Shifts in the spin axis of our planet are not only being driven by melting ice sheets but also changes in the relative amount of water stored on the continents, researchers have discovered.

Key points: Earth's spin axis drifted towards Canada from 1899 to 2000

Earth's spin axis drifted towards Canada from 1899 to 2000 In 2000 it changed direction, and is now drifting towards Greenwich, UK

In 2000 it changed direction, and is now drifting towards Greenwich, UK The shift is mainly due to melting ice sheets

The shift is mainly due to melting ice sheets Changes in the amount of water storage on land also affects polar drift



The Earth spins from west to east, which is why we have a night and day.

The spin axis — which is the line through the planet from one pole to another — constantly wobbles.

For 100 years after the wobble was first measured in 1899, the spin axis drifted in one direction as the North Pole headed southwards towards Hudson Bay in Canada.

However, in 2000 the spin axis did a dramatic turn and the North Pole began drifting east towards Greenwich in the United Kingdom.

"Since about the year 2000 there is a new drift direction, and this drift direction is along the central meridian," said Dr Surendra Adhikari, a theoretical glaciologist at the Jet Propulsion Laboratory, California.

Dr Adhikari said many scientists agreed that melting of the polar ice sheets — both Greenland ice sheet and the western part of the Antarctic ice sheet — is the major driver for the direction shift.

But analysis of gravitational data between 2003 and 2015 indicates changes in distribution of water stored on continents also contributed to polar drift, report Dr Adhikari and his colleague Dr Erik Ivins in the journal Science Advances.

"If you combine [the melting ice sheets] together they are not enough to produce the new drift direction. Something is missing," Dr Adhikari said.

Changes in gravity indicate changes in water

Using data from NASA's GRACE satellites, the researchers measured changes in gravity across the whole planet on a monthly basis.

"We have a pair of satellites that constantly provide us with the information about how the gravity of our planet is varying every single month, since about 2002," Dr Adhikari said.

The researchers observed differences in gravity at a regional level, which they said, are explained by the presence — or absence — of water.

"The logic is very simple. When you see a positive gravity anomaly you are getting more mass in that region in that particular month," Dr Adhikari said.

"And the only material transported on that huge scale is water."

This gravity anomaly could be related to drought, water melting in glaciers or even groundwater pumping, he said, but the distribution of water on land across the planet adds up to an overall pull on the Earth's spin axis.

"Over the last 13 years, the hydrological mass has been distributed in such a way that it's pulling the pole towards Eurasia — India," Dr Adhikari said.

Added together with the impact of the melting ice sheets, this mathematically accounts for the current change in direction of the movement of the spin axis.

"For the first time we have a view that there is another important component, which is related to the pattern of the global scale of land-water storage," he said.



Water storage could explain past wobbles

Looking back over all the data captured in the past 115 years, the scientists also observed smaller wobbles in the past.

Why was the North Pole heading towards Canada? Throughout the entire 20th century the North Pole was drifting slowly towards Hudson Bay, Canada

Throughout the entire 20th century the North Pole was drifting slowly towards Hudson Bay, Canada This drift was the direct result of the melting of the Laurentide ice sheet, which covered much of North America during the last ice age and finally disappeared 9,000 years ago

This drift was the direct result of the melting of the Laurentide ice sheet, which covered much of North America during the last ice age and finally disappeared 9,000 years ago Its melting resulted in a transfer of mass from northern North America to the oceans

Its melting resulted in a transfer of mass from northern North America to the oceans The Earth recalibrated to this shift in mass by slightly shifting its spin axis

"For all the 20th century the pole was heading towards Hudson Bay Canada," Dr Adhikari said. "But sometimes it's heading to the east or west of that general drift direction."

They were able to confirm that this wobble related to water storage on land on a roughly 10-year — or decadal — timescale.

"We have found a correlation between the decadal scale motion of the pole and the decadal scale redistribution of the continental land-water storage," he said.

This new information will provide insight into past climates, and possibly provide more confident predictions about future climate change, he said.

