We start
where the last post ended, the closure of the Panama gap around 2 million years
ago which led to a loss in transport of warm water to the north pole. Erosion of
the newly-formed mountain ranges such as the Himalayas slowly reduced the CO2
in the atmosphere and the earth slowly grew colder with ice building at the
poles [1]
The causes
of ice ages are not completely understood [2]. A strong theory is that slight
changes in solar radiation being received due to orbital changes can then
amplified through a number of positive feedback processes. The reflection of
sunlight back from ice and snow reduces the net heating of the sun, and the
formation of large ice sheet reduces the flows of oceans to and from the ice
sheets leading to further increase in ice formation. Analysis of trapped air
bubbles shows that greenhouse gases drop by a third (CO2) and a half (methane)
during glaciation. There is some dispute on whether CO2 levels lead or lag the
temperature changes[3], though there seems to be some consensus that in ice
ages it lags – the forcing is the solar radiation which drives temperature, and
the amount of CO2 in the atmosphere follows the change in temperature.
Building an
ice age can take 80,000 years, but getting rid of ice can occur much faster –
only 4,000 years. There are a number of reasons for this speed of deglaciation
but the major reason would seem to be the rise in sea levels undercutting the
ice sheet and causing more melting leading to a rapid melting of the ice sheet.
During this deglaciation the water level increases massively – lowest to
current is about 120m [4], so at some point the Mediterranean was a lake, then
a salt basin, then flooded again.
The
frequency of ice ages has been much analysed. They are clearly periodic but not
so regular as to make explanation easy. The most obvious source of a periodic
forcing variable is oscillations either in the earth’s orbit round the sun or
from the sun itself. There are three separate cycles that interact: changes in
eccentricity (the ratio of the length of the earth’s orbit to the width, the
orbit being an ellipse not a circle); changes in the tilt of the earth’s axis
of rotation, and precession of the tilt. These periodic oscillations known
collectively as Milankovitch cycles interact to make ice ages more or less
likely. The periods of oscillation of these aspects of the earth’s rotation are
96,000 years, 41,000 years and 27,000 years respectively. The combined effects
are sufficiently vague in frequency and amplitude to be capable of continued
re-interpretation [5]. Furthermore if all else fails then meteors or super-volcanoes
can always be invoked to explain a sudden cooling [6]
I’ve
attached a chart that illustrates the temperature and CO2 [7]. We are in the
thick blue blob on the RHS. Possibly the most interesting part of the chart is
the massive increase in CO2 on the RHS to a level not seen for the duration of
this chart (400,000 years) which I'll return to shortly.
As we
approach the Holocene (the name given to our current inter-glacial period
starting nearly 12,000 years ago) we get a feel for the way temperature climate
changes work – a forcing variable, then positive feedback mechanisms such as changes
in ice and CO2 that drive the change until it reaches a new equilibrium. Then a
switch back in the forcing variable until a new equilibrium is reached.
[2] http://www.bbc.co.uk/science/earth/water_and_ice/ice_age
[7] https://wattsupwiththat.com/2012/04/11/does-co2-correlate-with-temperature-history-a-look-at-multiple-timescales-in-the-context-of-the-shakun-et-al-paper/
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