CR
Cryospheric Sciences

Sea-ice

Image of the Week – ROVing in the deep…

Aggregates of sea ice algae seen from the ocean below by the ROV [Credit: Katlein et al. (2017)].

Robotics has revolutionised ocean observation, allowing for regular high resolution measurements even in remote locations or harsh conditions. But the ice-covered regions remain undersampled, especially the ice-ocean interface, as it is still too risky and complex to pilot instruments in this area. This is why it is exactly the area of interest of the paper from which our Image of the week is taken from!


This is sea ice… seen from the ocean

Traditionally, only divers (and maybe seals, fish, krill, belugas, etc.) have been able to see what is happening just under the sea ice, in the ocean. That is no routine activity – I personally have not been in a fieldwork campaign involving a diver. It is extremely dangerous to dive in such cold waters, and the diver is limited to a small area around the entry hole, which might refreeze really fast. The most common method is to drill small holes from the top of the sea ice to the ice-ocean interface at specific locations instead, and collect the bottom of the resulting ice core. There are obvious problems with this method:

  • drilling takes a lot of time and effort;
  • you cannot drill everywhere, since it becomes unsafe if the ice is too thin (you still have to be standing on the ice to do the drilling);
  • the location of your core has to be representative of what you are sampling.

This is why researchers are trying to more often use sea robots, which can take measurements over a large area while the researchers are safe somewhere else. But most robots that are now used to monitor the ocean are not adapted to ice-covered regions, and the few that are require a lot of specifically trained technicians to operate them and/or can only perform very specific tasks.

Our Image of the Week was taken by a new robot, “The Beast”, whose specificities are described in the recently published Katlein et al. (2017). In brief, it is ice-resistant, small, very manoeuvrable, can be operated by only one or two people from a cosy hut on the ice, and contains any possible sensor you can think of (even a small water bottle for sampling, and a net). It belongs to the family of Remotely Operated Vehicles (ROV), which means that it is connected to the operator by a cable – if anything goes wrong under the ice, just pull on the leash!

And thanks to ROVs, we can see (e.g. on this Image of the Week) that the thickness of the sea ice, hence the amount of light that goes through it and the whole sympagic communities vary a lot over small regions.

What the pilot sees when driving the ROV by a sea ice pressure ridge [Credit: Katlein et al. (2017)].

Why do we need such observations?

  1. Robustness: it will not totally replace the traditional ice coring, for some studies still need to get the actual ice. But it will ensure that the choice of locations make sense, or help extrapolate the localised coring results to a larger region.
  2. Validation: for basin-wide studies, we need satellites. But satellite retrievals, especially those for sea ice thickness, still need in-situ measurements for validation. ROVs can provide more validation points than traditional point-coring for the same mission duration, hence ultimately improving algorithms.
  3. Seeing is believing: for anything from outreach to future fieldwork preparation, videos captured by an ROV are an unvaluable tool. Ecologists can even see which species live there (or discover new ones).

 

Further reading

Edited by Clara Burgard

Image of the Week – The birth of a sea-ice dragon!

Image of the Week – The birth of a sea-ice dragon!

Dragon-skin ice may sound like the name of an episode of the Game of Thrones fantasy franchise. However, this fantasy name hides a rare and bizarre type of ice formation that you can see in our Image of the Week. It has been recently observed by the “Polynyas, ice production and seasonal evolution in the Ross Sea” (PIPERS) research team in Antarctica. This bizarre phenomenon caused by strong wind conditions has not been observed in Antarctica since 2007.


PIPERS expedition observed dragon-skin ice

In early April, the Nathan B Palmer icebreaker (see Fig. 2) began its 65-day voyage to Antarctica to study sea ice in the Ross Sea during the autumn period. This expedition, named PIPERS, was carried out by a team of 26 scientists from 9 countries. Its goal was to investigate polynyas, ice production, and seasonal evolution with a particular focus on the Terra Nova Bay and Ross Sea Polynyas (see Fig. 3).

Fig.2 : The Nathan B Palmer icebreaker caught in sea ice [Credit: IMAS ].

