winter’s snow until fall to melting all of it plus some of the previous accumulations
before fall. Moreover, since the Antarctic, during at least the past 5 million years, has
been continuously cold and always glaciated, the main changes are those of northern,
continental ice sheets. Thus, the aspect of insolation cycles that matters most is
variation of sunlight during the northern summer near the Arctic Circle, the zone from
which ice sheets advance.
(^) Once snow remains unmelted all summer and northern glaciers begin to grow, there
are important positive feedbacks. (i) Glaciers have high albedo (90% vs. 15% for
vegetation) and reflect light back to space, enhancing cooling. (ii) The great glaciers
of the ice ages slowly rose to altitudes of 1–3 km, lowering temperature on their
surfaces at the vertical lapse rate of about −6.5°C km−1, thus further protecting the
high core from summer melting. (iii) General cooling reduces the amount of water
vapor in the atmosphere, reducing its role as a greenhouse gas and cooling the
atmosphere further. (iv) Of main interest here, the reduction of CO 2 by 100 ppmv
allowed more 15 μm IR to pass through the atmosphere to space, reducing the mean
global temperature needed to sustain radiation balance. The list of feedback
suggestions is even more extensive, some speculative, some with counter-evidence.
(^) Summer insolation varies with cycles in four aspects of the Earth–sun rotation:
(^1) The tilt of the Earth’s axis of rotation relative to the ecliptic, the plane of
rotation around the sun, varies from 22.25 to 24.25° (currently 23.44°) with
a period of about 41 kyr. Summer warming is greater at greater tilt, since the
polar regions point more directly at the sun. The value at a given time is
termed the obliquity. Two degrees difference in tilt seems like a small
number, but its effects are large. At 24.25° versus 22.25° the Arctic Circle
shifts south 2°, summer daylengths are slightly shorter, and summer noon
sunlight spreads over 8% more area.
2 The tilted axis wobbles (axial precession) in the plane of the ecliptic with
a period of 25.7 kyr.
3 The ellipse of the Earth’s rotation around the sun also rotates around the
sun once about every 22 kyr, moving perihelion and aphelion with respect
to the seasons. Aspects (2) and (3) combine as “precession of the
equinoxes” (and the solstices) with a net period of around 23 kyr. Summer
warming is greater when the summer solstice occurs at perihelion (where
the ellipse lies closest to the sun), least at aphelion. Perihelion at present is
in the northern winter.
4 Ellipse eccentricity ([a^2 − b^2 ]1/2/a, where a and b are the long and short
axes) varies between 0.005 and 0.0607 (currently 0.0167), shifting the
annual variation in Earth–sun distance, with a period of about 100 kyr.
Greater eccentricity produces closer perihelion, and farther aphelion.