In 2000 (but using data from 1996), a review, two of whose authors had affiliations to ExxonMobil, concluded that sugarcane grown under Brazilian conditions could generate above-ground and harvestable biomass of 932 GJ/hectare/year; a total of 9.3 x 108 hectares of land could substitute the full global primary energy of fossil fuels (3.2 x 1011 GJ/year), even if only sugarcane stems were used as a fuel with a quan­titative extraction of the energy inside the plant material, that is, 343 GJ/hectare.81 Based on a total area of land used to grow crops worldwide of 1.4 x 109 hectares, the fossil fuel demand could, in principle, be met with 67% of cultivatable land dedi­cated to sugarcane as an energy crop, or only 24% if all of the harvestable material were used. If, however, the sugarcane stems were to be used for ethanol production, only a third of the useful energy would be converted to the biofuel, and more than twice the global area of land used to grow crops would be needed, that is, ethanol production could not substitute for the world’s appetite for fossil fuels.

This is an interesting but disingenuous set of calculations because ethanol had never been advocated as anything other than a convenient energy carrier as a replace­ment (partial or otherwise) for gasoline in automobiles. From the most recent data published by the International Energy Agency, oil used for transport is expected to continue being approximately 20% of the yearly fossil fuel demand or to slowly increase to more than 50% of the crude oil extraction rate (figure 5.8). To substitute the global demand for oil as a transport fuel, therefore, sugarcane grown for ethanol would need to occupy 40% of the world’s arable land (stems only as a feedstock) or 20-25% if the entire harvestable biomass were to be used for ethanol production. The long-perceived potential conflict between land use for food crop production and for bioenergy is highly likely to be a reality, and genetic engineering or other innovative

technologies cannot avoid this without massive increases over current bioprocess — limited abilities to transform biomass into ethanol (or any other liquid biofuel).82 Any conventional or molecular “improvement” of a major crop species must avoid greater dependence on fertilizer and other agrochemical application to achieve greater yields. A particular drawback is the reliance on energy crops with large inputs of nonrenew­able energy for the production of biofuels: corn-derived ethanol and, for biodiesel, rape (canola) and soybean seeds (see chapter 6, section 6.1).83

Equally, the use of agricultural residues for biofuel production has severe agro­nomic limitations. Although a worldwide potential residue harvest of 3.8 x 109 tonnes/year has been estimated, equivalent to 7.5 billion barrels of diesel, removing more than 30% of this could greatly increase soil erosion as well as depleting the soil organic carbon content. One solution to this problem is to establish new bioenergy plantations on uncultivated land, perhaps 250 million hectares worldwide.84 This could be supplemented by the exploitation of weed infestations, for example, water hyacinth in lakes and waterways, naturally occurring sugar-rich forest flowers, or other adventitious resources.85 86 Of the major candidate energy crops, switchgrass and other prairie grasses appear to offer the best balance among high net energy yields, low nutrient demands, and high soil and water conservation; with its high total root mass, switchgrass can replace soil carbon lost during decades of previous tilling within (perhaps) 20 years.87 Under U. S. farm conditions, there is mounting evidence that dedicated energy crops would significantly reduce erosion and chemi­cal runoff in comparison with conventional monoculture crops.88-90 Replacing con­tinuous corn cropping with a corn-wheat rotation and no-till field operations might maximize agricultural residue availability.91

Doubts persist, however, that any of these options represent true sustainability, even when consideration is extended to prolific plantations of acacia and eucalyptus
trees, both of which rapidly exhaust tropical soils of nutrients and fall far short of even 100 years of high productivity.79 The nearest approach to true sustainability could be the immediate-locale use of sun-dried wood from well-managed energy planta­tions, using highly efficient wood-burning stoves; the necessary high rates of biomass “mining” (probably requiring some method of energy-dependent biomass drying) to support predicted rates of growth of gasoline and biofuel consumption appears to be mathematically unsustainable.79 In any case, intensive energy crop plantations would certainly require a high degree of environmental monitoring and management to avoid biological collapse unless lavish amounts of fertilizers are applied and maxi­mum yields are ensured by equally large supplies of insecticides and pesticides.

A final twist is the impact of global climate change: could global warming (at least for a limited time, perhaps as long as 50-100 years) increase plant growth to such an extent that biomass extraction targets would be more closely met? Even accurately predicting biomass yields from monocultures of fast-growing tree species such as willow in the northeastern United States suffers from methodological weaknesses.92 The effects of changes in daily minimum and maximum temperatures are complex because they differentially influence crop yield parameters; this is a major area of uncertainty for projecting yield responses to climate change.93 For three major cereal species (wheat, corn, and barley), and an increase in annual global temperatures since 1980 of approximately 0.4°C, there is evidence for decrease in yield; the magnitude of the effect is small in comparison with the technological yield gains during the same period but suggests that rising temperature might cancel out the expected increase in yield because of increased CO2 concentrations.9495 Using the global environment as an uncontrolled experimental system has its methodological drawbacks, but the results of the one long-term field trial to attempt to isolate the effects of temperature on rice growth indicate that a 15% reduction in yield could result from each 1°C rise in temperature, a much greater effect than predicted by simulation models.96 The International Panel on Climate Change has, in stark contrast, predicted that CO2 benefits will exceed temperature-induced yield reductions with a modest rise in tem­perature. Because this conclusion has received wide media coverage, it is important to examine the detailed conclusions for food, fiber, and forest products: [53]

• Globally, commercial timber productivity rises modestly with climate change in the short to medium term, with large regional variability around the global trend.

Taken together, these International Panel on Climate Change prognoses suggest some short-term improvement in the productivity of a range of plant species, including timber grown as feedstocks for lignocellulosic ethanol, but the predictions become more unreliable as the geographical area narrows and related effects of climate are considered. This highlights the obvious conclusion that geostatistical surveys and controlled experiments should both be pursued vigorously as priority issues for agronomy and plant physiology in the next ten years.