A polynya is an area of open water or thin sea ice surrounded by thicker sea ice and is generally located in coastal areas [Stringer and Groves, 1991]. Ice formation in polynyas is strongly influenced by wind conditions whose action can lead to astonishing spatial patterns in sea ice appearance. Special wind conditions probably also lead to what the members of the PIPERS expedition had the opportunity to observe: ice patterns that resemble dragon scales, therefore called dragon-skin ice. Such a sighting is quite remarkable as the last one dates back from a decade. However, the sparsity of observations of dragon-skin ice phenomena is probably a consequence of the relatively small number of expeditions in Antarctica during the autumn and winter seasons…

Fig. 3: The Terra Nova Bay Polynya and Ross Sea Polynya explored by the PIPERS expedition. [Credit: PIPERS ].

Chaotic ice formation caused by strong winds

Dragon-skin ice is a chaotic result of the complex interplay between the ocean and the atmosphere. Coastal polynyas in Antarctica are kept open by the action of strong and cold offshore winds (see Fig. 4) known as katabatic winds, which blow downwards as fast as 100 km/h for several hours [McKnight and Hess, 2000]. Sea ice forming at the cold sea surface gets blown away by these strong winds, preventing a closed sea-ice cover in this area. As the ice is blown away, an area of open water gets in direct contact with the atmosphere, leading to strong cooling and new formation of ice, that gets blown away again, and so on… Therefore, in general, sea ice in polynyas consists of thin pancake ice (see Fig. 5) i.e. round pieces of ice from 0.3 to 3 meters in diameter, which results from the aggregation of ice crystals caused by the wave action. Due to the wind action, the pieces of ice are pushed out by the wind action to the edges of the polynya.  As these pieces push strongly against each other, dragon-like scales appear on sea ice giving birth to the so-called dragon-skin ice.

Fig.4: Formation of coastal polynyas due to the action of katabatic winds [Credit: Wikimedia Commons ].

Figure 5: Sea ice in polynyas takes the form of pancake ice due to the action of water waves [Credit: PIPERS ].

The importance of polynyas for ocean-atmosphere interactions

Besides providing us with dazzling pictures of the cryosphere, investigating sea-ice production and evolution in polynyas is essential to better understand the complex interactions between the ocean and the atmosphere.
As sea water freezes into sea ice, salt is expelled into the ocean, raising its local salinity. The incessant production of sea ice in polynyas leads to water masses with very high salinity inside the polynyas. As sea water cools down, it releases energy in the atmosphere, leading to a warming of the atmosphere in polar regions. Moreover, due to their high density, these masses of cold and salty water sink and mix with lower ocean layers.
First results from the PIPERS mission show that when sea ice is forming, polynyas release greenhouse gases to atmosphere, instead of capturing it, as it was previously assumed! But fully understanding what’s happening there will necessitate more time and analyses….

Further reading

 

Edited by Scott Watson and Clara Burgard
Modified by Sophie Berger on 3 July 2017 to account for remarks of Célia Sapart (Member of the PIPER expedition)


Kevin Bulthuis is a F.R.S.-FNRS Research Fellow at the Université de Liège and the Université Libre de Bruxelles. He investigates the influence of uncertainties and instabilities in ice-sheet models as a limitation for accurate predictions of future sea-level rise. Contact Email:kevin.bulthuis@ulg.ac.be.

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 – The ups and downs of sea ice!

Image of the Week – The ups and downs of sea ice!

The reduction in Arctic sea-ice cover has been in the news a lot recently (e.g. here) – as record lows have been observed again and again within the last decade. However, it is also a topic which causes a lot of confusion as so many factors come into play. With this Image of the Week we will give you a brief overview of the ups and downs of sea ice!


In general, Arctic sea ice is at its minimum extent at the end of the summer (September), and its maximum extent at the end of the winter (March). Our Image of the Week (Fig. 1) shows the summer and winter sea ice cover over the last year. In September 2016, the Arctic sea-ice minimum covered the second smallest extent since the beginning of satellite observations (38 years). Only 4.14 million square kilometres of the Northern Hemisphere were covered by sea ice on the day of minimum extent (September 10th). The maximum sea-ice extent was observed on March 7th 2017, only 14.42 million square kilometres of sea ice were observed: the lowest maximum since the beginning of satellite observations.

