What is Milankovitch Cycle?

Published on 1 November 2020

Photo by Pixabay on Pexels.com

In the 19th century, French mathematician, Joseph Adhemar, and Scottish scientist, James Croll, among many others, suggested that the earth’s climate variability was based on the changes in the earth’s orbit.

The core idea that changes in the rotation and orbit of the earth around the sun may trigger climate variation became widely accepted and further strengthened by Serbian geophysicist and astronomer, Milutin Milanković in the 1920s.  It came to be known as the Astronomical Theory of Climate Change or Milankovitch Cycle.

The three elements that make up the cyclical orbital movements or the Milankovitch cycle include eccentricity, obliquity and precession.

  1. Eccentricity is the shape of the earth’s orbit around the sun from circular to more elliptical.  There are two main periods – one cycle with an average period of over 100,000 years and another cycle with a longer period of over 413,000 years.
  2. Obliquity is the tilt of the earth’s axis with respect to its orbital plane.  It varies between 22.1 to 24.5 degrees over a period of 41,000 years.  The current tilt is 23.44 degrees. The tilt last reached its maximum in 8,700 BCE.
  3. Precession is the direction of the earth’s axis of rotation, like that of a spinning top, over a period of 19,000 to 23,000 years.

The variations in these three elements of the Milankovitch Cycle affect how much solar radiation, otherwise known as insolation, reaches the earth’s atmosphere and surface at a particular place and time.  These cycles drive earth’s long-term climate, triggering the beginning and end of ice ages.

History of climate change over longer time-scales can be understood and reconstructed by studying the ice cores and marine sediments.  Discovery of the ice age in the mid-19th century, paved the way for the science of climate history.  Northern polar regions play a central role in climate change.  Climatologist, Wladimir Köppen, and German meteorologist, Alfred Wegener, supported the theory of Milankovitch stating that melting of northern ice sheets were central to ice-age climate fluctuations.

Earth’s land-sea distribution modifies the temperature response to orbitally induced perturbations of the seasonal insolation.

David A. Short et al.

Of note is the sun cycle, where every 11 years, the sun’s magnetic field flips and its poles change position from north to south.  This also leads to changes on the surface of the sun, namely sunspots.  There is variation of the distance between the sun and earth, and variation in sunlight reaching earth.  The sun rotates on its axis once in 27 days, which is observed by the motion of sunspots.

Sunspots
Image Credits:NASA/Goddard/SORCE

When the disk of the sun is large more sunlight is received than when it appears small. Owing to the changing distance of Earth from the sun (the orbit being a slowly changing ellipse, with the sun in one of the focal points), and to the migration of the seasons along the orbit (“precession”) the sun’s disk changes in size through the seasons, on time scales of millennia. According to Milankovitch, if the disk is large in summer in high northern latitudes (that is, perihelion in northern summer), melting can occur, but if small (that is, perihelion in northern winter), ice buildup proceeds.  Since the seasons migrate along the orbit, completing a cycle roughly every 21,000 years, the melting opportunity has this cyclicity (actually, several cycles are involved, all of which are near this value). In addition, the tilt of the Earth’s axis changes through a range of somewhat less than three degrees on a cycle near 41,000 years (present intermediate tilt, 23º27’). The tilt (“obliquity”) determines how high the sun can rise during noon, in northern summer. A higher position translates into higher insolation in high latitudes, in summer.

W. H. Berger, Scripps Institution of Oceanography

The Dansgaard-Oeschger event or D-O event refers to “the fleeting global climatic swings characterized by abrupt warming and a period of slow cooling that occurred during the last ice age.  It is linked to perturbations in the ocean circulation pattern.  The climate of the last glacial period was extremely unstable and interrupted by about 24 distinct warming and cooling events.”

The scientists have found that at least five major ice ages have occurred throughout Earth’s history, the earliest being 2 billion years ago and the most recent approximately 3 million years ago which is continuing today.  Currently, it is the warm interglacial that began about 11,000 years ago. The summer insolation anomalies at high latitudes in the Northern Hemisphere drive ice ages.

Minimum summer insolation allows snow and ice accumulated in the cold season to survive, while maximum summer insolation tends to melt the ice sheets.  Earth is now closest to the sun in January, which favors warm winters and cool summers in the Northern Hemisphere, thus favoring growth of glaciers and ice caps in the Northern Hemisphere. However, that tendency is very weak during the current interglacial period because another more slowly varying orbital parameter, the eccentricity of Earth’s orbit, happens to be small during this interglacial period.  When the eccentricity is near zero Earth’s orbit is almost perfectly circular, so the date at which Earth is closest to the sun becomes irrelevant.  The remaining Milankovitch parameter, the tilt of Earth’s spin axis relative to the plane of the orbit, is now at an intermediate value, headed toward a minimum tilt that will occur in about ten thousand years.  Minimum tilt favors growth of ice sheets in the polar regions of both hemispheres.

James E. Hansen et al.

And so……according to Milankovitch Theory……..this is a great period to form glaciers!!!

If that is so, then why are the ice sheets currently melting?

Could climate feedback from ocean or land caused by Milankovitch cycles be responsible for climate change?

According to NASA research, Milankovitch cycles have not changed the amount of solar radiation absorbed by earth for the last 150 years, and in fact, it has reduced over a period of 40 years.  Ice sheets reflect back to the space sun’s incoming energy, which impacts earth’s temperature.

Climate scientists have stated that apart from the Milankovitch cycle driving climate change over long geological time scales, there are many other factors to consider such as tectonic uplift and associated sensitivity of ocean dynamics, continental drift, availability of methane ice on the seafloor, and changes in the carbon dioxide content of the atmosphere.

Photo by Pixabay on Pexels.com

Due to Milankovitch cycle, the concentration of carbon dioxide (CO2) in the atmosphere fluctuated from 180 to 280 parts per million (ppm) during the past glacial cycles.  Ever since the Industrial Age, CO2 in the earth’s atmosphere has increased to 412 ppm and global average temperature has risen by 1 degree Celsius or 1.8 degrees Fahrenheit.

The answer according to NASA research: “Since 1750, the warming driven by greenhouse gases coming from the human burning of fossil fuels is over 50 times greater than the slight extra warming coming from the Sun itself over that same time interval. If Earth’s current warming was due to the Sun, scientists say we should expect temperatures in both the lower atmosphere (troposphere) and the next layer of the atmosphere, the stratosphere, to warm. Instead, observations from balloons and satellites show Earth’s surface and lower atmosphere have warmed but the stratosphere has cooled.  Finally, Earth is currently in an interglacial period (a period of milder climate between Ice Ages). If there were no human influences on climate, scientists say Earth’s current orbital positions within the Milankovitch cycles predict our planet should be cooling, not warming, continuing a long-term cooling trend that began 6,000 years ago.

It is believed by 87% of scientists that human activities are contributing to climate change.  The anthropogenic intervention in Anthropocene (Age of Man) has led to global climate change.

Photo by Brent Olson on Pexels.com

Can it then be concluded that the rapid warming of the earth’s climate is attributed to human excess in addition to the very slow changes to climate caused by Milankovitch cycles?

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