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(a) 70% + oil demand (h) More coal extraction & conversion; 16% of

(b) Nuclear power plants power generation

(c) Carbon capture and sequestration (i) H2 mainly from natural gas reforming

(d) Increased biofuels production (see below) (j) U mining

(e) Wind onshore (+1600 GW/yr over baseline) (k) ~11 T hectares

(f) Deployment of FCVs in transportation (l) Pt for FCVs

(a) 70% + oil demand (h) More coal extraction & conversion; 16% of

(b) Nuclear power plants power generation

(c) Carbon capture and sequestration (i) H2 mainly from natural gas reforming

(d) Increased biofuels production (see below) (j) U mining

(e) Wind onshore (+1600 GW/yr over baseline) (k) ~11 T hectares

(f) Deployment of FCVs in transportation (l) Pt for FCVs

BLUE Map 2050 case, which has the highest level of biofuel consumption out of all the IEA scenarios, requires 6% of current global permanent pasture land, 16% of current global arable land, 6% of global renewable freshwater, 117% of current global water use by agriculture, and 82% of current total global water use.

It is useful to express the land and water requirements relative to the percent of energy demand satisfied by biofuels. For every 10% of the IEA-projected global ground transportation energy demand satisfied by cellulosic biofuels, the requirements are 2% of current global permanent pasture land, 6% of current global arable land, 2% of global renewable freshwater, 44% of current global water use by agriculture, and 31% of current total global water use.

Note that these percentages are calculated with respect to the current situation and do not reflect increases in demand for land and water in other sectors, particularly agriculture. Several studies project that total global water withdrawals could increase by more than 20% by 2025, leading to severe water stresses in several regions of the world (e.g., Seckler et al. 1999). However, even if future freshwater withdrawals for all uses other than biofuel feedstock production were to double by 2050, the addition of the water demand estimated for the IEA BLUE Map 2050 scenario still would result in a total water withdrawal of just under 20% of the total global renewable freshwater resource. Alcamo and Henrichs (2002) assume that when withdrawals are less than 20% of the available resource, there is low stress on water resources.

Thus, even though the land and water requirements of biofuels are very large with respect to both the requirements of current transportation energy systems and agricultural systems, at the global level there will be no obvious water and (pasture) land resource constraint on the development of bioenergy for several decades, unless the requirements of other sectors have been vastly underestimated. Water and arable land are not, however, distributed uniformly across the globe with respect to population or energy demand; thus, there can be severe constraints at the regional level on land and water availability. In parts of China, South Asia, West Asia, and Africa, current demands are already stressing water supplies, and this trend is expected to increase dramatically over the coming decades (Shah et al. 2000; Seckler et al. 1999; Serageldin 1995). Development of biofuel feedstocks in these areas could place intolerable stresses on water supplies.

Assuming that biofuels can be traded globally, the way petroleum fuels are, regional constraints on land and water need not impede the development of biofuels. FAO data (http://faostat.fao.org/faostat/) and the analysis of Berndes (2002) indicate that there are large regions of the world with ample land and water to produce biofuels: vast areas of North America, South America, Russia, Indonesia, and parts of Sub-Saharan Africa. If biofuel feedstocks can be grown in these resource-rich regions at reasonable cost and with minimal environmental impact,4 and if future demands for land and water by other sectors do not dramatically exceed present expectations—issues not examined here—then biofuel production need not be constrained by the global availability of land and freshwater. (For a similar, more detailed analysis and conclusion, see Berndes 2002.)

Nonrenewable Minerals Impacts from FCV Production

It i s clear that the production of millions of FCVs using platinum catalysts would increase demand for Pt substantially. Indeed, the production of 20 million 50-kW FCVs annually might require on the order of 250,000 kg of Pt—more than the total current world annual production of about 200,000 kg in 2008 (Yang 2009; USGS 2009, p. 123). How long this output can be sustained, and at what platinum prices, depends on at least three factors: (a) the technological, economic, and institutional ability of the major supply countries to respond to changes in demand; (b) the ratio of recoverable reserves to total production; and (c) the cost of recycling as a function of quantity recycled. Regarding the second factor, Spiegel (2004:364) writes that the International Platinum Association concludes that "there are suffi cient available reserves to increase supplies by up to 5-6% per year for the next 50 years," but does not indicate what the impact on prices might be. Gordon et al. (2006:1213) estimate that 29 million kg of platinum group metals are available for future use, and state that "geologists consider it unlikely that signifi cant new platinum resources will be found." This will sustain annual production of at least 20 million FCVs (with 12.5 g Pt per vehicle), plus production of conventional catalyst-equipped vehicles, plus all other current nonautomotive uses, for less than 100 years, without any recycling of Pt catalysts. Thus, the prospects for very long-term use and price behavior of platinum depend in large part on the prospects for recycling.

