Milutin Milanković
was a brilliant Serbian
mathematician
and climatologist.
In 1941 he claimed that
variations in Earth’s orbit
could push the planet’s
climate in or out
of an ice age.
Vital to that idea is the
amount of insolation
—incoming solar radiation—
at 65° N, just south
of the Arctic Circle.
At that latitude,
insolation can vary
seasonally by 25%.
Milanković argued that
reductions in summer
insolation allow some
winter ice to survive.
Each year, for thousands
of years, ice accumulates
around 65° N, and eventually
forms sheets large enough
to trigger an ice age.
Three scientists
joined forces
30 years later
to verify Milanković’s
theory using
deep-sea sediment
cores collected by the
international Ocean
Drilling Program.
James Hays,
Nicholas Shackleton and
John Imbrie published
a paper in 1976
showing that their
climate records
contained the
same cycles
as three
parameters
that describe
Earth’s orbit:
eccentricity,
obliquity, and
precession.
Milankovitch cycles
include three types
of variation in Earth’s
motion.
(1) Eccentricity
Eccentricity describes
the shape of Earth’s orbit
around the Sun.
As Earth experiences
a gravitational force
from Jupiter, its orbit
adjusts during
a 96, 000-year
period from nearly
a perfect circle
to an ellipse,
which causes
minor variations
in total isolar insolation.
(incoming solar energy).
(2) Obliquity
Obliquity is the tilt of Earth’s
axis of rotation.
It fluctuates during a period
of 41, 000 years, between
21.8˚ and 24.4˚, and is currently
at 23.4˚.
A larger obliquity generates
a greater difference in the
insolation Earth receives
during summer and winter.
(3) Precession
Precession consists of
the spin of Earth’s
rotational axis and
its orbital path
over time;
the combined effects
of those two
components produce
an approximately
21, 700-year cycle.
and influences Earth’s
closest approach
to the Sun.
During each hemisphere’s
summer, precession has
the greatest effect
in the tropics.
Tidal forces
of the Sun and Moon,
amplified by Earth’s
oblate spheroid shape,
cause one component
of precession.
Those forces exert
gyroscopic motion
on the planet that
changes the orientation
of its rotational axis.
The second component
of precession moves Earth’s
entire orbit around the Sun
in space and resembles
the petals of a flower.
The Recent Ice Ages:
Over the past 2.5 million years,
Earth has had 50 major ice ages,
and each has significantly
changed the planet’s climate.
During the last ice age peak,
21, 000 years ago, a nearly
continuous ice sheet
spanned North America.
Across Hudson Bay
it was more than
two miles deep, and
reached as far south
as New York City
and Cincinnati, Ohio.
The British–Irish ice sheet
spread as far south as Norfolk,
and the Scandinavian ice sheet
extended from Norway
to the Ural Mountains
in Russia.
In the Southern Hemisphere,
large ice sheets covered
Patagonia, South Africa,
southern Australia, and
New Zealand.
So much water
was locked in all those
ice sheets on land
that global sea level
dropped 120 meters
( about 400 feet ).
If all the Antarctic
and Greenland ice
melted today,
sea level would rise
only by 70 meters.
For the past million years,
ice sheets have taken
at least 80 ,000 years
to reach their maximum ice
( which last occurred
about 21, 000 years ago ).
Ice glaciers
seem to melt
much quicker
than they formed
-- 80% of expanded
ice sheets can be lost
in just 4,000 years.
Summer sunshine at 65° N
starts the melting of Northern
Hemisphere ice sheets.
Ultimately,
rising sea levels
reduce large ice sheets
because the coldest
that seawater can be
is −1.8 ˚C, while the
temperature of the
ice sheet’s base
is −30 ˚C.
As seawater melts
the ice sheets by
undercutting them,
ice calves (breaks off
near the glacier edge)
into the ocean.
The calving raises
sea level further and
causes more undercutting
(see Physics Today,
October 2019, page 14).
The last million years of
glacial–interglacial cycles,
each lasting about
100, 000 years, have a
pattern -- a long period
of cooling. followed by
a short, warm period
of rapid melting.
More than
a million
years ago,
the cycles
were smoother,
and each lasted
only 41, 000 years.
That period corresponds
to the length of the orbital
change associated with
obliquity, which controls
the heat transfer between
low and high latitudes,
and regulates ice growth.
For many years,
scientists struggled
to explain the
100, 000-year
glacial–interglacial cycles,
because the 96 ,000-year
eccentricity mechanism
has a similar length.
The 100, 000-year cycle
is actually a
statistical artifact:
The average length
of the last eight cycles
is 100, 000 years,
but each one varied
from 80 ,000 to
120, 000 years.
Every fourth or fifth
precessional cycle
is weak enough
that ice sheets can
grow bigger and thus
be more vulnerable
to sea-level rise
during deglaciation.
The timing of deglaciation
seems to better match
precession, but some
researchers have argued
that the long glacial
–interglacial cycles
may correspond to
every second or third
obliquity cycle.
The influence
of precession
further increases
every 96 ,000 years,
when eccentricity peaks,
and it is greatest once
every 413, 000 years,
when Earth’s orbit
reaches its most elliptical.
To predict the next ice age,
scientists are studying
planetary geometry and
greenhouse gas emissions.
The next major natural
temperature change
would be 80,000 years
of global cooling,
if past 100,000 year
cycles repeat.
Adding CO2 to the
atmosphere has been
claimed to be the cause
of mild warming since 1975
If that belief is true,
CO2 warming
will delay
the start
of the next
80,000 year
cooling trend,
which would be
very good news !