The Melting & Significance of the Greenland Ice Sheet

Greenland ice sheet

Located between the Atlantic and Arctic oceans, Greenland is home to a small population of people who live amidst some of the most incredible scenes of natural beauty found on Earth. In recent years though, Greenland has become a focus of climate and polar research. It is the Greenland Ice Sheet (incidentally the world’s second largest mass of ice) and its glaciers that are the principal subject of study. Both can be characterised by their melting at increasingly rapid rates, a process that’s accelerating global sea-level rise and contributing to unpredictable and extreme weather events around the world. The picture is particularly concerning since it’s one that many climatologists have taken to forecast what awaits other regions of the Arctic and Antarctic yet to undergo such massive changes.


Ice Sheet Formation 

An ice Sheet is defined as a mass of glacial land ice extending greater than 50,000 square km (20,000 square miles). They form as very dense, thick masses of ice, over the course of thousands of years as a result of enormous pressures caused by the weight of layer upon layer of snow that accumulates. During the last ice age, ice sheets were what covered most of North America and Scandinavia. Ice sheets remain in a constant state of motion – gradually moving under the force of gravity from their centre towards coastlines. As ice sheets reach coastal areas, the ice produces formations such as ice flows, glaciers, and ice shelves which move at greater speeds than the ice sheets from which they emanate.

What remains of the Greenland Ice Sheet has been in place for between two and three million years. Covering 1.7 million square kilometers, the ice sheet blankets about 81% of Greenland and at its thickest point is estimated to be 3200 meters deep. For some perspective, its size is roughly equal to three times that of Texas, or 14 times that of England.

ice-canyon-greenland-tourism

The Greenland Ice Sheet. Image rights: Alexi Trofimov.


The Effect of Climate Change on Ice Sheets

To date the impact of climate change has been far more pronounced in Greenland than the Antarctic. And while seasonal melting within Greenland is variable between eastern and western regions as well as between years, several clear trends have emerged in recent times. To frame the affect of climate change on the Greenland Ice Sheet itself, we can consider it across four levels: decline in volume of ice; the extent of surface melting; the lengthening of the melt season; and the retreat of glaciers.

It’s important to realise however, that temperatures in Greenland are not rising as would be expected from global warming trends. Rather, Greenland is subject to very nuanced weather patterns that have so far kept it from warming. This is a very fortunate circumstance, but not one that can be relied upon indefinitely – as greenhouse gas emissions continue to rise, the threshold at which Greenland’s temperatures will begin to rise will be reached.  At that point, even the massive amounts of melt we are witnessing now may seem mild by comparison.


Temperature Trends

It is perhaps surprising to learn that temperatures on the Greenland Ice Sheet and at coastal regions are not following the current trend of global warming. Though there have been consecutive years of warming at times, there is no evidence of persistent warming beyond what’s been determined as the natural variability of Greenland (ref. 1,2,3).

If anything, Greenland has actually been cooling over the second half of the last century. Coastal station temperatures are estimated as about 1 degree celsius below their 1940 levels; while at the ice sheet summit, records show a decrease in the summer average temperature at a rate of 2.2 degree celsius per decade since 1987 (ref. 1,2,3).

While a cooling trend of this sort may at first appear entirely contrary to the prevailing picture of global warming, it has been interpreted by scientists to demonstrate the substantial impact of a high natural variability in the regional climate of Greenland (ref. 1,2,3). Such cooling therefore should not be taken to dismiss the impact of climate change on Greenland, since anthropogenic change to the atmosphere is evidenced to impact Greenland in a variety of other manners. Equally, the prevailing view is that it remains only a matter of time before Greenland’s natural variation and cooling as a result of regional weather systems is no longer enough to outweigh northern hemispheric warming – at this point Greenland will begin to catch up with global temperature trends (Box et al., 2009).

Significant atmospheric components affecting Greenland’s regional climate system are the north polar vortex (a persistent cyclone system) and in particular the North Atlantic Oscillation (NAO, a powerful air pressure system).

Greenland’s location means its sensitive to positive NAO effects (namely cold northerly winds) that maintain cool temperatures on both coastal and inland areas. Specifically, there is a strong anti-correlation between the Greenland’s temperatures and the NAO: as the intensity of the NAO increases, warm, southerly air masses are diverted eastward away from Greenland and cool northerly airflow in west Greenland brings cooler, wetter and cloudier weather than normal, that moderates temperatures, and melting to some extent also (NOAA). So while there has been warming within decades (coincident with lessening of the NAO), it’s not been persistent enough to extend beyond a relatively short period of time.

