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Cryospheric Sciences

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Image of the Week – Kicking the ice’s butt(ressing)

Risk map for Antarctic ice shelves shows critical ice shelf regions, where local thinning increases the ice flow from the continent into the ocean [Credit: modified from Reese et al., 2018]

Changes in the ice shelves surrounding the Antarctic continent are responsible for most of its current contribution to sea-level rise. Although they are already afloat and do not contribute to sea level directly, ice shelves play a key role through the buttressing effect. But which ice shelf regions are most important for this?


The role of ice-shelf buttressing

Schematic ice-sheet-shelf system: buttressing arises when an ice shelf is laterally confined in an embayment or locally grounds at pinning points [Credit: Ronja Reese & Maria Zeitz]

In architecture, the term “buttress” is used to describe pillars that support and stabilize buildings, for example ancient churches or dams. In analogy to this, buttressing of ice shelves can stabilize the grounded ice sheet (see this blog article about the marine ice sheet instability). It can be understood as a backstress that the ice shelf exerts on the grounding line – the line that separates the grounded ice from the floating ice shelves. When an ice shelf thins or disintegrates, this stress can be reduced, then the ice flow upstream is less restrained and can increase.

This effect has been widely observed in Antarctica: the thinning of ice shelves in the Amundsen Sea is driven by the ocean and linked to ice loss there (see this blog article) and after the spectacular disintegration of Larsen A and B ice shelves the adjacent ice streams accelerated.

Which ice shelf regions are important?

Risk maps show how important each ice-shelf location is: if an ice shelf thins in this location, how much does the flux across the grounding line increase? We estimated this immediate increase using the numerical ice-flow model Úa. At first glance, one can see that all ice shelves have regions that influence upstream ice flow, and thus, provide buttressing. The highest responses occur near grounding lines of fast-flowing ice streams. Equally strong responses are found in the vicinity of ice rises or ice rumples – where the ice shelf re-grounds locally and is subject to basal drag. On the other hand, “passive” regions with negligible flux response are located towards the calving front, but also in spots close to the grounding line. Flux response signals can sometimes travel quite far – for example a perturbation near Ross Island accelerates the ice flow in almost the entire Ross Ice Shelf and reaches ice streams more than 900km away (not visible in the figure).

Risk maps for Antarctic ice shelves, as presented here, help us to get a better understanding of the critical ice shelf regions – if you are interested to read more, please see for example Gagliardini, 2018 and Reese et al., 2018.

Edited by Scott Watson and Sophie Berger


Ronja Reese is a postdoctoral researcher at the Potsdam Institute for Climate Impact Research, Germany, in the ice dynamics working group. She investigates ice dynamics in Antarctic with a focus on ice-ocean interactions and ice-shelf buttressing. She created the risk map together with Ricarda Winkelmann, Hilmar Gudmundsson and Anders Levermann. Contact Email: ronja.reese@pik-potsdam.de

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Image of the Week – Fifty shades of May (Glacier)

Image of the Week – Fifty shades of May (Glacier)

With over 198 000 glaciers in the world, you can always find a glacier that fits your mood or a given occasion. So why not for example celebrate the first Image of the Week of May with a picture of the aptly named May Glacier?


May Glacier is in fact not named after the month, but after Mr May, an officer onboard the Flying Fish during her expedition to the East Antarctic coast in the 1840s. Apart from that, there is not much to say about this 9 x 11 km glacier located in East Antarctica around 130°E, except that it is really hard to find a picture with the keywords “may” and “glacier”… So I let you enjoy the Image of the Week, which combines three satellite images of May Glacier (at the centre) and its surroundings exactly a month ago, when the skies were clear and the sea ice pretty:

See you in a month to talk about the June(au) ice field!

Image of the Week — Cavity leads to complexity

Aerial view of Thwaites Glacier [Credit: NASA/OIB/Jeremy Harbeck].

 

A 10km-long, 4-km-wide and 350m-high cavity has recently been discovered under one of the fastest-flowing glaciers in Antarctica using different airborne and satellite techniques (see this press release and this study). This enormous cavity previously contained 14 billion tons of ice and formed between 2011 and 2016. This indicates that the bottom of the big glaciers on Earth can melt faster than expected, with the potential to raise sea level more quickly than we thought. Let’s see in further details how the researchers made this discovery.


Thwaites Glacier

Thwaites Glacier is a wide and fast-flowing glacier flowing in West Antarctica. Over the last years, it has undergone major changes. Its grounding line (separation between grounded ice sheet and floating ice shelf) has retreated inland by 0.3 to 1.2 km per year in average since 2011. The glacier has also thinned by 3 to 7 m per year. Several studies suggest that this glacier is already engaged in an unstoppable retreat (e.g. this study), called ‘marine ice sheet instability’, with the potential to raise sea level by about 65 cm.

