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Cryospheric Sciences

energy balance

Image of the Week – Bioalbedo: algae darken the Greenland Ice Sheet

Image of the Week – Bioalbedo: algae darken the Greenland Ice Sheet

Most of the energy that drives glacier melting comes directly from sunlight, with the amount of melting critically dependent on the amount of solar energy absorbed compared to that reflected back into the atmosphere. The amount of solar energy that is reflected by a surface without being absorbed is called the albedo. A low albedo surface absorbs more of the energy that hits it compared to a high albedo surface. Our Image of the Week shows patches of dark grey-brown algal blooms on the Greenland Ice Sheet, giving the surface a surprisingly low albedo.


The colour of ice

Clean ice and snow are among the most reflective natural materials on Earth’s surface making them important ‘coolers’ in Earth’s climate system. The term ‘albedo’ describes how effectively a material absorbs or reflects incoming solar energy – it is the ratio of downwelling light arriving at a surface to the amount of upwelling light leaving it. The albedo of fresh, clean snow can be as high as 90%, meaning that out of all the solar energy reaching the surface only 10% is absorbed. However, the albedo of ice and snow can vary widely. This is important because the albedo determines how much of the incoming solar energy is retained within the snow or ice and used to raise the temperature or drive melting. It therefore controls snow and ice energy balance to a large extent.

There are several reasons why the albedo of snow and ice can vary. First, once ice crystals begin to melt they lose their delicate structures that efficiently scatter light and develop rounded granular shapes. Meltwater generated by snow or ice melt fills the gaps between the grains, promoting forward scattering of light deeper into the ice, rather than scattering back towards the surface. This increases the distance travelled through media where absorption can occur, and therefore lowers the albedo as the light is less likely to escape the material after it enters. The more melt, the greater this effect. Second, other materials such as dust or rock debris can enter the snow or ice. These ‘impurities’ generally absorb light more effectively than the ice crystals themselves and therefore reduce the albedo. However, this depends upon their concentration, optical properties and proximity to the surface. Additionally, whether the impurities are inside or outside the ice crystals, where on the planet the material is and the time of day are also important.

Any impurity that darkens a mass of ice or snow increases the amount of solar energy absorbed compared to when the material is impurity-free. This means that impurities promote melting, which is in itself an albedo reducing process. Therefore, the impact of impurities on albedo is non-linear and greater than the direct effect of their absorption alone. There are many different impurities that commonly lower the albedo of ice and snow, including mineral dusts and black carbon (e.g. from fossil fuel combustion). However, there is also a growing literature on another form of impurity that darkens ice and snow on glaciers and ice sheets on both hemispheres: biological growth (also see this previous post). Algae are the primary biological albedo-reducers on ice and snow. Photosynthetic microalgae bloom on the surface where light is abundant, which provides them with energy that they use to turn carbon dioxide and water into sugars. This in turn provides food for other microorganisms. In doing so, they darken the ice surface simply because the algal cells are more effective absorbers than the ice crystals. However, as the algae become exposed to increasing light intensities, they produce pigments that act as sun shields, protecting their cellular machinery from the damaging effects of too much light. This effect enhances the biological darkening and increases the energy absorbed within the snow or ice.

Biological darkening

There are several distinct microbial habitats on glaciers and ice sheets. Snow algae are a feature of melting snowpacks that colour snow surfaces green early in the year and red later because prolonged exposure to sunlight causes them to produce red ‘sunscreen’ pigments (see this previous post). Their influence on snow albedo has yet to be determined, although they have been shown to change the amount of visible light reflected from the surface (Lutz et al., 2014) and in Antarctica they have been shown to influence light absorption at depth within the snowpack (Hodson et al., 2017). Some bacteria have been identified feeding upon the algae, and the algal blooms also provide food for red coloured ice worms. This is probably why, in ‘The History of Animals’, Aristotle wrongly attributed the red discoloration of patches of snow to red worms rather than pigmented algae!