How long do we have until Arctic summer sea-ice cover is completely gone?

The Arctic Ocean is defined as ice-free, when the sea-ice area does not exceed 1 million km². Due to the close relationship between CO2 emissions and the sea-ice area (see one of our previous posts), it is likely that the summer Arctic sea-ice cover will fall below this threshold during the 21st century. Under the highest emission scenario (RCP 8.5 – IPCC, 2015), an almost ice-free Arctic in September is likely to occur before the middle of the century. It is, however, not easy to predict the exact year of an ice-free Arctic summer as the extent of the ice cover depends on many parameters influencing the freezing and melting of the ice.

On one hand, some parameters and their effect on the sea-ice cover are well understood and their future evolution can be projected quite well through climate models. For example, changes in the sea surface temperature tend to affect the starting date of the freezing period while changes in air temperature tend to affect the starting date of the melting period. As both air temperature and sea surface temperature are projected to increase in the long term, due to climate change, the period where ice can be present will be reduced more and more.

On the other hand, some parameters lead to several concurring effects, which are difficult to separate clearly and not always fully understood. Therefore, their future evolution and influence on sea ice is not totally clear. For example, the sea-ice loss leads to more open ocean areas, which absorb solar radiation, causing warming and therefore leading to faster sea-ice melting – a mechanism called “sea-ice albedo feedback”. At the same time, more open ocean areas also lead to more evaporation and therefore more clouds, which shield the ice from solar radiation and therefore lead to less warming of the ice and ocean surfaces.

Still, even if we knew the effect and long-term evolution of all these parameters, the exact date of ice-free Arctic could not be defined easily in advance. Why? The chaotic nature of the atmosphere leads to very short-term effects that influence the ice cover as well…

Be careful! A record minimum does not always mean a record maximum (and vice versa)!

On shorter time scales, sudden changes in the atmospheric circulation can have a large impact on sea-ice extent. Therefore, it is not guaranteed that a year with a record low maximum will have a record low minimum and vice versa. For example, heat waves and warm air outbreaks or high winds due to the transport of low pressure systems into the Arctic can lead to a more rapid decline of the sea-ice cover. The other way round, if the atmosphere from lower latitudes does not disturb the Arctic region, the sea-ice cover can stabilise again.

What about this year (2016/2017 season)?

Sometimes, it is not clear why sea-ice retreats rapidly. For example, the low 2016 minimum came as a surprise as the cover started with a very low minimum but then did not melt as fast as in previous years, due to average or below average temperatures. Only shortly before the minimum extent, stormy conditions came into play and led to the low extent that was observed (see Fig. 2).

Figure 2: Comparison of Arctic sea-ice extent between different years for summer (left) and winter (right). [Credit: Image courtesy of the National Snow and Ice Data Center]

The reasons for the record low 2017 maximum are better understood. The Arctic Ocean was not covered by much ice to begin. Then, the autumn and winter in the Arctic were very warm with air temperatures from October 2016 to February 2017 being from 2.5 to up to 5 degrees in some regions higher than on average.

From the Arctic to the Antarctic

In the last decades, although it recovered in some years between the record lows, the Arctic sea-ice cover has overall been declining. This is not the case on the other side of the planet, in Antarctica. Note that Antarctica is a complete different setting than the Arctic Ocean. The former being a continent surrounded by ocean and sea ice, the latter being an ocean with sea ice surrounded by continents.

Figure 3: Comparison of Antarctic sea-ice extent between different years for summer (left) and winter (right). [Credit: Image courtesy of the National Snow and Ice Data Center]

In recent decades, Antarctic sea-ice has been increasing very slowly (see Fig.3). Scientists were puzzled as such an evolution was not expected in a global warming framework. Explanations for this behaviour are that this is likely due to changing wind and surface pressure patterns around Antarctica. Contrary to this trend, this year (2016/2017) was a record low maximum and minimum in Antarctic sea-ice cover. This change is puzzling scientists even more. It remains unclear up to now if this is a permanent shift in the tendency of Antarctic sea ice or if this a single event. Be sure that the next months will be full of papers trying to explain this change in behaviour, it is going to be exciting!