Identifying cavities

With the help of airborne and satellite measurement techniques, the researchers that carried out this study have discovered a 10km-long, 4km-wide and 350m-high cavity that formed between 2011 and 2016 more than 1 km below the ice surface. In Figure 2B, you can identify this cavity around km 20 along the T3-T4 profile between the green line (corresponding to the ice bottom in 2011) and the red line (ice bottom in 2016). According to the researchers, the geometry of the bed topography in this region allowed a significant amount of warm water from the ocean to come underneath the glacier and progressively melt its base. This lead to the creation of a huge cavity.

Fig. 2: A) Ice surface and bottom elevations in 2014 (blue) and 2016 (red) retrieved from airborne and satellite remote sensing along the T1-T2 profile identified in Fig. 2C. B) Ice surface and bottom elevations in 2011 (green) and 2016 (red) along the T3-T4 profile. C) Changes in ice surface elevation between 2011 and 2017. The ticks on the T1-T2 and T3-T4 profiles are marked every km [Credit: adapted with permission from Figure 3 of Milillo et al. (2019)].

What does it mean?

In order to make accurate projections of future sea-level rise coming from specific glaciers, such as Thwaites Glacier, ice-sheet models need to compute rates of basal melting in agreement with observations. This implies a correct representation of the bed topography and ice bottom underneath the glacier.

However, the current ice-sheet models usually suffer from a too low spatial resolution and use a fixed shape to represent cavities. Thus, these models probably underestimate the loss of ice coming from fast-flowing glaciers, such as Thwaites Glacier. By including the results coming from the observations of this study and further ongoing initiatives (such as the International Thwaites Glacier Collaboration), ice-sheet models would definitely improve and better capture the complexity of glaciers.

Further reading

Edited by Sophie Berger


David 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 – Delaying the flood with glacial geoengineering

Figure 1: Three examples of glacial geoengineering techniques to mitigate sea-level rise from ice-sheet melting [Credit: Adapted from Figure 1 of Moore et al. (2018); Design: Claire Welsh/Nature].

As the climate is currently warming, many countries and cities are preparing to cope with one of its major impacts, namely sea-level rise. Up to now, the mitigation of climate change has mainly focused on the reduction of greenhouse gas emissions. Large-scale geoengineering has also been proposed to remove carbon from the atmosphere or inject aerosols into the stratosphere to limit the rise in temperature. But locally-targeted geoengineering techniques could also provide a way to avoid some of the worst impacts, like the sea-level rise. In this Image of the Week, we present examples of such a technique that could be applied to the Antarctic and Greenland ice sheets (Moore et al., 2018; Wolovick and Moore, 2018).


Sea level is rising…

The sea level of the world oceans has been rising at a mean rate of 3 mm per year since the 1990s, mainly due to ocean thermal expansion, land-ice melting and changes in freshwater storage (see this post). More than 90% of coastal areas could experience a sea-level rise exceeding 20 cm with a 2°C warming (relative to the pre-industrial period), which is likely to happen by the middle of this century (Jevrejeva et al., 2016).

The Antarctic and Greenland ice sheets constitute two huge reservoirs of ice and contain the equivalent of 60 and 7 m of sea-level rise, respectively, if completely melted. Although a complete disintegration of these two ice sheets is not on the agenda in the coming years, surface melting of the Greenland ice sheet and the flow of some major polar glaciers could be enhanced by different positive feedbacks (see this post on climate feedbacks and this post on marine ice sheet instability). These feedbacks would elevate the sea level even more than projected by the models.

… but could potentially be delayed by glacial geoengineering

In order to cope with this threat, reducing our greenhouse gas emissions might not be sufficient to delay the rise of sea level. One alternative has been suggested by Moore et al. (2018) and consists of using glacial geoengineering techniques in the vicinity of fast-flowing glaciers of the Antarctic and Greenland ice sheets. They propose three different ways to delay sea-level rise from these glaciers and these are presented in our Image of the Week (Fig. 1):

A.   A pumping station could be installed at the top of the glacier with the aim of extracting or freezing the water at the glacier base. This would slow down the glacier sliding on the bedrock and reduce its contribution to sea-level rise.

B.   An artificial island (about 300 m high) could be built in the cavity under the floating section of the glacier (or ice shelf). This would enhance the so-called buttressing effect (see this post) and decrease the glacier flow to the ocean.

C.   A wall of up to 100 m high could be built in the ocean bay right in the front of the ice shelf. This would block (partially or completely) any warm water circulating underneath the ice shelf and delay the sub-shelf melting (see this post).

In theory

Wolovick and Moore (2018) studied in detail the possibility of building artificial islands (proposal B above) underneath the ice shelf of Thwaites Glacier (West Antarctica), one of the largest glacier contributors to the ongoing sea-level rise. They used a simple ice-flow model coupled to a simple ocean model and considered different warming scenarios in which they introduced an artificial island underneath the ice shelf.

