A Discussion About Climate Change – Part 1: Milankovitch Cycles

The Great Debate

On television, radio, and in writing, we often hear two political ideologies debating the issue of climate change. One side swears the change is merely a natural cycle and the minuscule impacts of mankind cannot be detected. The other side places all of the responsibility on mankind and our irresponsible burning of fossil fuels in the quest for cheap energy. Often with a lack of credible references, they argue back and forth, their perspectives unwavering. The discussion ends with no real progress being made, and there remains an uncomfortable tension that permeates and divides the world.

We are conditioned to process information that has been distilled into an oversimplified format and this can lead to 'tunnel vision'. With arguments it is much easier to favor a straightforward explanation and 'pick a side' than it is to objectively examine both sides and acknowledge the validity that exists throughout. As with most things in life, the answer is neither black nor white, rather an ever-changing shade of gray. Only by honestly examining facts from all perspectives can we make our own conclusions about the reality of climate change.

Natural Cycles

The climate of our planet is unquestionably influenced by several predictable celestial cycles. Some of these cycles are intuitive like the daily rising and setting of the sun caused by the spinning rotation of the earth on it's axis, or the annual changing of the seasons due to the axial tilt and revolution of Earth around the sun. The axial spin and orbital revolution of the earth are not perfect,

^{Showing Earth (blue dot) in an elliptical orbit (white) and a circular orbit (red) around the sun (orange dot).}

and this error manifests itself through a number of gradual cycles collectively known as the Milankovitch cycles after scientist Milutin Milanković who first developed the theory. The cycles are based on changes in the tilt and position of Earth relative to the sun. Fluctuations in the amount of solar radiation received during these cycles directly influences the global temperature and climate.

Eccentricity of the Orbit

^{Earth's (blue dot) orbit (red ellipse) around the sun (orange dot) as it changes with time}

The earth orbits the sun in a path that is best described as an ellipse rather than a perfect circle. This slight elongation in the orbit is one of two primary causes for the seasons that our planet experiences. Over the course of 100,000 years the eccentricity of the orbit gradually increases and decreases, directly influencing the intensity of seasonal variation. Winters and summers become more extreme whenever the orbit's eccentricity is increased because the distance between Earth and the sun is seasonally skewed. A more circular orbit yields more temperate conditions globally. Currently our planet has a decreasing eccentricity that is slightly below the average.

Obliquity of the Axial Tilt

^{Minor changes in the tilt of the axis have large impacts on the amount of solar radiation the polar regions receive.}

The earth rotates on an axis that has a tilt of approximate 23.5 degrees relative to the plane of orbit. The axial tilt and the eccentricity of orbit are the primarily causes for annual seasonal change. While the earth is orbiting the sun, the tilt of it's axis relative to the sun gradually shifts from about 22 to 24.5 degrees. This cycle repeats approximately every 41,000 years, with the current axial tilt at about 23.4 degrees and decreasing. A decline in the obliquity of the tilt reduces the amount of direct sunlight that reaches the polar regions and causes more moderate global temperatures. Ice is able to accumulate faster in polar regions in these circumstances, and this contributes to the development polar ice caps.

Precession of the Axis

^{Two-dimensional representation of the effects of a circular precession. The pole is tilted ~23 degrees throughout this cycle; when the poles appear to be vertical they are actually tilting toward and away from the screen.}

While maintaining it's axial tilt, the axis of Earth also wobbles from its center over a cycle of about 26,000 years. This cycle is currently at one extreme, and as a result the Southern Hemisphere is pointed toward the sun when Earth's orbit is closest (summer) and away when the orbit is most distant (winter). This configuration creates more intense seasonal variation in the Southern Hemisphere. In the Northern Hemisphere, Earth receives less direct sunlight during the summer and more during the winter causing the seasons to be more mild. After another 13,000 years, the precession will be reversed and the Northern Hemisphere will experience more extreme seasonal variation.

Precession of the Orbit

The orbit of Earth has a wobbling precession, much like the axis. This wobble is caused by the gravitational pull of massive planets like Jupiter and Saturn and it influences the relative start dates of the seasons. The cycle takes about about 112,000 years to fully complete, but it synchronizes with axial precession to gradually alter the length of the seasons. The date that we currently consider to be the summer solstice will become the autumnal equinox in about 5,000 years.

Incline of Orbit

A final cyclical variation in the orbit of Earth has to do with the plane of orbit around the sun. Milankovitch was unaware of this cycle and did not study it, but like orbital eccentricity it lasts approximately 100,000 years. There is some speculation that the additive effect of these cycles may trigger the onset of global ice ages.

Accuracy of Data

The only way that historical climate data can be collected is through a proxy. The two most common proxies used to estimate prior climate conditions are the composition of gas bubbles trapped in ice cores and the intensity of different indicators in an analysis of deep ocean sediment.

While a great deal of information can be gleaned by these indicators, their precision is limited in accuracy due to the indirect nature of the measurements. It is impossible to know exactly how rapidly or intensely the climate has changed in the past. Cycles that seem impossibly gradual in human time are rather abrupt in geologic time.

End Part 1

I have elected to break this topic into multiple parts in the interest of encouraging readers to read through the entire article (and for my own convenience). In the next post, we will consider other possible factors in climate change and how they compare to the Milankovitch cycles. Please follow and stay tuned for future elaboration on these concepts!

All photos and animations by the author, except the lead photo from Pixabay. See sources and additional reading as embedded links in the article!

External Resources:
Milankovitch, M. "Canon of insolation of the earth and its application to the problem of the ice ages." Royal Serbian Academy Press, Cemian (1941): 1-626.