Two ice shelves that existed before Canada was settled by Europeans diminished significantly this summer, one nearly disappearing altogether, newly-published research reveals.

The loss is important as a marker of global warming, returning the Canadian Arctic to conditions that date back thousands of years, scientists say.

Floating icebergs that have broken free as a result pose a risk to offshore oil facilities and potentially to shipping lanes. The breaking apart of the ice shelves also reduces the environment that supports microbial life and changes the look of Canada's coastline.

Luke Copland, an associate professor in the geography department at the University of Ottawa who co-authored the research published on Carleton University's website, said the Serson Ice Shelf shrank from 79.15 square miles to two remnant sections five years ago and was further diminished this past summer.

Prof Copland said the shelf went from a 16-square-mile floating glacier tongue to 9.65 square miles, and the second section from 13.51 square miles to two square miles, off Ellesmere Island's northern coastline.

This past summer, Ward Hunt Ice Shelf's central area disintegrated into drifting ice masses, leaving two separate ice shelves measuring 87.65 and 28.75 square miles respectively, reduced from 131.7 square miles the previous year.

"It has dramatically broken apart in two separate areas and there's nothing in between now but water," said Prof Copland.

Prof Copland said those two losses were significant, especially since the Ward Hunt Ice Shelf had always been the biggest, the furthest north and the one scientists thought might have been the most stable.

"Recent (ice shelf) loss has been very rapid, and goes hand-in-hand with the rapid sea ice decline we have seen in this decade and the increasing warmth and extensive melt in the Arctic regions," said Ted Scambos, lead scientist at the National Snow and Ice Data Centre at the University of Colorado.

Prof Copland, who uses satellite imagery and has conducted field work in the Arctic each May for the past five years, said since the end of July, pieces equalling one and a half times the size of Manhattan Island have broken off.

Co-researcher Derek Mueller, an assistant professor at Carleton University's geography and environmental studies department, said the loss this past summer equals up to three billion tons.

Prof Copland said their findings have not yet been peer reviewed since the research is new, but a number of scientists contacted by The Associated Press reviewed the findings, agreeing the loss in volume of ice shelves was significant.

Mr Scambos said the loss of the Arctic shelves was significant because they were old and their rapid loss underscored the severity of the warming trend scientists see now relative to past fluctuations such as the Medieval Warm Period or the warmer times in the BC era.

Ice shelves, which began forming at least 4,500 years ago, are much thicker than sea ice, which is typically less than a few feet thick and survives up to several years.

Canada has the most extensive ice shelves in the Arctic along the northern coast of Ellesmere Island. These floating ice masses are typically 131ft thick - equivalent to a 10-storey building - but can be as much as 328ft thick. They thickened over time via snow and sea ice accumulation, along with glacier inflow in certain places.

The northern coast of Ellesmere Island contains the last remaining ice shelves in Canada, with an estimated area of 402 square miles, said Prof Mueller.

Between 1906 and 1982, there has been a 90% reduction in the areal extent of ice shelves along the entire coastline, according to data published by WF Vincent at Quebec's Laval University.

The former extensive "Ellesmere Island Ice Sheet" was reduced to six smaller, separate ice shelves: Serson, Petersen, Milne, Ayles, Ward Hunt and Markham. In 2005, the Ayles Ice Shelf whittled almost completely away, as did the Markham Ice Shelf in 2008 and the Serson this year.

"The impact is significant and yet only a piece of the ongoing and accelerating response to warming of the Arctic," said Dr Robert Bindschadler, emeritus scientist at the Hydrospheric and Biospheric Sciences Laboratory at the Nasa Goddard Space Flight Centre in Maryland.

Dr Bindschadler said the loss was an indication of another threshold being passed, as well as the likely acceleration of buttressed glaciers able to flow faster into the ocean, which accelerates their contribution to global sea level.

Prof Copland said mean winter temperatures had risen by about 1 degree Celsius (1.8 degrees Fahrenheit) per decade for the past five to six decades on northern Ellesmere Island.