Environmental Engineering Reference
In-Depth Information
Fig. 1.12
Estimated
average remaining
population sizes of
various animal and
plant taxa in relation to
intensity of land use.
The index commences
at 100 for all taxa in the
most natural 'protected'
situation, and repre-
sents the situation 300
years ago. There is a
more-or-less progressive
decline in all cases. The
surprising increase in
amphibian densities
associated with urban
land use occurs because
urban areas in semiarid
landscapes of southern
Africa provide artifi cial
water-fi lled habitats
needed by frogs and
their relatives. (From
the Millennium
Ecosystem Assessment,
2005b.)
Land use type
Protected
Light
use
Degraded
Cultivated Plantation Urban
120
100
80
60
Birds
Amphibians
Reptiles
Mammals
Plants
40
20
0
More
natural
More
artificial
Land use intensity
up before they reach the ocean. In addition, excess plant nutrients fi nd their way
into waterways, and chemical pesticides affect nontarget species. All these agricul-
tural problems look set to increase over the next 50 years as more land is converted
(Figure 1.13). And because greater human population growth is expected in species-
rich tropical areas, increased agricultural activity will place biodiversity at high risk.
The challenge for managers is to keep land conversion to a minimum (needed to
support the human population) and to promote agricultural 'best practices' that
minimize ecological fallout.
1.2.8
Global climate
change - life in the
greenhouse
The most far-reaching consequence of our use of fossil fuels has been an increase
in the concentration of carbon dioxide in the atmosphere. The level in 1750 (i.e.
before the Industrial Revolution), measured in gas trapped in ice cores, was about
280 ppm (parts per million), but this rose to 320 ppm by 1965 and stands at
about 380 ppm today (Figure 1.14). It is projected to increase to 700 ppm by the
year 2100.
The Earth's atmosphere behaves rather like a greenhouse. Solar radiation warms
up the Earth's surface, which reradiates energy outward, principally as infrared
radiation. Carbon dioxide - together with other gases whose concentrations have
increased as a result of human activity (nitrous oxide, methane, ozone, chlorofl uoro-
carbons) - absorbs infrared radiation. Like the glass of a greenhouse, these gases
(and water vapor) prevent some of the radiation from escaping and keep the tem-
perature high. The air temperature at the land surface is now 0.6
0.2˚C warmer
than in pre-industrial times. Note, however, that temperature change has not been
uniform over the surface of the Earth. Up to 1997, for example, Alaska and parts of
Asia experienced rises of 1.5-2˚C, while the New York area experienced little
change, and temperatures actually fell in Greenland and the northern Pacifi c Ocean.
Given the expected further rises in greenhouse gases, temperatures are predicted to
continue to rise by a global average of between 1.8˚C and 4.0˚C by 2100 (IPCC, 2007;
Millennium Ecosystem Assessment, 2005b), but to different extents in different
places. Such changes will lead to a melting of glaciers and icecaps, a consequent rise
±
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