(Note: The figures are at the end of this web document for easier online reading).
Industrial
ecology studies the totality of material relations among different industries,
their products, and the environment. Applications of industrial ecology should
prevent pollution, reduce waste, and encourage reuse and recycling of
materials. By displaying trends, scales, and relations of materials consumed,
emitted, dissipated, and discarded, metrics can expose opportunities to improve
the performance of industrial ecosystems.
Metrics
can indicate environmental performance at all levels: factory, firm, sector,
nation, and globe. National metrics focus attention on collective behavior,
particularly in a large country such as the United States whose economy sums
the actions of more than 250 million people and 3 million for profit
corporations. The federal government assembles national data on a vast array of
activities. The need is for a coherent set of metrics that enables efficient
diagnosis of national environmental conditions and provides help in considering
strategies for the future.
The
need to develop environmental metrics is particularly strong for materials.
National materials consumption indicates the structure of national industrial
activity and its extent. Environmentally important industries such as mining,
forestry, agriculture, construction, and energy production can be evaluated
based on their material requirements and outputs. Despite their ubiquity and
close association with environmental quality, materials have received little
systematic analysis, particularly as compared with energy. This inattention
stems in part from the heterogeneity of materials used in the modern economy
and the myriad enterprises involved in transforming, processing, and disposing
of materials and goods.
With the help of the Bureau of Mines, we have developed an
environmentally oriented framework for characterizing material flows in
the United States. 1 Choosing metrics requires a grasp of the
diversity and enormity of U.S. materials flows (Figure 1). Our framework
considers primarily three components: inputs to the economy (including
imports), outputs (including exports), and extractive wastes. We aim for
comprehensiveness in this framework in the sense that we do not want to
"lose" materials and would eventually hope to record the complete
materials balance. Our choice of inputs and outputs as major categories
derives from the simplest of materials-flow models. We group extractive
wastes separately because they represent immense mobilizations of
materials readily distinguished from commodities, products, and other
wastes. We use previously published data for all the values indicated
and generally adhere to existing classifications.
We
segment inputs into energy, construction minerals, industrial minerals, metals,
forestry products, and agricultural products. We class outputs as domestic stock,
2
atmospheric
emissions, other wastes, dissipation, and recycled materials. Imports and
exports represent the masses of major individual commodities and classes of
commodities crossing U.S. borders. Extractive wastes include residues from the
mining and oil and gas industries. We account for water in Figure I but not in
the material metrics because the weight and omnipresence of this resource would
obscure what remains. We also omit consumption of atmospheric oxygen for
biological respiration and in industrial processes.
3
We do not explicitly consider manufactured chemical products, but do include
the mass of feed stocks used for organic and inorganic chemical production.
Materials
have the advantage of offering a single unit of measure, weight, that allows
for direct comparison across a broad range of material types. Kilograms and
tons can hide variables such as volume, land disturbance, toxicity,
4
and other environmentally important qualities associated with materials that
weight measures do not reflect. Nevertheless, weight does provide a reasonable
starting point for appreciating the structure and scale of major activities
affecting national environmental quality.
National
material metrics do not obviate the need for monitoring environmental variables
locally. Rather, they complement smaller scale metrics that underscore the
spatial distribution of problems and needs. In this respect, they resemble
national economic indicators, such as gross domestic product (GDP). In
addition, national materials metrics offer the prospect of capturing
environmentally significant trends and relations not captured in the current
regulatory framework, which tends to emphasize reporting by media, especially
air and water, rather than along the functioning of the economic system.
We
propose eight general classes of metrics to indicate the current status and
salient trends in national materials use as they influence environmental
performance (Table 1). Most address either the productivity or the efficiency
of resource use. Others indicate trends in the size and composition of
materials use. Some metrics offer a means for quantifying aggregate
environmental changes resulting from current national activities. Although some
of the metrics are novel, others are already employed but gain meaning from the
more systematic context. Although imperfect, this initial classification is
intended to stimulate subsequent inquiry into the development of material
metrics and the logic sustaining them.
