Dr Karl Kruszelnicki's science conundrums: Why is the world's spin axis shifting?

We are only a little way into the 21st century, and the signs of global warming are clear. Even so, the latest finding really surprised me. By burning huge quantities of fossil fuels, we humans have actually tipped the Earth off its axis by a tiny amount.

Each year since 2005, our global warming activities have shifted the spin axis from its previous path (by centimetres).

We humans now shift as much dirt as all the rivers on Earth added together. It comes to about 20 billion tonnes a year – mostly coal, followed by iron ore. This is a huge amount – but by using carbon dioxide, we moved an amount of water 30 times greater. We melted ice on land to water, which then relocated all over the entire planet.

As the Earth spins on its own axis, the position of the north pole is not dead true – it wobbles a little. The Earth is not perfectly spherical. It's a bit flattened at the poles, and bulges at the equator.

In addition, the surface is not smooth – it's pretty bumpy. Mountains poke up towards space, while oceans dip down into the solid crust.

Furthermore, our planet is not perfectly rigid, but somewhat elastic – made mostly of molten rock and then liquid iron, with a core of solid iron.


As a result of these (and other) factors, when the Earth rotates on its own axis over the course of a day, that spin axis wobbles a little. There are lots of individual wobbles. 

A major one is the so-called Chandler Wobble, which the American astronomer Seth Carlo Chandler discovered in 1891.

Over about 433 days, the Chandler Wobble shifts the north pole over a rough circle or ellipse – about several metres across. (About two-thirds of the Chandler Wobble seems to be caused by ocean currents, and about one-third by winds in the Earth's atmosphere.)

From 1982 to 2005, we measured the location of the north pole as drifting slowly south towards Labrador, about six to seven centimetres each year. But in 2005, the various motions of the north pole suddenly flipped in three unexpected ways.

First, the north pole started heading east, parallel to the equator. Second, it more than tripled its drift speed – to about 24 or so centimetres a year. Third, the Chandler Wobble changed phase. So far, our scientists do not have a good answer as to why the Chandler Wobble changed phase.

However, they do have a good answer for the tipping of the spin axis. Rapid melting of ice on land turned it into water, which then relocated everywhere across our planet – changing its balance. This made the drift velocity of the north pole accelerate and also changed its direction of travel to the east.  

It's hard to imagine something so small as us humans being able to shift something as massive as our whole planet. But we used global warming as a force multiplier. We dumped billions of tonnes of carbon dioxide into the atmosphere, which then heated the atmosphere and oceans.

The combination of hotter atmosphere and ocean water then melted over half a trillion tonnes of ice, which then flowed as liquid water into the oceans. And this redistributed water shifted the north-south spin axis.

This is an extract from Dr Karl's Short Back & Science by Dr Karl Kruszelnicki, Macmillan Australia, RRP $32.99