CR
Cryospheric Sciences

Image of the Week

Image of the Week – Heat waves during Polar Night!

Fig. 1: (Left) Evolution of 2-m air temperatures from a reanalysis over December 2016. (Right) Time series of temperature at the location of the black cross (Svalbard). Also shown is the 1979-2000 average and one standard deviation (blue). [Credit: François Massonnet ; Data : ERA-Interim]

The winter 2016-2017 has been one of the hottest on record in the Arctic. In our Image of the Week, you can see that air temperatures were positive in the middle of the winter! Let’s talk about the reasons and implications of this warm Arctic winter. But first, let’s take a tour in Svalbard, the gateway to the Arctic…

A breach in the one of the world’s largest seed vaults

The Global Seed Vault on Svalbard (located at the black cross in our Image of the Week) is one of the world’s largest seed banks. Should mankind face a cataclysm, 800,000 copies of about 4,000 species of crops can safely be recovered from the vault. Buried under 120 m of sandstone, located 130 m above sea level, and embedded inside a thick layer of permafrost, the vault can withstand virtually all types of catastrophe – natural or man-made. This means, for example, that it is high enough to stay above sea level in case of a large sea-level rise, or that it is far enough from regions that might be affected by nuclear warfare. But is it really that safe? Last winter, vault managers reported water flooding at the entrance of the cave, after an unexpected event of permafrost melt in the middle of polar night. Not enough to put the seeds at risk (they are safely guarded in individual chambers deeper in the mountainside), but worrying enough to raise concern about how, and why such an event happened…

Fig. 2: Entrance of the Svalbard Global Seed Vault. [Credit: Dag Terje Filip Endresen, Wikimedia Commons ].

Soaring temperatures in the Arctic

The Arctic region is often dubbed the “canary in the coal mine” for climate change: near-surface temperatures there have risen at twice the pace of the world’s average, mainly due to the process of “Arctic Amplification whereby positive feedbacks enhance greatly an initial temperature perturbation. Increases in lower-troposphere Arctic air temperatures have occurred in conjunction with a dramatic retreat and thinning of the sea-ice cover in all seasons, a decrease of continental spring snow cover extent, and significant mass loss from glaciers and ice sheets (IPCC, 2013)

Winter temperatures above freezing point

The last two winters (2015-2016 and 2016-2017) have been particularly exceptional. As displayed in our Image of the Week for winter 2016-2017 and here for 2015-2016 (see also two news articles here and here for an accessible description of the event), temporary intrusions of relatively warm air pushed air temperatures above freezing point in several parts of the Arctic, even causing sea ice to “pause” its expansion at a period of the year where it usually grows at its fastest rate (see Fig. 3).

Fig.3 : Mean Arctic sea ice extent for 1981 to 2010 (grey), and the annual cycles of 1990 (blue), and 2016-2017 (red and cyan, respectively). [Credit: National Snow and Ice Data Center. Interactive plotting is available here ]

Cullather et al. (2016) and Overland and Wang (2016) conducted a retrospective analysis of the 2015-2016 extreme winter and underlined that the mid-latitude atmospheric circulation played a significant role in shaping the observed temperature anomaly for that winter (see also this previous post). Scientists are still working to analyse the most recent winter temperature anomaly (2016 – 2017).

Unusual?

How unusual are such high temperatures in the middle of the boreal winter? It is important to keep in mind that the type of event featured in our Image of the Week results from the superposition of weather and climate variability at various time scales, which must be properly distinguished. At the synoptic scale (i.e., that of weather systems, several days), the event is not exceptional. For example, a similar event was already reported back in 1975! It is not surprising to see low-pressure systems penetrate high up to the Arctic.

At longer time scales (several months), the observed temperature anomaly in the recent two winters is more puzzling. The winter 2015-2016 configuration appears to be connected with changes in the large-scale atmospheric circulation (Overland and Wang, 2016). To understand the large-scale atmospheric circulation, scientists like to map the so-called “geopotential heightfield for a given isobar, that is, the height above sea level of all points with a given atmospheric pressure. The geopotential height is a handy diagnostic because, in a first approximation, it is in close relationship with the wind: the higher the gradient in geopotential height between two regions, the higher the wind speed at the front between these two regions. The map of geopotential height anomalies (i.e., deviations from the mean) for the 700 hPa level in December (Fig. 4) is suggestive of the important role played by the large-scale atmospheric circulation on local conditions. The link between recent Arctic warming and mid-latitude atmospheric circulation changes is a topic of intense research.

Fig.4: Anomaly in 700 hPa geopotential height, December 2016 (with regard to the reference period 1979-2000) [Credit: François Massonnet; Data: ERA-Interim]

Finally, at climate time scales (several years to several decades), this event is not so surprising: the Arctic environment has changed dramatically in the last few decades, in great part due to anthropogenic greenhouse gas emissions. With a warmer background state, there is higher probability of winter air temperatures surpassing 0°C if synoptic and large-scale variability positively interact with each other, as seems to have been the case during the last two winters.

What does this mean for future winters?