In
1990, each American mobilized on average about 20 metric tons of materials, or
over 50 kg/day. The breakdown in Figure 2 equates with Figure I on national
flows at the level of the individual American. This sum may be similar in other
industrial nations. For example, estimates of Japanese materials use in 1990
total 52 kg per capita per day, a number closely comparable to the U.S.
estimate (Gotoh, 1997).
The
dynamics of per capita resource use as well as the efficacy of various policy
initiatives aimed at affecting it could be gauged by comparing this number over
time and across nations. More detailed metrics would took at consumption of
classes of materials, such as energy fuels or agricultural minerals, and
environmentally significant individual materials, such as lead.
Composition
of Material Inputs to the National Economy
With
economic development and technical change, the demand for materials evolves.
Input composition reveals economic structure and dynamics and helps anticipate
environmental consequences.
For
example, environmental import attaches to the evolving ratio of the three
fossil fuels used for energy, coal, oil, and gas, or in more elemental terms to
the balance of hydrogen and carbon used to power and heat the nation
(Marchetti, 1989; Nakicenovic, 1996). Although not used for energy,
nonrenewable organic materials derived from petroleum and natural gas such as
petrochemicals, plastics, asphalt, fibers, and lubricants comprise an
appreciable fraction, about 6 percent, of total hydrocarbon consumption (Bureau
of Mines, 1991a). The endpoints for these materials matter environmentally and
as such merit their own distinct measure as a fraction of all hydrocarbon
consumption.
The
choice of structural materials indicates trends relevant to national
environmental performance as well. Demand for properties in industrial and
consumer goods influences selection among the major classes of structural
materials: metals, ceramics and glasses, and polymeric materials including wood
(Ashby, 1979). These materials range widely in their ability to bear loads,
resist fracture, and operate in harsh thermal conditions. They also differ in
typical densities (Figure 3). Similarly, they possess varying environmental
attributes such as the energy needed, waste generated, and toxins released to
the environment during extraction and processing. Comparing the energy needs
for processing an equal mass of aluminum, steel, cement, and polystyrene yields
an approximate ratio of 85:10:2:1 (Agarwal, 1990; Hocking, 1991). Of course,
materials rarely substitute for one another in products in a 1:1 mass ratio.
Historically,
substantial scientific and engineering effort has been directed at improving
the properties of metal alloys. Future gains may come in the area of polymers
stiffened in the direction of loading, ceramics toughened to resist fracture,
and composite materials designed to accentuate the best qualities (i.e., light,
strong, and tough) of each material class. Although advanced materials may be
difficult to reprocess, recyclability is not the single measure of
environmental friendliness. This property must be weighed against gains derived
from shifting to materials that perform functions using less mass, require less
energy to process, and generate less incidental waste.
The
composition of the food we consume, directly or indirectly, impacts the
environment. Reduced national meat consumption accompanied by a rise in fruit,
grain, and vegetable consumption diminishes the acreage used for grazing and
feed in favor of less land-extensive crops. Cultivation of legumes and rice
affects nitrogen fixation rates and atmospheric methane concentrations,
respectively. Fertilizer and pesticide use rates are tailored to specific
crops. In this case as with the others, input composition metrics clarify the
environmental dimension of varying the mix of materials society consumes and
shed light on paths for future development.
Intensities
of Use
Intensity-of-use
metrics show the evolution of individual materials used in the national economy
by indexing primary, as well as finished, materials to GDP (Figure 4; also, see
Malenbaum, 1978). These measures inform policy choices relating to natural
resources by helping to gauge developmental status and to define realistic
goals that integrate economic growth and improved environmental quality. In the
energy sector, the declining intensity of carbon use, "decarbonization," of the
U.S. economy relative to economic activity as well as energy use has been well
established (Figure 5).
Intensity-of-use
metrics also can show physical resource efficiency. For example, in 1990, the
ratio of agricultural produce (e.g., grain, hay, fruit, and vegetables) to
fertilizer inputs (e.g., nitrogen compounds and phosphates) was roughly 10:1
(Bureau of Mines, 1991b; United States Department of Agriculture, 1992). The
ratio of food actually consumed by humans to mineral inputs is considerably
lower. Other sectors using raw inputs as well as auxiliary materials for
production (e.g., iron ore, coke, and lime for steel; wood and chemicals for
paper) could apply similar environmental performance measures.