Figure 2 below illustrates an example coming from their analysis. In the beginning (Fig. 2b), the grounding line (separation between the grounded ice sheet in blue and the floating ice shelf in purple) is located on top of a small mountain range. When running the model under a global warming scenario, the grounding line retreats inland and the glacier enters into a ‘collapsing phase’ (Fig. 2c; marine ice sheet instability). The introduction of an artificial island under the ice shelf with a potential to block half the warm ocean water allows the ice shelf to reground (Fig. 2d; the ice-shelf base touches the top of the small island below). The unprotected seaward part of the ice shelf shrinks over time, while the protected inland part thickens and regrounds (Fig. 2e-f), which overall decreases the glacier mass loss to the ocean.

Figure 2: Example of a model experiment realized on Thwaites Glacier by Wolovick and Moore (2018). Different times are presented and show the (b) initial state, (c) the collapse underway, (d) the initial effect of the construction of the artificial island below the ice shelf, (e) the removal of the seaward ice shelf and thickening of the landward ice shelf, (f) the stabilization of the glacier [Credit: Figure 5 of Wolovick and Moore (2018)].

In practice

The model experiments presented above show that delaying sea-level rise from glacier outflow is possible in theory. In practice, this would mean substantial geoengineering efforts. For building a small artificial island under the ice shelf of Pine Island Glacier (West Antarctica), 0.1 km3 of gravel and sand would be necessary. That same quantity would be sufficient to build a 100 m high wall in front of Jakobshavn Glacier (Greenland) to prevent warm water from melting the ice base. For building such a wall in front of Pine Island Glacier, a quantity of 6 km3 (60 times more than Jakobshavn) of material would be needed.

In comparison, the Three Gorges Dam used 0.03 km3 of cast concrete, the Hong Kong’s airport required around 0.3 km3 of landfill, and the excavation of the Suez Canal necessitated 1 km3 of material. Thus, the quantities needed for building glacial geoengineering structures are comparable in size to the current large engineering projects.

However, many other aspects need to be considered when implementing such a project. In particular, the construction of such structures in cold waters surrounded by icebergs and sea ice is much more difficult than in a typical temperate climate. A detailed study of physical processes in the region of the glacier, such as ocean circulation, iceberg calving, glacier sliding and erosion, and melting rates, is needed before performing such projects. Also, the number of people needed to work on a project of this scale is an important factor to include.

Potential adverse effects

Beside all the factors that need to be considered to implement such a project, there is a list of potential adverse effects. One of the main risks is to the marine ecosystems, which could be affected by the constructions of the islands and walls. Also, if not properly designed, the geoengineering solutions could accelerate the sea-level rise instead of delaying it. For instance, in the case of water extraction (proposal A above), the glacier might speed up rather than slow down if water at the glacier’s base is trapped in pockets.

Wolovick and Moore (2018) do not advocate that glacial geoengineering is done any time soon, due to the different factors mentioned above. Instead, they suggest that we start thinking about technological solutions that could delay sea-level rise. Other studies also look at different glacial geoengineering ideas (see this post).

In summary

Glacial geoengineering techniques constitute a potential way to cope with one of the greatest challenges related to global warming, namely sea-level rise. In theory, these projects are possible, while in practice a series of technical difficulties and potential ecological risks do not allow us to implement them soon.

While important to keep thinking about these solutions, the most important action that humanity can take in order to delay sea-level rise is to mitigate greenhouse gas emissions. And scientists like us need to keep carefully studying the cryosphere and the Earth’s climate in general.

Further reading

Edited by Jenny Turton


David 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 – Why is ice so slippery?

Ice can be slippery! [Credit: giphy.com]

Having spent most of my life in places where the temperature hardly ever falls below zero, my first winter in Sweden was painful. Especially for my bum, who met the ice quite unexpectedly. Reading the news this week, from reports of emergency services overwhelmed after so many people had slipped to a scientific study on how no shoes have a good enough grip, via advice on how to walk like a penguin, I understand I am far from alone in having a problem with ice. But why is ice so slippery anyway? This is what we will talk about in this Image of the Week.


Did you know that you lacked friction?

To understand why one might fall sometimes, let us start with why one usually can walk without falling: friction! Friction is a resistive force that can have three causes:

  • Adhesion (think about glue or tape)

  • Surface roughness (think about sandpaper)

  • Deformation (think about dragging a suitcase over a gravel path)

Each of these types of friction is nicely explained on this website, so I will concentrate on our walking question. Note that if you are standing still, it is a different story; then we are talking about static (instead of dynamic) friction. And everything is actually a bit more complicated than the distinction between the three causes, since adhesion and roughness are somehow related. I will not get into that, but if that stirred your interest, you could have a look at this paper. Anyway, back to walking.

