A new NASA-funded study finds that the Greenland and Antarctic ice sheets are losing mass at an ever-accelerating pace.

Total ice sheet mass balance between 1992 and 2009, as measured for Greenland (top), Antarctica (middle) and the cumulative sum of both ice sheets (bottom), in gigatonnes per year. A straight line in these figures (with negative mass balance) would be a constant loss of ice, the slope indicates that the loss is increasing over the last twenty years.

The team found that for each year over the 18-year study, the Greenland ice sheet lost mass faster than it did the year before, by an average of 21.9 gigatonnes a year. In Antarctica, the year-over-year speedup in ice mass lost averaged 14.5 gigatonnes. The two ice sheets lost a combined average of 36.3 gigatonnes more than they did the year before. Lead author Eric Rignot, jointly of NASA’s Jet Propulsion Laboratory, Pasadena, CA and the University of California, Irvine said this means sea levels will rise more than previously thought:

What is surprising is this increased contribution by the ice sheets is already happening. If present trends continue, sea level is likely to be significantly higher than levels projected by the United Nations Intergovernmental Panel on Climate Change in 2007.

The authors conclude that if this trend continues, ice sheets will be the dominant contributor to sea level rise in the 21st century.

Due to a lack of basis in published literature, the IPCC’s models did not consider ice sheet flow. The IPCC’s 4th Assessment Report estimated that sea level rise would be 18 to 38 cm (7 to 15 inches) in a low scenario and 26 to 59 cm (10 to 23 inches) in a high scenario.

Study results are published this month in Geophysical Research Letters.

A number of stories appeared in the media last week about one of Greenland’s largest of  “outlet” glaciers (glaciers ending in the sea) calving an enormous ”ice island” of more than 100 square miles in size.

None of the stories had a good graphic showing what happened. Fortunately, WWF Climate Blog has posted this one:

The glacier has lost about a quarter of its floating ice shelf.

A 2009 survey of 34 of the widest Greenland marine-terminating glacier outlets from the inland ice sheet found the loss rate has been nearly constant since 2000.

Above: cumulative annual area changes for 34 of the widest Greenland ice sheet marine-terminating outlets. Source: Byrd Polar Research Center.

To put the Petermann Glacier’s latest ice island in perspective, the island’s area of at least 260 km² is well over twice what all 34 glaciers surveyed by the  Byrd Polar Research Center have been losing annually (-106 km² per year).

Scientists recently documented the breakup up of a 7 km² (2.7 square mile) section of another glacier in the region, the Jakobshavn Isbrae glacier. The calving front – where the ice sheet meets the ocean is now further inland than at any time previously observed.

Location of the successive calving fronts of the Jakobshavn Isbrae glacier between 1851 and 2009, overlain on a Landsat image from 7/29/2009. Source: NASA/Goddard Space Flight Center Scientific Visualization Studio. Historic calving front locations courtesy of Anker Weidick and Ole Bennike, Geological Survey of Denmark and Greenland.

A British research team studying the Greenland ice sheet has discovered evidence of a rapidly accelerating rate of melt: the ice sheet in the region they are studying has dropped six meters in just one month.

The shrinking of the glaciers is caused by surface melt, a vicious cycle in which melted ice brings about further thawing of the cap beneath it. Frozen ice has an “albedo”, or reflectivity, of around 80%, whereas open water reflects only around 20% of the sun’s rays. So as the ice melts and turns into water, its surface reflectivity decreases. It then absorbs even more of the heat from the sun, further accelerating the melting.

Black soot particles are also contributing, as shown in this photo in an article at Sky News.

Dr Hubbard shows how soot has pock-marked the ice

Dr. Alan Hubbard explains:

What you see in there is a bunch of particles of aeolian dust . . . and black soot and just particles that are in the air caused by industry or combustion engines. Because they are dark in color, compared to the ice that surrounds it, which is white, they absorb more of the sunlight and a lot more of its energy, which creates a positive feedback and so they effectively drill down into the surface of the ice sheet.

Ice loss from the Greenland ice sheet is now moving up its northwest coast, according to a new study led by the Denmark Technical Institute’s National Space Institute in Copenhagen and involving the University of Colorado at Boulder.

An article in Physorg.com quotes lead author Shfaqat Abbas Khan:

Our results show that the ice loss, which has been well documented over southern portions of Greenland, is now spreading up along the northwest coast.

The paper, titled Spread of ice mass loss into northwest Greenland observed by GRACE and GPS, will appear in the journal Geophysical Research Letters. From the abstract:

Greenland’s main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade. Recent work has shown that Greenland’s mass loss is still increasing. Here we show that the ice loss, which has been well-documented over southern portions of Greenland, is now spreading up along the northwest coast, with this acceleration likely starting in late 2005. . . .  In addition to showing that the northwest ice sheet margin is now losing mass, the uplift results from both the GPS measurements and the GRACE predictions show rapid acceleration in southeast Greenland in late 2003, followed by a moderate deceleration in 2006. Because that latter deceleration is weak, southeast Greenland still appears to be losing ice mass at a much higher rate than it was prior to fall 2003.