The rapid transformation of the Arctic is already having profound implications on ecosystems (Descamps et al., 2016) and indigenous populations (e.g., SWIPA report). To a larger extent, it can potentially affect our own weather: we polar scientists like to say that “what happens in the Arctic, does not stay in the Arctic”. The unusual summers and winters that large parts of Europe, the U.S. and Asia have experienced in recent years might be related to the rapid Arctic changes, according to several scientists – but there is no consensus yet on that matter. One thing is known for sure: the last two winters have been the warmest on record, but this might just be the beginning of a long chain of more extreme events…

Further reading

Edited by Scott Watson and Clara Burgard


François Massonnet is a F.R.S.-FNRS Post-Doctoral Researcher at the Université catholique de Louvain and affiliated at the Barcelona Supercomputing Center (Spain). He is assessing climate models as tools to understand (retrospectively and prospectively) polar climate variability and beyond. He tweets as @FMassonnet. Contact Email: francois.massonnet@uclouvain.be

Image of the Week – When the dirty cryosphere destabilizes!

Image of the Week – When the dirty cryosphere destabilizes!

Ice is usually something you see covering large ocean areas, mountain tops and passes or as huge sheets in polar regions. This type of ice is clearly visible from space or with the naked eye. There is, however, a large volume of ice that is less visible. This ice is distributed over the polar and high alpine permafrost regions; and is the ice hidden below ground. It might be hidden, but that doesn’t mean we should ignore it. If this below-ground ice melts, the ground might collapse!


On solid ground?

To change the surface of a landscape usually requires wind or water, which actively erodes the material around it. In permafrost areas, however, different mechanisms are at work. In these areas, the ground or parts of the ground, are frozen all year round and the formation and melting of below-ground ice changes the landscape in a complicated way. Below-ground ice can have many shapes and sizes depending on moisture availability, sediment type and thermal regime (French, 2007). Because a gram of ice has 9 % higher volume than a gram of water, simply freezing, thawing and re-freezing soil water can make the surface “wobbly” and irregular. Since ice doesn’t drain from a saturated soil, as water does, a permanently frozen soil can also contain moisture in excess of the absorption capacity of the soil – excess ice. This means that ice might take up the majority of the ground volume in ice-rich areas.

Our Image of the Week (Fig. 1) was taken in NE Greenland. The phenomenon shown is a result of ground, which has been frozen for many years, being destabilized. In this photo, the below-ground ice has begun to melt, and the decrease in ice volume has caused the ground to collapse, forming what is known as a thermokarst development (Fig. 1). This is just one type of feature that can be caused by below-ground ice mass loss. Kokelj and Jorgenson (2013) give a nice overview of recognized thermokarst features including: retrogressive thaw slumps, thermokarst lakes and active layer detachment slides. Ice melt might also simply be expressed as a lowering of the land surface (thermal subsidence), as observed in peat (Dyke and Slaten, 2010) and in areas with ice wedge polygons (Jorgenson et al., 2006), or in upraised plateaus (Chasmer et al., 2016).

the decrease in ice volume has caused the ground to collapse

The spatial scales of these types of collapse features span from depressions of 10 cm depth to areas of several square kilometers, with thermokarst features many meters deep. The rates of surface change also vary from sudden detachment and slide of the unfrozen upper active layer on slope, to features developed over centuries and even millennia (e.g. Morgenstern et al., 2013).

The most dramatic surface changes often happen where ground ice content is high, such as in the coastal lowland tundras of Siberia (e.g. Morgenstern et al., 2013) or coastal northern Canada (Fortier, et L., 2007). However, thermokarst development is found also in coastal Greenland (Fig. 1) and even the McMurdo Dry Valleys of Antarctica (Levy et al., 2013).

Why does the ground ice melt?

Many factors can lead to the destabilization of below-ground ice bodies. Notable ones are:

  • Erosion of the surface allows for atmospheric energy to penetrate deeper into the ground.
  • Thermal contraction or other types of cracks might create an easy access to deeper layers for water and energy.
  • Persistent running water might erode physically as well as transfer fresh energy into the system.

Fig. 2 shows a recently opened crack in the ground, close to the karst formation shown in Fig.1. The crack reveals a large body of massive (pure ice) below-ground ice. The opening of the crack, however, also creates a highway for heat energy into the now unstable ice body, which will start degrading.

Figure 2: Looking into a recently opened crack revealing a large ice body just below the summer thaw layer, NE Greenland [Credit: Laura Helene Rasmussen]

“And so what?”

The surface changes somewhat. No big deal. Why investigate where and how and how much and how fast?

For people living in permafrost areas thermal subsidence might happen below the foundation of their house or destabilize the one road leading to their local airport (Fortier, et al., 2011).

Figure 3: Taking a closer (!) look at below-ground ice, NE Greenland [Credit: Line Vinther Nielsen].

Thermal subsidence might also change the hydrology of the area, causing surface water to find new routes (Fortier, et al., 2007) or pool in new places. When water pools in the depressions above frozen ground, the exchange of energy between the atmosphere and the permafrost is altered.

There is increased heat transport downward into the ground in summer (Boike et al., 2015), which can then lead to more melting. Similarly, thermokarst development itself exposes more frozen ground to above-zero temperatures, leading to further thawing (Chasmer et al., 2016)

and crucially mobilising otherwise dormant carbon stored in the permafrost (Tarnocai, et al., 2009).

Reports of an increase in rates of thaw have been linked to recent climatic warming (Kokelj and Jorgenson, 2013), and changes in precipitation patterns (e.g. Kokelj et al., 2015). So expect to see this “dirty“ cryospheric component receiving more attention, and don’t be surprised if you see an increasing number of strange scientists figuratively or literally (!) sticking their heads into cracks in the ground…

Edited by Emma Smith and Clara Burgard


Laura Helene Rasmussen is a Danish permafrost scientist working at the Center for Permafrost, University of Copenhagen. She has spent many seasons in Greenland, working with the Greenland Ecosystem Monitoring Programme and is interested in Arctic soils as an ecosystem component, their climate sensitivity, functioning and simply understanding what goes on below.

