Environmental Engineering Reference
In-Depth Information
nations. And all this has taken its toll on the Earth's ecosystems, the use of nonrenewable
resources, and the alteration of the Earth's biogeochemical cycles in a period of time that is a
blink of an eye when compared with the planet geological times. So the question now is: How
long do humans think this expansion will go on? One answer would be, as long as fossil fuels
and natural resources are available. However, the tremendous alteration of natural systems by
human activity may introduce constraints that will limit the business-as-usual approach well
before those resources are depleted.
THE EARTH AS A SYSTEM
With exception of some meteors that contact the Earth sporadically (thus becoming meteorites)
and aircrafts that are deployed for the purpose of exploration, Earth, for all practical purposes,
can be considered a closed system in terms of matter, or mass, with boundaries located at the
outer edge of the atmosphere. In terms of energy, on the other hand, the Earth is an open
system that receives solar radiation during the day and rejects energy to the atmosphere at
night. Within the boundaries of the planet, there are four subsystems of living and nonliving
things called spheres that work in coordination with each other. These are the lithosphere,
hydrosphere, atmosphere, and biosphere (Fig. 2.1).
The
lithosphere
is the outermost layer of the earth that includes the crust and uppermost
layer of the mantle. The lithosphere is irregular in shape and depth, but on average is about
62 miles (100 km) thick (United States Geological Survey [USGS], 2009).
●
The
hydrosphere
includes water in all three states (liquid, solid, and gas). Liquid water lies
on top of and into the lithosphere and as fog and rainfall in the atmosphere. Solid water
●
Atmosphere
Hydrosphere
Circulation
of mass and
energy
Biosphere
Lithosphere
Anthrosphere
Figure 2.1
Interactions among the four Earth spheres: hydrosphere, atmosphere, lithosphere, and
biosphere (with the anthrosphere shown as a subsystem of the biosphere).
