COPENHAGEN - Global warming has increased the speed at which glaciers in Greenland are melting by fivefold over the last 20 years, scientists from the University of Copenhagen said on Friday.

Greenland's ice melt is of particular concern, as the ancient ice sheet holds enough water to raise sea levels by at least 20 feet (6 meters) if it were to melt away entirely.

A study of a thousand glaciers in the area showed the rate of melting has entered a new phase over the last two decades, Anders Anker Bjork, assistant professor at the department of geosciences and natural resource management at the University of Copenhagen told Reuters.

"There is a very clear correlation between the temperature we experience on the planet and the changes we observe in how rapidly the glaciers are melting," Bjork said.

The glaciers on average decrease by 25 metres annually, compared with 5-6 metres around two decades ago, scientists concluded after studying the development of the glaciers over 130 years through satellite imagery and 200,000 old photos.

The world has already warmed by nearly 1.2C (2.2F) above pre-industrial temperatures, and 2023 is "virtually certain" to be the warmest in 125,000 years, scientists from the European Union said earlier this month.

Lowering temperatures would require a global effort to minimise greenhouse gases in the atmosphere, said Jørgen Eivind Olesen, Institute Director of the Climate Institute at Aarhus University.

"I believe we can prepare for those glaciers to continue to melt at increasing speeds," Olesen said.

Glaciers in Greenland are often used to anticipate the effects of climate change on Greenland's ice sheet.

"If we start to see glaciers losing mass several times faster than in the last century, it can make us expect that the ice sheet will follow the same path just on a slower and longer time scale," William Colgan, senior researcher at the Geological Survey of Denmark and Greenland (GEUS) said.

The Greenland ice sheet contributed 17.3% of the observed rise in sea level between 2006 and 2018 and glaciers have contributed 21%. There are around 22,000 glaciers in Greenland.

(Reporting by Johannes Birkebaek; editing by Barbara Lewis)