Image of the Week – Far-reaching implications of Everest’s thinning glaciers

Fig. 1: Surface lowering on the debris-covered Khumbu Glacier, Nepal derived from differencing two digital elevation models. (a) The debris-covered surface looking down-glacier. (b-d) Surface elevation change 1984−2015. [Credit: Scott Watson and Owen King]

From 1984 to 2015, approximately 71,000 Olympic size swimming pools worth of water were released from the melting Khumbu Glacier in Nepal, which is home to Everest Basecamp. Find out how Himalayan glaciers are changing and the implications for downstream communities in this Image of the Week.


Himalayan glaciers supply freshwater

Himalayan glaciers supply meltwater for ~800 million people, including for agricultural, domestic, and hydropower use (Pritchard, 2017). They also alleviate seasonal variations in water supply by providing meltwater during the dry season. This freshwater resource is rapidly depleting as glaciers thin and glacial lakes begin to form (Bolch et al., 2008; Watson et al., 2016; King et al., 2017). Additionally, outburst floods from these lakes (see those previous posts on the topic) threaten downstream impacts for communities and infrastructure (Rounce et al., 2016).

Debris-covered glaciers thin, rather than retreat

Erosion in the rugged mountain topography leads to high quantities of rocky debris accumulating on the glacier surface, which changes the glacial response to climatic warming. The debris-layer (which can be several metres thick at the lower terminus) insulates the ice beneath, leading to highest melt rates up-glacier of the terminus. Therefore these debris-covered glacier thin, rather than retreat up-valley.

This thinning is actually a complex process of sub-debris melt, and mass loss associated with supraglacial ponds and ice cliffs, which form pits on the glacier surface and are ‘hot-spots’ of mass loss. Since the highest rates of surface lowering are up-glacier from the terminus, the surface slope of the glacier reduces and meltwater increasingly ponds on the surface, which can ultimately form a large glacial lake.

Khumbu Glacier

Fig 2 : Khumbu Icefall viewed from Kala Patthar. [Credit: Scott Watson]

The image of this week (Fig 1) shows surface elevation change on Khumbu Glacier, which flows down from Everest and is home to Everest Base Camp in Nepal. Parts of the glacier surface have thinned by up to 80 m 1984−2015 and over 197,600,000 m³ of ice melted over study period, which is approximately 71,000 Olympic size swimming pools worth of water! The thinning is clearly visible in the vertical offset between the contemporary glacier surface and the Little Ice Age moraines (a) and is highest in the mid-section of the glacier (b).

Mountaineers ascending Mount Everest climb the Khumbu icefall (Fig 2) and camp on the glacier surface. Additionally, popular trekking routes also run alongside and across the glacier, which are used by thousands of tourists every year. The accessibility of both these mountaineering and trekking routes is changing in response to glacier mass loss.

Stagnating glaciers are unhealthy glaciers

Accumulation of snowfall in the highest reaches of the glacier would typically compress to form new ice and replenish mass loss on the lower glacier as the glacier flows downstream. However, trends of reduced precipitation (Salerno et al., 2015) and decreasing glacier surface slopes promote a reduction in glacier velocity. Figure 3 shows glaciers stagnating in their lower reaches, where water is also visibly ponding on the glacier surface. For Khumbu and Ngozumpa glaciers, this contributes to the development of large glacial lakes. If these lakes continue to grow, once fully established they can rapidly increase glacier mass loss as a calving front develops (e.g. at Imja Lake).

Fig. 3: Surface velocity of glaciers in the Everest region derived from feature tracking on ASTER satellite imagery. [Credit: Scott Watson]

Edited by Sophie Berger

References/further reading

  • Bolch, T Buchroithner, MF Peters, J Baessler, M and Bajracharya, S. 2008. Identification of glacier motion and potentially dangerous glacial lakes in the Mt. Everest region/Nepal using spaceborne imagery. Nat. Hazards Earth Syst. Sci. 8: 1329-1340. 10.5194/nhess-8-1329-2008
  • King, O Quincey, DJ Carrivick, JL and Rowan, AV. 2017. Spatial variability in mass loss of glaciers in the Everest region, central Himalayas, between 2000 and 2015. The Cryosphere 11: 407-426. 10.5194/tc-11-407-2017
  • Pritchard, HD. 2017. Asia’s glaciers are a regionally important buffer against drought. Nature 545: 169-174. 10.1038/nature22062
  • Rounce, DR McKinney, DC Lala, JM Byers, AC and Watson, CS. 2016. A new remote hazard and risk assessment framework for glacial lakes in the Nepal Himalaya. Hydrol. Earth Syst. Sci. 20: 3455-3475. 10.5194/hess-20-3455-2016
  • Salerno, F Guyennon, N Thakuri, S Viviano, G Romano, E Vuillermoz, E Cristofanelli, P Stocchi, P Agrillo, G Ma, Y and Tartari, G. 2015. Weak precipitation, warm winters and springs impact glaciers of south slopes of Mt. Everest (central Himalaya) in the last 2 decades (1994–2013). The Cryosphere 9: 1229-1247. 10.5194/tc-9-1229-2015
  • Watson, CS Quincey, DJ Carrivick, JL and Smith, MW. 2016. The dynamics of supraglacial ponds in the Everest region, central Himalaya. Global and Planetary Change 142: 14-27. http://dx.doi.org/10.1016/j.gloplacha.2016.04.008

Scott Watson is a PhD student at the University of Leeds, UK. He studies glaciers in the Everest region and specifically the surface interactions of supraglacial ponds and ice cliffs, which act as positive feedback mechanisms to increase glacier mass loss. He also investigates glacial lake hazards and the implications of glacial lake outburst floods.

