This weekly newsletter delves into a different captivating topic from the world of geoscience, exploring Earth’s fascinating phenomena in bite-sized, easy-to-understand segments each week.
Monday 12th of February, an ice age is a long period of time in Earth’s history when much of the planet’s surface is covered in ice sheets and glaciers. Most importantly, an ice age is a natural process. You see the Earth follows a rhythm of ‘breathing’ where the atmospheric concentration of carbon dioxide fluctuates. As seen in the data below from the Earth Observatory, NASA the Earth experiences this rhythm over hundreds of thousands of years and there is a direct correlation between carbon dioxide concentration in the atmosphere and temperatures on Earth. We call the colder periods of time ‘glacial periods’ and warmer periods of time ‘interglacial periods’.
This alternating cycle of interglacial and glacial periods are caused by phytoplankton growth which may have increased the amount of carbon the ocean took out of the atmosphere, volcanoes, solar energy, variation in the Earth’s orbit around the Sun and the axial tilt of the Earth. These variations in Earth’s orbit and axial tilt are collectively known as Milankovitch cycles which are considered to be one of the most important factors for the formation of an ice age. To fully understand Milankovitch cycles we need to split the topic into three parts: eccentricity, obliquity and precession.
Eccentricity
When the Earth orbits the Sun, the eccentricity of the Earth’s orbit changes slowly over time becoming more eccentric (oval). This causes the distance between the Sun and the Earth to vary. At perihelion (Earth’s orbit is closest to the Sun), temperatures on Earth increase and on the other hand at aphelion (Earth’s orbit is furthest from the sun) the Earth’s temperatures decrease. The Earth’s orbit changes from 0.005 eccentricity to a maximum eccentricity at 0.057 and the cycle takes between 90,000 and 100,000 years. At perihelion the amount of insolation (the thermal radiation received from the Sun per unit surface area of the Earth) would be 20 to 30 percent greater than at aphelion resulting in a very different climate. This occurs because of the gravitational pull from the gas giants, Jupiter and Saturn. The diagrams below show the exaggerated difference in orbital eccentricity.
Obliquity
The axial tilt of the Earth’s axis increases and decreases from 22.1 and 24.5 degrees in a cycle that takes 40,000 years. Today, the Earth’s axis is titled about 23.5 degrees from the plane of its orbit around the sun. When the Earth tilts, the seasons become exaggerated meaning with high tilt we get warmer summers and colder winters and at low tilt we get colder summers and warmer winters. Obliquity plays a very important role in the formation of ice ages which we will get onto in more detail later. But via a positive feedback loop, the colder summers at low tilt allows the development of snow and ice in high latitudes year-on-year which has a high albedo (measure of how reflective a surface is) typically reflecting over 90% of sunlight causing additional cooling. On the other hand, high tilt is associated with periods of deglaciation but the effects aren’t uniform globally as the higher latitudes receive a larger change in total solar radiation then regions closer to the equator. The photo below demonstrates the variation in the Earth’s axial obliquity.
Precession
Finally, axial precession is a slightly weird concept. Effectively as the Earth rotates, it wobbles slightly upon its rotational axis due to tidal forces caused by the gravitational forces of the Sun and Moon that cause the Earth to bulge at the equator which affects its rotation. This bulging affect is what also dictates tides around the world. The cycle of precession is 26,000 years however when combined with apsidal precession the overall precession cycle spans about 23,000 years. Apsidal precession is when the entire orbital ellipse wobbles due to interactions with Jupiter and Saturn — not just the rotational axis - with a cycle of about 112,000 years. Precession makes seasonal contrasts more extreme in differing hemispheres. The image bellow demonstrates the Earth’s precession.
