Earth has spun on its axis since it emerged out of a primordial cloud of rock, gas and dust. The endless pivot has given us a predictable pattern of night and day, and when combined with its orbit around the sun, seasons and climate. But that natural rhythm, it appears, is changing.
According to a study published Thursday in the journal Geophysics Research Letters, pumping groundwater to the surface to irrigate humanity’s farms has sent Earth’s pole drifting.
The study found redistributing water from aquifers to the ocean has knocked the pole almost 80 centimetres eastward and boosted the effects of sea level rise.
“There’s a steady depletion of groundwater,” said Clark Wilson, a professor emeritus in geophysics at the University of Texas at Austin.
“When you move water from one place to another, you change the gravity field of Earth.”
Somewhere under the Arctic ice, the Earth’s axis shifts in a roughly 10-metre circle every year.
At the same time, across the planet, massive amounts of ice, water and rock are on the move.
Since being freed from the ice sheets of the last glaciation, most of Canada’s landmass has been rising in a phenomenon known as isostatic rebound. That has combined with the pressures of human-caused climate change — including the melting of modern glaciers and ice shelves — to throw the planet into a deeper wobble.
Since the late 1800s, all that movement has helped send Earth’s rotational axis adrift by 10 or 15 metres, said Wilson, one of the study’s co-authors.
But Wilson said something in that calculation was always missing, and the numbers didn’t make sense until they considered the long-term effects of heavy groundwater removal.
Between 1992 and 2010, humans removed 2,150 gigatonnes of groundwater (in frozen terms, a gigatonne of water is equivalent to a one-kilometre squared block of ice).
Draining into the sea, that water appears to have thrown off the planet's spin — like adding weight to one side of a spinning top.
Wilson and his colleagues combined those numbers with computer models and highly accurate satellite data from the Global Positioning System (GPS), which launched in 1993.
Sucking that much water out of the Earth and draining it into the seas has also raised global sea levels, the study concluded.
Sea level increased in most of the world’s oceans by about 10 millimetres. But in northwestern India and western North America, which both saw significant decreases in groundwater storage, sea level actually dropped.
That’s because, unlike a bathtub, the planet has a gravitational field that reacts when that much mass moves across its surface.
Because poles shift on the order of metres per year, the impact of groundwater will unlikely impact the planet’s seasonal swings, Clark said. But knowing where the Earth spins on its axis and where that pole is going is crucial for modern technology.
Ride-hailing services like Uber, shipping, and flying an airplane or drone all rely on a near-exact measures of time and location.
“It’s all based on having very high-precision atomic clocks in the satellites. It allows you to have timing with micro-second precision networking,” said Clark.
“If you don’t know that pole position, you’re using a reference frame uncoupled to the Earth.”