Tweets @CScottWatson. Outreach: www.rockyglaciers.co.uk

Image of the Week – Ice lollies falling from the sky

Lolly in the sky. [Credit: Darwin Bell via flickr]

You have more than probably eaten many lollipops as a kid (and you might still enjoy them). The good thing is that you do not necessarily need to go to the candy shop to get them but you can simply wait for them to fall from the sky and eat them for free. Disclaimer: this kind of lollies might be slightly different from what you expect…


Are lollies really falling from the sky?

Eight years ago (in January 2009), a low-pressure weather system coming from the North Atlantic Ocean reached the UK and brought several rain events to the country. Nothing is really special about this phenomenon in Western Europe in the winter. However, a research flight started sampling the clouds in the warm front (transition zone where warm air replaces cold air) ahead of the low-pressure system and discovered hydrometeors (precipitation products, such as rain and snow) of an unusual kind. Researchers named them ‘ice lollies’ due to their characteristic shape and maybe due to their gluttony. The microphysical probes onboard the aircraft, combined with a radar system located in Southern England, allowed them to measure a wide range of hydrometeors, including these ice lollies that were observed for the first time with such concentration levels.

How do ice lollies form?

A recent study (Keppas et al, 2017) explains that ice lollies form when water droplets (size of 0.1 to 0.7 mm) collide with ice crystals with the form of a column (size of 0.25 to 1.4 mm) and freeze on top of them (see Fig. 2).

Fig 2: Formation of an ice lolly: water droplet (the circle) collides with an ice crystal (the column) [Credit: Fig. 1a from Keppas et al., (2017)].

Such ice lollies form in ‘mixed-phase clouds’, i.e. clouds made of water droplets and ice crystals and whose temperature is below the freezing point (0°C). At these temperatures, water droplets can be supercooled, meaning that they stay liquid below the freezing point.

Figure 3 below shows the processes and particles involved in the formation of ice lollies. Ice lollies are mainly found at temperatures between 0 and -6°C, in the vicinity of the warm conveyor belt, which represents the main source of warm moist air that feeds the low-pressure system. This warm conveyor belt brings water vapour that participates in the formation and growth of supercooled water droplets. Ice crystals formed near the cloud tops fall through the warm conveyor belt and collide with the water droplets to form ice lollies.

Fig 3: Processes involved with the formation of ice lollies, which mainly form under the warm conveyor belt [Credit: Fig 4 from Keppas et al., (2017)].

Are these ice lollies important?

Ice lollies were observed more recently (September 2016) during another aircraft mission over the northeast Atlantic Ocean but no radar coverage supported the observations. At the moment of writing this article, the lack of observations prevent us from determining the importance of these ice lollies in the climate system. However, future missions would provide more insight. In the meantime, we suggest you to enjoy a lollipop such as the one shown in the image of this week 🙂

This is a joint post, published together with the atmospheric division blog, given the interdisciplinarity of the topic.

Edited by Sophie Berger and Dasaraden Mauree

Reference/Further reading

Keppas, S. Ch., J. Crosier, T. W. Choularton, and K. N. Bower (2017), Ice lollies: An ice particle generated in supercooled conveyor belts, Geophys. Res. Lett., 44, doi:10.1002/2017GL073441

 


DavidDavid Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.

Image of the Week – How geometry limits thinning in the interior of the Greenland Ice Sheet

Image of the Week –  How geometry limits thinning in the interior of the Greenland Ice Sheet

The Greenland ice sheet flows from the interior out to the margins, forming fast flowing, channelized rivers of ice that end in fjords along the coast. Glaciologists call these “outlet glaciers” and a large portion of the mass loss from the Greenland ice sheet is occurring because of changes to these glaciers. The end of the glacier that sits in the fjord is exposed to warm ocean water that can melt away at its face (a.k.a. its “terminus”) and force the glacier to retreat. As the glaciers retreat, they thin and this thinning can spread into the interior of the ice sheet along the glacier’s flow, causing glaciers to lose ice mass to the ocean as is shown in our Image of the Week. But how far inland can this thinning go?

Not all glaciers behave alike

NASA’s GRACE mission measures mass changes of the Earth and has been used to measure ice mass loss from the Greenland ice sheet (see Fig. 1a). The GRACE mission has been extremely valuable in showing us where the largest changes are occurring: around the edge of the ice sheet. To get a closer look, my colleagues and I use a technique called photogrammetry.

Using high-resolution satellite photos, we created digital elevation models of the present-day outlet glacier surfaces. The imagery was collected by the WorldView satellites and has a resolution of 50 cm per pixel! When we compared our present-day glacier surfaces with surfaces from 1985, with the help of an aerial photo survey of the ice sheet margin (Korsgaard et al., 2016), we found that glacier thinning was not very uniform in the West Greenland region (see our Image of the Week, Fig. 1b). Some glaciers thinned by over 150 meters at their termini but others remained stable and may have even thickened slightly! Another observation is that, of the glaciers that have thinned, some have thinned only 10 kilometers into the interior while others have thinned hundreds of kilometers inland (Felikson et al., 2017).