Joseph Romm at Climate Progress points out the staggering ice loss seen in Greenland since 2003 is all the more worrisome because it was not predicted by the IPCC’s climate models.  In 2001, the IPCC thought that neither Greenland nor Antarctica would lose significant mass by 2100. But they both already are – 100 years ahead of schedule.

Another new study finds the carbon pollution threshold or “tipping point” for collapse of the Greenland ice sheet is lower than previously thought, in the range of 400 to 560 ppm.

Here’s the abstract:

This paper uses updated data on bedrock topography and ice thickness to produce more accurate modelling results of Greenland ice sheet behaviour. They model how the Greenland ice sheet will respond to three different scenarios with atmospheric CO2 held at 400 ppm, 560 ppm and 1120 ppm. The simulations are run over a 400 year period. Although not completely collapsed, the 400 ppm ice-sheet loses ice mass in the north of the island, with a total reduction in ice volume ranging between 20 to 41%. Note – due to the large inertia of the Greenland ice sheet, this mass loss doesn’t happen at the moment CO2 levels reach 400 ppm but over a period of centuries. Under a 560 ppm climate, the Greenland ice sheet loses between 52 to 87% of its ice volume. If CO2 reaches 1120 ppm, there is almost complete elimination of the Greenland ice sheet with loss between 85 to 92%. The important result from this paper is that there is a critical threshold where the Greenland ice sheet becomes unstable somewhere between 400 and 560 ppm.

We’re currently at about 390 parts per million atmospheric concentrations of CO2, rising about 2 ppm a year.

Sustained high rates of Greenland ice loss could lead to rising sea levels that would be devastating to coastal communities. Already, rising seas are causing an environmental crisis in the Nile Delta and swallowing islands in the Bay of Bengal.

Many recent studies support a conclusion that Greenland and Antarctic ice sheets are very sensitive to sustained warming. The last time Earth was as warm as it is likely to be by mid-century if we stay near our current emissions path, sea levels were probably more than 26 feet higher than today.

Romm points out the IPCC “business as usual” scenario has CO2 levels reaching 1000 ppm by 2100, and even its most optimistic scenario sees CO2 levels hitting 500 ppm by 2100.

So how are we doing?

It’s not looking good.

A recent post reported on scientists’ findings that Greenland’s glaciers are melting from the bottom up. Findings from another team of scientists help explain why: subtropical waters from warmer latitudes are reaching Greenland’s glaciers, driving melting and likely triggering an acceleration of ice loss.

Credit: Jack Cook, Woods Hole Oceanographic Institution

The research team, led by Fiamma Straneo, a physical oceanographer at Woods Hole Oceanographic Institution, found that subtropical waters are reaching Greenland’s glaciers, driving melting and likely triggering an acceleration of ice loss. Melting ice also means more fresh water in the ocean, which could flood into the North Atlantic and disrupt a global system of currents, known as the Ocean Conveyor.

Science Daily quotes Straneo:

This is the first time we’ve seen waters this warm in any of the fjords in Greenland. The subtropical waters are flowing through the fjord very quickly, so they can transport heat and drive melting at the end of the glacier.

The Greenland ice sheet’s contribution to sea level rise over the last decade has doubled due to increased melting and especially to the widespread acceleration of outlet glaciers.

The research teamconducted two extensive surveys during July and September of 2008 in Sermilik Fjord, a 100-kilometer long glacial fjord in East Greenland connecting Helheim Glacier with the Irminger Sea. In 2003 alone, Helheim Glacier retreated several kilometers and almost doubled its flow speed.  Deep inside the fjord, researchers found subtropical water as warm as 39 degrees Fahrenheit (4 degrees Celsius). The team also reconstructed seasonal temperatures on the shelf using data collected by 19 hooded seals tagged with satellite-linked temperature depth-recorders. The data revealed that the shelf waters warm from July to December, and that subtropical waters are present on the shelf year round.

A study published in Nature Geoscience finds that submarine melting is causing Greenland’s glaciers to melt from underneath and calve off.  As the glaciers thin and become unpinned from their moorings on the sea bed, they then flow more rapidly into the sea.

Rates of submarine melting are two orders of magnitude (100 times) larger than surface melt rates. The rate of submarine melting is comparable to rates of iceberg discharge.

Here’s the abstract:

Widespread glacier acceleration has been observed in Greenland in the past few years, associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean. These glaciers thin both at the surface, from warm air temperatures, and along their submerged faces in contact with warm ocean waters. Little is known about the rates of submarine melting and how they may affect glacier dynamics. Here we present measurements of ocean currents, temperature and salinity near the calving fronts of the Eqip Sermia, Kangilerngata Sermia, Sermeq Kujatdleq and Sermeq Avangnardleq glaciers in central West Greenland, as well as ice-front bathymetry and geographical positions. We calculate water-mass and heat budgets that reveal summer submarine melt rates ranging from 0.7±0.2 to 3.9±0.8 m d?¹. These rates of submarine melting are two orders of magnitude larger than surface melt rates, but comparable to rates of iceberg discharge. We conclude that ocean waters melt a considerable, but highly variable, fraction of the calving fronts of glaciers before they disintegrate into icebergs, and suggest that submarine melting must have a profound influence on grounding-line stability and ice-flow dynamics.