Agriculture Reference
In-Depth Information
Canada. These strange phenomena were attrib-
uted to a major eruption of the Tambora volcano
in Indonesia in 1815.
parts of the world are infl uenced by ocean
currents more than others. The coast of Peru and
other adjoining regions are directly infl uenced by
the Humboldt current that fl ows along the coast-
line of Peru. The El Niño event in the Pacifi c
Ocean can affect climatic conditions all over the
world.
Another region that is strongly infl uenced by
ocean currents is the North Atlantic. If we com-
pare places at the same latitude in Europe and
North America, the effect is immediately obvi-
ous. Take a closer look at this example - some
parts of coastal Norway have an average temper-
ature of −2 °C in January and 14 °C in July, while
places at the same latitude on the Pacifi c coast of
Alaska are far colder: −15 °C in January and only
10 °C in July. The warm current along the
Norwegian coast keeps much of the Greenland-
Norwegian Sea free of ice even in winter. The rest
of the Arctic Ocean, even though it is much fur-
ther south, remains frozen.
Ocean currents have been known to change
direction or slow down. Much of the heat that
escapes from the oceans is in the form of water
vapor, the most abundant greenhouse gas on
earth. Yet, water vapor also contributes to the for-
mation of clouds, which shade the surface and
have a net cooling effect.
2.1.3
The Earth's Tilt
The earth makes one full orbit around the sun
each year. It is tilted at an angle of 23.5° to the
perpendicular plane of its orbital path. For one
half of the year when it is summer, the northern
hemisphere tilts towards the sun. In the other half
when it is winter, the earth is tilted away from the
sun. If there was no tilt, we would not have expe-
rienced seasons. Changes in the tilt of the earth
can affect the severity of the seasons - more tilt
means warmer summers and colder winters; less
tilt means cooler summers and milder winters.
The earth's orbit is somewhat elliptical, which
means that the distance between the earth and the
sun varies over the course of a year. We usually
think of the earth's axis as being fi xed, after all, it
always seems to point towards Polaris (also
known as the Polestar and the North Star).
Actually, it is not quite constant: the axis does
move, at the rate of a little more than a half-
degree each century. So Polaris has not always
been, and will not always be, the star pointing to
the North. When the pyramids were built, around
2500 BC, the pole was near the star Thuban
(Alpha Draconis). This gradual change in the
direction of the earth's axis, called precession, is
responsible for changes in the climate.
2.2
Human Causes
Climate change can also be caused by human
activities, such as the burning of fossil fuels and
the conversion of land for forestry and agriculture.
Since the beginning of the industrial revolution,
these human infl uences on the climate system
have increased substantially. In addition to other
environmental impacts, these activities change the
land surface and emit various substances to the
atmosphere. These in turn can infl uence both the
amount of incoming energy and the amount of
outgoing energy and can have both warming and
cooling effects on the climate. The dominant
product of fossil fuel combustion is carbon diox-
ide, a greenhouse gas. The overall effect of human
activities since the industrial revolution has been a
warming effect, driven primarily by emissions of
2.1.4
Ocean Currents
The oceans are a major component of the climate
system. They cover about 71 % of the earth and
absorb about twice as much of the sun's radiation
as the atmosphere or the land surface. Ocean cur-
rents move vast amounts of heat across the
planet - roughly the same amount as the atmo-
sphere does. But the oceans are surrounded by
land masses, so heat transport through the water
is through channels.
Winds push horizontally against the sea
surface and drive ocean current patterns. Certain
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