Milankovitch Cycles and the Big Chill CGF3MI Sunday, June 5, 2016

Milankovitch’s Big Chill Milankovitch cycles are cycles in the Earth’s orbit that influence the amount of solar radiation striking different parts of Earth at different times of the year. Milankovitch cycles are cycles in the Earth’s orbit that influence the amount of solar radiation striking different parts of Earth at different times of the year. To explain this we are interested in three types of variations in Earth’s orbital patterns: eccentricity of the orbit, obliquity (axial tilt), and precession. and precession.

Milankovitch’s Big Chill So what is The Milankovitch Theory?

Milankovitch’s Big Chill The theory is named for Serbian astronomer Milutin Milankovitch( ), who calculated the slow changes in the earth's orbit by careful measurements of the position of the stars, and through equations using the gravitational pull of other planets and stars. The Milankovitch or astronomical theory of climate change is an explanation for changes in the seasons which result from changes in the earth's orbit around the sun.

Milankovitch’s Big Chill Milankovitch used the results of previous scientists to compute the geographic and seasonal distribution of sunlight for the past million years and showed that the effect of the tilt on climate is very important.

Joseph Alphonse Adhemar (1797 — 1862) was a French mathematician. He was the first to suggest that ice ages were controlled by astronomical forces in his 1842 book Revolutions of the Sea. Adhemar's theory was initially rejected, but later further developed and greatly modified, by James Croll. The essence of his theory was that the amount of radiation received by the southern hemisphere was less than that received in the north, thus accounting for the Antarctic ice sheet. Milankovitch’s Big Chill

James Croll ( ) was largely self-educated became an apprentice wheelwright, but gave that up due to ill-health. a tea merchant… manager of a temperance hotel…insurance agent He was appointed as a janitor in the museum at Glasgow, in Milankovitch’s Big Chill

Croll’s theory built on Adhemar’s and considered of the effects of variations of the earth's orbit on climate cycles. Suggested ice ages occur on 22,000 year cycles in each hemisphere, and alternate between southern and northern hemispheres, lasting approximately 10,000 years each. Milankovitch’s Big Chill idea was that decreases in winter sunlight would favour snow accumulation first to couple this to the idea of a positive ice- albedo feedback to amplify the solar variations. This is wrong, but that was not known then!

Feedback Loop: Reinforcement which either accelerates (positive feedback) or retards (negative feedback) a process. Feedback Loop: Reinforcement which either accelerates (positive feedback) or retards (negative feedback) a process. Milankovitch’s Big Chill The accumulation of glacial snow and ice increasing the albedo of the surrounding region, thus cooling the air, thus accelerating glacier growth. The accumulation of glacial snow and ice increasing the albedo of the surrounding region, thus cooling the air, thus accelerating glacier growth.

By the end of the 19th century, Croll's theory was widely disbelieved. However, the basic idea of orbitally-forced insolation variations influencing terrestrial temperatures was further developed by Milankovitch. Milankovitch’s Big Chill

Then the precession effect, which affects the position of the solstices in the annual orbit. Milankovitch’s Big Chill Milankovitch determined that the earth "wobbles" in its orbit. The earth's "tilt" is what causes seasons, and changes in the tilt of the earth change the strength of the seasons. The seasons can also be accentuated or modified by the eccentricity (degree of roundness) of the orbital path around the sun.

And now the theory…. Milankovitch’s Big Chill

Earth’s orbit around the sun is a flattened ellipse, so there are times when it is closer to the sun (perihelion) and times when it is farther from the sun (aphelion). Currently, perihelion occurs in January and aphelion in July. Sun Earth Eccentricity of the Orbit

Milankovitch’s Big Chill Eccentricity of the Orbit When e=0, the ellipse is a circle. When e is very near 1, the ellipse is very long and skinny.

Milankovitch’s Big Chill Eccentricity of the Orbit

Milankovitch’s Big Chill Eccentricity of the Orbit Variation in orbit affects the amount of energy different areas on Earth receive by season. Over time, the elliptical shape of the orbit is more circular, or flattens. The change in the ellipse increases the effects the variation.

Milankovitch’s Big Chill Eccentricity of the Orbit When orbit is highly elliptical, one hemisphere will have hot summers and cold winters. The other hemisphere will have warm summers and cool winters. When orbit is nearly circular, both hemispheres will have similar seasonal variations in temperature.

Milankovitch’s Big Chill Eccentricity of the Orbit The amount of variation in energy receipt will be less than about 0.2%, but has an important impact on expansion or melting of ice sheets. The variation in the shape of the orbit occurs on the order of every 100,000 years and is known as Orbital Eccentricity.