"Virginity”
and Recycling Indices
A
virginity, or raw materials, index measures the ratio of national raw materials
use to total national inputs. It monitors the distance a society must go to
stop extracting materials from the earth and sustain itself through its
above-ground materials endowment and recycling. For 1990, recycled material
accounted for about 5 percent of all inputs to the U.S. economy by weight
(Rogich, 1993). Impeding the increase of this fraction are the heterogeneity of
materials in the waste stream, industrial demand for materials with highly
specific properties, and cumbersome regulations. These factors combine to
shrink the pool of resources that can be used as inputs to production (Frosch,
1994; Wernick, 1994).
Among
specific materials of interest are metals and wood. The fraction of secondary
to total metals consumption indicates both the efficiency of metals reuse from
new scrap generated within industry and the success in recycling old scrap
recovered from obsolete products such as automobiles. Recycling today accounts
for over half the metals consumed in the United States (Figure 6; Rogich,
1993). However, recovery remains below 10 percent for arsenic, barium,
chromium, and other biologically harmful metals listed in the Toxic Release
Inventory (Allen and Behmanesh, 1994). The difference between annual forest
growth and removal of growing stocks offers a simple measure of incremental
changes in forest volume.
5
For
the period 1970-1991, U.S. forests gained an average of over 150 million cubic
meters of timber annually, augmenting existing timber volume at an annual rate
of about 0.7 percent (United States Department of Agriculture, 1992).
Waste
(Emission) Intensities
Waste
intensities measure residuals and emissions per unit of output in physical or
economic terms. Corporate practice increasingly evaluates the ratio of wastes
to total firm output, including products and salable by-products (3M
Corporation, 1991) and seeks uses for wastes (Ahmed, 1993; Edwards, 1993) as
efficiency measures. National indicators would assess "green" productivity by
evaluating the amount of materials considered as waste against various output
categories. Figure 7 shows long-term trends of U.S. municipal solid-waste (MSW)
generation, sulfur dioxide emissions, and emissions of nitrogen oxides indexed
to economic activity. Industrial wastes are strong candidates for analysis
using this metric. However, dry weight data on industrial wastes rarely exist
or are hard to obtain (United States Congress, Office of Technology Assessment,
1992).
Leak
Indices
Leak
indices measure the ratio of outputs emitted and dissipated to total outputs,
thereby quantifying the proportion of materials lost to further productive use
and dispersed into the environment. Applying this measure allows for easier
identification and isolation of "holes" in the system and focuses efforts to
plug them.
Geographical
information on nutrient and heavy-metals loadings aids improvement of accounts
of dissipated materials. National efforts in this area are well established
but incomplete. The National Oceanic and Atmospheric Administration (1993)
estimates coastal discharges of nutrients (nitrogen, phosphorus), heavy metals
(e.g., arsenic, lead, cadmium), and petroleum hydrocarbons in U.S. estuaries in
the National Coastal Pollution Discharges Inventory. Estimates of inland
nutrient discharges and metals deposition rates are sparse at best. Extending
these measures to the entire nation would be laborious but worthwhile from the
perspective of national environmental management.
Environmental
Trade Index
An
environmental trade index indicates the degree to which the nation is retaining
or displacing pollution through international trade. Exporting raw materials
consumes national resources and scars the domestic landscape. Using domestic
industry to convert imported materials into finished goods and prepare
indigenous materials for export can damage the environment in other ways.
Despite intense interest in the monetary balance of U.S. foreign trade, the
environmental profile of trade flows has received scant attention until
recently, in the context of trade with Mexico.
By
weight, commodities dominate trade. The mass of manufactured products traded
contributes little to the total but may be responsible for domestic waste
generation and discharges to the environment. During 1990, exports were
dominated by agricultural products (33 percent), coal (23 percent), and
chemicals (10 percent), all goods associated with domestic pollution. In the
same year, crude oil and petroleum products accounted for over 60 percent of
U.S. imports by weight, with metals and minerals accounting for another 20
percent (Bureau of the Census, 1993). We lack ready means to assess how the
spatial redistribution of economic functions would affect environmental quality.
