Earth's tilt key to extinction

2006-10-12 13:59

Paris - Climate change, naturally induced by tiny shifts in Earth's rotational axis and orbit, periodically wipes out species of mammals, a study published on Thursday says.

Palaeontologists have long puzzled over fossil records that, remarkably, suggest mammal species tend to last around two and a half million years before becoming extinct.

Climate experts and biologists led by Jan van Dam at the University of Utrecht in the Netherlands, overlaid a picture of species emergence and extinction with changes that occur in Earth's orbit and axis.

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 which goes through cycles of 21 000 years.

In addition, the axis is also tilted, and this tilt 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 radiation - heat and light - which Earth receives from the Sun. The effect can be amplified, causing global cooling, affecting precipitation patterns and even creating Ice Ages in higher latitudes.

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.

Fade-out of species

What they looked for was 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 which 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," van Dam said. "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 on Thursday in the British journal Nature, is confident that the species turnover among rodents also happened to other mammals and possibly other biological groups too.

Previous research has also established that Earth's climate system can be affected by massive volcanic eruptions or the impact of an asteroid or comet.

These can kick up so much dust that the planet receives less sunlight, and in turn plant species suffer.

In addition to natural factors, the world's climate system and its biodiversity are also being affected by the burning of fossil fuels.

Oil, gas and coal, and to a lesser degree agriculture, release carbon gases into the atmosphere, creating a "greenhouse effect" that traps solar radiation and causes Earth's surface temperature to warm.

- AFP