The strength of Greenland’s natural variability is revealed by a warming period during the 1920s when temperatures rose between 2 and 4 degree celsius in just 10 years (Box et al., 2009). Since this rise occurred before considerable increase in global temperatures due to greenhouse gases, it demonstrates both the significant extent of natural temperature variability and the significance of NAO in Greenland.

A 2011 study of Greenland’s temperature variability over the past 4000 years concluded that current mean temperatures have not exceeded the margins of natural temperature variability set over this time period (Kobashi et al., 2013).

Crucially though, several factors have been identified as contributing to loss of Greenland’s ice despite lowering in peak air temperatures: increases in precipitation, increases in ice outflow, increases in surface albedo, and the lengthening of the melt season. These, together with other facets of climate variability will be presented alongside the effects they are contributing too.


Decline in Volume of the Ice Sheet

In contrast to a long period of relative balance where ice loss was compensated for by new ice formation (1961-1990), since 1990 the Greenland Ice Sheet’s mass has shown a decreasing trend (Box et al., 2012). GRACE satellite gravity data revealed that from the end of April 2012 to the end of April 2013, the cumulative ice sheet loss was 570 Gt – over twice the average annual loss rate of 260 Gt set between 2003 and 2012, and the largest annual loss rate for Greenland during the GRACE record (NOAA). Ice loss is the result of increased surface melt and runoff, peripheral thinning of ice flows and glacial discharge.

Greenland 2002 - 2013 ice volume

Monthly changes in the total mass (in Gigatonnes) of the Greenland Ice Sheet estimated from GRACE measurements since 2002. The blue and orange asterisks denote April and July values, respectively. Image rights: Arctic Report Card 2013.


The Extent of Surface Melting 

A reliable index for changes in polar regions is the measurement of the area of land which is subject to melting during the summer. The National Snow and Ice Data Center (NSIDC) report that maximum surface-melt area on the ice sheet increased on the average by 16% from 1979-2002. Adding in new data, to account up until 2006, that increase was 30% – that’s an area nearly twice the size of Sweden.

The summer of 2012 stands out as unprecedented for several reasons. For one, because over a four-day period between 8 and 12 July, measurements showed an rapid increase in the extent of surface melt area from 40% to 97% of the ice sheet surface – the highest on record. And secondly because melting reached up beyond 2000 meters in elevation, which it has done on few occasions before (Steffan research group).

In 2007 melting proved sufficient enough to reveal a previously unknown island. Named Uunartoq Qeqertaq (which in English translates to Warming Island) – most members of the scientific community attribute the island’s discovery to be a direct result of climate change.

NSID greenland melt 2011-13

These figures show the number of anomalous melt days (when melting was higher or lower than average) in 2011,  2012, and 2013.  The extraordinary circumstances of 2012 are plain to see. Image rights: NSIDC.

Greenland 2010 - 2014 temp

These graphs show the percentage of the ice sheet that was melting per month over 2012 – 2013 (left) and 2014 (right) compared to the 1981-2010 average. Image rights: NSIDC.

Summer 2014 saw the extent of melt returning closer to the 1981-2010 average, but in June and July it was still above average, particularly along the northwestern coast where there were more than 15 days of above average melt. Total melt area was close to 40% of the ice surface area by mid-June – this turned out to be the maximum melt extent for the summer (all: NOAA).

Surface melt is of interest to researchers, who not only see it as a key index to Greenland’s melting, but are interested in the knock-on effects it has on processes that accelerate further melt.

A principal consequence of surface melt is its affect on the reflectiveness of the ice sheet – or its albedo. The NSIDC note that for clean new snow, a 2% decrease in reflectivity represents a 15-20% increase in energy absorption. In recent years, increases in surface melt has produced thousands of small lakes and channels across the surface of the ice, that collectively reduce the albedo of the ice sheet. In this way, lowered albedo creates a positive feedback loop, where a lowering of albedo leads to greater absorbance of solar radiation that accelerates melting and further lowers the albedo of the ice sheet. We return to the concept and importance of albedo in the section ‘Dark Snows’ below.