Extractive
Waste Ratios
Extractive
waste ratios measure resource efficiency in the mining industry. Recalling
Figure 1 confirms the massiveness of wastes generated in this sector. Rock
removed to expose mineral and ore bodies accounts for most of this waste. This
material may be harmless, but exposing raw earth to wind and water can raise
local acidity levels and allows for transport of trace elements. The sheer
amounts of materials mobilized in mining and the economic incentive to minimize
wastes combine with environmental objectives to advocate metrics of efficiency.
Geological characteristics primarily determine overburden and tailings
generated, but judgmental variables also affect mine wastes. One measure,
subject to some physical constraints, is the amount of mine wastes per ton of
mineral or ore mined, or primary metal produced. A separate useful measure,
already used at the company level, looks at other inputs such as water and
energy use per ton of finished product (Chiaro and Joklik, 1997). Measures of
the recovery of by-products (e.g., methane in coal seams, sulfuric acid from
smelter emissions, and metals from flue dusts) provide further examples of
environmental indicators for the mining and mineral processing sector.
DISCUSSION
Industry
operates and people behave within a system that evolves to satisfy human wants
and uses a dynamic set of means to achieve them. As a discipline, industrial
ecology discourages reducing the system to components and examining them in
strict isolation. The challenge for national material metrics, as well as other
national environmental metrics, is to quantify and integrate relevant data that
elucidate the primary structure and development of the system from an
environmental perspective.
National
material metrics rely on empirical data. Various agencies of the federal
government collect relevant data for one purpose or another. However, unless
coordinated, the data do not fully support existing metrics and limit the scope
for future ones. Procedural changes aimed at synchronizing data collection
among various federal departments and agencies to build a single base (year)
would amplify the benefits of existing collection efforts. Equally important
from an environmental perspective is the development of standardized
definitions for classifying material commodities to erase confusion leading to
omissions and double counting of material components.
Accurate
data on wastes are the hardest to obtain. Companies collect little or no data
for many waste streams due to the actual or perceived absence of economic
value. High disposal costs and regulatory requirements have improved waste
accounting practices at many firms, but wastes have yet to receive the respect
that marketability confers. Among the main goals of industrial ecology is
exploring potential markets for waste materials. Currently, the dearth of
reliable information available for wastes is one of the factors blocking
progress. Better information would improve the market climate for wastes and at
the same time help to develop metrics that assess their relative impact
nationally.
Although
improved national environmental metrics go hand in hand with better databases,
metrics are not meant simply to compile information. Their purpose is to embed
the data in a context that recognizes the larger system and is relevant to how
it works. Good environmental indicators exist, but too often remain detached
from each other and from an unambiguous framework. Appropriate metrics should
correlate individual indicators and clarify the relation of each one to the
whole. To illustrate, citing fertilizer usage rates without reference to
agricultural productivity is misleading and causes unwarranted alarm.
Conversely, extolling the environmental virtue of a lighter consumer product
without examining the life-cycle implications of its fabrication and disposal
is premature. To enhance their value and minimize misuse, commentary and
interpretation should accompany the publication of metrics.
To
adequately respond to complex questions of environmental performance requires
both context and an array of metrics. For example, is the nation beginning to
"dematerialize," that is, effectively decouple overall materials consumption
from continued economic growth? For the U.S. energy sector the answer has been
in the affirmative. Efficiency gains and the shift away from heavy
manufacturing have modified the traditional relation between energy consumption
and economic growth in the United States. Single indicators (i.e., kilowatt
hours consumed/$GDP) elegantly illustrate this development. To have similar
confidence regarding materials will require a more elaborate set of measures
that are sensitive to the diverse structure of contemporary materials use and
the many forces affecting its dynamics (Wernick et al., 1996). National
materials metrics would refine how such questions are articulated and provide
the basis for more convincing answers than are now available.
Looking
to the future, national materials metrics help order the national research
agenda for materials science and engineering (National Academy of Sciences,
1989). At over 50 kg per day per American, even the rough profile developed
here demonstrates the need for meshing environmental and materials research.
Metrics highlight the locations and relative urgency of incorporating
environmental goals into materials research programs. Significantly, these
goals often overlap with factors affecting the bottom line such as reducing
inputs, improving efficiency, recycling, and complying with environmental
regulations.
