A short article I wrote for the EGU blog about biological darkening of ice and snow was posted last month. The article was built around an aerial view of our 2016 field camp on the Greenland Ice Sheet, where large areas of dark ice are clearly visible.
The dark colour is due to a collection of dusts, soot and algal cells, with the algal cells doing the bulk of the darkening. A second figure in the article shows the algal cells under the microscope along with the spectra of reflected light from the algal ice surfaces. This was one of several EGU blog posts about icy biology, including this one and this one!
Summer 2017 seriously challenged the idea that summer in SW Greenland has a reliably stable, clear, dry meteorology. Our field work was characterized by unpredictable swings between weather extremes from blizzards dropping 1ft of snow in an evening to bright sunshine and low wind, to rain and tens of centimeters of surface lowering in a few hours. Most of this was inconsequential and actually scientifically very interesting since we experienced what would normally be a year’s worth of surface change in a few weeks. However, we did have to deal with a particularly vicious couple of days of unexpected storm… Here are the notes from my field diary…
Wind steadily increased through afternoon with frequent periods of heavy rain. No real work got done b/c too windy for drones and spectrometer needs to stay dry. As dinner time approached winds continued to strengthen. Tedstone cooked a killer dahl while Stefan and I redrilled the stakes holding down all the tents and added extra guy lines to the mess on the windward side. Side of mess pushing in towards middle of mess during dinner. The fabric was looking a bit delicate and the flex in the tent wall was knocking things off the cooking table – boxes and stove etc gradually moved into the middle of the tent over about an hour as we ate. Downloaded data from AWS – winds averaging 48 kmph with much stronger gusts. Getting a little concerned about the longevity of the mess.
By 2300 the mess was pressing in and becoming quite concave during stronger gusts. Avoided going outside because of rain, but some tent maintenance was now essential. Intense surface lowering around the ply under the mess has caused poles to float in space – tent not so geodesic now! To try to counter this, poles on opposite sides of the tent were tied together with accessory cord to try to maintain dome shape. Outside tent, tags were tied up to the fly sheet to try to stop poles coming out. Predict chance of mess tent survival 40%, so all contents packed down into Zarges boxes and/or tied down, gas disconnected from stove, electrics and batteries dry-bagged and stored. Essentials moved to personal tents or stashed in dry bags for moving later.
Tedstone went to bed, but almost immediately came back with ‘bad feeling’… Bang on. On cue, a strong gust ripped the tent fabric on the windward side, which was now bending inwards to touch the plyboard floor in the centre of the tent. Now no chance of maintaining tent shape. We evacuated the tent, thankfully the rain had died down, and watched the tent collapse inwards. Seeing poles bending and breaking, we pulled as many as possible out of their tags to allow them to flop safely downwards rather than pinging dangerously as they or the fabric snapped under tension. Zarges boxes pulled onto the edges of the fabric to stop tent flying away entirely.
Now early morning and personal tents also looking in poor condition, with surface lowering causing stakes to bob uselessly in shallow drill holes and strong winds bending the tents out of shape. No sign of storm passing – front after front lining up on horizon and winds only getting stronger. Tom and Stefan looking very cold, so sent to their tents to get warm. Buddy system established: in event of any problems with personal tents, warmth etc Tom would get into my tent and vice versa, and the same for Andrew and Stefan. Tedstone and I extracted the drill and flights from the wrecked mess and redrilled holes to stake down all of the personal tents. Agreed that if one personal tent goes down, we call in search and rescue. Rationale was that once a personal tent goes, the others will follow and we then have no shelter. With no sign of storm abating the risk of exposure and hypothermia was not justifiable. However, both know chances of heli getting here soon are slim. No panic yet – personal tents standing up OK and everyone dry and warm. At 0120, Tedstone and I went to our personal tents with agreement to reconvene and check all tents again in 2 hours, and also call back to the UK for up to date forecast.
0330 Reconvened with Tedstone – tents looking ok but storm still raging. Called Martyn (project PI) on satellite phone to ask for urgent weather forecast. Text response indicated clear weather after this storm, but could be a further 6-8 hours. Still satisfied with safety of personal tents, so 0430 back to tent to sleep with agreement to meet at 0730.
