In late July, polar scientist Martin Stendel was sweltering at his desk in Copenhagen as Europe suffered its worst ever heatwave.

Key points: Greenland's ice cap is melting, and that water is draining into the ocean, contributing to sea level rise

Greenland's ice cap is melting, and that water is draining into the ocean, contributing to sea level rise In July this year, it's estimated more than 30 billion tonnes of ice melted in three days

In July this year, it's estimated more than 30 billion tonnes of ice melted in three days Australian scientists have been able to use NASA satellites to accurately weigh how much ice is melting

As temperatures climbed to more than 15 degrees Celsius above average, the meteorologist realised the record heat was about to hit the arctic.

"I looked at the forecast, and one could see that this heatwave, or this anomalous temperature, was on its way to Greenland," Dr Stendel said.

Greenland holds the second-largest reserves of fresh water on the planet, after Antarctica.

But year in, year out, the Greenland ice cap has been melting, and that water is draining into the ocean, contributing to sea level rise.

Second greatest ice melt event

Danish climate scientist Martin Stendel's team estimated over 30 billion tonnes of ice melted in a three-day event over summer. ( Supplied: Martin Stendel )

Over the next week, Martin Stendel looked on as Greenland suffered the second greatest melt event in recorded history.

"Gigatonnes and gigatonnes melted. And actually, this contributed in a measurable way to sea level rise," Dr Stendel said.

Martin Stendel's team at the Danish Meteorological Institute estimated that more than 30 billion tonnes of ice melted in just three days.

Even more troubling, the highest point in Greenland — above 3,000m in altitude — reached temperatures above zero.

"This has happened only seven times, seven times over the past 2,000 years," Dr Stendel said.

"So, in this 2,000-year record, we've had two events in the past decade — in 2012 and now in 2019."

GRACE works by measuring the distance between two satellites as they change speed due to minute changes in gravity ( Supplied: NASA )

Australian scientists weigh Greenland

When the heatwave hit Greenland in early August, Paul Tregoning from the Australian National University in Canberra was waiting for the first data from NASA's new GRACE-FO mission.

GRACE-FO stands for Gravity Recovery and Climate Experiment Follow On and it uses a pair of satellites that literally weigh the Earth's water from space by detecting tiny changes in gravity, caused by changes in mass on Earth.

Dr Paul Tregoning at the Australian National University uses twin satellites to accurately measure the change in weight of ice on Greenland. ( Supplied: Paul Tregoning )

"One satellite is following 200 kilometres behind the other," Dr Tregoning explained.

"The change in distance between the satellites is measured really accurately, down to a 10th of a thickness of a human hair, or about a micro-metre.

"If there is less ice in a location, then there'll be weaker gravity that will affect the change in distance between the satellites, which is what we measure."

Like a huge ice cube, as Greenland melts it weighs less. These changes in mass affect how the satellites orbit Earth, which can be detected in the measurements made by GRACE-FO.

GRACE data shows the steady decline in the mass of the Greenland ice cap. ( Supplied: Paul Tregoning )

Dr Tregoning's team have devised a new way of calculating the weight of large masses like the Greenland ice cap, using data from GRACE-FO.

The first GRACE satellites were launched in 2002, the mission going on to far outlive its five-year design lifespan.

But in 2017 the first GRACE satellites finally stopped working and climate scientists lost one of their most powerful tools to track ice cap melt.

New GRACE Follow-On satellites were launched in 2018, with technology on board based on designs by the ANU's Daniel Shaddock.

In recent months, the first verified data from the new GRACE-FO mission was released publicly by NASA. But it required expert knowledge to turn instrument data into estimates of mass loss of ice sheets.

"We've got our eyes back in the sky. We can see again what's going on," Dr Tregoning said.

Enough melted ice to cover Tasmania

Professor Daniel Shaddock and colleagues preparing to test the GRACE Follow-on retroreflector ( Supplied: Australian National University )

Using the fresh data, Dr Tregoning measured that Greenland this September weighed almost a third of a trillion tonnes less than it did the previous month.

He said the melting over the entire summer was more than 10 times the 30 gigatonnes melted in the 3-day heatwave event, and painted this picture.

"The mass loss in Greenland between August and September 2019 would amount to 42 millimetres of water covering the whole of Australia, or 4.7 metres over the whole of Tasmania."

These figures closely track the amounts calculated by Martin Stendel from the Danish Meteorological Institute, who used computer models to estimate 329 billion tonnes of ice was lost in Greenland in the year from September to September.

The GRACE missions have now been documenting the melting of the Greenland ice cap for 17 years.

Ice loss varies from year to year, but the trend is relentlessly downward.

"From mid-2003 to mid-2019 there has been the equivalent of about 2 metres of water scraped off the entirety of the Greenland ice sheet," Dr Tregoning said.

"If you then convert that into the effect that it would have had on global sea level, the amount of ice that has melted from Greenland since the beginning of the GRACE mission has added about 12 or 13 millimetres of water across the whole of the global oceans."

Given that we are looking at an average 20-centimetre sea level rise historically since the industrial period, that is significant.

"Greenland was not contributing to global sea level rise a few decades ago," Ian Fenty from NASA's Oceans Melting Greenland (OMG) team said.

"And now it's a major and possibly accelerating contributor to the rise."

Retreating ice, rain at Christmas

Ian Fenty was on a ship off the coast of Greenland when the heatwave struck in July.

"Well, it was weird to be in a t-shirt in the middle of a heatwave at 76 degrees north," Dr Fenty said.

"It makes me think this is the new normal in Greenland."

He said the people of Greenland were watching their environment change around them.

"A few years ago, it rained on Christmas Day and it's extraordinary, right? You would think of the dead of winter in Greenland, you're definitely getting snow. To have a wet, rainy Christmas seems extraordinary," Dr Fenty said.

"People know that the ice is retreating."

Terminus of outlet glacier in west Greenland. Supplied as supporting material for Nature paper. December, 2018. ( Woods Hole Oceanographic Institution: Matt Osman )

OMG — Oceans Melting Greenland

Dr Fenty said warmer air temperatures were only half the story when it came to melting glaciers. His cheekily named OMG team was looking at how warmer sea temperatures, not just warmer air temperatures, were also melting Greenland's ice caps.

Where glaciers meet the sea, warmer sea water was melting and eroding the ice, leading to glaciers moving faster and melting further.

"We lose about half of the ice loss from the atmosphere and the other half along the margins, and the ocean plays a first-order role in determining how much ice is lost on the margins," Dr Fenty explained.

It was this eroding of glaciers by warmer oceans that most worried the ANU's Paul Tregoning.

"Now, a 13 millimetre rise in 18 years may not sound like it's dramatic, but the concern that we should have is that, particularly in the Antarctic ice sheet, there is an enormous amount of potential large global sea level that is locked up below sea level and may be released if we continue to warm our climate and oceans as we have been doing recently."

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