changes in developed economies away from manufacturing toward services. This tendency has been most pronounced in the United States, in which the energy intensity of the economy fell from about 70 megajoules (MJ) per constant dollar of gross domestic product (GDP) in the mid-19th century to about 20 MJ today (Schrattenholzer, 1998).
The second transition comprises a change in market share among the various commercial fuels; this change has favored fuels with lower ratios of carbon to hydrogen. In general, solid fuel has lost market share to liquid fuel, especially in transportation, where the greater energy density (energy per unit of volume) of the liquids offers significant advantages. More recently, the share of natural gas has grown steadily, though chiefly in stationary applications in which the lower energy density of natural gas presents no disadvantage. As an unintended consequence of this interfuel competition, the more carbonaceous fuels such as wood and coal have been superseded by less carbonaceous fuels such as oil and methane.
This substitution, together with the rise of knowledge-based industries, has caused a general reduction in the carbon intensity of the global economy—the amount of carbon released to the atmosphere per unit of primary energy—as shown in Figure 2-5. Even if no changes are made to the current energy infrastructure, this decline will probably continue into the future, driven by continued interfuel substitution and by the ongoing shift in the balance of value creation from heavy industry to a knowledge-based economy. Nevertheless, world carbon emissions continue to rise, despite this drop in carbon intensity, as economic growth outpaces business-as-usual improvements in both energy efficiency and carbon intensity (see Figure 2-6; EIA, 2003). The amount of carbon emitted varies widely around the globe, but its survival time in the lower atmosphere is sufficiently long that it is spread around by wind and becomes evenly mixed spatially across latitudes and longitudes (NRC, 2001b). The remainder of this chapter and the rest of the report, however, concentrate on hydrogen technology policies specifically for the United States.
Two public goals—environmental quality, especially the reduction of greenhouse gas emissions, and energy security—provide the policy foundation for the hydrogen programs of the DOE (DOE, 2003a). The first of these goals