Page 77 - The World About Us

Eyja allajökull
1.1.1
1.1.1
2.2.3
2.3.2

The Milankovitch Cycles debated. In the first 1.5 million years of the Quaternary,
glacials and inter-glacials tended to match the 41,000
In the 1920s the Serbian geophysicist, Milu n Milankovitch, year axial lt cycle. However, in the last million years
suggested how a series of cyclical changes in the Earth’s rota on the pa ern of colder and warmer periods has more
around the Sun impacted on global temperatures. These closely mirrored the 100,000 year eccentricity cycle.
‘Milankovitch Cycles’ affect the total amount of solar radia on
One issue for climatologists remains. However
received on Earth. The cycles, each opera ng on a different useful the Milankovitch Cycles are for explaining
mescale, affect the seasons, as well as long-term climate pa erns. changes of climate within the Quaternary, they cannot
Figure 209 shows the three most important Milankovitch Cycles: explain the onset of the colder Quaternary period itself.
eccentricity, precession and axial lt.
This is because the Milankovitch Cycles existed for
The precise inter-rela onships between these three cycles are s ll millions of years prior to the onset of the Ice Age.

Eccentricity Over 100,000 years the Earth's orbit around million years, glacials and inter-glacials have tended to
the Sun changes from being almost circular match changes in the Earth's orbital eccentricity.
to being mildly ellip cal. Climate records suggest that global Precession describes a wobble
temperatures are at their warmest when the Earth's orbit is more Precession in the Earth's rota on caused
ellip cal and coolest when the orbit is almost circular. Over the last
by the gravita onal pull of the Moon and the Sun
(imagine the wobble you see in a spinning top as it
fig.209 The Milankovitch Cycles. Not to scale. slows down). A complete cycle, or wobble, takes
Angles and elipses
are exaggerated. around 26,000 years.
Changes in precession affect how much of the
Sun's energy reaches each part of the planet. This can
Eccentricity has a affect the length of days and nights, as well as the
100,000 year cycle
severity of the seasons in high la tudes.
Axial Tilt The Earth was knocked off its ver cal
24.5 axis by the collision 4.5 billion years
22.1 ago which created the Moon. At present, the lt is
23.5 but this is slowly moving through a 41,000 year
Precession has a cycle. Across the cycle the lt ranges from 21.5 to
23,000 year cycle
24.5 . Global temperatures tend to be higher when
axial lt is at its greatest. For much of the early
Axial lt has a 41,000 year cycle
Quaternary the pa ern of glacials and inter-glacials
tended to match the changes in Earth’s axial lt.

Significant eruptions Fig.210 Segara Anak, Lombok. The 1991 erup on of Mount
Pinatubo, on the Filipino island of
Gunung Samalas on the Indonesian Luzon, ejected 10km³ of volcanic
island of Lombok erupted in 1257. The material and 20 million tonnes of
erup on blew 2,000 metres off the sulphur dioxide. Global temperatures
mountain crea ng the 7.5km by 6km fell by 0.5 C in the following two years.
wide crater lake, Segara Anak. The ash
and sulphur from this erup on may Fig.211 Calbuco, southern Chile, 2015.
have ini ated the Li le Ice Age.
Kuwae Island, in modern-day
Vanuatu, was split in two by an erup on Europe with bi erly cold winters. The
in 1452 crea ng an undersea caldera. erup on led to a famine in Russia
Up to 30km³ of magma was ejected. where two million people (one third of
The ash cloud acted as a 'second pulse' the popula on) starved to death.
to the Li le Ice Age (fig.201, p.73)
In 1883 the Indonesian island of
Huaynapu na is a stratovolcano in Krakatoa was blown apart in an
southern Peru. Its 1600 erup on was erup on heard over 3,000 miles away.
the largest in modern South American Global temperatures fell by 1.2 C and
history. Ash fell 500km away in Chile did not recover un l 1888.
and Bolivia. Its effects were felt in

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The climate has changed from the start of the Quaternary period.