Written by: Jenn Xu
Image from pinterest.com
Around every 100,000 years, Earth enters a period of frigid coldness. Temperatures drop, glacial expansion takes place frequently, and ice sheets form. Ice ages can last up to hundreds of millions of years, and if you weren't aware of it, we’re currently in an ice age now! In this article, we’ll look into what exactly an ice age is and what triggers one to occur.
An ice age is defined as a long period of a drop in global temperatures, in which ice sheets and glaciers form and expand. Furthermore, for a period in time to be considered an ice age, at least one ice sheet has to be present on Earth—that’s why we’re still in one! There are currently two ice sheets present on the planet: the Antarctic ice sheet and the Greenland ice sheet. Ice ages are caused by the lowering of temperatures, which glaciers and ice sheets are formed by the compacting of snow. The cooler temperatures freeze the bottom layer of snow into ice and, over time, a glacier is formed.
What exactly prompts these freezing temperatures all of a sudden? The answer lies in the concept of Milankovitch cycles. These cycles are part of a theory called the Milankovitch Theory, coined by Serbian geophysicist and astronomer Milutin Milakovitch around a century ago. Milankovitch theorized that the combination of small changes in Earth’s orbit over time would dictate the planet’s climate in the long run, including the generation of ice ages. There are 3 Milankovitch cycles that affect Earth’s climate: eccentricity, obliquity, and precession.
Eccentricity is the change in the shape of Earth’s orbit around the Sun. This usually follows a cycle every 100,000 years, where Jupiter and Saturn's gravitational pull on Earth cause the planet's almost perfectly circular orbit around the Sun to become a little more elliptical. This first cycle explains the variation in the lengths of our seasons throughout the years.
As its name may suggest, obliquity is the angle in which Earth’s axis is tilted. We're taught in schools that the Earth's tilt is a 23.5° angle, but that's just the average angle. In reality, our planet's tilt varies from 22.1° to 24.5°. As most of us know, Earth's tilt (or obliquity) gives us our seasons. The tilt allows different concentrations of solar radiation to hit the planet's surface, making the difference hemispheres warmer or colder depending on which hemisphere is tilted towards or away from the Sun during Earth's orbit. The greater the tilt, the greater the effects of our seasons. This means that when the planet's obliquity is greater, summers would be hotter than usual and winters would be colder. The cycle of obliquity is around 41,000 years.
The last Milankovitch cycle precession, deals with the direction that Earth’s rotational axis is positioned. The planet doesn't just spin on its axis—it actually wobbles around the axis, much like a spinning top. This cycle is around 26,000 years long. The first cycle under the umbrella of precession is caused by the gravitational forces of the moon and the Sun which affect the tides. The idea is that it creates somewhat of an expansion near the equator and this changes the Earth's axial rotation. This is known as axial precession. Another cycle under this umbrella is apsidal precession, in which Jupiter and Saturn once again cause changes to Earth's orbit around the sun. However, in this case, it causes a wobble in its orbit itself rather than its shape.
All of these cycles pay into the creation of an ice age. First and foremost, eccentricity and a change to a more elliptical orbit may cause a prolonged winter in a certain hemisphere. Then, smaller obliquity would promote the creation of an ice age, as milder changes in temperature and warmer winter air retains more moisture—something ice sheets and glaciers need to grow. And finally, precession causes greater seasonal extremes.
All through history, scientists have been able to pinpoint 5 major ice ages. In order from oldest to newest, they are the Huronian, the Cryogenian, the Andean-Saharan, the Karoo, and the Quaternary ice ages (where we are now!). Some have wondered when the next ice age would come. Back in 2015, there was a prediction that within the subsequent 15 years, there would be a “mini ice age” due to predictions that solar magnetic activity would lower by 60% within the years 2030-2040. However, this has been refuted as greenhouse gas emissions and anthropogenic climate change continues to be on the rise, warming Earth’s temperature. Human impact on Earth’s temperatures are much more powerful than natural solar magnetic activity changes. While we won’t be experiencing any noticeable ice ages any time soon, we can all appreciate the science behind them in the meantime.
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