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GRAPEGROWING
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heat-trapping or "greenhouse" gases such as
carbon dioxide (CO
2 ) in the Earth's
atmosphere naturally absorb or trap
some of it and radiate it back toward
Earth, raising the temperature the way
a radiator warms a room. This "natural
greenhouse effect" is a good thing
for life on Earth. Without the atmosphere,
the average temperature of the
planet would be a frigid -18°C (0°F),
instead of the actual average of 15°C
(59°F).22
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WHAT CAUSES CLIMATE CHANGE?
Over the billions of years of
Earth's history, it has experienced a
wide range of natural climate changes.
Over the lastmillion years, a dominant
driver has been subtle fluctuations in
the shape of Earth's orbit, which have
resulted in a cyclical progression of ice
ages. The last ice age ended about
18,000 years ago.
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Since the Industrial Revolution,
however, human activity has become a
global-scale "force of nature," especially
by adding greenhouse gases to
the atmosphere faster than they can be
removed by natural processes. This
"enhanced greenhouse effect" is
increasing the temperature of the
atmosphere, much as pulling on a second
blanket on a cool night absorbs
more of the heat from your body and
radiates it back to you.
The role of greenhouse gases in
affecting the earth's climate has been
understood for a long time. A Swedish
scientist, Svante Arrhenius, published
a paper in 1896 demonstrating that
CO 2 from burning coal could warm
the atmosphere.1 He used very simple
calculations to show that increasing
the concentration of atmospheric CO2
by 2 to 2.5 times could cause a warming
of 3.4°C (6.1°F), a number close to
the middle of the current range of projections
(see "What are the projections
for the future?"). The accuracy of this
calculation is partly a result of lucky
assumptions, but it is also partly a
demonstration of the relatively simple
nature of the basic physics of the
greenhouse effect.
We have records of the amount of
CO 2 in the atmosphere over the last
600,000 years. Atmospheric CO2 has
been carefully and continuously measured
at Mauna Loa, Hawaii since
1957. Before 1957, we can infer atmospheric
CO2 from bubbles of air
trapped in continental ice sheets on
Greenland and Antarctica.
Before the Industrial Revolution,
CO2 levels in the atmosphere were
around 285 parts per million (ppm).
When the Mauna Loa measurements
began, the levels were around 315
ppm. Today they are over 380 ppm, a
33% increase over pre-industrial levels,
and they are increasing about 2
ppm per year.16 There is no doubt that
the increase in atmospheric CO 2 since
the start of the Industrial Revolution is
due to human activities.
Where does this human-induced
CO
2 come from? In the early decades
of the 20th century, most of the CO2
from human activities came from the
clearing of forests.
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Figure I. Relative contributions of greenhouse gases in the atmosphere from nine regions (EU
= European Union; D1 = other developed countries such as Canada; FSU = Former Soviet
Union; D2 = developing countries such as Brazil and Mexico; D3 = least developed countries,
such as most of sub-Saharan Africa). Cumul = cumulative emissions since the start of the
industrial revolution to 2004; flux = average emissions from 2000-2004; growth = five-year
growth rate in emissions from 2000 to 2004; Pop = current population.
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