There are variations in climate due to precession and orbital
perturbations. The point is that over the last ~100 years, changes are
occurring that are beyond that expected from natural causes. These
points are discussed in An Inconvenient Truth. And I pointed this out
the last time you posted a similar comment.
"Because the direction of the Earth's axis determines when the seasons
will occur, precession will cause a particular season (for example,
northern hemisphere winter) to occur at a slightly different place in
the Earth's orbit from year to year. At the same time, the orbit itself
is subject to small changes, called perturbations. The Earth's orbit is
an ellipse, and there is a slow change in its orientation, which
gradually shifts the point of perihelion in space. The two effects - the
precession of the axis and the change in the orbit's orientation - work
together to shift the seasons with respect to perihelion. Thus, since we
use a calendar year that is aligned to the occurrence of the seasons,
the date of perihelion gradually regresses through the year. It takes
21,000 years to make a complete cycle of dates.
We would not expect the 21,000-year cycle to be very important
climatologically because the Earth's orbit is almost circular - the
distance to the Sun at perihelion is only about 3% less than its
distance at aphelion. That is, whether perihelion occurs in January or
July, it seems unlikely that our seasons would be much affected. At
least, that is the case now; but the eccentricity of the Earth's orbit
(how elliptical it is) also changes over very long periods of time, from
almost zero (circular orbit) to about three times its current value. The
eccentricity of the orbit varies periodically with a time scale of about
100,000 years. So, it would be reasonable to suppose that if the
21,000-year perihelion shift cycle were to have any effect on climate at
all, it would only be during the more widely-spaced epochs when the
orbital eccentricity was relatively large. That is, climatologically,
the 100,000-year cycle of eccentricity should modulate the 21,000-year
cycle of perihelion."
<http://aa.usno.navy.mil/faq/docs/seasons_orbit.html>
-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of JMGreen
Sent: Sunday, March 25, 2007 4:18 PM
To: Forum for Physics Educators
Subject: [Phys-l] Precession
What I do not hear is Gore et al saying that this current warming is
despite the Earth's precession and we _can_ do something to stop it
Can someone please clarify this for me?