The Physics of Ice Ages

1.5 million years ago, the Earth experienced its most recent ice age, namely the Pleistocene glaciation. An ‘Ice age’ is a period, which can be as long as hundreds of millions of years, during which the global temperature is abnormally low. How cold exactly? Imagine the Chicago skyline under almost 3km of ice; that’s what the landscape looked like at the peak of the last ice age. 

To our knowledge, Earth has experienced 5 significant ice ages, for which there are a couple of theories. One theory proposes that ice ages are brought on when a squirrel tries to protect its nuts, which is explored in the film series ‘Ice Age’. We’ll be delving into the other theory of Milankovitch cycles, first hypothesized by Serbian astrophysicist Milutin Milankovitch in the 1940s. By analysing weather data, Milankovitch noticed a correlation between the amount of sunlight that reaches the Northern hemisphere and the average global temperature. 

Each winter, whilst the Earth’s North and South poles are faced away from the Sun, they turn to ice. Each summer, much of this ice melts. The proportion that melts depends on the amount of summer sunlight reaching Earth, which varies year upon year. When the earth receives less sunlight, more ice remains on the poles. Ice is a very good reflector, readily reflecting a large amount of sunlight and heat back into space. This further cools the Earth’s temperature, creating a positive feedback loop

The North pole has much more ice than the South pole because it has far more land. Land has a far lower specific heat capacity than water, enabling it to change temperature more easily. In addition, land transfers heat by conduction rather than convection; the former mechanism is significantly less efficient. As a result, the proportion of ice melted in the North pole is a far more significant factor in determining the state of the Earth’s climate.

Milankovitch proposed that the amount of sunlight reaching Earth predominantly depends on 3 key positional cycles, each with their own cycle length. These include: the eccentricity of Earth’s orbit, obliqueness and precession, which dictate where and how much solar radiation reaches Earth. Collectively known as the Milankovitch cycle, this model is still used today to understand past trends and predict future climate.

The Earth’s orbit varies from elliptical to almost circular on a cycle of 100,000 years. A marks aphelion, the point of furthest separation between the Sun and Earth, whilst p marks perihelion, the point of closest distance between Sun and Earth.
(Note: orbits in diagram are greatly exaggerated)

Eccentricity describes the shape of the Earth’s elliptical orbit around the Sun, which in turn influences how close the Earth passes by the Sun. The degree of the orbit’s ‘oval-ness’ varies on a 96,000 year cycle. At maximal eccentricity, there is 20-30% more incoming sunlight when Earth is at its closest point to the Sun (perihelion) compared to its furthest point (aphelion). In contrast, at minimal eccentricity (when the orbit is most circular), there is minimal difference in incident sunlight between these points.

Constituting 4% of the mass of the solar system, Jupiter produces a strong gravitational field that perturbs Earth’s orbit. These perturbations, combined with similar, smaller perturbations from Saturn’s gravitational field, lead to subtle variations to Earth’s orbital eccentricity on a 100,000 year cycle.

A subsequent effect of perturbations caused by these aforementioned Giant planets, as well as other masses in the Solar System (including moons, asteroids and the Kuiper belt, to name a few), is precession of the Earth about its axial tilt. Precession refers to the direction in which the Earth’s rotational axis points – or how much Earth ‘wobbles’ about its axis of rotation. Precession causes a change in the orientation of the Earth’s axis with respect to perihelion and aphelion; the hemisphere that points toward the Sun during perihelion (and away during aphelion) will experience a greater variation in seasonal weather.

Axial tilt (left) is known as ‘obliqueness’, which causes for seasonal variation. Precession (right) can be thought of as the Earth’s ‘wobble’ about its rotational axis.

Last but not least, we have obliqueness, which describes Earth’s tilt. When the Earth is less tilted, there is less variation in the amount of sunlight reaching Earth across the seasons; conversely, a larger tilt leads to more extreme seasonal changes in weather. When axial tilt is minimal, Summers are cooler, allowing increased build up of snow and ice on the polar caps.

This means that ice ages are brought on when these cycles align such that the Earth’s orbit is more elliptical, its axis has a low tilt, and the Northern hemisphere’s summer is kept cool due to being positioned on the orbit so that it faces farther away from the Sun. It is important to note that, whilst these factors are instrumental to understanding long-term variation in global climate, variations in these factors alone is not enough to bring on an ice age. There are several other factors that will have played a part in the conception and ending of each ice age, such as volcanic activity and atmospheric composition.

In accordance with historic trends, the Earth was set to see its next ice age in the next 1,500 years. However, research shows that climate change due to human activity has likely disrupted the natural Milankovitch cycle forever.  Atmospheric CO2 has an extremely long life-time, which is why global warming is expected to have a significant long-term impact, capable of postponing the next ice age by at least 100,000 years. Solar geoengineering is a controversial, theoretical approach to reversing these effects, which would involve the eruption of a ‘man-made’ volcano to replicate the cooling effects that are seen following natural volcanic eruption.

Disruption to Milankovich cycles, as well as the potential for solar geoengineering, are perfect examples of the Anthropocene Epoch, the most recent, unofficial era that marks this new period in which human activity is drastically changing our planet’s climate and ecosystems.

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