Geology Reference
In-Depth Information
changes in temperature, strongly hinting at their central involvement. But there is a fur-
ther message in these traces that we have already encountered: Gaia's temperature has
reached the same maximum at each warming, the same minimum at each cooling. These
tight bounds, beyond which she hasn't strayed for the last two million years, speak of
the presence of a tightly coupled self-regulating dance between life, rocks, atmosphere
and ocean.
The recent ice ages have to do with the fact that Gaia, although apparently alone in
the vastness of space, is deeply sensitive to the animate presences of the other planets
in our solar system, and of course to the heat stress she is experiencing from the sun,
which is now hotter and brighter than ever before. Gaia engages in conversations with
the community of planets; she is privy to a larger realm of interactions, mediated by that
mysterious attraction to which we so glibly ascribe the word 'gravity'. There, out in the
vastness of space lie the great gas giants, Jupiter and Saturn, and closer than us to the
sun lie Venus and Mercury. And nearby, at a mere stone's throw by galactic reckoning,
is our own moon. Each is an animate presence, a character in its own right, and each
expresses itself in part by drawing our planet closer to its own ambit, much as an Abori-
ginal lover in the Great Sandy Desert beckons to his beloved using the ancient technique
of
yilpinji,
or love magic.
These conversations and attractions cause Gaia's orbit around the sun to vary from
perfectly circular in three important ways. First there is eccentricity of her trajectory
though space, which expands and contracts from egg-shaped to almost circular with a
periodicity of 100,000 years. Then there is the tilt of her spin axis, which returns to a
maximum of 24.5 degrees from a minimum of 22.5 degrees every 41,000 years. Fin-
ally, there is precession, referring to the top-like wobble of her spin axis that takes about
25,700 years to complete. Currently, the North Pole points towards
Polaris
, the North
Star, but 5,000 years ago it pointed to a different North Star,
Alpha Draconis
, observed
in wonder by the ancient Egyptians from their pyramids.
These orbital variations determine the distribution of solar energy reaching our plan-
et. Precession and tilt by themselves cannot alter the absolute amount of solar energy
reaching Gaia's surface, they only alter its distribution, but the changes in eccentricity
do make a difference in this regard because the absolute distance of the Earth from the
sun is altered. Even so, the amount of extra energy received is tiny—only 0.2% less at
maximum eccentricity relative to the almost circular minimal eccentricity.
The fact that the periodicity of Gaia's orbit matches that of the swings from glacial
to interglacial conditions is taken as evidence that eccentricity is the main driver of the
cycle. But how can such a small change in the amount of solar energy reaching the plan-
et make such a difference? The answer seems to be that a suite of global-level positive
feedbacks are acting as amplifiers.
