Milankovitch, 1937 Orbital Theory of Ice Ages 3 dominant pacemakers of climate change Eccentricity:...
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Transcript of Milankovitch, 1937 Orbital Theory of Ice Ages 3 dominant pacemakers of climate change Eccentricity:...
Milankovitch, 1937
Orbital Theory of Ice Ages
3 dominant pacemakers of climate change
Eccentricity: 100,000 year cycle
Obliquity: 41,000 year cycle
Precession: 23,000 year cycle
Milankovitch, 1937
Orbital Theory of Ice Ages
3 dominant pacemakers of climate change
Eccentricity: 100,000 year cycle
Obliquity: 41,000 year cycle
Precession: 23,000 year cycle
• Eccentricity – 100,000 & 400,000 yrs
• Obliquity – 41,000 yrs
• Precession – 23,000 yrs
Earth wobbles in space so that its tilt changes between about 22 and
25 degrees on a cycle of about 41,000 years.
Changes in tilt change the severity of the seasons - more tilt means more severe seasons - warmer summers and colder
winters; less tilt means less severe seasons - cooler summers and milder
winters.
It is the cool summers which are thought to allow snow and ice to last from year to
year in high latitudes, eventually building up into massive ice sheets (moderate
winters – warm).
There are positive feedbacks in the climate system as well, because an Earth covered
with more snow reflects more of Sun's energy into space, causing additional cooling. In addition, it appears that the
amount of Carbon Dioxide in the atmosphere falls as ice sheets grow, also
adding to the cooling of the climate.
e = (a2 - b2)1/2 / a
Aphelion – when Earth is furthest from Sun.
~21,000 yr.
These three “orbital parameters” operate simultaneously, influencing the distribution of solar radiation on Earth (Insolation)
Milankovitch cycles are “Pacemakers of the Ice Ages”
Cool summers in the northern hemisphere, where most of Earth's land mass is located,
appear to allow snow and ice to persist to the next winter, allowing the development of large
ice sheets over hundreds to thousands of years. Conversely, warmer summers shrink ice sheets
by melting more ice than the amount accumulating during the winter.
The combination of the 41,000 year tilt cycle and the The combination of the 41,000 year tilt cycle and the 22,000 year precession cycles, plus the smaller 22,000 year precession cycles, plus the smaller eccentricity signal, affect the relative severity of eccentricity signal, affect the relative severity of
summer and winter, and are thought to control the summer and winter, and are thought to control the growth and retreat of ice sheets.growth and retreat of ice sheets.
Orbital changes occur over thousands of years, and the climate system may also take thousands of years
to respond to orbital forcing.
Theory suggests that the primary driver of ice ages is the total summer radiation received in northern latitude
zones where major ice sheets have formed in the past, near 65 degrees north.
Past ice ages correlate well to 65N summer insolation. Astronomical calculations show that 65N summer insolation should increase gradually over the next 25,000 years, and that no 65N summer insolation
declines sufficient to cause an ice age are expected in the next 50,000 - 100,000 years
Obliquity (41 ka cycle) dominates most of Earth history
but
Eccentricity (100 ka cycle) dominates the last 700 ka with higher amplitude changes and the “sawtooth”
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5
million years before present
100 ka cycles (eccentricity) dominate the
late Pleistocene
41 ka cycles (obliquity) dominate thePliocene and early Pleistocene
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5
million years before present
100 ka 41 ka
Orbital parameters have been operating throughout Earth history
But the energy changes between “glacial” and “interglacial” are actually very small, so they cannot explain all climate change
Why have we not always had ice ages?
Global climate of the Pleistocene and Holocene appears to be more susceptible to rapid change than in most of Earth history.
Thus the world today may be highly sensitive to things like atmospheric CO2 concentration