A Cambridge scientist has warned that the "decaying" of the Greenland ice sheet risks pushing up the sea level and threatens coastal cities around the world.

Peter Wadhams, professor of ocean physics and head of the Polar Ocean Physics Group at the University of Cambridge, said he had observed first hand drastic changes to conditions in the Arctic.

The 71-year-old, who has led 55 expeditions to the region during his career, compared flowing melt water to the "Niagara Falls" as ice in the region disappeared.

Speaking to the Press Association from near the settlement of Kangerlussuaq on the south-west edge of Greenland, Mr Wadhams said there had been "large changes" to the area since his last visit five years ago.

"It's certainly a far more rapid rate of ice loss going on now than at any time in the past," he said.

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Mr Wadhams added: "The rate of global sea level rise ... is really completely dependent now on the loss from the Greenland ice sheet, that's going to be going up quite rapidly.

"The first time I was here 30 years ago, there was never any melt from the Greenland ice sheet even in summer.”

The professor, who first visited the Arctic in 1969, said the melting of ice had moved from the low altitude edges of the Greenland ice sheet to include its surface at the centre.

"It's coming down more or less like Niagara Falls, down through holes all over the ice sheet," he said.

His comments came after a period of record-breaking temperatures that hit the world in recent months.

According to the World Meteorological Organisation (WMO), July 2019 had at least equalled and possibly exceeded the record for the hottest month in history.

It followed data showing the world had experienced the warmest June on record.

Belgium, Germany, Luxembourg, the Netherlands and the UK all saw new national temperature records on 25 July.

The Met Office said it took a recording of 38.7C at Cambridge Botanic Garden, officially the highest temperature recorded in the UK.

Wildfires struck areas of Siberia in Russia in July, with the plumes of smoke visible from space.

According to the US National Snow and Ice Data Centre, the loss of ice extent in the Arctic in the first half of July matched rates from 2012, the year with the lowest September sea ice extent in the satellite record.

Mr Wadhams said 300 cubic kilometres of ice was lost from the Greenland sheet every year.

If the entire Greenland ice sheet melted the sea level would rise by seven metres, flooding most of the world's coastal cities, the professor warned.

"It's decaying and decaying quite rapidly," he said.

WMO secretary-general Petteri Taalas said of global weather conditions: "This is not science fiction. It is the reality of climate change.

"It is happening now and it will worsen in the future without urgent climate action."

Greenland hit by ‘extreme’ temperatures as Europe’s heatwave moves north

In Greenland, Mr Wadhams said there was now "a huge epidemic of icebergs being emitted" as water flow drives glaciers out to sea.

"Most of the other glaciers in the world are already more or less gone," he said.

"So there's not much extra water to add from glaciers from other parts of the world.

"Greenland is now the driver for global sea level rise and will be until the ice has gone from [the country].”

He described witnessing "a raging torrent" passing beneath a bridge in Kangerlussuaq and the appearance of "black ice" as centuries old dirt is left behind by melting ice.

"Some of it is really dark, it means that it reflects less of the radiation of the sun that falls on it," the professor said.

"That means it's absorbing radiation and warming it faster still."

Mr Wadhams is in Greenland to help scope out a suitable location for a new global electric car rally.

Extreme E will see teams race head-to-head in the Arctic, the Himalayas, the desert, the Amazon rainforest and an island in the Indian Ocean.

The race will take place in locations already damaged by the effects of climate change and aims to highlight the environmental challenges facing the world.

Mr Wadhams lamented the changes to the Arctic, admitting that people previously held a "false picture" that it would never change.

"It's very sad," he said.

"You will never again see things that I was able to see when I was young."

He argued the solution to the melting ice was to remove carbon dioxide from the atmosphere, rather than just reducing carbon emissions.

Mr Wadhams added: "The trouble is nobody has suggested any practicable way of bringing back ice.

"That's what we have to do … to cool the whole planet."