Tuesday 7th April 12:53 pm
Global warming shifts spin axis of Earth
Tuesday 7th April 2015 12:53 pm
Even though we are only a little way into the 21st century, the signs of global warming are obvious and many. There are droughts in East Africa, stranded polar bears in the Arctic, bleached coral reefs in the tropics, and retreating glaciers on land.
But the latest sign was a real surprise to me. By burning huge quantities of fossil fuels, we humans have actually tipped the Earth off its axis — by a tiny amount.
When I say “tiny”, let me emphasise how tiny. It’s centimetres per year, not hundreds or thousands of kilometres each year.
The north-south spin axis of the Earth runs, of course, through the North and South Poles — the North Pole in the Arctic, the South Pole in the Antarctic.
From 1982 to 2005, we measured the location of the North Pole as drifting slowly southwards towards Labrador around six to seven centimetres each year.
But in 2005, the North Pole suddenly, and without any warning, did two new things. First, it chucked a leftie and started heading east, parallel to the Equator. Second, the North Pole more than tripled its speed to about 24-or-so centimetres per year.
We’ve been measuring the position of North Pole for over a century, but the more accurate methods came with the introduction of satellites, such as the GRACE satellite, (‘GRACE’ standing for Gravity Recovery and Climate Experiment).
GRACE measures how the gravity on different parts of the planet changes over time, following variations in polar ice sheet, mountain glaciers, land hydrology, the atmosphere and the oceans.
So let me first step back and look at the Earth as it spins on its own axis. The Earth is not perfectly spherical, but instead it’s a bit flattened at the poles, and a bit bulging at the equator. The surface is not smooth, but instead it’s pretty bumpy. And our planet is not perfectly rigid, but somewhat elastic, being made mostly of molten rock and then liquid iron all surrounding a core of solid iron.
As a result of these factors, when the Earth rotates on its own axis about once a day, that spin axis wobbles a little. One major wobble is the so-called ‘Chandler wobble’, which the American astronomer, Seth Carlo Chandler, discovered over a century ago in 1891.
Over a period of slightly more than a year (about 430 days), the Chandler wobble shifts the north-south spin axis of the Earth about three to six metres. It seems to be caused by pressure effects — about two-thirds due to the ocean pressing on the sea floor, and about one-third due to changes in the Earth’s atmosphere.
But here’s something odd. Back in 2005, the same year that the north-south spin axis of the Earth abruptly turned left and accelerated, the Chandler wobble changed phase. So far, our scientists do not have a good answer why the Chandler wobble changed phase.
However, they do have a good answer for the tipping of the spin axis. Plain and simple, rapid melting of ice on land has driven Earth’s North Pole to the east. This solid ice used to be on land, but is now liquid water spread everywhere across the planet.
We’ve been measuring this change to the land ice with satellites beginning in the early 1990s, right up to our current CryoSat-2, which was launched in 2010. Over the decades, the satellites have taken many tens of millions of height measurements. The most recent analysis tells us that between 2011 and 2014, Greenland and Antarctica between them were losing about 500 billion tonnes of land ice per year – about three-quarters of it from Greenland. This was an increase of two-to-three times over the previous loss rate as measured between 2003 and 2009.
This is actually quite astonishing. Prior to this, I would not have thought that anything we humans did could shift the spin axis of our planet. But we used global warming as a force multiplier. We dumped billions of tonnes of carbon dioxide into the atmosphere, it heated the atmosphere and the oceans, which then melted half-a-trillion tonnes of ice which then flowed as liquid water into the oceans and in doing so shifted the North-South spin axis.
Why, in 2005, did both the Chandler wobble and the spin axis shift suddenly instead of gradually? Again, we know that they did, but we don’t why – yet.
Perhaps it’s like pushing a pencil towards the edge of a table. You push and you push and you push, and it’s still on the table. You give it just one more tiny push, and its centre of gravity is no longer supported by the table, and it suddenly falls to the floor.
So if we push and push at the balance of our planet, it may well respond with by throwing a real wobbly of its own …
This blog first appeared on Dr Karl's Great Moments in Science
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