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Not a steady-state concept
Does the composition of the asset base matter? In principle, this depends on
the potential for substitutability among assets (see the section titled
"The importance of a range of assets"). In the environmental economics
literature (Pearce and others 1989) a distinction is made between weak
constraints on growth, known as "weak sustainability" (which presumes that
assets are fully substitutable) and strong constraints on growth, known as
"strong sustainability" (which holds that assets are not fully substitutable
because some natural assets, or more precisely some of the functions performed
by these assets-such as global life support-cannot be replaced by others).
Limits-to-growth type arguments focus on strong sustainability, while arguments
in favor of indefinite growth focus on weak sustainability. So far the former
arguments have not been very convincing because the substitutability among
assets has been high for most inputs used in production at a small scale. There
is now, however, a growing recognition that different thresholds apply at
different scales-local to global. Technology can be expected to continue to
increase the potential substitutability among assets over time, but for many
essential environmental services-especially global life support systems-there
are no known alternatives now, and potential technological solutions cannot be
taken for granted (box 2.1)
Box 2.1
Not yet able to fully duplicate natural processes
Biosphere 2-a sealed glass ecosystem that was built in Oracle, Arizona, at a
cost of some $200 million in 1991-attempted to create a completely
self-contained, human-made system to support eight people for two years. It
could not.
There is still debate on how to conduct such an experiment. The idea was that
there would be no exchange with the outside world except for the energy
supplied to run appliances. The people inside the biosphere would grow all
their own food. And the system would operate with a fixed volume of air and
water, recycled and reused as they are on Earth, the original biosphere.
A year and a half after the sphere was sealed, the oxygen content of the
atmosphere had fallen from 21 percent to 14 percent, a level normally found at
17,500 feet and barely sufficient to keep people in the biosphere functioning.
Carbon dioxide (CO2) and nitrous oxide levels surged. All pollinators became
extinct, so agricultural production could not be sustained. Worse still, the
drop in oxygen and rise in CO2 meant that the biosphere's systems could not
replicate the carbon cycle, the most essential cycle for life.
Source: Heal (2000).
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The limits to substitutability among assets are likely to be greater for those
assets that enter consumption untransformed (for example, natural forest
scenery versus natural desert scenery) rather than as a produced output using
the same materials (for example, a wooden window shutter or a glass pane).
Ensuring that the well-being of future generations does not decline requires
maintaining sufficient levels of some assets for the future, particularly when
the drawdown or degradation entails irreversible losses and there is a
possibility that these assets matter directly for the well-being of future
generations. Of course, the mix of assets that supports improvements in human
well-being is likely to change over time, as people's preferences and
technologies change. So the concept of sustainability will itself evolve over
time.
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