What’s with the Greenland Ice Sheet?

Image: The Greenland Ice Sheet © Christophe Milhères

The fact that sea levels globally are rising in response to ongoing climate change is well known. But what is known about the biggest source of this rise? The Greenland ice sheet is currently the single greatest contributor to global sea level rise, and its rate of mass loss is increasing (Velicogna & Wahr, 2005; Rignot et al., 2011; Pritchard et al., 2009; Khan et al., 2010, 2014; Shepherd et al., 2012). While this trend is predominantly driven by the warming climate, it is by no means the whole story. The Greenland ice sheet’s degradation, and its key role in sea level rise (Hanna, 2012) as well as other severe repercussions, is driven by both the ocean and the atmosphere around it – it is suffering an assault from all angles.

Warming air temperatures over the Greenland ice sheet are causing unprecedented surface melting. Extreme melting events, such as in July 2012 when ~97% of the ice sheet surface was actively melting (Ngheim et al., 2012; Tedesco et al., 2013), and in the spring of 2016 when the earliest recorded onset of the summer melt season began in April (Mottram, 2016), are not in themselves indicators of change in state, but the increasing frequency of such events, and an increase in less extreme melting, indicates a clear acceleration of surface melting of the ice sheet in response to ongoing anthropogenic climate change (Hanna et al., 2012). Even without any clear trend for increased melting, extreme events like 2012 and 2016 are cause for great concern, highlighting the vulnerability of the ice sheet and its obvious sensitivity and susceptibility to climatic variations.

Increased surface melting is not simply due to rising temperatures over the ice sheet. An important factor is albedo. Albedo is effectively a measure of how reflective a surface is. Light coloured, reflective surfaces, such as snow and ice have a high albedo, reflecting the majority of short-wave radiation that reaches them directly back to space (fresh snow can reflect as much as 90% of short wave radiation from the sun). Darker surfaces, such as open ocean, bare rock and vegetation, absorb more radiation than they reflect, and thus are warmed by the sun far more efficiently than high albedo surfaces. Unlike many perceptions, the surface of the Greenland ice sheet is not pristine snow and ice with consistently high albedo. The ice sheet surface is darkened by coloured microbial life within the snow and ice, particularly during algal blooms, as well as by pollution particulates deposited there (most notably black carbon from fossil fuel combustion and wild fires). Furthermore, meltwater pooling on the ice sheet surface in lakes lowers albedo, which leads to greater melting and further growth of lakes throughout the melt season. Seemingly slight changes to surface albedo have significant effects on melt rates across the ice sheet, and are an essential consideration when attempting to understand and quantify mass loss and sea level rise contributions (Nolan, 2016).

The Greenland ice sheet is not only vulnerable to melting driven by warm air over its surface. With many of its outlet glaciers terminating in the sea at fjords, the oceans surrounding Greenland have a huge impact on its mass loss.

Various oceanic water masses interact with the margins of the Greenland ice sheet. Of these, relatively warm Atlantic water, generally found at depths below ~200 m due to its density (as a result of its salinity) (Farmer & Freeland, 1983; Cottier et al, 2010; Inall & Gillibrand, 2010; Stigebrandt, 2012; Straneo & Cenedese, 2015), is responsible for much of the submarine melting observed around Greenland. Furthermore, meltwater discharging into fjords at outlet glacier termini causes major submarine melting, and is responsible for much of the rapid retreat of Greenlandic glaciers observed in recent years (Mortensen et al., 2011; Sole et al., 2012; Chauché et al., 2014; Inall et al., 2014; Lea et al., 2014a, 2014b; Porter et al., 2014).

Where outlet glaciers sit at points of stability or resistance within their fjords (be it a narrowing or at a shallow sill on the fjord floor), a small retreat of the terminus beyond this point can trigger a much larger and rapid retreat until a new stable position is reached upstream. Submarine melting is particularly damaging to outlet glaciers as they retreat beyond shallow sills on the fjord floor. This allows the inflowing Atlantic water to over-top the sill, occupying the space behind in contact with the glacier, greatly increasing the area of ice exposed to warm water, resulting in extensive melting at the ice-ocean interface (Holland, 2010; Rignot et al., 2010; Straneo & Heimbach, 2013).

This is all very depressing, but why should we care about the steady demise of the Greenland ice sheet really? Rising sea levels are a relatively well understood threat in science, policy and public circles, and their impacts on vulnerable coastal and low lying regions far from the Arctic are well known. Receiving less attention outside of scientific research discourse is the impact of Greenland ice sheet mass loss on the climate itself, particularly in the North Atlantic region, as it interacts with both the atmosphere and the oceans.

The Greenland ice sheet plays a key role in the North Atlantic regional and global climate system. The huge ice mass cools air masses that sit or pass over it, creating an area of high pressure. This area of high pressure, quantified by the Greenland Blocking Index (GBI) (Hanna et al., 2016), influences the flow of the jet stream, altering its path. As such, major mass loss from the ice sheet may result in a weakened GBI, altering the air masses, and thus weather, present in major populated regions of Europe and North America.

In addition to atmospheric interactions, ongoing mass loss from the ice sheet freshens oceans around Greenland and the North Atlantic region by effectively dumping huge volumes of cold, fresh water into the sea. Ocean currents are largely controlled by the relative densities of different water masses, dictated by temperature and salinity. In freshening the seas around Greenland, mass loss from the ice sheet is changing the characteristics and structure of water masses, and has the potential over time to disrupt, divert or even shut off existing ocean currents in the region. Ocean currents, such as the Gulf Stream which is responsible for maintaining north-western Europe’s warm climate relative to its latitude, have major influence on regional and global climate systems and weather, and changes to them will have profound effects. While a sudden shut down of the Gulf Stream, plunging Europe into a new ice age is not imminent (in human terms), the impacts of the ongoing freshening of oceans around Greenland cannot be ignored.

Both atmospheric and oceanic interactions with the changing Greenland ice sheet demonstrate how there are potentially severe repercussions of mass loss beyond sea level rise. Catastrophist rhetoric is unhelpful in efforts to increase understanding and awareness of the very real issues associated with ongoing climate and environmental change in Greenland or elsewhere, and the risk of imminent change and disaster for most of us is small. Nonetheless, mass loss from the Greenland ice sheet is a big issue with potential major repercussions for us all. It must not be forgotten that for many, far from Greenland or its regional vicinity, attempting to defend vulnerable communities from rising sea levels, the contributing impacts the ice sheet’s degradation are already coming to fruition.

The gloomy picture painted here is confounded by the fact that there appears little that can be done to curb the ongoing degradation of the ice sheet, aside from the continued global fight against climate change. However, even if global ambitions to limit global temperature rises to 1.5 or 2 °C are realised, the ongoing impacts on the ice sheet under present conditions look set to continue and increase.

The significant retreat and shrinking of outlet glaciers, combined with the seemingly relentless acceleration of surface melting that drives Greenland ice sheet mass loss, earns it its title as the current greatest single contributor to sea level rise globally, but the impacts of change in Greenland are complex and difficult to predict. The ice sheet takes punishment from both the atmosphere and the ocean, with the effects of this constant assault exacerbated by various positive feedbacks or shifts from stability. The Greenland ice sheet in its current state is able to withstand this onslaught to a certain extent, and is not in imminent danger of collapse triggering a return to glacial conditions across the North Atlantic region. It nonetheless remains vulnerable, and is clearly both a major victim and influencer of ongoing global and regional climate change.


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