Coastal Processes, Hazards, and Society

Extrinsic Drivers of Sea Level Change


Extrinsic Drivers of Sea Level Change

Earth Orbit, Solar Insolation Variability & Sea Level Change

Sea level over time. Generally above average until 3 m.y. ago then below average but getting closer to 0 the past 100000 years.

Reconstruction of Sea level change over three time periods: a. 35 million years, b. 5.33 million years, c. 550,000 years. From Hansen, Sato, Russell, and Kharecha.

Take look at the graphs showing changes in sea level through the history of the Earth. The top figure covers 35 million years, the middle one covers a little more than 5 million years, and the bottom one zooms in to the past 500,000 years. What do you notice? Is there a regularity in the pattern of sea level ups and downs?

Through Earth’s history, there appears to have been a regular spacing of glacial maximum events, at roughly 120 thousand years (ky). When variations in Earth's orbit produce repetitive changes in climate and sea level, the observed cycles are often referred to as Milankovitch Cycles. Many sedimentary rock sequences have been shown to have stacking patterns that reflect these time scales, as do ice core data.

see caption. (L-R): orbit around sun, tilt of the earth, orbit at the poles & equator

This figure is a composite of diagrams designed to show each of the three types of cycles outlined here and in the National Geographic Video. From left to right these are Eccentricity, Obliquity, and Precession.
Credit: Public Domain downloaded from Wikipedia: Milankovitch Cycles

Milankovitch Cycles

Mathematician Milutin Milankovitch proposed an explanation for the changes in the way the Earth orbits the sun. These changes define the sequence of ice ages and warm periods.

  1. The Earth’s orbit changes from being nearly circular to slightly elliptical (eccentricity). This cycle is affected by other planets in the solar system and has a period of around 100,000 years.
  2. The angle of tilt of the Earth’s axis changes from 22.1° to 24.5° (obliquity). This cycle has a period of 41,000 years.
  3. The direction of the tilt of the axis changes (precession) on a cycle of 26,000 years.
  4. These changes influence the length of the seasons and the amount of solar radiation received by the Earth.

Optional Video

Please watch Milankovitch Cycles in 5 Minutes (5:00), if you are not already familiar.

Click for the "Milankovitch Cycles in 5 Minutes" video transcript.
The Sun is the Earth's main energy source. In fact, it provides 99.96 percent of all the energy that drives the Earth's climate. Some of the energy produced by nuclear fusion in the sun's interior will eventually strike the top of the Earth's atmosphere. The amount of energy that does strike the atmosphere depends on two main factors: the total amount of energy produced and transmitted by the Sun, and the orbital cycles of the Earth with respect to the Sun. The energy transmitted by the Sun is in a constant state of flux depending on solar activities such as sunspots, solar flares, coronal loops, and coronal mass ejections. The relationship between the Earth's orbital cycles and climate change was proposed by Milutin Milankovitch. Milankovitch was a Serbian engineer, and during the 1930s, he proposed that the changes in the intensity of solar irradiation received on the Earth were affected by three fundamental factors: precession, obliquity, and eccentricity. These factors are now collectively known as the Milankovitch cycles. The Milankovitch cycles are widely accepted by climate change scientists and are well documented by, for example, the IPCC. A more detailed description of the cycles is available by clicking on the tab above, but the remainder of this video will provide an excellent overview. The Earth rotates on its axis every 24 hours. Around once in 27 days, the Sun also rotates on its axis. Its average distance from the Earth is approximately 150 million kilometers (93 million miles). It is an average distance because this Earth's orbit around the Sun is not fixed. Its orbit cycles from being almost a circle to that of an ellipse to almost a circle again. The cycle takes place over a period of around 100,000 years. The rotation of the Earth is at an angle to the vertical, and this angle changes over time. It moves from 22.1 degrees to 24.5 degrees and back again. This is over a time span of approximately 41,000 years. The Earth also goes through a cyclic wobble. It moves from its current position of the north pointing to the star Polaris to where the North points to the star Vega and returns to pointing at Polaris. The full cycle takes place between 19 to 26 thousand years. The combined effects caused the seasons to gradually cycle relative to the perihelion and aphelion, this over a time span of about 21 thousand years.[music]

How do these variations in Earth’s orbit affect climate and sea level? Collectively, variations in Earth's orbit (eccentricity, obliquity, and precession) can either reinforce signatures of cooling or warming, or they can work to counteract each other and produce less severe or ameliorated climate change. When the multiple variables reinforce each other, the amount of climate change and, as a result, sea level change can be significant.