0730 Reconvene with Tedstone. Storm still strong and still looks heavy all the way to horizon. Back to tents to sit it out. Tried to snooze.
1000 Fetch stove and emergency dehy food from wrecked mess tent and cooked in porch of my personal tent. Tedstone delivered very odd breakfasts to very hungry researchers in their tents. Personal tents now looking ropey, so agreed to sit out until next break in rain, then repitch. 4 hours until next break in weather. By this time calmer weather was on the way. Cooked a second dehy meal for team and waited another 2 hours. Rain stopped and wind calmed through day. Once manageable, wrecked mess was packed down and entire camp rebuilt. No science done today!
A new paper, led by Johnny Ryan, shows that a consumer grade digital camera mounted to a drone can be used to estimate the albedo of ice surfaces with an accuracy of +/- 5%. This is important because albedo measurements are fundamental to predicting melt, but satellite albedo data is limited in its spatial and temporal resolution and ground measurements can only be for small areas. Methods employing UAV technology can therefore bridge the gap between these two scales of measurement. The work demonstrates that this is achievable using a relatively simple workflow and low cost equipment.
The full workflow is detailed in the paper, involving processing, correcting and calibrating raw digital images using a white reference target, and upward and downward shortwave radiation measurements from broadband silicon pyranometers. The method was applied on the SW Greenland Ice Sheet, providing albedo maps over 280 km2 at a ground resolution of 20 cm.
This study shows that albedo mapping from UAVs can provide useful data and as drone technology advances it will likely provide a low cost, convenient method for distinguishing surface contaminants and informing energy balance models.
Our new discussion paper, led by Black and Bloom PDRA Andrew Tedstone, examines in detail why there is a stripe of dark, fast-melting ice on the Greenland Ice Sheet, particularly in the south-west. This ‘dark zone’ is clearly visible in satellite imagery of the Greenland Ice Sheet and is important because darker ice melts faster. It is crucial to understand what causes the ice to be dark there because if it grows or darkens in a warming climate then we can expect the deglaciation of Greenland to accelerate more than is currently predicted. There are two main competing hypotheses that could explain the presence of the dark zone: 1) dust melting out from ancient ice is darkening the ice; 2) algae are growing on the ice sheet and changing its colour.
The paper shows that the dark zone changes its shape, size and duration each year. This appears to be most strongly controlled by the sensible heat flux (air temperature) between June and August, number of days with air temperatures above zero, and timing of the snow-line retreat.
These findings provide some insights into which surface processes are most likely to explain the dynamics of the dark zone. The spatial distribution of the dark ice is best explained by the melting out of dust particles from ancient ice, although these particles are not dark enough to explain the colour change of the dark zone. However, these dusts may be crucial nutrients and substrates for ice algae, suggesting that the dusts control where the dark zone is, and the algae determine how dark it gets. Our other recent TCD paper showed how algae can darken ice and snow; however, there are also meteorological conditions required for algal growth including sufficient sunlight and liquid water. We suggest in the paper that the most likely hypothesis is that dust melts out from ancient ice and stimulates the growth of algae when meteorology allows it. Algae need the dust to grow, and the dust is not dark without the algae.
We are now well into planning 2017 field work so I revisited some archive footage from previous trips. The short clip below provides a good summary of the great diversity of microbial habitats that exist, even within a very small area of ice. These include cryoconite holes, a cryo-pond (the big cryoconite and water filled pool), algal blooms on the ice surface, dispersed cryoconite, streams, cryoconite ‘alluvium’ stranded on the stream banks, weathered ice and the snowpack. The clip also shows how hummocky and non-uniform the ice surface is near the margin of the ice sheet.
To get a better idea of how these habitats are arranged spatially we also flew a small UAV (unmanned aerial vehicle) with a downwards-looking HD camera. The clip below shows some of the footage. The winds were pretty strong and you can actually see the landing gear bow into shot every so often. We’ll have a more sophisticated UAV system in Greenland in 2017 that will collect images at specific wavelengths of light.
Finally, here is a short clip of the 2016 team at the S6 camp enjoying a beautiful full moon over the ice sheet. This site is well into the ‘dark zone’ where impurity loading is very high. We’ll be back there this summer to measure the effect of this on the reflectivity and therefore melt rate of the ice sheet.
