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:
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
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.
Figure 1. Eccentricity describes how elliptical the Earth’s orbit is. I’ve just realised how much these diagrams look like fried eggs.
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).