But atmospheric and ocean temperatures are changing on much larger scales – they can’t be the cause of these huge differences in thinning that we observe between glaciers. So what could be the cause of the differences in glacier behaviour? My colleagues and I used kinematic wave theory to help explain how each glacier’s unique shape (thickness and steepness) can control how far inland thinning can spread…

A kinematic wave of thinning

As a glacier’s terminus retreats, it thins and this thinning can spread upglacier, into the interior of the ice sheet, along the glacier’s flow. This spreading of thinning can be modeled as a diffusive kinematic wave (Nye, 1960). This means that the wave of thinning will diffuse in the upglacier direction while the flow of ice advects the thinning in the downglacier direction. An analogy for this process is the spreading of dye in a flowing stream. The dye will spread away from the source (diffusion) and it will also be transported downstream (advection) with the flow of water.

The relative rates of diffusion and advection are given by a non-dimensional value called the Peclet number. For glacier flow, the Peclet number is a function of the thickness of the ice and the surface slope of the ice. Where the ice is thick and flat, the Peclet number is low, and thinning will diffuse upglacier faster than it advects downglacier. Where the ice is thin and steep, the Peclet number is high, and thinning will advect downglacier faster than in diffuses upglacier.

Let’s take a look at an example, the Kangilerngata Sermia in West Greenland

Figure 2: Thinning along the centreline of Kangilerngata Sermia in West Greenland. (a) Glacier surface profile in 1985 (blue), present-day (red), and bed (black). (b) Dynamic thinning from 1985 to present along the profile with percent unit volume loss along this profile shown as colored line. (c) Peclet number along this profile calculated from the geometry in 1985 with Peclet number running maxima highlighted (red). [Credit: Denis Felikson]

There, dynamic thinning has spread from the terminus along the lowest 33 kilometers (see Fig. 2). At that location, the glacier flows over a bump in the bed, causing the ice to be thin and steep. The Peclet number is “high” in this location, meaning that any thinning here will advect downglacier faster than it can spread upglacier. Two important values are needed to further understand the relationship between volume loss and Peclet number. On the one hand, we compute the “percent unit volume loss”, which is the cumulative thinning from the terminus to each location normalized by the total cumulative thinning, to identify where most of the volume loss is taking place. On the other hand, we identify the “Peclet number running maxima” at the locations where the Peclet number is larger than all downglacier values. These locations are critical because if thinning has spread upglacier beyond a local maximum in the Peclet number, and accessed lower Peclet values, then thinning will continue to spread until it reaches a Peclet number that is “large enough” to prevent further spreading. But just how large does the Peclet number need to be to prevent thinning from spreading further upglacier?

Figure 3: (a) Percent unit volume loss against Peclet number running maximum for 12 thinning glaciers in West Greenland. (b) Distances from the termini along glacier flow where the Peclet number first crosses 3. Abbreviations represent glacier names [Credit: Denis Felikson]

If we now look at the percent unit volume loss versus Peclet number running maxima for not only one but twelve thinning glaciers in the region, we see a clear pattern: as the Peclet number increases, more of the volume loss is occurring downglacier (see Fig. 3). By calculating the medians of the glacier values, we find that 94% of unit volume loss has occurred downglacier of where the Peclet number first crosses three. All glaciers follow this pattern but, because of differences in glacier geometry, this threshold may be crossed very close to the glacier terminus or very far inland. This helps explaining the differences in glacier thinning that we’ve observed along the coast of West Greenland. Also, it shows that the Peclet number can be a useful tool in predicting changes for glaciers that have not yet retreated and thinned.

Further reading

Image of the Week – Ice Ice Bergy

Image of the Week – Ice Ice Bergy

They come in all shapes, sizes and textures. They can be white, deep blue or brownish. Sometimes they even have penguins on them. It is time to (briefly) introduce this element of the cryosphere that has not been given much attention in this blog yet: icebergs!


What is an iceberg?

Let’s start with the basics. An iceberg, which literally translates as “ice mountain”, is a bit of fresh ice that broke off a glacier, an ice shelf, or a larger iceberg, and that is now freely drifting in the ocean. As an approximation, you can consider that since an iceberg is already in the water (about 90% under water even), its melting does not contribute to sea-level rise. However, if you remember our Sea Level “For Dummies” post, you know that the melting of fresh ice reduces the ocean’s density and makes it expand. Icebergs are found at both poles, although they tend to be larger in the Southern Ocean. The largest iceberg ever spotted there was 335 by 97 km, which represents an area larger than Belgium !

Modelled trajectories of icebergs around Antarctica. The different colours represent different size classes, ranging from 0-1 km² (class 1) to 100-1000 km² (class 5). [Credit: subset of Fig 2 from Rackow et al (2017)]

Icebergs can drift over thousands of kilometres (Rackow et al., 2017), during several years. A more thorough account of the life of an iceberg will be given in a future post, but be aware that among other things, as it drifts:

  • The iceberg is eroded by the waves and melted by the relatively warm ocean;
  • It can split in several pieces because of this melting and mechanical stress;
  • Sea ice can freeze around it, trapping it in the pack ice.

This means that the iceberg changes shape a lot, and can be tricky to monitor (Mazur et al, 2017).

Why do we want to monitor icebergs?

You may have heard of the Titanic, and hence are aware that icebergs pose a risk for navigation not only in the polar regions but even in the North Atlantic. Icebergs also are large reservoirs of freshwater, and depending on how and where they melt, this inflow of melted freshwater can really affect the ocean; it even dominates the freshwater budget in some Greenland fjords (Enderlin et al., 2016).

