050, humanity would need to at least double the world’s annual harvest of plant material in all its forms. Those increases would have to come on top of the already large increases needed to meet growing food and timber needs.
Today, the best estimates are that agriculture and some kind of forestry use three-quarters of all the world’s vegetated land, and agriculture consumes around 85% of the freshwater people withdraw from rivers, lakes or aquifers. Seen in this context of land and water scarcity, the quest for bioenergy at a meaningful scale—even assuming large future increases in efficiency—is both unrealistic and unsustainable.
Even assuming large increases in efficiency, the quest for bioenergy at a meaningful scale is both unrealistic and unsustainable.
Why does a small share of energy require such vast amounts of biomass? Although photosynthesis is an effective means of producing food, wood products, and carbon stored in vegetation, it is an inefficient means of converting the energy in the sun’s rays into a form of non-food energy useable by people.
Fast-growing sugarcane on highly fertile land in Brazil, for example, converts only around 0.5 percent of incoming solar radiation into sugar, and only around 0.2 percent ultimately into ethanol. For maize grown in Iowa, the energy conversion rate is around 0.3 percent into biomass and 0.15 percent into ethanol. Even assuming highly optimistic estimates of future yields and conversion efficiencies, fast-growing grasses on productive U.S. farmland would only do slightly better, converting around 0.7 percent of sunlight into biomass and around 0.35 percent into ethanol. Such low conversion efficiencies explain why it takes a large amount of productive land to yield a small amount of bioenergy.
Is bioenergy nevertheless good for climate? Burning biomass, whether directly as wood or in the form of ethanol or biodiesel, emits carbon dioxide, just like burning fossil fuels. In fact, burning biomass directly emits at least a little more carbon dioxide than fossil fuels for the same amount of generated energy. But most calculations claiming that bioenergy reduces greenhouse gas emissions relative to burning fossil fuels do not include the carbon dioxide released when biomass is burned. They exclude it based on the theory that this release of carbon dioxide is matched and implicitly “offset” by the carbon dioxide absorbed by the plants growing the biomass feedstock. Yet if those plants were going to grow anyway, simply diverting them to bioenergy does not remove any additional carbon from the atmosphere and therefore does not offset emissions from burning that biomass.
In 2010, biofuels provided roughly 2.5% of the energy in the world’s transportation fuel (the fuel used for road vehicles, airplanes, trains, and ships). On a net basis, these 108 billion liters of biofuel provided roughly half a percent of global delivered energy. These liters came overwhelmingly from food crops: ethanol distilled mainly from maize, sugarcane, sugar beets, or wheat (88.7 billion liters), and biodiesel refined from vegetable oils (19.6 billion liters).
The United States, Canada, and Brazil accounted for about 90% of ethanol production, while Europe accounted for about 55% of biodiesel production.10 Overall, excluding feed byproducts, about 3.3 exajoules (EJ)11 of energy in crops were grown around the world for biofuels in 2010, using 4.7% of the energy content of all crops.
WHAT ABOUT FAST-GROWING GRASSES OR TREES FOR CELLULOSIC BIOFUELS?
Some biofuel proponents suggest that switching biofuels away from food crops to various forms of “cellulose”— sometimes referred to as “second generation” biofuels— would avoid competition with food. Cellulose forms much of the harder, inedible structural parts of plants, and researchers are devoting great effort to find ways of converting cellulose into ethanol more efficiently. In theory, almost any plant material could fuel this ethanol, including crop residues and much garbage. Such “waste” would not compete with food and, in a later section, we discuss the merits, demerits, and potential for its use. Yet the potential for wastes to provide energy on a large scale is sufficiently limited that virtually all plans for future large-scale biofuel production assume that most of the biomass for bioenergy would come from fast-growing trees and grasses planted for energy.
For these reasons, most studies of sustainable bioenergy— including biofuel—potential assume that bioenergy crops will not be grown on existing cropland. But yields on poorer, less fertile land tend to be substantially lower. More fundamentally, using less fertile land for bioenergy still uses land. Land that can grow bioenergy crops reasonably well will typically grow other plants well, too—if not food crops, then trees and shrubs that provide carbon storage, watershed protection, wildlife habitat, and other benefits. In Appendix A, we address various claims of the availability of such non-croplands for bioenergy. We argue that studies that find large bioenergy potential systematically “double count” land for biofuels that is already producing vegetation meeting other important human needs.
Unfortunately, growing trees and grasses well requires fertile land, resulting in potential land competition with food production. In general, growing grasses and trees on cropland generates the highest yields but is unlikely to produce more biofuel per hectare than today’s dominant ethanol food crops (i.e. 1 hectare of maize produces 1,600 gallons of ethanol). For cellulosic ethanol production to match this figure, the grasses or trees must achieve almost double the national cellulosic yields estimated by the U.S. Environmental Protection Agency (EPA), and two to four times the perennial grass yields farmers actually achieve today. Although there are optimistic projections for even higher yields, they are unrealistically predicated on small plot trials by scientists—sometimes only a few square meters. Scientists can devote greater attention to crops than can real farmers, and field trials for all types of crops nearly always produce far higher yields than those that farmers achieve in practice.
Some of the bioenergy literature calls for the use of “marginal” or “degraded” lands, relying on studies that use large-scale maps. However, these areas that appear to be unused and available for bioenergy using a coarse satellite map often turn out to be in some use upon closer examination. If millions of potentially productive hectares were truly both unused and not storing carbon, it should be easy to identify them specifically, but thus far no closer examinations have done so.
The International Energy Agency (IEA), among others, has suggested a goal of supplying 20% of the world’s energy use in the year 2050 from bioenergy. Since the Organisation for Economic Co-operation and Development (OECD) projects global primary energy use in 2050 to be 900 EJ per year, a 20% target equates to 180 EJ per year. How much plant material would that require? To get a sense of how much, consider that in 2000 the total amount of energy in all the crops, plant residues, and wood harvested by people for all applications (e.g., food, construction, paper) and in all the biomass grazed by livestock around the world was roughly 225 EJ. This amount of energy could in theory be liberated by perfect combustion of this biomass. But combustion is not perfect. Factoring in relative energy conversion efficiencies, this 225 EJ of biomass would optimistically replace about 180 EJ of primary energy from fossil fuels. Thus, it would take the entirety of human plant harvests in the year 2000 to meet a 20 percent bioenergy target in the year 2050.