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Earth's wobble linked to extinctions

Spinning top

The way the Earth wobbles on its axis, like a poorly balanced spinning top, influences our climate and therefore species' survival (Image: iStockphoto)




The Earth's orbit is not a perfect circle. It is slightly elliptical, and the ellipticality itself goes through cycles of change that span roughly 100,000 and 400,000 years.

Its axis, likewise, is not perfectly perpendicular but has a slight wobble, rather like a poorly-balanced child's top, which goes through cycles of 21,000 years.

In addition, the axis, as schoolbooks tell us, is also tilted, and this tilt also varies in a cycle of 41,000 years.

These three shifts in Earth's pattern of movement are relatively minor compared with those of other planets.

But they can greatly influence the amount of heat and light the Earth receives from the Sun.

The effect can be amplified, causing global cooling, affecting precipitation patterns and even creating ice ages in higher latitudes, when two or all the cycles peak together.

Van Dam's team looked at 132 species of mammal fossils recovered from three sites in Spain, in layers of soil that dated from 24.5 million years ago to 2.5 million years ago.

What the researchers looked for were the fossilised teeth of rodents, mainly because these could be spotted and identified much quicker than the remains of other animals.

They found that rodent species bit the dust in two regular waves.

One wave of extinction was roughly every 2.4 million years or so and the other was about every million years or so, coinciding with extremes in the cycles of ellipticality, wobble and tilt.

These were not swift, massive die-outs of the kind that famously wiped out the dinosaurs 65 million years ago, but rather a fade-out of species that could not cope with habitat loss or competition, especially when ice ages kicked in.

As they became extinct, other species emerged.

"At any given time, there would be around 15 [rodent] species in residence," says van Dam.

"During the extinction waves, about five or six species would disappear over a period of about 100,000 years or so."

The paper, which appears today in the journal Nature, is confident that the species turnover among rodents also happened to other mammals and possibly other biological groups too.


Tags: climate-change, astronomy-space, ecology, weather