Here is a brief field report from our 2016 field season which i also posted on the Arctic Club website (here).
2016 Greenland Field Work Report
Our field work aimed to deepen our understanding of the processes darkening the Greenland Ice Sheet. This is important because the colour of the ice sheet is one of the main drivers of its melt rate because it controls how much sunlight the ice sheet reflects or absorbs. The more sunlight absorbed, the more energy is available for melting ice.
In 2016 a team of researchers from Bristol, Sheffield, Leeds, Potsdam, Aberystwyth and NASA JPL camped on the ice sheet throughout the summer melt season in order to measure and monitor the changing colour of the ice and determine the causes of the darkening. The camp was inhabited in two month-long shifts. The first team comprised Joseph Cook (University of Sheffield), Chris Williamson (University of Bristol), Johan Nilsson (NASA JPL), Ewa Sypianska (Cardiff University), Tom Gribbin (Bristol University), Tris Irvine Fynn (Aberystwyth University) and Jim McQuaid (University of Leeds). Three weeks in, we were joined by Liane Benning, Steffi Lutz and Jenine McCutcheon (all University of Leeds). The team and all the camping and scientific kit was delivered in two flights on an Air Greenland Sikorsky S-61 helicopter.
The camp was built around two large Mountain Hardware “Space Station” tents, one of which was used as a mess tent (with a dining table and chairs, gas hob and food storage) and the other was a laboratory (kitted out with microscopes, spectrometers, filtration units, gas analysers, and all the usual lab consumables). The lab tent was also our power station, with the batteries, inverters and tracking system for our solar array. The long daylight hours and low temperatures helped the solar arrays to perform extremely well and we were able to charge all our scientific equipment, as well as laptops and satellite phones any time without issue. We were even able to run extension cables from the solar array to the mess tent to provide power across the camp! Around these two large tents were our own sleeping tents. Each person had a 3-man tent to provide room for bags and belongings.
A big problem is that the tents can melt the underlying ice, so we pitched on top of layers of white ‘polfelt’ and plyboard that both insulated the floor and provided a flat(ish) surface to walk on. However, this insulation also meant that after a few days the tents rested upon large ice pinnacles so needed to be repitched regularly!
For most of the season the weather was very friendly, with clear skies and very little precipitation – typical of summer on the SW Greenland ice Sheet. However, there was a significant rainfall event early on that washed away the crunchy, weathered ice layer and left a slick, slippery surface that was impossible to walk on without sharp crampons. It is also hard to dry out wet clothes and equipment in cold, overcast conditions. The rain also caused lots of glacier surface sediment (called ‘cryoconite’) to be washed onto the ice surface, instead of being held at the bottom of ‘cryoconite holes’. The combination of washed cryoconite and the loss of the crunchy, white ice made the surface noticeably darker.
We were particularly interested in the role of algae on the colour of the ice, and therefore our microbiology team was hard at work characterising the biology of the ice surface, including identifying the species present, their productivity, abundance and colouration. It seems that algae can bloom very densely and have a severe darkening effect on the ice surface. Coupled with this were detailed measurements of the reflectivity of the surface and the deposition of dark particulates from the atmosphere.
After the first month, the ‘in’ team decamped and was replaced by the project’s head-honcho Martyn Tranter, Alex Anesio, Alex Holland and Andrew Tedstone. Jenine also stayed out there with the second team. By the end of the season, the temperature had dropped significantly – large streams were freezing up completely every evening and remaining frozen until the middle of the day. What were almost 24 hour days at the start of the season became shorter and shorter and the team was treated to spectacular sunrises and sunsets over the ice sheet. In the far distance was a plume of water that, upon close inspection in the helicopter, turned out to be spray from a huge meltwater river crashing round a tight bend. Cryoconite holes grew, coalesced, divided and migrated around the camp.
The field season was successful in terms of the science and the team also reported feeling both awestruck at the scale of the ice sheet and simultaneously surprised by its sensitivity. The growth of microscopic algae and deposition of nanoscale particles of dust and soot influence the rate at which the vast ice sheet melts, and may therefore amplify climate changes and accelerate sea level rise. The challenge now is to quantify these processes and integrate them into future melt predictions.