Environmental Engineering Reference
In-Depth Information
7.1
Natural Threats
This effect is known as the precession of the
equinox. The precession of the equinox has a
cycle of approximately 26,000 years. According
to illustration (A), the Earth is closer to the Sun
in January (perihelion) and farther away in July
(aphelion) at the present time. Because of pre-
cession, the reverse will be true in 13,000 years
and the Earth will then be closer to the Sun in
July. This means, of course, that if everything
else remains constant, 13,000 years from now
seasonal variations in the Northern Hemisphere
should be greater than at present (colder winters
and warmer summers) because of
The natural factors that affect the process of
erosion and accretion of habitats that sustain
coastal vegetation as a whole may be broadly
divided into
ve heads.
A. Changes in the Earth
s orbit
B. Alteration of atmospheric carbon dioxide
C. Volcanic activities
D. Variations in solar output
E. Plate tectonics
F. Natural disasters and extreme weather events.
A. Changes in the Earth
'
the closer
s Orbit
How it is possible that variation in the Earth
'
proximity of the Earth to the Sun.
The third cyclical variation is related to the
changes in the tilt (obliquity) of the Earth
s
orbit regulates the distribution and growth of
coastal vegetation? This is a general question that
will peep out from the mind of readers. The
answer is simple. The variation in the Earth
'
s axis
of rotation over a 41,000 years period. During the
41,000 year cycle,
'
the tilt can deviate from
s
orbit, which affects the intensity of solar energy
reaching the surface of the Earth, regulates the
vegetation pattern. It has been documented that
regions with considerable solar energy coincide
with higher
'
approximately 22.5
-
24.5
°
. At the present time,
the tilt of the Earth
. When the tilt
is small, there is less climatic variation between
the summer and winter seasons in the middle and
high latitudes. Winters tend to be milder and
summers cooler. Warmer winters allow for more
snow to fall in the high-latitude regions. When
the atmosphere is warmer, it has a greater ability
to hold water vapour and therefore more snow is
produced at areas of frontal or orographic uplift.
Cooler summers cause snow and ice to accu-
mulate on the Earth
'
s axis is 23.5
°
oral diversity.
There are several factors that regulate the
alteration of the Earth
fl
rst cyclical
variation, known as eccentricity, controls the
shape of the Earth
'
s orbit. The
s orbit around the Sun. The
orbit gradually changes from being elliptical to
being nearly circular and then back to elliptical in
a period of about 100,000 years. The greater the
eccentricity of the orbit (i.e. the more elliptical it
is), the greater is the variation in solar energy
received at the top of the atmosphere between the
Earth
'
s surface because less of this
frozen water is melted. Thus, the net effect of a
smaller tilt would be more extensive formation of
glaciers in the polar latitudes.
Periods of a larger tilt result in greater sea-
sonal climatic variation in the middle and high
latitudes. At these times, winters tend to be
colder and summers are warmer. Colder winters
produce more snow because of lower atmo-
spheric temperatures. As a result, more snow and
ice accumulates on the ground surface. More-
over, the warmer summers produced by the lar-
ger tilt provide additional energy to melt and
evaporate the snow that fall and accumulate
during the winter months. In conclusion, glaciers
in the Polar Regions should be generally reced-
ing, with other contributing factors constant,
during this part of the obliquity cycle.
'
s closest (perihelion) and farthest (aphelion)
approach to the Sun. Currently, the Earth is
experiencing a period of low eccentricity. The
difference in the Earth
'
'
s distance from the Sun
between perihelion and aphelion (which is only
about 3 %) is responsible for approximately a 7 %
variation in the amount of solar energy received at
the top of the atmosphere. When the difference in
this distance is at its maximum (9 %), the differ-
ence in solar energy received is about 20 %.
The second cyclical variation results from the
fact that, as the Earth rotates on its polar axis, it
wobbles like a spinning top changing the orbital
timing of the equinoxes and solstices (Fig.
7.1
).
