Geoscience Reference
In-Depth Information
ar variations as well. There is a regular solar cycle of 11 years which sees a rise and fall
in sunspot activity, in turn reflecting the cycle of solar magnetic activity which produces
storms and the solar wind. Other Sun-like stars seem to spend about a third of their time
free of sunspots, a state called a Maunder minimum. That happened to our Sun between
1645 and 1715
AD
. Solar power only dropped by about 0.5%, but this was enough to plunge
northern Europe into what has become known as the Little Ice Age, with a series of partic-
ularly severe winters. The River Thames in London froze over, and markets and frost fairs
were held on it.
Hot air
The Sun distributes its warmth unevenly, warming up equatorial regions the most. As the
air warms it tries to expand, increasing atmospheric pressure. To try to restore equilibrium,
winds begin to blow and the air circulates. Whilst all this goes on, the Earth continues to
rotate, giving the air angular momentum. That is greatest at the equator and results in the
so-called Coriolis effect. The atmosphere is not firmly coupled to the solid planet, so, as
winds blow away from the equator, they have a momentum that is independent of the ro-
tating surface beneath. This means that, relative to the surface, the winds curve to the right
in the northern hemisphere and left in the south. This leads to rotating systems of high and
low air pressure, the weather systems that bring us rain or sunshine.
Land masses and mountain ranges influence the circulation of heat and moisture too. Until
the Himalayan mountain range began to rise, there was no Indian monsoon, for example.
And most importantly, the oceans play a huge part in storing heat and transporting it around
the globe. The top 2 metres of the ocean have the same heat capacity as the entire atmo-
sphere. At the same time, heat circulates in ocean currents. But currents on the surface are
only half the picture. A good example is the Gulf Stream in the North Atlantic. That car-
ries warm water north and east from the Gulf of Mexico and is one of the reasons why the
climate of northwest Europe is much milder in winter than that of northeast America. As
the warm water heads north, some evaporates into the clouds, which always seem to fall
on British holidaymakers. The remaining surface waters in the ocean cool and become pro-
gressively more salty. As a result, they also become denser and eventually sink down to
flow back south in the deep Atlantic, completing the conveyor belt of the ocean circulation.
Sudden freeze
About 11,000 years ago, the Earth was emerging from the last Ice Age. Ice was melting,
sea levels were rising, and the climate was getting generally warmer. Then, suddenly, in the
space of a few years, it turned cold again. The change was particularly marked in Ireland,
where pollen in sediment cores shows that the vegetation suddenly reverted from temperate