Milankovitch’s Big Chill Variation in the eccentricity of the Earth's orbit over the last 750,000 years. The blue line traces the eccentricity. The orange line shows today's value for comparison. The data are from Berger and Loutre (1991). Eccentricity of the Orbit Periodicity: ~100,000 years

Milankovitch’s Big Chill The second part of the theory deals with the tilt of Earth’s axis. Over time, the tilt varies. Today it is about 23.5 degrees, but ranges from 21.6 to 24.5 degrees. Obliquity of the Axis

What happens if the tilt decreases?

Milankovitch’s Big Chill Closely linked with Precession of Equinoxes, causing up to 15% variation of energy in high latitudes that greatly affects growth and melting of ice sheets. Obliquity of the Axis

Milankovitch’s Big Chill Changes in tilt cause large changes in seasonal distribution of energy at high latitudes, and variations in length of polar night. Has very little effect on low latitudes Obliquity of the Axis

Milankovitch’s Big Chill Obliquity of the Axis Periodicity: ~41,000 years

Milankovitch’s Big Chill Precession of the Equinoxes The third part of the theory deals with Precession of the Equinoxes. Equinox is the time when the sun is directly over the equator. March 21 and September 21 at present.

Milankovitch’s Big Chill Over time, Earth’s axis wobbles like a spinning top, and this changes the timing of the equinoxes. Changing timing of wobble mixed with changes in timing of perihelion and aphelion affect the seasonal balance of energy receipt. Precession of the Equinoxes

Milankovitch’s Big Chill  At present the Earth passes through the perihelion during the northern winter (January), with the Earth's proximity to the Sun during this period lessening somewhat the harsh winter cold in this hemisphere.  the Earth reaches its aphelion during the northern summer (July), the greater distance between the planet and Sun at this time softens the summer heat. Precession of the Equinoxes

Milankovitch’s Big Chill Precession of the Equinoxes

Milankovitch’s Big Chill Precession of the Equinoxes Periodicity: ~23,000 years

Milankovitch’s Big Chill Putting it all together…

Adding up the “little bits” Orbital Eccentricity 100,000 yrsvariation in energy receipt ~0.2%, Obliquity of the Axis 41,000 yrs15% variation of energy in high latitudes Precession of the Equinoxes 23,000 yrsSlight variation in seasonal intensity

Milankovitch’s Big Chill In discussion with Koppen (climate distribution over Earth) and Wegener (plate tectonics), he found that summer snow melt is most affected by changes. Putting it all together…

 A 1976 study, published in the journal Science examined deep-sea sediment cores and found that Milankovich's theory corresponded to periods of climate change. Indeed, ice ages had occurred when the earth was going through different stages of orbital variation.

Milankovitch’s Big Chill Any decrease in intensity of summer sunlight would inhibit melting, making the annual snow budget positive and leading to growth of the ice sheets. Putting it all together…

Milankovitch’s Big Chill Putting it all together… Formulated a mathematical relationship between summer radiation and the altitude of snowline (elevation at which there is at least some snow year round). Determined the increase in snow cover that would result from any given change in summer radiation.

Milankovitch’s Big Chill Putting it all together… Summer insolation at 65 degrees North is the most important latitude and season to model Great ice sheets grew near this latitude and that cooler summers might reduce summer snowmelt, leading to a positive annual snow budget and ice sheet growth.

Milankovitch’s Big Chill Putting it all together…accepting the theory. 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 (Imbrie 1982).

Milankovitch’s Big Chill Putting it all together… Astronomical calculations show that 65 degree north summer insolation should increase gradually over the next 25,000 years No declines in 65 degree north summer insolation sufficient to cause an ice age are expected in the next 100,000 years (Hollan 2000).

For about 50 years, Milankovitch's theory was largely ignored. Milankovitch’s Big Chill Putting it all together…accepting the theory. 1976, a study published in the journal Science examined deep-sea sediment cores and found that Milankovitch's theory did in fact correspond to periods of climate change (Hays et al. 1976). the authors were able to extract the record of temperature change going back 450,000 years

Milankovitch’s Big Chill Oxygen isotope data from deep sea cores confirm the existence of the astronomical cycles with periods of 100, 43, 24, and 19 thousand years. These correspond to peaks in eccentricity, tilt and two in precession. Putting it all together…accepting the theory.

Milankovitch’s Big Chill Indeed, ice ages had occurred when the Earth was going through different stages of orbital variation! They found that major variations in climate were closely associated with changes in the geometry (eccentricity, obliquity, and precession) of Earth's orbit. Putting it all together…accepting the theory.

Milankovitch’s Big Chill Then, what does Milankovitch Theory say about future climate change (and consequently, our future!)? Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond to orbital forcing. Putting it all together…accepting the theory.

Milankovitch’s Big Chill Putting it all together…accepting the theory. It would appear that we are in an interglacial rather than a post glacial world.