The roughness of our roads and pavements, along with that of your shoes and their deformation ability, is, of course, crucial. But in the case of water after the rain or rotten autumn leaves, adhesion can be the deciding factor between casually walking and experiencing a sudden unexpected loss of altitude: not that much adhesion between your foot and what you walk on, but rather between what you walk on and the rest of the world. And that is exactly the problem with ice.

Frozen lake [Credit: Nilay Dogulu (distributed via imaggeo.egu.eu)]

Water really is a weird material

Coming from a place where people rarely worry about ice, I had never heard the commonly accepted reasons why ice is slippery. A quick internet search informed me that a common belief is that ice is slippery because, by walking on it, we melt the very surface of the ice through the pressure of our weight and/or the heat of the friction. As a result, we end up with a dangerous layer of liquid water between our foot and the ice, lose adhesion, and … boom! A study published this summer has a different explanation: water in its solid form is made of chains of molecules attached to three other water molecules. But the chain has to stop somewhere, so, at the very surface, molecules are only attached to one or two others, and can, as a result, be easily detached from the rest of the ice. When that happens, they just hang around on top of the ice, “like marbles on a dancefloor“.

However, it cannot be seen as a layer of liquid water, rather as a gas, the authors of that new study say. Not that it makes a big difference when you are on the floor… The good (?) news is, this strange property of ice depends on temperature. They report that ice is the most treacherous at -7°C, but then becomes safer as the temperature decreases.

EGU Cryosphere friendly advice: how to walk around -7°C

Personally, I avoid roads and pavements like the plague and walk on frozen paths and grass, which retain some roughness unless covered by a lot of snow. Since it is not always possible, adopt the technique of our favourite polar animal:

  • put your centre of mass ahead of you by slightly bending your torso forward

  • go slowly

  • move your foot next to each other, instead of in front of one another

  • or give up and slide on your belly!

One of our favourite polar animals [Credit: Giuseppe Aulicino (distributed via imaggeo.egu.eu)].

Further reading

Edited by Clara Burgard

Image of the Week – What’s Hot in the Cryosphere? A 2018 review

Image of the Week – What’s Hot in the Cryosphere? A 2018 review

Every year, humanity understands more and more about a remote and unforgiving component of the Earth system – the cryosphere. 2018 has been no exception, and in this blog post we’ll take a look at some of the biggest scientific findings of cryospheric science in 2018. We will then look forward to 2019 and beyond, to see what the future holds for these rapidly changing climate components.


The Cryosphere at 1.5°C warming

In 2018, the IPCC (Intergovernmental Panel on Climate Change) released their report that looked at the impact of 1.5 and 2.0°C of global warming by 2100 on the Earth system. In the Arctic, warming is already in excess of 2.0˚C, driving a very strong decreasing trend in the summer sea-ice extent. The IPCC suggest that sea-ice-free summers will occur once per century at 1.5°C, but this increases to once per decade at 2.0°C. Limiting warming to 1.5˚C will also save 1.5-2.5 million km2 of permafrost thaw (preventing the release of ancient carbon into the atmosphere), 10 cm of sea-level rise contribution from ice sheets and glaciers, and reduce the risk of the irreversible collapse of the ice sheets. Read more about the cryosphere under 1.5°C warming in this previous post.

 

Mass Balance of the Antarctic Ice Sheet

Compiling 24 independent estimates of mass balance, from a number of different remote sensing and modelling techniques, the IMBIE team produced the best estimate of how Antarctica is responding to continued climate warming. The mass balance refers to the net change in ice mass, accounting for all of the inputs and outputs to the ice. They quantify that ice mass loss from West Antarctica has increased three-fold between 1992 and 2017, largely due to melting from a warmer ocean. On the Antarctic Peninsula, the collapse of ice sheets has led to an increase ice mass loss by a factor of 4. East Antarctica is gaining mass slightly, although this is highly uncertain, by 5 ± 46 billion tonnes per year. Overall, Antarctica has lost 2,720 ± 1,390 billion tonnes of ice in this 25-year time period, and this mass loss is accelerating. Read more about these results in this previous post.

Mass loss from the Antarctic ice sheet is accelerating, largely due to ocean warming impacting West Antarctica. East Antarctica is very slightly gaining mass, but this doesn’t go anywhere near balancing out mass loss across the continent [Credit: NASA Goddard].

A polluted cryosphere

It’s easy to think of the cryosphere as a pristine, beautiful, untouched landscape. However, research from 2018 has shown us that the remoteness of Polar Regions has not protected them from man-made pollution. In one litre of melted Arctic sea-ice, 234 particles of plastic and over 12,000 particles of microplastics were found, which will only go onto adversely impact Arctic wildlife by spreading through the ecosystem. Radioactive material from the Chernobyl accident has also been found to be concentrated in dark sediments found on Swedish glaciers. As these glaciers melt, this concentration of radioactive material may be released in meltwater. In Greenland, lead pollution found in ice cores has provided exciting new insight into wars, plagues and invasions during the Roman Empire.