Fig. 2: (a) Albedo for clean snow, bare ice and ice with an algal bloom measured on the Greenland Ice Sheet in July 2017. (b) Microscope image of melted surface ice from the Greenland ice sheet. The red oval shaped particles are ice algae and the angular, clear particles are mineral dust fragments. [Credit: A: J. Cook, B: C. Williamson]

On ice, a different species of algae exists in a thin liquid water film on the upper surface of melting ice crystals. These algae are also photosynthetic but are not bright green or red, but rather grey, brown or purple. They produce a purple pigment that acts as a UV shield that protects their delicate intracellular machinery from excessive light energy. The side effect of this is that the algae become very dark and have an albedo-lowering effect on the ice surface (see our Image of the Week). Ice with algae has a lower albedo than clean ice (Fig 2a) but, up to now, the magnitude of the biological darkening effect has not been quantified because of difficulties isolating algal darkening from that of mineral dusts, soot and the changing optical properties of the ice itself. This also limits our capability to map these algae using remote sensing. Samples of dark coloured ice examined under the microscope clearly show the presence of an algal community darkening the ice (Fig 2b).

In addition to surface-dwelling ice algae, microbial life exists in small pits known as cryoconite holes (see also this previous post). At the bottom of these holes exists a thin layer of granules comprising living microbial cells, dead cells, biogenic molecules, mineral fragments and soot. The organic matter in these granules is very dark, so they warm up when illuminated by the sun and melt into the ice. The relationship between cryoconite and ice surface albedo is complex because, although the cryoconite is dark, the hole geometry hides the granules beneath the ice surface.

Implications for the future of glaciers and ice sheets

The challenge facing scientists now is to quantify the bioalbedo effect by determining the optical properties of individual algal cells and remotely assessing their spatial coverage at the scale of entire glaciers and ice sheets. This will require new methods to be developed for detecting living cells from the air or space. Then, we must understand the factors controlling their growth, so we can predict biological darkening of ice in future climate scenarios. It is possible that algal coverage will increase as glaciers and ice sheets waste away because algae bloom where there is liquid melt water. Because of the darkening effect, an increasingly widespread algal ecosystem in a warming climate will accelerate the demise of its own habitat by enhancing glacier and ice sheet retreat.

Further reading

Edited by Scott Watson and Clara Burgard


Joseph Cook is a Postdoctoral Research Associate on NERC’s Black and Bloom project based at the University of Sheffield, UK where his remit is the measurement and modelling of surface albedo on the Greenland Ice Sheet. His background is in biotic-abiotic interactions on ice. He tweets as @tothepoles and blogs at http://tothepoles.wordpress.com. Contact Email: joe.cook@sheffield.ac.uk

Katabatic winds – A load of hot (or cold) air?

Katabatic winds – A load of hot (or cold) air?

It might seem obvious that a warming world will lead to a reduction in glacial ice cover, but predicting the response of glaciers to climatic change is no simple task (even within the short term). One way to approach this problem is to come up with relationships which describe how glaciers interact with the world around them, for example, how the ice interacts with the air above it. Our post today delves into the world of ice-air interaction and describes some of the problems encountered by those who are investigating it, in particular the problem of modelling katabatic winds! Not sure what we are talking about…then read on to find out more! 


What are katabatic winds?

Anyone who has stood on, or in front of a glacier on a clear, sunny day has no doubt felt the bitter chill of a katabatic wind, forcing them to don a warm jacket and lose their chance at that lovely “glacier tan”. Katabatic winds (derived from the Greek word katabasis, meaning ‘downhill’) develop over snow and ice surfaces because the 0°C ice surface cools the air just above it. This cold, dense air then flows downhill under the force of gravity (Fig. 1 and Fig. 2). This is not recent news and such wind chill has no doubt punished glaciologists and explorers for the last century or more –  Mawson’s Description of the 1911-1914 Australian Antarctica Expedition is aptly named “The Home of the Blizzard“. However, despite being well known, this phenomenon still causes much uncertainty when it comes to modelling the melting of glacier ice surfaces around the world.

Soon gusts swept the tops of the rocky ridges, gradually descending to throw up the snow at a lower level. Then a volley raked the Hut, and within a few minutes we were once more enveloped in a sea of drifting snow, and the wind blew stronger than ever. – Mawson, 1915, The Home of the Blizzard

Figure 2: The view from the upper reaches of Tsanteleina Glacier in the western Italian Alps (Val d’Rhemes, Aosta). Katabatic winds generally flow in a down-glacier direction – here, from right to left [Credit: T Shaw].