The prospects for economical recycling are difficult to quantify. In 1998, 10 metric tons of Pt were available from recycling automobile catalysts (USGS 1999). Carlson and Thijssen (2002) report that recycling of automotive catalysts is between only 10% and 20%, but they note that economic theory predicts that recycling will increase as demand increases. Spiegel (2004:360) states that "technology exists to profitably recover 90% of the platinum from catalytic converters," and in his own analysis of the impact of FCV platinum on world platinum production (but not price), he assumes that 98% of the Pt in FCVs will be recoverable. However, Gordon et al. (2006) assume that only 45% of the Pt in FCVs will be recovered. Our belief is that enough platinum

In this respect, note that the estimates of water requirements presented here do account, roughly, for the extra water needed to dilute polluted agricultural water to acceptable levels; for further discussion, see Dabrowski et al. (2009).

will be recycled to supply a large FCV market, until new, less costly, more abundant catalysts or fuel cell technologies are found. Indeed, catalysts based on inexpensive, abundant materials may be available relatively soon. Lefevre et al. (2009) report that a microporous carbon-supported iron-based catalyst is able to produce a current density equal to that of a platinum-based catalyst with 0.4 mg Pt/cm2 at the cathode. They note, however, that further work is needed to improve the stability and other aspects of iron-based catalysts; still, this research suggests that a worldwide FCV market will not have to rely indefinitely on precious metal catalysts.

Summary and Recommendations

Perhaps the most challenging aspect of measuring sustainability is to develop an operational definition of sustainability itself. Our experience here suggests that the detailed definition of sustainability emerges out of the scenarios and from the unique perspective that each represents. Nonetheless, an overarching characteristic that reappeared throughout our discussions was that sustainabil-ity concerns itself with the assurance of the well-being of current and future generations. Our recognition and response to the constraints imposed by ourselves and natural systems combine to limit the range of strategies by which we might achieve a particular degree of well-being.

Effectively managing the inevitable transitions in resource utilization depends on our ability to measure critical system characteristics related to those constraints. This approach can lead to specifi c strategies for anticipating the need for developing social, economic, and technical mechanisms to manage these transitions. Our goal is to avoid catastrophic transitions. This requires us to understand the evolution of supply systems, demand for services, technology approaches, and the full life-cycle environmental impact on land, water, air and nonrenewable mineral resources. Perhaps, most critically, we must recognize the need to develop approaches that are resonant with the world views suggested by integrated social science. A failure to find the common ground that exists at the intersection of these diverse viewpoints will almost certainly lead to suboptimal or even counterproductive responses. The premise is that if we can see a transition on the horizon, then we can take steps to mitigate its impacts. These could include alternative investment strategies, particularly in R&D, moderating economic dislocations, avoiding suboptimal, short-term supply decisions, developing mechanisms to enhance the rate of energy intensity improvement, and provide more effective feedback on the consequences of our choices of energy services.

Our experience indicates that improvements are needed in many measurement domains, including data acquisition as well as cost and impact analysis. We recognize that there may be other resource systems (e.g., human resources) and other critical constraints (e.g., restrictions on access and geopolitical concerns) that need to be taken into account as well. Geopolitical concerns, for example, include energy security (Greene 2009). The foregoing analysis highlights both the high degree of complexity and uncertainty in analyzing energy system sustainability. One source of this complexity is that the constraints imposed within the various systems interact. For example, land use practices designed to produce fuels with a reduced CO2 impact in the energy system can result in increased CO2 impacts in the land resource system.

The following recommendations are intended to assist in developing more robust strategies that address this complexity, in an effort to reduce some of the uncertainty associated with specific constraints. This list is not exhaustive and will evolve over time as implementation is attempted:

• Complete the detailed evaluation of the impact matrix: this would help identify resource constraints in the domain bounded by the three scenarios.

• Examine the reciprocal impacts on energy resulting from resource use in land, water, and nonrenewable minerals: this would help identify energy resource and CO2 (and air quality) constraints.

• Improve our understanding of how to measure the links between various energy services and well-being.

• Identify and elaborate additional resource linkages (e.g., human resources).

• Identify and elaborate additional constraint systems (e.g., access, geopolitical concerns).

Next Steps

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