In 2018, we saw a glimpse of the geological secrets that Greenland hides beneath its ice sheet. However, there is still a hidden world that future field-based campaigns or airborne radar missions will help to unravel [Credit: NASA Goddard].

What secrets is Greenland hiding?

In 2018, we got our best ever look beneath the Greenland ice sheet. Scientists from the British Antarctic Survey and NASA found that the hotspot (a thermal plume in the Earth’s mantle) currently under Iceland was once beneath Greenland, between 80 to 50 million years ago. This hotspot was discovered by studying the magnetism of minerals beneath the ice. Using airplanes, radio waves and sediment that’s washed out from underneath the ice sheet has also revealed a massive 31 kilometre wide meteorite crater underneath Hiawatha glacier. Given it’s beneath three kilometres of ice, the age of this crater is unknown, but given the interest and speculation in connecting this event to an abrupt cooling period 12,000 years ago (the Younger Dryas), we may know very soon.

 

Blast Off!

Satellites remain one of the most popular methods of monitoring the vast, hostile cryosphere. In 2018, a new generation of earth observation missions launched. ESA’s Sentinel-3B continues the Copernicus programme, monitoring the reflectivity of the ice, elevation and sea-ice thickness. NASA’s GRACE FO mission continues the successful first GRACE mission, which used gravimetry to ‘weigh’ different regions of ice. NASA also launched ICESat-2, which will provide global elevation data at unprecedented spatial resolution on a 91-day repeat orbit. Each satellite is being finely tuned to make sure it’s working exactly as intended, and we’ll get the first science from them in 2019. Stay tuned!

Remote sensing data has provided us with answers to some of the biggest questions in the cryosphere. We use it to help quantify mass loss, sea-level rise and glacial retreat. In 2019, new missions will take our knowledge of cryospheric sciences to new heights! [Credit: Liam Taylor]

A look ahead to 2019

On the ground, getting inside the ice will continue to provide fascinating insights into the history of the cryosphere – from reconstructing winds in sub-Antarctic islands using ice cores, to further insights deep inside the world’s highest glacier. As permafrost continues to thaw, we are likely to hear of more discoveries of woolly mammoths, ancient diseases and carbon release. The IPCC will also publish their special report devoted to The Ocean and Cryosphere in a Changing Climate, which will provide the best overall state of the cryosphere to date. And, of course, the infamously named ‘Boaty McBoatface’ will provide us with incredible data from beneath sea-ice and ice shelves when the RRS Sir David Attenborough is launched. 2018 has been a truly exciting year to be a cryospheric scientist, and 2019 looks set to be another hot one!

 

Edited by Adam Bateson


Liam Taylor is a PhD student at the University of Leeds and Centre for Polar Observation and Monitoring. His research looks at identifying novel remote sensing methods to monitor mountain glaciers for water resource and hazard management. He is passionate about climate change and science communication to a global audience, as an educator on free online climate courses and through his personal blog. You can find Liam on Twitter.

Image of the Week – Alien-iced

Image of the Week – Alien-iced

What do Chile and Jupiter’s moon Europa have in common? If you like astronomy, you may reply “space missions!” – Chile’s dry air and clear skies make it an ideal location for telescopes like the VLT or ALMA, while Europa’s inferred subsurface ocean will be studied by the upcoming mission to Jupiter JUICE, due to launch in 2022. But Chile’s high altitude Atacama desert and Europa’s frozen surface also have another feature in common, as you can see in this Image of the Week: ice spikes!   


Penitentes is the word

The official name of these ice spikes is “Penitentes”, Spanish for penitents. Why? As you might see (with quite some imagination) on the Image of the Week, there is some resemblance with a kneeling and praying procession.

Fields of penitentes ranging from a few centimetres to five metres can be found above 4000 m altitude both in the Andes and Himalayas, the only places on Earth where the right conditions exist for their formation. Because although it looks as if the snow is just blown into penitentes by unidirectional winds, in reality everything is due to thermodynamics…

I promise I will not write the equations this time (see this previous post); instead, I invite you to read them in this paper. In summary, penitentes form where snow is in contact with very dry and very cold air. As the sun shines, the snow absorbs the energy and heats up from inside, so much and so fast that the only way to be rid of that heat is by changing phase, directly from solid to water vapour (this is called sublimation). Since snow is anything but a smooth surface, sun rays will in fact be more concentrated at given locations on the snow, so that sublimation occurs only at specific points. But it is a self-amplifying mechanism: sublimation will leave a little crater behind in the snow, whose shape will concentrate even more the sun rays and lead to further sublimation. And this is how the penitentes get their shape.

