Geography Reference
In-Depth Information
further cooled the climate by becoming a large reflective body of
the Sun's energy. The northern hemisphere continents were also
huddled around the Arctic Ocean and the ocean circulation systems
were fixed into new positions. This then allowed other climate
forcing factors to become important.
During the Quaternary the climate has cooled and warmed
many times and this has been associated with major advances and
retreats of ice sheets. This has shaped the land surface, carving new
features out of rock, depositing sediments and landforms and there-
fore leaving evidence of past climates behind. When ice sheets
grow sea levels fall because water is taken out of the oceans and
locked up on land. The last major ice advance peaked at around
18,000 years ago and then retreated. At this time it was possible to
walk from Britain to Europe across land because the North Sea and
English Channel were dry since sea levels had dropped by 120
metres. The last 10,000 years have been relatively warm and this
warm period within the Quaternary is known as the Holocene .
The cold periods when ice advances are known as glacials and the
warmer periods in between are known as interglacials . The
Holocene is our present interglacial. Several processes have been
proposed to explain Quaternary climate changes and many of these
involve internal feedback mechanisms within the Earth's climate
system. However, it has been shown that changes in the receipt of
the Sun's energy on Earth helps to partly explain the glacial cycles
that have occurred during the Quaternary. This is known as orbital
forcing or the 'Milankovitch theory' which is named after an early
twentieth century mathematician who examined the Earth's orbit
around the Sun.
Orbital forcing theory is based on the idea that the amount of
energy reaching different parts of the Earth from the Sun varies
through time, but in a regular and predictable way. It varies with
three factors as shown in Figure 2.1. First the shape of Earth's orbit
around the Sun (its eccentricity) varies over around 100,000 year
cycles as it moves from being more circular to more elliptical and
back again. Second, the Earth's axis around which it rotates is cur-
rently tilted at 23.5°. But this tilt varies through time from 21.8° to
24.4° over a 41,000 year cycle. Third, the Earth has a slow wobble
on its spinning axis caused by the gravitational pull exerted by the
Sun and the Moon. This happens over two cycles of 19,000 and
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