Even though we can’t feel it, most of us are aware that our planet spins like a top during its interminable journey around the Sun. Rather than revolving about a vertical axis, however, Earth has always been off-kilter, spinning about an axis (or pole of rotation) that’s tilted at 23.44°.
The amount of tilt – also known as obliquity – is never constant, and displays natural short-term oscillations and longer-term cycles.
Earth’s tilt can also be changed by shifting huge amounts of mass around the planet. And this is happening right now, on an extraordinary scale, due to us.
As global heating drives the melting of the polar ice sheets, it decants colossal volumes of water into the oceans. In a recent paper, scientists have revealed that this redistribution of mass is very slightly modifying the world’s tilt.
But there’s more. In itself, polar melting isn’t sufficient to account for all of the tilt change – something else has to be happening too. The answer, the authors of the paper say, is beneath our feet.
In recent decades, the world’s ever-growing population has resulted in a huge increase in demand for freshwater, particularly for use in crop irrigation. This need has been largely met by sucking up an estimated 23 million cubic kilometres of groundwater that was once locked away in aquifers beneath Earth’s surface.
The authors have shown that the tilt change can be completely explained if, over the period of study, a water mass of around 2,150 billion tonnes (equal to 2,150 cubic kilometres) was abstracted from aquifers and used in one way or another, before finding its way, ultimately, into the oceans.
As Prof Ki-Weon Seo, a geophysicist at South Korea’s Seoul National University, and his co-authors, observe, it’s “like adding a tiny bit of weight to a spinning top; Earth spins a little differently as water is moved around.”
Between 1993 and 2010, this transfer of water mass raised global sea levels by more than 6mm (0.2in), shifting the rotational pole by nearly 80cm (2.6ft).
We have a deeper impact than we think
Should we be worried? Such a tiny change in tilt isn’t going to have an impact on the climate, so the answer should be no. On the other hand, this finding shows that how we live today has a measurable impact on a planetary scale.
Prof Benedikt Soja of ETH Zurich in Switzerland noted in a recent interview that “we can see our impact, as humans, on the whole Earth system, not just locally, like the rise in temperature, but really fundamentally, altering how it moves in space and rotates.”
One of the more outlandish claims of the denier community is that humanity is too puny to have any real effect on the climate and how Earth operates in general. But this latest evidence is another nail in the coffin of this dangerous idea.
In addition to modifying tilt, the redistribution of mass from polar regions into the ocean basins has also contributed towards slowing the planet’s rotation. This is estimated to be happening at 1.3 milliseconds a century; a value that could more than double by 2100 if we fail to curb emissions.
Like a spinning figure skater throwing out their arms to slow down, this huge transfer of mass, from polar regions close to Earth’s axis to the wide expanse of the world’s oceans, will continue to slow the planet’s rotation further.
Compounding prodigious volumes of water behind China’s Three Gorges Dam has had the same knock-on effect, slowing Earth’s rotation and increasing day length by 60 billionths of a second.
It's a matter of time
While these changes don’t have a detectable impact on our physical environment, they certainly present a challenge for electronic systems and networks predicated on accuracy and precise time measurement.
These include: the GPS that allows planes and ships to navigate, the algorithms that underpin today’s financial markets and global timekeeping. When a 'leap' second was added to the world’s atomic clocks in 2015, glitches were reported across multiple networks.
As groundwater aquifers are drained further, and polar melting continues, repeated tweaks to timekeeping and positioning systems – to account for new tilt and day length variations – will be essential to prevent their potentially catastrophic failures.
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