Icebergs have traditionally been rather understudied, so we are only now discovering how important they are and how they interact with the rest of the climate system: increasing sea ice production (A. Mazur, PhD thesis, 2017), biological activity (Vernet et al., 2012), and even carbon storage (Smith et al., 2011). And sometimes, they have penguins on them!

All eyes in the CryoTeam are now turned to the Antarctic Peninsula, where a giant iceberg may detach from the Larsen C ice shelf soon. To learn how we know that, check this video made by ESA. And of course, continue reading us – we’ll be reporting about the birth of this monster berg!

An iceberg by Antarctica [Credit: C. Heuzé]

Edited by Sophie Berger

Further reading

  • Enderlin et al. (2012), Iceberg meltwater fluxes dominate the freshwater budget in Greenland’s iceberg-congested glacial fjords, Geophysical Research Letters, doi:10.1002/2016GL070718

  • Mazur et al. (2017), An object-based SAR image iceberg detection algorithm applied to the Amundsen Sea, Remote Sensing of Environment, doi:10.1016/j.rse.2016.11.013

  • Rackow et al. (2017), A simulation of small to giant Antarctic iceberg evolution: Differential impact on climatology estimates, Journal of Geophysical Research: Oceans, doi: 10.1002/2016JC012513
  • Smith et al. (2011), Carbon export associated with free-drifting icebergs in the Southern Ocean, Deep Sea Research, doi: 10.1016/j.dsr2.2010.11.027
  • Vernet et al. (2012), Islands of Ice: Influence of Free-Drifting Antarctic Icebergs on Pelagic Marine Ecosystems, Oceanography, doi:10.5670/oceanog.2012.72

Image of the Week – Antarctica’s Flowing Ice, Year by Year

Fig 1: Map series of annual ice sheet speed from Mouginot et al. (2017). Speeds range from 0 (purple) to 1000+ (dark brown) m/yr. [Credit: George Roth]

Today’s Image of the Week shows annual ice flow velocity mosaics at 1km resolution from 2005 to 2016 for the Antarctic ice sheet. These mosaics, along with similar data for Greenland (see Fig.2), were published by Mouginot et al, (2017) last month as part of NASA’s MEaSUREs (Making Earth System Data Records for Use in Research Environments) program.


How were these images constructed?

The mosaics shown today (Fig 1 and 2) were built by combining optical imagery from the Landsat-8 satellite with radar (SAR) data from the Sentinel-1a/b, RADARSAT-2, ALOS PALSAR, ENVISAT ASAR, RADARSAT-1, TerraSAR-X, and TanDEM-X sensors.

Although the authors used the well-known techniques of feature and speckle tracking to produce their velocities from optical and radar images, respectively, the major novelty of their study lies in the automation and integration of the different datasets.

Fig.2: Mosaics of yearly velocity maps of the Greenland and Antarctic ice sheet for the period 2015-2016.Composite of satellite-derived yearly ice sheet speeds from 2005-2016 for both Greenland and Antarctica. [Credit: cover figure from Mouginot et al. (2017)]

How is this new dataset useful?

Previously, ice sheet modellers have used mosaics composed of satellite data from multiple years to cover the entire ice sheet. However, this new dataset is one of the first to provide an ice-sheet-wide geographic scale, a yearly temporal resolution, and a moderately high spatial resolution (1km). This means that modellers can now better examine how large parts of the Greenland and Antarctic ice sheets evolve over time. By linking the evolution of the ice sheets to the changes in weather and climate over those ice sheets during specific years, modellers can calibrate the response of those ice sheets’ outlet glaciers to different climate conditions. The changes in the speeds of these outlet glaciers have important consequences for the amount of sea level rise expected for a given amount of warming.

How can I start using this data?

The yearly MEaSUREs data is hosted at the NSIDC in NetCDF format. The maps shown in the animated image were made using Quantarctica/QGIS (for more information on Quantarctica, check out our previous post E). QGIS natively supports NetCDF files like these mosaics with no additional import steps. Users can quickly calculate new grids showing speed, changes in velocities between years, and more by using the QGIS Raster Calculator or gdal_calc.

References/ Further Reading

Mouginot, J., Rignot, E., Scheuchl, B., & Millan, R. (2017). Comprehensive Annual Ice Sheet Velocity Mapping Using Landsat-8, Sentinel-1, and RADARSAT-2 Data. Remote Sensing, 9(4), 364. http://dx.doi.org/10.3390/rs9040364

Image of the Week – Quantarctica: Mapping Antarctica has never been so easy!

Image of the Week – A high-resolution picture of Greenland’s surface mass balance

Written with help from Jelte van Oostsveen
Edited by Clara Burgard and Sophie Berger


George Roth is the Quantarctica Project Coordinator in the Glaciology group (@NPIglaciology) at the Norwegian Polar Institute. He has spent the last several years helping researchers with GIS, cartography, and remote sensing in both the Arctic and Antarctic.

Image of the Week – A rather splendid round-up of CryoEGU!

Image of the Week – A rather splendid round-up of CryoEGU!

The 2017 edition of the EGU general assembly was a great success overall and for the cryospheric division in particular. We were for instance thrilled to see that two of the three winning photos of the EGU Photo contest featured ice! To mark the occasion we are delighted to use as our image of this week,  one of these pictures, which  shows an impressive rapid in the Pite River in northern Sweden. Congratulations to Michael Grund for capturing this stunning shot.  You can find all photos entered in the contest on imaggeo — the EGU’s  open access geosciences image repository.