Challenges for modelling

Air temperature is really important in determining how much a glacier melts and we need to know as much about it as possible to provide accurate predictions now and into the future. This is particularly relevant because the warmer it gets, the more energy is available to melt ice and seasonal snow. Unfortunately though, we don’t have an infinite supply of meteorological observations (e.g. air temperature, wind speed etc) at many locations we are interested in. As a result, we have to make simple assumptions about what the weather is doing at a remote, far away glacier. One such simple assumption is based upon the fact that air temperature typically decreases with increasing elevation, and so if we know the elevation of a location we are interested in, we can assume a ‘likely’ temperature. The rate of change in temperature with elevation is known as a ‘lapse rate’.

Air temperature is really important in determining how much a glacier melts…the warmer it gets, the more energy is available to melt ice and seasonal snow.

When predicting glacier melt, it is common practice to use a lapse rate which stays constant in time and space. This is convenient as we often don’t know the actual lapse rate at a given location, but this often ignores things happening at the surface of the Earth. An important example of this is when we have katabatic winds over glaciers!

When conditions are warm, and skies are clear, the cooling of the air above the ice surface, means that the application of a lapse rate is fairly useless, or close to it [Greuell and Böhm, 1998]! That is because the cooling from the surface continues as air flows down the glacier, typically creating colder temperatures at lower elevations, the opposite of the typical lapse rate assumption that models will apply.

‘Bow-shaped’ temperature vs. elevation relationships

To complicate matters for people trying to model the air temperature over glaciers, the effect of surface cooling is not just dependent on the amount of time an air parcel is in contact with the ice surface but also the characteristics of the ice surface it has been in contact with. In fact, after cooling on their descent down-glacier, air parcels have been documented to warm again, leaving interesting slightly “bow-shaped” curves to the temperature-elevation relationship. This effect has been found for the Swiss Haut Glacier d’Arolla and the Italian Tsanteleina Glacier (Fig. 3c,d). A new model approach to tackling this bow-shaped problem has been presented by recent research [Ayala et al., 2015] and offers a means of accounting for katabatic winds in glacier models. Nevertheless, more data and more work are still needed to generalise these models [Shaw et al., in review].

Figure 3: Relationship between elevation and air temperature on three different glaciers in the western Alps. Miage (Italy), Tsanteleina (Italy) and Arolla (Switzerland). Glaciers are represented using the mean of all data available (green), the top 10% of off-glacier temperatures (P90 – red) and the bottom 10% of off-glacier temperatures (P10 – blue), plus one standard deviation. The debris-covered Miage Glacier does not demonstrate a classic katabatic flow regime and therefore temperature corresponds well to elevation even under warm conditions [Credit: T Shaw, unpublished].

after cooling on their descent down-glacier, air parcels have been documented to warm again, leaving interesting slightly “bow-shaped” curves to the temperature-elevation relationship.

Air temperatures across debris-covered glaciers

As you may have read in our previous post on the topic, debris-covered glaciers behave in a different way to those with a clean ice surface. Detailed observations of air temperature across a debris-covered glacier show that the glacier responds to the heating of surface debris in the sunlight and a consequent warming of the lower atmosphere [Shaw et al., 2016]. Because of this, air temperature conforms very strongly to the elevation dependency that is assumed when using a lapse rate. Although very local variations of air temperature on other debris-covered glaciers cannot be well estimated by a lapse rate [Steiner and Pellicciotti, 2016], the insulating effect of thick debris cover means that the current approach to using simple lapse rates for estimating air temperature over debris-covered glaciers could be suitable.

Nevertheless, challenges for accurately representing air temperature above glaciers without debris cover remain. The fact that globally averaged temperatures are expected to rise over the current century (areas at high latitudes have shown a stronger warming trend) [Collins et al, 2013], the applicability of using lapse rates could further diminish. Recent patterns of warmer-than-average temperatures also suggest a difficulty of accurately estimating on-glacier temperatures in the short-term. For example, for the period of May 2015 – August 2016, every month beat the previously held record for warmest globally average temperature (GISTEMP). Imagine the bow-shaped problem to that!

Edited by Matt Westoby and Emma Smith


Thomas Shaw is a PhD student in the Department of Geography at Northumbria University, UK. His research is focused on the spatial and temporal variance in near-surface air temperature across debris-covered and debris-free glaciers in the western Italian Alps. As well as conducting research in the Alps, he is also very interested in glaciers and their processes on Svalbard (Norwegian Arctic) and has spent plenty of time studying above, or within (!), ice at high latitudes. Contact e-mail: thomas.shaw@northumbria.ac.uk