Environmental Engineering Reference
In-Depth Information
Chapter 7: Global Change
With an increasing number of people inhabiting this planet, the atmosphere is experiencing rapid changes,
which impacts all life. In this chapter, the focus is on global change, starting with the protective ozone layer
and followed by global warming, climate change, and the greenhouse effect. Finally, the impacts on other spe-
cies and biodiversity are presented, along with conservation efforts. Each of these components of the Earth has
been altered by human activities. Although changes in the ozone, climate, and biodiversity all occur naturally,
human involvement has increased the rate at which destruction and alteration is occurring.
Stratospheric Ozone
Formation of Stratospheric Ozone
Stratospheric ozone shields life on Earth from the sun's ultraviolet (UV) radiation. There are three forms of UV
radiation: UVA (long wavelength), UVB (medium wavelength), and UVC (short wavelength). Ozone (O
3
) is
formed in the upper stratosphere by a photochemical reaction between an existing oxygen molecule (O
2
) and
an additional oxygen atom (O), with the catalyst of ultraviolet (UV) radiation. UVC, the strongest of the three
types of UV radiation, has enough energy to
photolyze
(break apart with light) the oxygen molecule.
The UVC splits apart the oxygen molecule into atomic oxygen. O
2
+ UVC → O + O
Atomic oxygen then reacts with the oxygen molecule to form ozone. O + O
2
→ O
3
This is a reversible reaction. When UVC strikes the ozone molecule, atomic oxygen and molecular oxygen are
formed. Atomic oxygen can react with an ozone molecule to form two oxygen molecules.
The UVC splits apart the ozone molecule into atomic oxygen and the oxygen molecule.
O
3
+ UVC → O + O
2
O + O
3
→ 2 O
2
Atomic oxygen now reacts with an ozone molecule to form two oxygen molecules.
For millions of years, the formation and destruction of ozone remained balanced so that the amount of ozone in
the atmosphere stays fairly constant. It filters out much of the UV radiation reaching to the Earth from the sun.
Keeping the UV radiation relatively constant for millions of years, has allowed life to evolve on Earth that is
equipped to handle this (and only this) level of UV radiation.
The layer of the Earth's atmosphere known as the
stratosphere
extends from about 6 miles to about 31 miles
above the Earth's surface. The highest part of the stratosphere houses 97 percent to 99 percent of the atmo-
sphere's ozone molecules; thus, it is often referred to as the
ozone layer.
The remaining small percentage of
ozone in the ground-level atmospheric layer known as the
troposphere
is considered a pollutant. The majority
of ozone is formed above the tropics near the equator where the sun is almost directly overhead most of the
year. Ozone is produced above the temperate and polar regions, too, but in less volume and concentration. As
you would expect, there are strong seasonal variations in the production of ozone (more in the summer months
and less in the winter months).
Ozone does not necessarily remain in the area where it is produced. Air currents distribute the ozone away
from the tropics toward the North Pole and South Pole. Generally, winds carry ozone toward the North Pole
during the Northern Hemisphere's winter and toward the South Pole during the Southern Hemisphere's winter.