But being the most photogenic division (at least the ice itself is…not sure about the division team itself!) was not our only cryo-achievement during the conference. Read on to get the most of (cryo)EGU 2017!


EGU 2017 in figures

  • 17,399 abstracts in the programme (including 1179 to cryo-related sessions)
  • 14,496 scientists from 107 countries attending the conference
  • 11,312 poster, 4,849 oral and 1,238 PICO presentations
  • 649 scientific sessions and 88 short courses
  • 53% of early-career scientists

Polar Science Career Panel

During the week we teamed up with APECS to put on a Polar Science Career Panel. Our five panellists, from different backgrounds and job fields, engaged in a lively discussion with over 50 session attendees. With many key topics being frankly and honesty discussed by our panelists, who had some great comments and advice to offer. Highlights of the discussion can be found on the @EGU_CR twitter feed with #CareerPanel.

At the end we asked each panellist to come up with some final words of advice for early-career scientists, which were:

  • There is no right and wrong, ask other people and see what you like
  • Remember you can shape your own job
  • Take chances! Even if you are likely to fail and think outside the box
  • Remember that you are a whole human being… not only a scientist and use all your skills
  • And last but not least… come and work at Carbon Brief (thanks Robert McSweeney!!)

However, the most memorable quotation of the entire panel is arguably from Kerim Nisancioglu :

Social media

One of the things the EGU Cryosphere team has been recognised for is its great social media presence. We tweeted away pre-EGU with plenty of advice, tips and information about events during the week and also made sure to keep our followers up-to-date during the week.
If it is not yet the case, please consider following us on twitter and/or facebook to keep updated with the latest news about the cryosphere division, the EGU or any other interesting cryo-related news!

We need YOU for the EGU cryosphere division

Conferences are usually a great way to meet new people but did you know that getting involved with the outreach activities of the division is another way?

Each division has an ECS (early-career scientist) representative and a team to go with that and the Cryosphere division is one of the most active. Our new team of early-career scientists for 2017/18 includes some well known faces and some who are new to the division this year:

Nanna Karlsson : outgoing ECS representative and incoming coordinator for posters and PICOs awards

Emma Smith : incoming ECS representative and outgoing co-chief editor of the  cryoblog

Sophie Berger: chief-editor of the cryoblog and incoming outreach officer

Clara Burgard : incoming co-chief editor the cryoblog

 

 

 

We also have many more people (who aren’t named above) involved in the blog and social media team AND the good news is that we are looking for new people to either run our social media accounts and/or contribute regularly to this “award winning” cryoblog. Please get in touch with Emma Smith (ECS Representative and former blog editor) or Sophie Berger (Chief Blog Editor and Outreach Officer) if you would like to get involved in any aspect of the EGU Cryosphere team. No experience is necessary just enthusiasm and a love of bad puns!

And here is your “Save the Date” for EGU 2018 – which will be held between 8th – 13th April 2018.

Co-authored by Emma Smith and Sophie Berger

Image of the Week — We’re heading for Vienna

Image of the Week — We’re heading for Vienna
Tatata taaa tatatatata Tatata taaa tatatatatatatata
We’re heading for Vienna (Vienna)
And still we stand tall
‘Cause maybe they’ve seen us (seen us)
And welcome us all, yeah
With so many miles left to go
And things to be found (to be found)
I’m sure that we’ll all miss that so
it’s the … 
…congratulations, you’ve recognise the song…..it is the Final Countdown (slightly adapted!)

With the EGU general assembly starting in two days only, we hope that your presentations are almost ready that you haven’t forgotten to include in your programme all the cool stuff listed in our cryo-guide!

 

However, if you don’t have time to read it all, please make sure you’ve heard of these 3 events :
  1. the pre-icebreaker meet up on Sunday 23rd from 16:00 aida (close to Stefanplatz)
  2. the Cryoblog lunch on Tuesday 25th 12:15 in front of the entrance.
    If you like this blog, are curious about it and would like to contribute to it  — directly and/or indirectly — please come and meet us on Tuesday (for more information please email sberger@ulb.ac.be or emma.smith@awi.de)
  3. the cryo night out on Thursday 27th from 19:30 at Wieden Braü

 

See you in Vienna!

PS: We take no responsibility for anyone who finds they have Final Countdown stuck in their head all week! (♪ Tatata taaa tatatatata Tatata taaa tatatatatatatata ♫)

Edited by Emma Smith

Image of the Week – A high-resolution picture of Greenland’s surface mass balance

Image of the Week – A high-resolution picture of Greenland’s surface mass balance

The Greenland ice sheet – the world’s second largest ice mass – stores about one tenth of the Earth’s freshwater. If totally melted, this would rise global sea level by 7.4 m, affecting low-lying regions worldwide. Since the 1990s, the warmer atmosphere and ocean have increased the melt at the surface of the Greenland ice sheet, accelerating the ice loss through increased runoff of meltwater and iceberg discharge in the ocean.


Simulating the climate with a regional model

To understand the causes of the recent ice loss acceleration in Greenland, we use the Regional Atmospheric Climate Model RACMO2.3 (Noël et al. 2015) that simulates the evolution of the surface mass balance, that is the difference between mass gain from snowfall and mass loss from sublimation, drifting snow erosion and meltwater runoff. Using this data set, we identify three different regions on the ice sheet (Fig. 1):

  • the inland accumulation zone (blue) where Greenland gains mass at the surface as snowfall exceeds sublimation and runoff,

  • the ablation zone (red) at the ice sheet margins which loses mass as meltwater runoff exceeds snowfall.