 

Penitentes and the Atacama Pathfinder EXperiment (APEX) telescope. Photo: Babak Tafreshi/ ESO

Where is the link with Europa?

Hopefully by now, you are happy because you have just learnt about yet another weird-but-wonderful cryospheric phenomenon on Earth. But, remember how the post was about about Europa in the beginning? This is because researchers have recently analysed data from the past mission to Jupiter Galileo that might suggest that the conditions are right on Europa for penitentes to exist. They had to use the careful phrasing because the data resolution was not good enough to see the actual individual penitentes and had instead to rely on their thermic signature.

As reported in the media storm of these last two weeks (see here, here or here for example), this is an important discovery for the planning of future space missions. Which landing site to use? Play it safe and land far from these ice blades, or go and study them but risk destroying your lander? Either way, we shall continue reporting about the cryosphere, from this world and beyond…

Reference/Further reading

 

Edited by Clara Burgard

Image of the Week – The 2018 Arctic summer sea ice season (a.k.a. how bad was it this year?)

Sea ice concentration anomaly for August 2018: blue means less ice than “normal”, i.e. 1981-2010 average. Credit: NSIDC.

With the equinox this Sunday, it is officially the end of summer in the Northern hemisphere and in particular the end of the melt season in the Arctic. These last years, it has typically been the time to write bad news about record low sea ice and the continuation of the dramatic decreasing trend (see this post on this blog). So, how bad has the 2018 melt season been for the Arctic?  


Yes, the 2018 summer Arctic sea ice was anomalously low

Before we give you the results for this summer, let us start with the definitions of the three most common sea ice statistics:

  • Sea ice concentration: how much of a given surface area (e.g. 1 km2) in the ocean is covered by sea ice. The concentration is 100% if there is nothing but sea ice, 50% if half of this area is covered by ice, and 0% if there is nothing but open water. Read more about how satellites measure sea ice concentration on this blog here.
  • Sea ice extent: typically defined as the ocean area with at least 15% sea ice concentration.
  • Sea ice volume: the whole volume of sea ice, i.e. total area times thickness of sea ice. This is probably the most difficult of the three statistics to measure since satellite measurements of sea ice thickness are only starting to be trustworthy.

So, how did summer 2018 perform regarding these three statistics?
As shown on today’s Image of the Week, the sea ice concentration has been anomalously low in most parts of the Arctic, with many areas in dark blue showing they had more than 50% less sea ice than normal (1981-2010 average).

The resulting extent was anomalously low as well (see figure below), but not record-breaking low. The volume however was the fourth lowest recorded or 50% lower than normal, with 5000 km3 of sea ice missing. In a more meaningful unit, that is one trillion elephants of ice, or 64 000 elephants per km2 of the Arctic Ocean.

But as we discussed in a previous post, talking about the Arctic as a whole is not enough to understand what happened this summer. So let us have a closer look at the area north and east of Greenland.

Summer 2018 Arctic sea ice extent up till 19th September (blue) compared to the “normal” extent (grey) and the all-time record of 2012 (green dashed). Credit: NSIDC.

North of Greenland: open water instead of multiyear ice

Until recently, most of the Arctic Ocean was covered by multiyear / perennial ice. That is, most sea ice would not melt in summer and would stay until the next winter. But with climate change and the warming of the Arctic, the multiyear ice cover has shrunk and became limited to the area north of Greenland.

The situation has been even more dramatic this summer. For the entire month of August 2018, there was open water north of Greenland where there should have been thick multiyear ice (see picture below). As nicely explained here, that area had already unexpectedly melted in February this year when the Arctic was struck with record high air temperatures; when the sea ice closed again, it was thinner and more brittle than it should have been, and did not withstand strong winds in August. Therefore, this unusual winter melting could have contributed to the formation of open water north of Greenland.

It is really bad news, and it does feel like yet another tragic milestone: even the last areas of multiyear ice are melting away. Most worryingly, we do not know what the consequences of this disappearance will be on the ecosystem and the entire climate. Or rather, we know that everything from local sea ice algae to European weather patterns will be affected, but more research is needed over the coming years before we can assess the full impact over our complex fully coupled climate system.

Optical satellite image of the northern half of Greenland, 19 August 2018. Dark colour is open water, and should not have been here. Credit: NASA.

Reference/Further reading

For near real time analysis of the sea ice conditions: https://nsidc.org/arcticseaicenews/

For checking sea ice data from home: https://seaice.uni-bremen.de/databrowser/

For simple visualisations of sea ice statistics: http://sites.uci.edu/zlabe/arctic-sea-ice-volumethickness/

 

Edited by David Docquier

Image of the Week – The shape of (frozen sea) water

 

Figure 1: Annual evolution of the sea ice area with two different floe shape parameters of 0.44 (red) and 0.88 (blue). The model is spun-up between 2000 – 2006 and then evaluated for a further ten years between 2007 – 2016 and the mean values over this period displayed by the thick lines. Thin lines show the results for individual years. [Credit: Adam Bateson]

Polar sea ice exists as isolated units of ice that we describe as floes. These floes do not have a constant shape (see here for instance); they can vary from almost circular to being jagged and rectangular. However, sea ice models currently assume that all floes have the same shape. Much focus has been paid to the size of floes recently, but do we also need to reconsider how floe shape is treated in models?