So now we know all about Milankovitch cycles and the factors which cause an ice age let’s move onto the how an ice age is formed. There is no simple answer. Ice ages are caused by a chain reaction of positive feedback loops involving the release/decrease of carbon dioxide exacerbating/reversing the warming of the Earth. We’re currently in a warm interglacial period that began 11,000 years ago in which the last glacial period peaked about 20,000 years ago but why did the the ice advance and retreat at different times during glaciation and why did the ice eventually disappear? Remember not everything is black and white and there are almost too many different factors to consider but this phenomena can be described through the ocean-control theory.
Ocean-Control Theory
Continental drift is the gradual movement of the continents across the earth’s surface through geological time. Continental drift affects ocean and atmospheric circulation patterns. The plate tectonic movements cause continents to be arranged such that warm water from the equator which flows to the poles can be blocked resulting in the formation of ice sheets. Scientists believe this was one of the main factors for the formation of today’s ice age, the land bridge between North and South America known as the Isthmus of Panama exchanged tropical water between the Atlantic and Pacific Oceans which altered the ocean currents leading to the formation of ice sheets. The warmer waters moving towards the poles causes precipitation as warm water vapour rises, it cools then condenses around a condensation nucleus (such as tiny particle of dust or soot). These then coalesce (combine) to create clouds which ultimately falls as snow as the climate begins to get colder marking the onset of a new glacial period. This means that precipitation falling as snow forms glacier ice on the land which slows down the hydrological cycle causing water to be cycled from the oceans through evaporation and condensation only to be stored in the ice. This creates a positive feedback loop and cold runoff of the land begins to cool the oceans. This leads to eustatic sea level fall (changes in the volume of water in the ocean) globally and ice is stored on the land leading to isostatic fall (local changes caused by subsidence of the crust related to changes in the amount of ice on the land). This happens when the weight of the ice displaces the asthenosphere (mantle that can flow) regulating eustatic sea level fall however during glacial periods the level of sea is 110 m lower than in interglacial periods meaning this regulation doesn’t have much of an effect. This is the glacial maximum — for example, the last glacial maximum occurring 20,000 years ago during the last phase of the Pleistocene epoch, glaciers covered 25% of Earth’s land area. Compared to today, glaciers cover 11% of Earth’s land area and this is unnaturally shrinking yearly due to human induced climate change.
When the oceans get colder this leads to lower levels of evaporation leading to less precipitation meaning more snow is melted than it is built up exacerbating the glaciers retreat. Warm runoff begins to warm the oceans. This is an example of how the oceans control the advance and retreat of glaciers and ice ages. For example, in Antarctica according to study published by AGU, recent ice sheet mass loss has been attributed to an influx of warm ocean waters which drove episodic ground-line retreat and ice thinning. In terms of the ocean-control theory, this causes eustatic sea levels to rise and also contributes to isostatic rebound as the weight of ice is removed due to the warming of the climate. When the oceans become warmer, the rate of evaporation increases leading to higher levels of precipitation and the feedback loop of the Ocean-Control Theory of ice age formation is complete and the process will repeat over thousands of years.
Conclusion
In conclusion, the occurrence of ice ages is intricately tied to both the Ocean-Control Theory and Milankovitch cycles. The Ocean-Control Theory emphasises the crucial role of oceanic circulation patterns in redistributing heat across the planet affecting dynamics and the Milankovitch cycles contribute to changes in the distribution of solar radiation reaching Earth’s surface. Importantly, the temporal formation of ice ages occurs over thousands of years and is the result of very small compound variations in geodynamics and orbital changes which overtime equates to the formation of ice ages. I hope you enjoyed learning about ice ages today and gained a deeper appreciation for the remarkable processes that shape our planet’s climate. If you want to learn more about geosciences come back every Monday for a new topic which is just as interesting.
Thank You
Thank you for reading, all information is taken from reputable sources and are linked below. All images are free to use under copyright laws.
Earth Observatory, NASA ‘ Global Climate Change, NASA ‘ Global Monitoring Laboratory ‘ Utah Geological Survey ‘ USGS, USA