  • the equilibrium line (white) that separates these two areas.

From 11 km to 1 km : downscaling RACMO2.3

To cover large areas while overcoming time-consuming computations, RACMO2.3 is run at a relatively coarse horizontal resolution of 11 km for the period 1958-2015. At this resolution, the model does not resolve small glaciated bodies (Fig. 2a), such as narrow marginal glaciers (few km wide) and small peripheral ice caps (ice masses detached from the big ice sheet). Yet, these areas contribute significantly to ongoing sea-level rise. To solve this, we developed a downscaling algorithm (Noël et al., 2016) that reprojects the original RACMO2.3 output on a 1 km ice mask and topography derived from the Greenland Ice Mapping Project (GIMP) digital elevation model (Howat et al., 2014). The downscaled product accurately reproduces the large mass loss rates in narrow ablation zones, marginal outlet glaciers, and peripheral ice caps (Fig. 2b).

Fig. 2: Surface mass balance (SMB) of central east Greenland a) modelled by RACMO2.3 at 11 km, b) downscaled to 1 km (1958-2015). The 1 km product (b) resolves the large mass loss rates over marginal outlet glaciers [Credit: Brice Noël].

 

The high-resolution data set has been successfully evaluated using in situ measurements and independent satellite records derived from ICESat/CryoSat-2 (Noël et al., 2016, 2017). Recently, the downscaling method has also been applied to the Canadian Arctic Archipelago, for which a similar product is now also available on request.

Endangered peripheral ice caps

Using the new 1 km data set (Fig. 1), we identified 1997 as a tipping point for the mass balance of Greenland’s peripheral ice caps (Noël et al., 2017). Before 1997, ablation (red) and accumulation zones (blue) were in approximate balance, and the ice caps remained stable (Fig. 3a). After 1997, the accumulation zone retreated to the highest sectors of the ice caps and the mass loss accelerated (Fig. 3b). This mass loss acceleration was already reported by ICESat/CryoSat-2 satellite measurements, but no clear explanation was provided. The 1 km surface mass balance provides a valuable tool to identify the processes that triggered this recent mass loss acceleration.

Fig. 3: Surface mass balance of Hans Tausen ice cap and surrounding small ice bodies in northern Greenland before (a) and after the tipping point in 1997 (b). Since 1997, the accumulation zone (blue) has shrunk and the ablation zone (red) has grown further inland, tripling the pre-1997 mass loss [Credit: Brice Noël].

 

Greenland ice caps are located in relatively dry regions where summer melt (ME) nominally exceeds winter snowfall (PR). To sustain the ice caps, refreezing of meltwater (RF) in the snow is therefore a key process. The snow acts as a “sponge” that buffers a large amount of meltwater which refreezes in winter. The remaining meltwater runs off to the ocean (RU) and contributes to mass loss (Fig. 4a).

Before 1997, the snow in the interior of these ice caps could compensate for additional melt by refreezing more meltwater. In 1997, following decades of increased melt, the snow became saturated with refrozen meltwater, so that any additional summer melt was forced to run off to the ocean (Fig. 4b), tripling the mass loss.

Fig. 4: Surface processes on an ice cap: the ice cap gains mass from precipitation (PR), in the form of rain and snow. a) In healthy conditions (e.g. before 1997), meltwater (ME) is partially refrozen (RF) inside the snow layer and the remainder runs off (RU) to the ocean. The mass of the ice cap is constant when the amount of precipitation equals the amount of meltwater that runs off. b) When the firn layer is saturated with refrozen meltwater, additional meltwater can no longer be refrozen, causing all meltwater to run off to the ocean. In this case, the ice cap loses mass, because the amount of precipitation is smaller than the amount of meltwater that runs off [Credit: Brice Noël].

  In 1997, following decades of increased melt, the snow became saturated with refrozen meltwater, so that any additional summer melt was forced to run off to the ocean, tripling the mass loss.

We call this a “tipping point” as it would take decades to regrow a new, healthy snow layer over these ice caps that could buffer enough summer meltwater again. In a warmer climate, rainfall will increase at the expense of snowfall, further hampering the formation of a new snow cover. In the absence of refreezing, these ice caps will undergo irreversible mass loss.

What about the Greenland ice sheet?

For now, the big Greenland ice sheet is still safe as snow in the extensive inland accumulation zone still buffers most of the summer melt (Fig. 1). At the current rate of mass loss (~300 Gt per year), it would still take 10,000 years to melt the ice sheet completely (van den Broeke et al., 2016). However, the tipping point reached for the peripheral ice caps must be regarded as an alarm-signal for the Greenland ice sheet in the near future, if temperatures continue to increase.

Data availability

The daily, 1 km Surface Mass Balance product (1958-2015) is available on request without conditions for the Greenland ice sheet, the peripheral ice caps and the Canadian Arctic Archipelago.

Further reading

Edited by Sophie Berger


Brice Noël is a PhD Student at IMAU (Institute for Marine and Atmospheric Research at Utrecht University), Netherlands. He simulates the climate of the Arctic region, including the ice masses of Greenland, Svalbard, Iceland and the Canadian Arctic, using the regional climate model RACMO2. His main focus is to identify snow/ice processes affecting the surface mass balance of these ice-covered regions. He tweets as: @BricepyNoel Contact Email: b.p.y.noel@uu.nl

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