Tuesday, November 15, 2016

Why is Earth’s axis shifting?

[Cosmos Magazine] We are only a little way into the 21st century, but signs of a warming planet are already evident around the globe: More frequent droughts in East Africa; stranded polar bears in the Arctic; bleached coral reefs in the tropics; and retreating glaciers in the high latitudes. Along the coasts, sea levels are rising.
Even so, a new study really surprised me. By burning huge quantities of fossil fuels, we humans have tipped the Earth off its axis by a tiny amount. Let me emphasise how tiny the tipping is. Each year since 2005, we have shifted the spin axis from its previous path by centimetres – not kilometres.
The north-south spin axis of the Earth runs through the North Geographical Pole in the Arctic Ocean, and the South Geographical Pole in the Antarctic. (I’m not talking about the North and South Magnetic Poles, just the Geographical Poles.)
But as the Earth spins on its own axis, the position of the North Pole is not dead true – it wobbles a little, for several reasons.
For one, the Earth is not perfectly spherical. Instead, it’s a bit flattened at the poles, and a bit bulging at the equator. And the surface is not smooth – it’s pretty bumpy. Mountains poke up towards space, while oceans dip down into the solid crust.
Our planet is not perfectly rigid, either – it’s somewhat elastic. Yes, it does have a solid crust at the surface – but it’s very thin. Earth is made mostly of molten rock and then liquid iron, with a core of solid iron.
So even today, parts of the crust that carried heavy ice sheets 20,000 years ago are still slowly rising (an effect known as the “isostatic rebound” or “post‑glacial rebound”). As a result of these (and other) factors, when Earth rotates on its own axis over the course of a day, that spin axis wobbles a little. Read More