By Lesley Wroughton

ABOARD THE HDMS THETIS, Greenland (Reuters) - Standing near Greenland's Jakobshavn glacier, the reputed source of the iceberg that sank the Titanic over a century ago, U.S Secretary of State John Kerry saw evidence of another looming catastrophe.

Giant icebergs broken off from the glacier seemed to groan as they drifted behind him, signaling eventual rising oceans that scientists warn will submerge islands and populated coastal region.

Briefed by researchers aboard a Royal Danish Navy patrol ship, Kerry appeared stunned by how fast the ice sheets are melting. He was struck by the more dire warnings he heard about the same process underway in more remote Antarctica.

"This has been a significant eye-opener for me and I've spent 25 years or more engaged in this issue," Kerry said on the deck of the ship with Danish Foreign Minister Kristian Jensen during a two-day visit that ended late on Friday.

Kerry made his first visit to this part of the Arctic to witness the effects of climate changes and press the need to implement the Paris climate accord. He has called it "the world's most fearsome weapon of mass destruction".

The United States chairs the Arctic Council, a forum created in 1996 to tackle issues arising from increased Arctic activity.

The landmark Paris agreement included commitments by most nations to reduce carbon emissions contributing to climate change but lacked any enforcement mechanism, leaving open who will pay costs that will rise into the trillions of dollars.

Current U.S. targets for cuts in greenhouse gases by the Obama administration for 2025 fall far short of what scientists say is needed to rein in rising temperatures.

"What we did in Paris ... is critical now to be implemented, but it's not even enough," he said. "We have to all move faster in order to embrace new energy policies that are sustainable, that are clean, all of which are there for the using if governments and private sector make the right choices."

U.S. Republican Presidential nominee Donald Trump has said he would renegotiate America's role in the climate agreement if he becomes president.

HUMAN CONTRIBUTION TO CLIMATE CHANGE

By visiting Greenland and the Arctic research post at Ny-Alesund in Norway's far-north, Kerry focused on some of the most visible impacts of climate change.

"There is no mistaking that we are contributing to climate change, we human beings have choices that can undo the damage," said Kerry. "There is profound change throughout the Arctic."

Jakobshavn is one of the world's biggest glaciers and the most active in the Arctic, where ice sheets are melting at a rate faster than ever before.

David Holland, a New York University scientist studying changes on Jakobshavn, explained that the glacier could retreat by about 62 miles (100 kilometers) over the next 100 years if the thawing of its ice sheet continues at its current pace.

If Greenland's ice sheets all melted, that would raise sea levels by about 6 meters (20 feet) over thousands of years. That is modest compared to what could happen if Antarctica thaws, said Holland.

Two developments in recent days show the magnitude of the challenge. For the first time in 4 million years, levels of carbon dioxide - a heat-trapping gas produced by burning fossil fuels - hit 400 parts per million in Antarctica, according to the National Oceanic and Atmospheric Administration.

The threshold shows the rising levels of climate pollution.

Last week, temperatures in Greenland's capital hit a record 24.8 degrees Celsius (76.6 Fahrenheit) for a single day in June, according to records dating back to 1958. Worldwide, 2016 has set repeated monthly records after a record warm 2015.

"GIGANTIC TRANSFORMATION"

"This is gigantic transformation taking place," said Kerry. "You can see it with the naked eye, as you see where the ice has retreated from just in the last 15, 20 years, where the marks are still left."

The Arctic is warming at about twice the global average, partly because the melting of the ice cover has revealed darker ground and water underneath that soak up even more heat.

"Things are changing and we are perhaps the last generation that can do something about it," said Jensen.

This new access to the ground underneath has opened the Arctic to increased political and commercial competition, including exploration for oil and minerals by countries that used to have no access to the region.

Aboard a research vessel at Ny-Alesund, the northernmost non-military post in the world, Kerry warned that exploiting newly accessible resources would undermine the carbon-reduction strategy of the Paris accord. He also said the public was still not sufficiently conscious of the challenge ahead.

"Even where there is awareness, the steps people are taking are not big enough, fast enough. We have a huge distance to travel," he said while visiting Ny-Alesund.

Temperatures there are now between six to 11 degrees warmer than normal, said Jan-Gunner Winther, director of the Norwegian Polar Institute, who wonders whether the changes signal a tipping point.

"We have more questions than answers," says Winther, "We are in the midst of a change that we have no comparison with in history because it is so much more rapid," he said.

(Editing by Tom Heneghan)