Why might floe shape matter?

In recent years, sea ice models have started to examine more and more how individual floes influence the overall evolution of sea ice.

A particular focus has been the size of floes (see here and here) and the parameterisation of processes which influence floe size (see here for example). However less attention has been given to the shape of the floe. The shape of the floe is important for several reasons:

  • Lateral melt rate: the lateral melt rate describes how quickly a floe melts from its sides. Two floes with the same area but different shape can have a different perimeter; the lateral melt rate  is proportional to the floe perimeter.
  • Wave propagation: a straight floe edge will impact propagating waves differently to a curved or jagged floe edge. The distance waves travel under the sea ice and hence the extent of sea ice that waves can fragment will be dependent on these wave-floe edge interactions.
  • Floe mechanics: an elongated floe (i.e. much longer in one direction than another) will be more likely to break from incoming waves if its longer edge is aligned with the direction the waves are travelling.

How do models currently treat floe shape?

One approach used within sea ice models to define floe shape is the use is the use of a parameter, α. The smaller the floe shape parameter, the longer the floe perimeter (and hence, the higher the lateral melt rate). A standard value used for the parameter is 0.66 (Steele, 1992). Figure 2 shows how this floe shape parameter varies for some common shapes.

Figure 2: The floe shape parameters for some common shapes are given for comparison to the standard value of 0.66. [Credit: Adam Bateson]

The standard value of the floe shape parameter, 0.66, was obtained from taking the mean floe shape parameter measured over all floes greater than 1 km from a singular study area of 110 km x 95 km at one snapshot in time. Despite the limited data set used to estimate this shape parameter, it is being used for all sea ice throughout the year for all floe sizes. However, this would only be a concern to the accuracy of modelling if it turns out that sea ice evolution in models is sensitive to the floe shape parameter.

 

Model sensitivity to floe shape

To investigate the model sensitivity to the floe shape parameter two simulations have been run: one uses a floe shape parameter of 0.88 and the other uses 0.44, chosen to represent likely extremes. The two simulations are run from 2000 – 2016, with 2000 – 2006 used as a spin-up period. Figure 1 displays the mean total ice area throughout the year and results of individual years for each simulation. Figure 3 is an equivalent plot to show the annual evolution of total ice volume for each simulation.

The results show that the perturbation from reducing the floe shape parameter is smaller than the variation between years within the same simulation.  However, the model does show a permanent reduction in volume throughout the year and a 10 – 20 % reduction in the September sea ice minimum. The impact of the floe shape is hence small but significant, particularly for predicting the annual minimum sea ice extent and volume.

Figure 3: Annual evolution of the sea ice volume with two different floe shape parameters of 0.44 (red) and 0.88 (blue). The model is spun-up between 2000 – 2006 and then evaluated for a further ten years between 2007 – 2016 and the mean values over this period displayed by the thick lines. Thin lines show the results for individual years.

More recent studies on floe shape

In 2015, Gherardi and Lagomarsino analysed the floe shape behaviour from four separate samples of satellite imagery from both the Arctic and Antarctic. The study found different distributions of floe shapes in different locations, however there was no correlation between floe shape and size. This property would allow models to treat floe shape and size as independent properties. More recently, in 2018, Herman et al. analysed the results of laboratory experiments of ice breaking by waves. It was found that wave break-up influenced the shape of the floes, tending to produce straight edges and sharp angles.  These features are associated with a smaller floe parameter i.e. would produce an increased lateral melt rate.

What next?

More observations are needed to identify whether the use of a constant floe shape parameter is justified. The following questions are important:

  • Do further observations support the finding that floe size and shape are uncorrelated?
  • What range of values for the floe shape parameter can be observed in reality?
  • Do we see significant variations in the floe shape parameter between locations?
  • Do these variations occur over a large enough scale that they can be represented within existing model resolutions?

Further reading

Edited by Violaine Coulon and Sophie Berger


Adam Bateson is a PhD student at the University of Reading (United Kingdom), working with Danny Feltham. His project involves investigating the fragmentation and melting of the Arctic seasonal sea-ice cover, specifically improving the representation of relevant processes within sea-ice models. In particular he is looking at lateral melting and wave induced fragmentation of sea-ice as drivers of break up, as well as the role of the ocean mixed layer as either an amplifier or dampener to the impacts of particular processes. Contact: a.w.bateson@pgr.reading.ac.uk or @a_w_bateson on twitter.

