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NASA |
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Air moving from a high pressure zone around
the poles to lower pressures at the equator would move in
a straight line if there were no rotation. But because of
the rotation, the air moves to the right of its straight path
as it moves equatorward.
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Two important things determine the Earth's climate. One is the amount
of heat the sun delivers to the Earth. Also important is where the Earth's
continents are located relative to the equator. Continental ice sheets
cannot develop unless one or more continents are located at high latitudes.
The Earth revolves around the sun once a year. The orbit is almost a
circle. If the Earth were the only planet, its orbit around the sun would
be almost unchanging. The other planets exert a pull on the Earth. Although
the pull is small, it makes the Earth's orbit much more complicated. The
orbit changes slightly in several different ways. The changes occur over
periods that range from about twenty thousand years to about a hundred
thousand years. These changes cause slight differences in how much of
the sun's heat the Earth receives, in winter vs. summer and at high latitudes
vs. low latitudes.
The astronomical theory of the ice ages holds that the small changes
in the Earth's orbit trigger the advance and retreat of ice sheets. Most
scientists now accept this theory. The details of how the changes trigger
the ice sheets are still only partly understood. For example, the very
fast melting of the ice sheets, compared to the long times needed for
them to form, is still a mystery.
The astronomical theory is only part of the story. Climate is known to
change on time scales as short as centuries, and the cause (or causes)
of these changes are still not clear. Here's a big question, and an important
one. Has the increase in temperature since the beginning of the twentieth
century been caused by human activity, or is it just another natural upward
"spike" like several during the past two millennia? Most climatologists
think that the upward trend in temperature during the twentieth century
is at least partly caused by human activity.
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| © Michael
Collier |
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This view of windmills in the Tehachapi,
California area shows one of the world's leading wind energy
producers. They offset 1.1 billion pounds of greenhouse gases
each year.
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Several gases in the Earth's atmosphere are called greenhouse gases.
The most important is water vapor, but carbon dioxide is important also.
Carbon dioxide has always been part of the Earth's atmosphere, but it
has been increasing more and more rapidly in recent times. Coal, oil,
and natural gas are called fossil fuels, because they come from plant
and animal material that was buried in the Earth's sediments. When they
are burned, they add carbon dioxide gas to the atmosphere. Along with
the other greenhouse gases, carbon dioxide absorbs some of the heat that
the Earth's surface sends out to space, and then sends it back to the
surface. That increases the Earth's average surface temperature. The effect
is the same as with the glass of a greenhouse, although the process itself
is not exactly the same.
Several groups of climatologists have been developing computer models
of the Earth's climate. They try to build in all of the important processes
of climate into the model. Then they start the model with the present
climate and let it run to see what the future climate will be. The models
are not perfect, because it is very difficult to build in some of the
most important influences on climate. The behavior of clouds is especially
tricky to model. The models have one thing in common, though. They predict
that the Earth's surface temperature is likely to increase by as much
as two degrees Celsius between 2000 and 2050 if the increase in atmospheric
carbon dioxide continues. A rise of two degrees would make the Earth's
temperature much warmer than during even the warmest periods in human
history.
If the predictions about global warming come true, many things about
the Earth's climate, aside from just temperature, are likely to change.
Some regions will get more rainfall, and other regions will get less.
The frequency and strength of severe storms are likely to increase. As
the world's glaciers continue to melt, sea level around the world will
rise, by as much as half a meter to a meter. That might sound like a lot
to you, but think of the flooding that it would cause in coastal cities
around the world! Will the predictions of the computer models come true?
That is likely, although not certain. All that science can do is try to
make likely predictions. How to act upon the predictions is for human
society to decide.
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