We hear a lot in the news about accelerated climate change due to human activity, and for very good reasons. Just have a glance at the first half Intergovernmental Panel on Climate Change’s 2014 report (IPCC 2014 Summary) if you want to know how we’ll all be affected by climate change in our own lifetimes.
But there’s also been quite a bit of coverage recently on how natural climate change nearly 2 million years ago may have made us what we are today. Readers in the UK can still catch Brian Cox’s documentary here for another 25 days (and counting). For international readers, it’s probably available somewhere on the internet.
It all seems to revolve around this fairly recent and wittily-titled paper by Mark Cuthbert & Gail Ashley concerning the water supply in the Great Rift Valley around 1.8 million years ago:
Their thinking goes that relatively abrupt swings between wetter and drier climates between 1.85 million and 1.74 million years ago catalysed the evolution of hominins that we classify as our own genus Homo in eastern Africa. These climate swings were caused by natural variations in the pattern of the Earth’s orbit and rotation which affect how much heat from the sun reaches different parts of our planet, or insolation, which in turn determines long-term global weather conditions.
The Orbital Cycles
The Earth’s orbit varies in three distinct ways that affect global climate: eccentricity of the orbit, obliquity of the Earth’s axis and precession of the equinoxes. Today, I’m going to break these terms down for you.
This describes the shape of the path the Earth travels as it orbits the sun. This path is not a perfect circle but is somewhat elliptical thanks to the gravity of the other planets (mainly Jupiter). A more circular, or regular, orbit alternates with a more elliptical , or eccentric, orbit (fig. 1) over a timescale of around 400, 000 years, with smaller fluctuations every 100, 000 years. Additionally, the sun does not lie at the exact centre of Earth’s orbit, and switches ends of the ellipse as the shape of the orbit wobbles back and forth. This means that greater eccentricity places the Earth further from the sun at the same point in its orbit, year after year, for hundreds of thousands of years (fig. 1). You may have gathered that this reduced insolation eventually has the potential to make our planet decidedly chilly.
The point in the orbit when Earth is furthest from the sun is termed aphelion (fig. 1 , left) and the point closest to the sun is termed perihelion (fig. 1, right).
A line is described as oblique if it runs at an angle to another line. In this case, the line of Earth’s axis is oblique from a vertical line by 23.5 degrees on average (fig. 2). I say “on average” because the angle varies between 22 degrees and 24.5 degrees over the course of about 41, 000 years. The Earth’s axis is, of course, the one that it rotates around once in 24 hours to give us day and night.
At times of greatest obliquity insolation during northern hemisphere summer is at its highest and southern hemisphere winter is at its lowest – temperature differences are very high. At times of lowest obliquity, this trend is reversed and the temperature differences are lower. This effect can really heat up the northern hemisphere if an obliquity maximum occurs at perihelion, or really cool the northern hemisphere if an obliquity minimum occurs at aphelion. Therefore, eccentricity is said to modulate the effects of obliquity.
Having said that, one other factor controls insolation throughout the year:
Precession of the equinoxes
This is the term given to the gyroscopic wobble of the Earth as it rotates around its axis, and occurs with a cycle length of around 21,000 years (fig. 3). This has the effect of altering the position of the seasons in relation to perihelion and aphelion, meaning that precession is modulated by both eccentricity and obliquity of the Earth’s orbit and axis.
Although it is easier for us to understand the position of the seasons in relation to the orbit, the position of the equinoxes will also shift according to precession. Since the equinoxes are more specific and easier to define than ‘summer’, this cycle is properly called precession of the equinoxes.
In particular, Cuthbert & Ashley (2014) suggest that the precessional cycle may have been the major controlling factor on periods of wet and arid climate between 1.85 and 1.74 million years ago. This interpretation fits well with evidence that global ice coverage over the last 3 million years correlates very strongly with orbital cycles, and that insolation at 65 degrees north of the equator is very important in determining global climate (Maslin & Ridgwell, 2005). Especially over the last 1 million years, fluctuations in global ice content have correlated strongly with every fourth or fifth precessional cycle, further suggesting that the longer-term climate has been determined by modulation of precession by the eccentricity cycle.
And here’s a quick graph showing how the values assigned to eccentricity and precession have interacted over the last 5 million years:
Seeing what pattern these cycles take is one thing, but what are these climate scientists using as evidence for the climate change itself? Find out next time!
Astronomy: Precession of Earth. http://astro.wsu.edu/worthey/astro/html/lec-precession.html
Cuthbert, M. O., & Ashley, G. M. (2014). A spring forward for hominin evolution in East Africa. PloS One, 9, e107358. doi:10.1371/journal.pone.0107358. Freely available here.
Maslin MA and Ridgwell AJ. 2005. From Early-Middle Pleistocene Transitions: The land-Ocean Evidence. Eds Head MJ and Gibbard PI. Geological Society London, Special Publications. Vol 247, pp. 19-34.