Image of the Week – Climate feedbacks demystified in polar regions

Figure 1: Major climate feedbacks operating in polar regions. Plus / minus signs mean that the feedbacks are positive / negative. Yellow and red arrows show solar shortwave and infrared radiation fluxes, respectively. Orange arrows show the flux exchanges between the different components of the climate system (ocean, atmosphere, ice) for several feedbacks. TOA refers to ‘top of the atmosphere’ [Credit: Fig 1 from Goosse et al. (2018)].

Over the recent decades, the Arctic has warmed twice as fast as the whole globe. This stronger warming, called “Arctic Amplification“, especially occurs in the Arctic because ice, ocean and atmosphere interact strongly, sometimes amplifying the warming, sometimes reducing it. These interactions are called “feedbacks” and are illustrated in our Image of the Week. Let’s see why these feedbacks are important, how we can measure them and what their implications are.


Climate feedbacks in polar regions

When it comes to climate science, feedback loops are very common. A climate feedback is a process that will either reinforce or diminish the effect of an initial perturbation in the climate system.

If the initial perturbation, for instance the warming of a region, is amplified by this process, we talk about a “positive feedback”. A positive feedback can be seen as a “vicious circle” as it will lead to an ever-ongoing amplification of the perturbation. The most prominent positive feedback in the Arctic is the “ice-albedo feedback“: as the surface warms, ice melts away, exposing darker surfaces to sunlight, which absorb more heat, leading to even more melting of the ice around.

On the contrary, if the initial perturbation is dampened by the process, we talk about a “negative feedback”. An example for a negative feedback is the “ice production-entrainment feedback”. In winter, when sea ice forms, it rejects salt into the ocean. As a result, the top ocean layer becomes denser and starts to sink. As the surface water sinks, it leaves room for warmer water below to rise to the surface. This warmer ocean surface then inhibits the formation of new sea ice.

The main climate feedbacks at play in polar regions involve the atmosphere, ocean and sea ice. They are represented in our Image of the Week. Plus and minus signs in this figure mean that the feedbacks are positive and negative, respectively.

 

How can we measure these feedbacks?

All the climate feedbacks depicted in our Image of the Week are far from being totally understood and are usually measured using different methods. That is why a new study (from which our Image of the Week is taken) proposes a common framework to quantify them.

In this framework, the feedback factor is the ratio between the changes due to the feedback only and the response of the full system including all feedbacks. It is positive for a positive feedback and negative for a negative feedback. In order to compute this feedback factor, we need to identify:

  1. the perturbation
  2. the reference variable involved in the feedback loop
  3. the full system, which includes all feedbacks
  4. the reference system in which the feedback under consideration does not operate.

 

If we take the example of the “ice production-entrainment feedback” (explained above):

  1. the perturbation is a given amount of sea-ice production
  2. the reference variable is sea-ice thickness
  3. the full system is sea ice and the ocean column with the entrainment process
  4. the reference system is sea ice and the ocean column without entrainment.

 

The feedback factor related to the “ice production-entrainment feedback” is then the ratio between the changes in ice thickness due to the feedback only and the total changes in ice thickness following a given amount of ice production. As it is a negative feedback, the related feedback factor is negative. As illustrated in Fig. 2, this feedback factor becomes even more negative, i.e. the strength of the feedback increases, with higher ice production. Therefore, this feedback is highly nonlinear, which is typical of feedbacks in polar regions.

Figure 2: Feedback factor related to the ice production-entrainment feedback as a function of ice production. It is computed from mean temperature and salinity profiles in the Weddel Sea for January-February 1990-2005 [Credit: Fig. 5 from Goosse et al. (2018)].

The advantage of this framework is that you can apply it to all feedbacks present in our Image of the Week. Therefore, it is possible to compute their effects in a similar way, making the comparison easier.

 

Reducing uncertainties in model projections

Accounting for all those climate feedbacks is difficult, as they involve several components of the climate system and interactions between them. Therefore, their misrepresentation (or lack of representation) is one of the sources of error in model projections, i.e. climate model runs going up to 2100 and beyond. Climate feedbacks are therefore one explanation why models largely disagree when it comes to projecting global temperature and sea-ice evolution.

This means that, if we want to better predict what is going to happen in the polar regions, we must better measure what the feedbacks do in reality and better represent them in climate models.

On the modelling side, the main problem is that feedbacks are often described qualitatively to understand climate processes, and many models cannot evaluate these feedbacks quantitatively. There is therefore a clear motivation to use the common framework presented in this study to compute climate feedbacks in models.

However, additionally to improving model projections, identifying the critical climate feedbacks at play in polar regions is also a way to better target observational campaigns, such as the Year of Polar Prediction (YOPP) and the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC).

 

References

Edited by Sophie Berger and Clara Burgard


David 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.