Surface melting also increases surface runoff. Channels flow over the ice surface toward fractures in the ice that serve as vertical shafts plummeting down to the subglacial land on which ice flows move. Researchers surveying these fractures – called Moulins – in Greenland recorded that on one day, 42 million litres of fresh water flowed through just one Moulin, with a peak flow rate of 9.4 miles per hour. This was of course just one Moulin, and perhaps an extreme example, but there are perhaps thousands like it.

moulin water

Surface melt water pours out into a crevasse on the Greenland Ice Sheet. Image rights: National Geographic.

Moulins are a sure sign of things not being well – not only illustrating the sheer amounts of melt water, but compounding the melting of ice sheets by impacting processes beneath. A recent study (September 2014) reviews new insights on melting mechanisms and positions surface melt as a driving factor for increased ice loss. Conducted by scientists in Cambridge, the study also presents a novel model describing the relationship between the known two sources of net ice melt.  While previous models were based off the notion that ice sheets move along a bed of rock, the new Cambridge model predicts that as melt water seeps into the ground below, it saturates the ground, producing a less resistant foundation over which ice sheets flow. Saturated subglacial land of this type is considered to be leaving the Greenland ice flows more vulnerable to melting than was first theorised.


Lengthening of the Melt Season

Just as the spatial extent of melting is important, so too is the length of the melt season. There has been a clear trend in Greenland towards a longer melt season that has been demonstrated between 1978 and 2013. Melting has begun 4 to 12 days earlier per decade; while the end of the melt season has come later, between 8 to 16 days per decade (NSIDC).


Glacial retreat

Glaciers stretch out towards the edges of ice sheets, and their calving and retreat provide some of the most dramatic illustrations of melting in polar regions. In recent years the glaciers in Greenland have been receding at increasing rapid rates. From 1996 to 2005 the loss of many of Greenland’s glaciers increased from 90 cubic km (22 cubic miles) per year to 220 cubic km (53 cubic miles) per year.

Glacier box

A glacier in Southern Greenland. Annual average air temperatures at nearby Narsarsuaq have been above the melting point for 39 of the past 51 years. Image rights: Jason Box, MeltFactor.

More recently, on May 28, 2008, Adam LeWinter and Director Jeff Orlowski filmed a historic breakup at the Ilulissat Glacier in Western Greenland. Incredible, humbling and terrifying all at once, in the course of this calving event the glacier retreated a full mile across a calving face three miles wide – it’s considered to be one of the largest ever calving events recorded. As you watch the video below, consider that the ice is about 3,000 feet thick, with 300-400 feet showing above water.

As another example of the magnitude of of melting in Greenland, consider that over the course of 100 years (1900-2000) the Ilulissat Glacier retreated 8 miles; but between 2000 to 2010 it retreated 9 miles. This staggeringly swift melting of the glacier, while extreme, isn’t an isolated event – rather, it falls into a larger pattern of glacial retreat that’s come to epitomise the situation in Greenland.


Dark Snows & Lowering Albedo

Earlier, the concept of albedo (the reflectance of snow and ice) was introduced. We know that albedo has a major bearing on surface melt, due largely to it’s self-perpetuating nature that hastens further surface melt (a so called positive feedback loop). Lower albedo of the ice sheet also contributes to the lengthening of melt seasons. This year, the Greenland Ice Sheet was the darkest its ever been, adding to a declining trend recorded since 1981. Jason Box, a researcher for Geological Survey of Denmark and Greenland stated: “In 2014 the ice sheet is precisely 5.6 percent darker [than 2013], producing an additional absorption of energy equivalent with roughly twice the US annual electricity consumption” .

Greenland 2000 - 2014 albedo copy

Average Greenland Ice Sheet albedo through summer (June, July, August) 2000 to 2014, derived from MODIS (Moderate-resolution Imaging Spectroradiometer). Image rights: Jason Box

UAV image Greenland

Images taken from high-altitudes reveal sinuous melt streams and linear fractures, as well as small cryoconite potholes about 0.5 to 1m in diameter, littered across the ice sheet. Image rights: Jason Box.

The dark surface of the Greenland Ice Sheet. Image rights: Jason Box.

The dark surface of the Greenland Ice Sheet. Image rights: Jason Box.

