The human ecology of climate change - Climate Change: Biological and Human Aspects - page 368

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1950 1970 1985 2000

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World

Less-developed Developed

Fig. 7.3 Thedeclineinhouseholdsize.AfterKeilman(2003).

instance the environmental cost is not covered by energy sales and so is external to
the price of energy. In economic terms such environmental impacts are known as
environmental externalities.
Accurately quantifying environmental externalities globally is extremely difficult.
There are two main reasons for this: assessing the cost of the impact and assessing
affluence. Assessing the cost of an impact is difficult because, for example, how does
one price the loss of a species, let alone an ecosystem? The financial value of a
species that has no known economic value is obviously going to be hard. Valuing an
ecosystem in one sense is a little easier. Ecosystems have function. For example, a
wooded valley will retain rainwater falling on it and release it slowly, so providing a
more steady water supply through the catchment. The same valley devoid of life with
just bare rock will see the water run off immediately, so scouring the lower catchment
and exposing it to periods of drought. This particular function of the ecosystem can be
part quantified in the value of the water that would have been purchased commercially
to replace that lost in the drought as well as the erosion-protection mechanisms to
prevent scouring.
This idea of ecosystem services is not purely theoretical. Furthering the above
ecosystem function example, in 1996 New York City invested between US$1 and
1.5 billion in ecosystem management in the Catskill Mountains, which provide its
main water supply. Up to the 1990s plant root systems, soil structure and its ecology
largely served to purify the water to US standards. However, sewage, fertilizer use
and pesticide use caused water standards to fall. New York either had to restore the
Catskill ecology and ecosystem function to what it had been or to build a water-
purification plant at an estimated capital cost of $6-8 billion and annual running
costs of $300 million. By comparison the ecosystem-management option was more
attractive (Chichilnisky and Heal, 1998).