The Earth is getting drier and may have hit a tipping point for how much water is stored in soil because of climate change.

So great is the decline in soil moisture that it has outpaced Greenland's melting ice sheets in its contribution to sea level rise and changes to the wobble in Earth's rotation.

That's according to a new study in the journal Science, which suggests more than 2,614 gigatonnes of moisture was lost from our planet between 2000 to 2016.

It's a trend that scientists think led to a major shift in land-based water storage — sources like groundwater, rivers, lakes, soil moisture and ice — from 1992.

The researchers estimated between 2000 to 2002, soil moisture loss was about 1,614 Gt, equivalent to a 1.95 millimetre per year rise in sea level.

That's compared to a 900 Gt loss of ice in Greenland from 2002 to 2006, which contributes to about 0.8mm of sea level rise annually.

Global soil moisture levels have not recovered, and a further 1,009 Gt was lost from 2003 to 2016.

Yearly soil moisture changes and the estimated sea level variation in corresponding years. (Supplied: Earthu H Oh)
One of the study's lead co-authors Dongryeol Ryu, a hydrologist and remote sensing specialist at the University of Mebourne, said the planet was seeing its land-based water storage drop without recovering.

"It's pretty similar to inflation," he said.

"If you are purchasing consumer goods the same way, somehow your wallet becomes slimmer.

"That means that if we use the water in agriculture and other sectors as we did before ... we will have much less water left on the land."

University of Mebourne hydrologist Dongryeol Ryu. (Supplied: Dongryeol Ryu)
How researchers estimated Earth's loss of moisture
For some years, scientists have been aware of changes to Earth's hydrological cycle — the movement of water between the surface and the atmosphere — at a regional level.

But getting an idea of whether there has been a longer term global drying trend tied to a warming planet has been more difficult to calculate.

The twin satellite program Gravity Recovery and Climate Experiment (GRACE) started collecting data in 2002, and has helped scientists to measure groundwater depletion, sea level changes and ice sheet loss.

These sources can be used to calculate changes in the planet's land-based water storage.

To develop a data set pre-2002, however, the researchers estimated land-based water storage by comparing GRACE's data with historical sea level changes and the wobble in Earth's planetary rotation.

The Gravity Recovery and Climate Experiment twin satellite program has collected data from 2002. (Illustration: NASA Jet Propulsion Laboratory)
Professor Ryu said global rain precipitation changed every year, oscillating with ups and downs.

"Even if you have large rainfall or drought in one year, it doesn't mean you see that impact right away in soil moisture because it's a delayed response," he said.

"We had quite a strong El Niño in 1997 and 1998 and also global temperature increasing steeply from the 1980s.

"They all worked together to create this unusual drop in soil moisture in 2000 to 2002."

At the same time, global precipitation dropped, he said.

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GRACE measures anomalies in gravity strength with red indicting stronger areas and blue for weaker areas. (Graphic: NASA Jet Propulsion Laboratory) (GIPHY)
Georgy Falster, a University of Adelaide climatologist who was not involved in the study, said soil moisture observations around the globe were unfortunately sparse.

That meant reanalysis data used in the study relied heavily on how well modelling simulated changes in soil moisture.

"The satellite and reanalysis data match quite well from 2003 to 2019, providing confidence in these results," Dr Falster said.

"Unfortunately, the satellite data are only available from 2003 so the rapid change across 2000–2002 cannot easily be verified."

What's global warming got to do with it?
Professor Ryu said, on average, it seemed the Earth was losing land-based water storage because of an increase in air temperature and vapour pressure deficit (VPD).

VPD measures the gap between how much moisture is in the air compared to how much moisture the air could have when fully saturated.

Saturated air leads to things like cloud formation and increased precipitation.

Professor Ryu said water was lost in the soil faster if VPD was greater.

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"If you look at the global data … both ground and model data show a very clear and steep increase in VPD from around the year 2000," he said.

"At a regional scale like in Australia, in the Northern Territory and Queensland, soil is wetter now than 20 or 30 years ago.

"But on average, it seems [globally] the land is losing water and that's largely because of the increase in temperature and an increase in VPD."

Areas which have seen greater groundwater depletion than other regions include northern India, central California and eastern China.

Dr Falster said the increasing VPD in recent decades was the result of human-caused global warming.

"[It's] making the recovery of lost soil moisture increasingly difficult," she said.

"The authors suggest that droughts over [recent] decades have been so widespread and severe that more than 2,600 Gt of water has permanently been transferred from the continents to the ocean.

"The equivalent of more than 4,000 Sydney Harbours."
Dr Falster said the findings highlighted the importance of continued investment in monitoring variables like soil moisture and evapotranspiration — the movement of water into the atmosphere from evaporation and plant transpiration — both globally and in Australia.

Professor Ryu said further research could look at agricultural water-use to get a better understanding of its impact on water storage depletion.

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