Agriculture Reference
In-Depth Information
1.2.2
Human Causes
Factors that cause climate change can be
divided into two categories - those related to nat-
ural processes and those related to human activ-
ity. In addition to natural causes of climate
change, changes internal to the climate system,
such as variations in ocean currents or atmo-
spheric circulation, can also infl uence the climate
for short periods of time. This natural internal
climate variability is superimposed on the long-
term forced climate change.
Climate change can also be caused by human
activities, such as the burning of fossil fuels and
the conversion of land for forestry and agricul-
ture. Since the beginning of the industrial revolu-
tion, 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 sub-
stances 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 dioxide, a greenhouse gas. The overall
effect of human activities since the industrial
revolution has been a warming effect, driven pri-
marily by emissions of carbon dioxide and
enhanced by emissions of other GHGs.
The buildup of GHGs in the atmosphere has
led to an enhancement of the natural greenhouse
effect. It is this human-induced enhancement of
the greenhouse effect that is of concern because
ongoing emissions of GHGs have the potential to
warm the planet to levels that have never been
experienced in the history of human civilization.
Such climate change could have far-reaching
and/or unpredictable environmental, social, and
economic consequences.
1.2.1
Natural Causes
The earth's climate can be affected by natural fac-
tors that are external to the climate system, such as
changes in volcanic activity, solar output, and the
earth's orbit around the sun. Of these, the two fac-
tors relevant on timescales of contemporary cli-
mate change are changes in volcanic activity and
changes in solar radiation. In terms of the earth's
energy balance, these factors primarily infl uence
the amount of incoming energy. Volcanic eruptions
are episodic and have relatively short-term effects
on climate. Changes in solar irradiance have con-
tributed to climate trends over the past century, but
since the industrial revolution, the effect of addi-
tions of GHGs to the atmosphere has been about
ten times that of changes in the sun's output.
Solar radiation is the driving force of global
climate. A portion of the radiation reaching the
earth's surface is scattered or refl ected by clouds,
aerosols, dust, and other particles. Radiation
reaching the planet is partly absorbed, causing
the earth to emit thermal radiation, and part of
the radiation is refl ected back to the atmosphere.
Water vapor and radiatively active CO 2 , CH 4 ,
N 2 O, and O 3 , among others, partly trap the
refl ected radiation to warm the surface tempera-
ture from a frigid 18 °C to about 15 °C, a natural
phenomenon known as the “greenhouse effect.”
Human activities have contributed to an increase
in the concentration of radiatively active gases
and added new GHGs such as halocarbons (like
chlorofl uorocarbons) and hexafl uoride (IPCC
1997 ). Together with changes in land cover, this
may have contributed to an enhanced greenhouse
effect to cause global warming and other cli-
matic changes.
1.2.3
Biggest Threats of Climate
Change
If the projected 2 °C rise in average temperatures
comes to pass, then:
• Southern Europe may become too hot and arid
to grow its present crops.
• Northern Europe will be the best place to grow
typically Mediterranean crops.
• Scandinavia and Scotland may be the prime
wine-producing areas.
• Much of Siberia will be a major cereal-
growing area.
• Southern Africa could lose up to 30 % of its
main staple crop - maize - by 2030.
Yields for rainfed agriculture could be reduced
by up to 50 % by 2020 (IPCC AR4).
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