Future
materials fluxes, including both products and by-products, may even exceed
contemporary ones in size. To make them environmentally compatible, we need
better methods for analyzing their current condition and anticipating future
changes. To achieve the goal of a more circular economy, society needs to
consider its materials legacy as a dowry to future generations, rich in
valuable ore. By capitalizing on the "mines above ground" or scrap piles for
materials, wastes from extraction and disposal grow dispensable. We can imagine
an industrial ecosystem in which emissions, including carbon and water vapor,
are captured and complex waste streams are separated to recover the value and
utility of their components. The discipline of creating national materials
metrics is a useful start to creating a consistent, realistic long-range
technical vision.
ACKNOWLEDGMENTS
We
are indebted to Donald Rogich, Jim Lemons, and Grecia Matos at the Bureau of
Mines for data and ideas on materials taxonomy.
FIGURES
FIGURE 1 U.S. materials flows, circa 1990. All values are in million
metric tons per year. Consumptive water use is defined as water that has
been evaporated, transpired, or incorporated into products and plant or
animal tissue and is therefore unavailable for immediate reuse. For a
detailed description of this figure and data sources see Wernick and
Ausubel (1995).
FIGURE 2 Per capita material flows, United States, circa 1990. All
values are in kilograms per day. See caption for Figure 1 for further
explanation.
FIGURE 3 Range of physical properties for structural materials.
Young’s modulus is a measure of material elasticity. Toughness is
a measure of resistance to fracture. Toughness is measured in units of
joules per square meter of fracture surface (G) and is here normalized
to Young’s modulus (E) times atomic size (a). SOURCE: After Ashby
(1979). Other sources include Carter and Paul (1991) and Hodgman
(1962).
FIGURE
4
Materials
intensity of use in the United States, 1900-1990. This metric conveys the
evolving materials requirements of an economy over time. Consumption data are
indexed to annual GDP in constant 1982 dollars. (For example, in 1900, U.S.
phosphate consumption was 1,515,425 metric tons and gross national product was
$261.5 billion, equivalent to about 5.8 metric tons per million dollars GDP. In
1990, 4,692,919 metric tons of phosphate were consumed and GDP was $4,120
billion, equivalent to about 11.2 metric tons per million dollars GDP.) All
intensity-of-use values are normalized to unity at 1940 with the exception of
plastics, which is indexed to 1942. SOURCES:
Modern
Plastics Magazine
(1960);
Bureau of the Census (1975, 1992). Data on U.S. production of plastics resin
are from Broyhill, Statistics Department, Society of the Plastics Industry,
Washington, D.C., personal communication, August 20, 1993.
FIGURE 5 Diminishing carbon intensity of per capita GDP in the United
States, 1800-1988. Carbon intensity is carbon consumed for energy
divided by annual GDP in constant 1985 dollars. SOURCE: After Gruebler
and Fujii (1991).
FIGURE 6 Ratio of secondary to primary metal consumption, United
States, 1962-1991. SOURCE: Rogich (1993).
FIGURE 7 Waste intensities in the United States, 1940-1990. Municipal
solid-waste (MSW) discards, and sulfur dioxide and nitrogen oxide
emissions, indexed to GDP in constant 1987 dollars. SOURCES: Bureau of the
Census (1975, 1994).
NOTES
- In this paper we draw on other work by the authors (Wernick
and Ausubel, 1995) that contains detailed data supporting the metrics
presented here.
-
Domestic
stock refers to materials embedded in structures and products not discarded for
a period longer than 1 year.
- We include atmospheric nitrogen fixed into NOx emissions as well as for ammonia
production. We omit estimates of the mass of soil eroded during agricultural
operations.
- A clear example of this is annual total U.S. dioxin and furan emissions, which
are counted in kilograms rather than tons, yet have considerable environmental
impact (Thomas and Spiro, 1995).
- A complete net carbon balance for forests includes annual carbon flows in trees,
soil, forest floor, and understory vegetation. Since 1952, the amount of carbon
stored in U.S. forests has grown 38 percent, adding about 9 billion metric tons
of carbon (Birdsey et al., 1993).
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URL: http://phe.rockefeller.edu/NatMatMetIndusEcol/