The exact causes of dark material – cryoconite – accumulation remain uncertain, but it’s suspected that a combination of wind-blown dust, grit, algae, and industrial emissions are involved. In 2014 there was a another, particularly strange, contribution to the cryoconite – soot. Surprisingly, forest fires have become an increasingly common occurrence in the Arctic region – climatologist Jason Box has reported that over the last three years or so, Arctic fires have been burning at a rate double to that of a decade ago. Some 3.3 million hectares burned in Canada’s Northwest Territories  (9 times the long term average) in 2014. A massive landfill fire lasting four months in the city of Iqaluit in one of Canada’s most northerly territories is thought to have added to the 2014 albedo low also.


Significance of the Greenland Ice Sheet

Our understanding of the mechanisms involved in Greenland’s melting are still evolving; while measurement of temperatures, volume loss of ice, and glacial outflow are all highly complex issues. Nevertheless, we’re now more sure than ever of the state of play in Greenland and know with considerable reliability that the situation today is unprecedented.

A key example of the global significance of the Greenland Ice Sheet is found in its connection to sea levels.  Scientists estimate that if the ice sheet melted away entirely, sea level would rise about 6m. Currently it’s estimated to account for nearly one-fifth of annual sea level rise, about 3mm each year. This may seem a small figure, but it’s significant enough to be affecting ocean currents, including reducing the intensity of the Atlantic thermohaline circulation (THC) which is offset by fresh water deregulating this salt-water based current.  The Atlantic THC is a critical ocean thermal system that, if destabilised, would have very serious consequences for European climates. What’s more, the projections for continued ice loss carry grave concerns for increases in the incidence of flooding and typhoons around the world.

It would be remiss not to note Greenland’s role as a regulator of global air masses. Together with the greater Arctic region, the Greenland Ice Sheet’s mass is sufficient to cool nearby wind streams. This cooling constitutes a regulatory mechanism for global air masses that go on to circulate around the world. Simply put, Greenland keeps the atmosphere’s temperatures far cooler than they would otherwise be. As Greenland’s ice mass continues to lower, its role as a regulator of global temperatures is jeopardised. Once again, there are concerns that if a tipping point is reached in destabilising this dynamic, it could have radical and unpredictable consequences for global weather.

Lastly, another consequence of ice melting is what it leaves in its wake. Much of the land in the Arctic is permafrost – frozen earth that contains organic matter, carbon and methane (a greenhouse gas some twenty times more powerful than carbon dioxide). As ice melts and temperatures rise, that land begins to thaw; releasing the greenhouse gases it’s currently trapping. Permafrost isn’t just a layer several meters deep though – it’s massive, covering upwards of 24% of the northern hemisphere, and in places it’s well over a kilometer in depth. Climatologists fear that as permafrost melts, the massive and sudden release of methane could have devastating consequences, with knock-on effects that remain unclear. Even more worrying is that due to complexities in measurement, many climate change models have focused on carbon dioxide emissions, and haven’t appropriately factored in the implications for permafrost melt and methane emissions in the event of widespread permafrost melt.

origin_54969488

The edge of the Greenland ice shelf. Image rights: Andrew Davies.

What is happening in Greenland is already deeply troubling. It is simply a matter of time, if trends continue, before Greenland’s melting reaches tipping points that carry massive consequences over several levels. It is foreseeable that fresh waters pouring out of Greenland could destabilise ocean currents to a point at which there are abrupt and irreversible changes that will bring with them extreme and unpredictable weather events and massive changes to our climates. In the mean time, sea levels continue to rise at greater pace, and the Greenland Ice Sheet and glaciers in all their magnificence are melting away into history, taking with them their innate capacity for regulation and cooling of the planet – mechanisms we perhaps rarely think of, but so desperately depend upon, perhaps now more than ever.


References and Resources

1) Box et al., (2009) Greenland Ice Sheet Surface Air Temperature Variability: 1840–2007. Journal of Climate.

2) Chylek, P., Box, J.E., &  Lesins, G. (2004) Global Warming and the Greenland Ice Sheet. Climatic Change.

3) Kobashi et al., (2013) Causes of Greenland temperature variability over the past 4000 yr: implications for northern hemispheric temperature changes. Climate of the Past.

Archive of peer-reviewed papers on Greenland warming

National Snow and Ice Data Center (NSIDC)

National Oceanic and Atmospheric Administration (NOAA), Arctic Report Card, 2013

Jason Box, Meltfactor