The next group of charts, Figures 3.5a-f, shows materials consumption in the US during the 20th century as measured in terms of mass and exergy in relation to economic activity (GDP). Though the exergy embodied in any given material is proportional to its mass, the mass/exergy ratio is not necessarily constant for groups of materials (for example, construction materials or fuels) due to shifts in the mix or composition of the group. Thus, Figures 3.4a and 3.5a for fossil fuels exhibit not-quite parallel curves for mass/GDP and exergy/GDP. Both curves peak in the early 1920s, and decline more or less monotonically thereafter.
The ratio E/GDP is sometimes called the Kuznets curve, although it is properly attributable to others (Schurr and Netschert 1960). It is often observed that, for many industrialized countries, the E/GDP (or E/Y) ratio appears to have a characteristic inverted-U shape, at least if E is restricted to commercial fuels. However, when the exergy embodied in firewood is included, the supposedly characteristic inverted-U shape is much less pronounced. When non-fuel and mineral resources, especially agricultural phytomass, are included, the inverted-U form is no longer evident. Figure
3.6 shows the two versions plotted from 1900 to 2004.
Similar peaks have been observed in the energy/GDP curves for a number of other countries, but at different times. The earliest peak (for the UK) was higher, while later ones for Germany, Japan, China etc. are progressively lower. This peak, followed by a declining trend, has been interpreted as a measure of relative industrialization. However, when biomass (including wood as a fuel) and other materials are included, as in Figure 3.6, the US curve did not peak after 1900. In fact, it apparently reflects a long-term substitution of commercial fuels for non-commercial biomass (fuelwood).
Similarly, comparing exergy/GDP and mass/GDP for fossil fuels (Figures 3.3a, 3.4a), it is evident that the mass/exergy ratio keeps decreasing.10 This is due to a long-term shift from coal, at the beginning of the
Figure 3.5b Major inputs to GDP of chemicals, major organic and inorganic: mass/GDP andexergy/GDP (USA, 1900-2004)
Figure 3.5d Major inputs to GDP of metals: mass/GDP and exergy/GDP (USA, 1900-2004)
• mass/GDP ratio (eft scale) exergy/GDP ratio (right scale)
Fossil fuels: coal, petroleum, natural gas, NGL Metals: iron, steel, copper, lead, zinc, aluminum
Construction: cement, gypsum, brick, lumber, sand and gravel, stone, clay Inorganic: sulfur, lime, phosphate, chlorine, ammonia Organic: ethylene, methanol, butadiene, propylene, benzene
Biomass: dry weight of harvested and non-harvested components of major non-fuel crops
Figure 3.5f Total major inputs to GDP (fuels, metals, construction, chemicals and biomass): mass/GDP and exergy/GDP (USA, 1900-2004)
century, to petroleum and increasingly to natural gas. This shift reflects the increasing hydrogen fraction of fuels being used, and it is interpreted by some as the ongoing 'hydrogenation' of the economy. A similar shift in construction materials (Figures 3.4b and 3.5b) reflects the substitution of inert (non-flammable) materials for wood. And a comparable shift towards lighter and more flammable materials (i.e. organics) can be observed in the case of chemicals (Figures 3.4c, 3.5c). On the other hand, from the charts for metals (Figures 3.4a, 3.5a), it can be seen that the shift toward lighter metals, notably aluminum, is much less pronounced.
The other noteworthy long-term trend in the data is the decline, in every group including chemicals, in consumption per unit of GDP, although the turning point occurred earlier for fuels and metals, later for construction materials and still later for chemicals. Total mass/GDP (Figure 3.4f) also tends to exhibit declines (albeit with some exceptions for specific materials during certain periods). The overall decline from 1905 to 1995 is almost exactly by a factor of three. Since 1950 the decline has been a little faster (a factor of two). This is interpretable, in part, from efficiency gains in extraction and primary processing and in part from the overall shift from products to services in the economy. Another way of saying the same thing is that GDP has increased faster than either population growth or mass or exergy consumption. This decline has sometimes been interpreted as evidence of dematerialization (for example Greenspan, cited in Cairncross
1997). However, the most important conclusion from the evidence is that the consumption of mass per capita (except for inert construction materials) is not declining significantly.
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