Climate of extremes - Antarctica: Global Science from a Frozen Continent - page 105

in the Antarctic, approximately 80% will be re ected back to space. This albedo

effect then further enhances the energy, and thus temperature contrasts between
the polar regions and the tropics.
There are seasonal variations in the amount of solar radiation reaching the top
of the atmosphere. In the tropics, again as a result of the geometry of the Earth

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orbit around the Sun, there is little variation in the amount of solar radiation
entering the atmosphere from one season to the next. As a result the temperature
in the tropics remains relatively constant throughout the year. Contrast that
with a location such as the South Pole. Here solar radiation enters the top of the
atmosphere continuously for 6months of the year followed by 6months when no
solar radiation is received. This large difference in the amount of solar radiation
entering the polar atmosphere between summer and winter results in a large
seasonal temperature range.
In looking at the net energy gain or loss from the atmosphere we must also
consider the longwave radiation lost from the top of the atmosphere. The amount
of longwave radiation lost is determined by the temperature of the atmosphere,
with warmer locations emitting more longwave radiation than cooler locations, so
that more is lost in the tropics than in the polar regions. The difference in the
amount of longwave radiation leaving the atmosphere between the tropics and the
poles is much smaller than the difference in the amount of solar radiation
entering the atmosphere between the tropics and the poles, so the differences
in solar radiation dominate the energy differences between the poles and the
tropics.
Considering these gains and losses we find that there is a net gain of energy in

the tropics, which extends to approximately 37ºN and 37ºS. Poleward of these
locations there is a net loss of energy by radiation at the top of the atmosphere.
It therefore comes as no surprise that the tropics are warm and the polar regions
are cold, but the gain of energy in lower latitudes year after year should result in
these regions getting progressively warmer every year, as more and more energy
enters this part of the atmosphere. Similarly, the high latitudes should become
colder and colder with each passing year, as more and more energy is lost from
these areas. But this is not what is observed, so what then can explain this
apparent contradiction?
Winds in the atmosphere and currents in the ocean act to transport energy
from warm regions to cold regions. In effect the circulation of the atmosphere
through winds and storms and of the oceans by currents counters the imbalance
in the radiation budget at the top of the atmosphere. These atmospheric and
oceanic circulations exist because of the temperature gradient between the tropics
and the poles, and ultimately work to remove this temperature difference. This
temperature difference never disappears, as the radiation imbalance always drives