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spin doctor



There has been some debate regarding the influence that the rotation,
etc of ther Earth may have on climate, and hence its bearing on
extinctions, were dinos hot/cold blooded, etc.

The Earth does change its angle of tilt over time, and
the general orbital and rotational parameters change also.  These
changes take place over timescales of between 1000 years and 10
Million years.
In general, palaeoclimate modelling needs to take account of past
plate reconstructions, since land masses have an influence on cloud
development and atmospheric circulation.  Some account needs to be
made for the change in tides due to the lunar orbit, but in general
the present-day figures for perturbations in the Earth's orbit are
applicable to the past (at least as far as 100 million years ago).


It is interesting to note that the oblateness of the Earth also
changes over time.  This is achieved by the transfer of mass from the
oceans to the ice sheets during glaciations, and can change the
oblateness by as much as one percent.  (oblateness refers to the
flatenning of the earth at the poles and the bulging at the equator,
caused by the centrifugal force generated by the Earth's spin).

The change in oblateness itself causes a change in the obliquity (the
tilt of the earth's axis.  The tilt varies between 22 degrees to 24.5
degrees, and it varies over a period of 40,000 years.  Present value
is 23.5 degrees.  It is the obliquity which controls the seasons, and
the larger the obliquity the greater the range of seasonality.  The
total recieved radiation does not change, however.

The Earth also precesses in its orbit around the Sun.  This means
that the point in the Earth's orbit which is closest to the Sun
(perihelion) changes over time (about 23000 years).  The current
perihelion is January, but in 15000 years time it will occur in July.
 Again, this does not change the total recieved radiation by all that
much.

All of the above relates to MILANKOVITCH Cyclicity, which is one
of the external factors which contribute to climate change.  It does
not, however, offer a good explanation for large scale changes in
climate.  For this we must look at internal factors, and the only one
of any consequence is volcanic eruption.  (This would also include
desertification and deforestation, but these are probably related in
part to volcanic eruption anyway, at least in the geologic past).

So, yes, the Earth does change its tilt, its shape and even its
position relative to the Sun.  But these variations alone do not
explain all of the observed climate change.  Also, these variations
cannot be detected directly by geologists.  We can see evidence for
major volcanic episodes and bolide impacts.

Of course, this brings us back to the old question of the cause of
major extinctions...

Glen Heward
University of Greenwich
Dept. Of Eath Science
London