Students also need to be
aware of periodic variations in climate both over the long and the
short term. Studying longer term changes like the onset of an Ice
Age will allow
students to appreciate longer term periodic changes in climate.
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Long Term climate change could include
Global Warming (human induced / 'relatively shorter' time scale) or
Glacial and Interglacial cycles (natural change / longer time scale)
Different time scales of periodic
climate change.
Source: Nagle
Long term climate changes are indicated
by features like changes in sea-level ,fossil landscapes, and special rocks
such as sandstones formed in desert conditions. There is further
evidence of more recent changes in historical records, such as the
travels of Brendan the navigator, Dickens' accounts of frost fairs on
the Thames and cave paintings in the Sahara.
After the last glacial, conditions warmed and peaked at about 5000 years
BP (Before the Present). Around 2000 BP conditions cooled again. Cold
periods are associated with the abandoning of mediaeval villages, and
the Little Ice Ages (1550-1750). Thereafter, the climate warmed between
1750-1945, and after a cooler spell until 1970, the climate warmed
again. Four different scales of climate change are shown in the diagram
above. In recent decades the human influence on climate change appears
to have been increasing markedly ...
The human influence is significant in
most recent times - since 1760 onwards.
Is
Climate Change Normal/Natural?
It looks like the climate is continually changing. Are we in a brief
warm period now? Will the trend reverse? What’s the long-term picture?
Here’s a graph of estimated global temperature for the past 425,000
years. This temperature record was computed from analysis of ice cores
taken at Vostok, a Russian research base in Antarctica, starting in
1970. The deepest core reached 3,623 m into the ice sheet. The ice at
the bottom has been undisturbed for about half a million years. During
this time there have been four ice ages
Milankovitch: Although it is
accepted that climatic fluctuations occur on a variety of timescales, as
yet there is no single explanation for the onset of major ice ages or
for the fluctuations within each ice age. The most feasible of theories
to date is that of Milutin Milankovitch, mathematician and astronomer.
Between 1912 and 1941, he performed exhaustive calculations which show
that the Earth's position in space ('wobble'),
its tilt and its orbit ('stretch')
around the Sun all change. These changes he claimed affect
incoming radiation from the Sun and produce three main cycles of
c.100,000 years; c. 41,000 years and c. 21,000 years. His theory, and
the timescale of each cycle, has been given considerable support by
evidence gained, since the mid-1970s, from ocean floor cores. As yet,
although the relationship appears to have been established, it is not
known precisely how these celestial cycles relate to climatic change.
Diagram above: The 21000 year 'wobble'
- as the earth slowly wobbles in space its axis describes a circle every
21000 years. 1) At present, the orbit puts the Earth closest to the Sun
in the northern hemisphere's winter and furthest away in summer. This
tends to make winters mild and summers cool. 2) The position was
reversed about 12,000 years ago and at the end of the last Ice Age when
winters were freezing cold.
Source: Waugh
Other Suggested Factors for creating
Ice Ages:
Variations in sun spot activity may
increase or decrease the amount of solar radiation received by the
Earth.
Injections of volcanic dust into the
atmosphere can reflect and absorb radiation from the Sun.
changes in atmospheric carbon
dioxide gas could accentuate the greenhouse effect. Initially extra
carbon dioxide traps heat in the atmosphere, possibly raising world
temperatures by an estimated 3*C. In time the oceans could absorb
some of this carbon dioxide, reducing the amount in the atmosphere
and causing world temperatures to drop leading to the onset of
another ice age.
The movement of tectonic plates -
either into colder latitudes or at constructive margins, where there
is an increase in altitude - could lead to an overall drop in world
land temperatures.
Changes in ocean current or in jet
streams.
Produce an annotated
Pictorial diagram giving reasons for long term changes in climate based
on the example below.
Source:
http://en.wikipedia.org/wiki/Ice_core Graph of CO2
(green), reconstructed temperature (blue) and dust (red) from the Vostok
ice core for the past 420,000 years