Further reading

Edited by Emma Smith

Image of the Week — The ice blue eye of the Arctic

Image of the Week — The ice blue eye of the Arctic

Positive feedback” is a term that regularly pops up when talking about climate change. It does not mean good news, but rather that climate change causes a phenomenon which it turns exacerbates climate change. The image of this week shows a beautiful melt pond in the Arctic sea ice, which is an example of such positive feedback.


What is a melt pond?

The Arctic sea ice is typically non-smooth, and covered in snow. When, after the long polar night, the sun shines again on the sea ice, a series of events happen (e.g. Fetterer and Untersteiner, 1998):

  • the snow layer melts;

  • the melted snow collects in depressions at the surface of the sea ice to form ponds;

  • these ponds of melted water are darker than the surrounding ice, i.e. they have a lower albedo. As a result they absorb more heat from the Sun, which melts more ice and deepens the pond. Melt ponds are typically 5 to 10 m wide and 15 to 50 cm deep (Perovich et al., 2009);

  • eventually, the water from the ponds ends up in the ocean: either by percolation through the whole sea-ice column or because the bottom of the pond reaches the ocean. Sometimes, it can also simply refreeze, as the air temperatures drop again (Polashenski et al., 2012).

Melt ponds cover 50-60% of the Arctic sea ice each summer (Eicken et al., 2004), and up to 90% of the first year ice (Perovich al., 2011). How do we know these percentages? Mostly, thanks to satellites.

Monitoring melt ponds by satellites

Like most phenomena that we discuss on this blog, continuous in-situ measurements are not feasible at the scale of the whole Arctic, so scientists rely on satellites instead. For melt ponds, spectro-radiometer data are used (Rösel et al., 2012). These measure the surface reflectance of the Earth i.e. the proportion of energy reflected by the surface for wavelengths in the visible and infrared (0.4 to 14.4 μm). The idea is that different types of surfaces reflect the sunlight differently, and we can use these data to then map the types of surfaces over a region.

In particular for the Arctic, sea ice, open ocean and any stage in-between all reflect the sunlight differently (i.e. have different albedos). The way that the albedo changes with the wavelength is also different for each surface, which is why radiometer measurements are taken for a range of wavelengths. With these measurements, not only can we locate the melt ponds in the Arctic, but even assess how mature the pond is (i.e. how long ago it formed) and how deep it extends. These values are key for climate change predictions.

Fig. 2: Melt pond seen by a camera below the sea ice. (The pond is the lighter area) [Credit: NOAA’s climate.gov]

Melt ponds and the climate

Let’s come back to the positive feedback mentioned in the introduction. Solar radiation and warm air temperature create melt ponds. The darker melt ponds have a higher albedo than the white sea ice, so they absorb more heat, and further warm our climate. This extra heat is also transferred to the ocean, so melt pond-covered sea ice melts three times more from below than bare ice (Flocco et al., 2012). This vicious circle heat – less sea ice – more heat absorbed – even less sea ice…, is called the ice-albedo feedback. It is one of the processes responsible for the polar amplification of global warming, i.e. the fact that poles warm way faster than the rest of the world (see also this post for more explanation).

The ice-albedo feedback is one of the processes responsible for the polar amplification of global warming

But it’s not all doom and gloom. For one thing, melt ponds are associated with algae bloom. The sun light can penetrate deeper through the ocean under a melt pond than under bare ice (see Fig. 2), which means that life can develop more easily. And now that we understand better how melt ponds form, and how much area they cover in the Arctic, efforts are being made to include more realistic sea-ice properties and pond parametrisation in climate models (e.g. Holland et al., 2012). That way, we can study more precisely their impact on future climate, and the demise of the Arctic sea ice.

Edited by Sophie Berger

Further reading

Image of the Week – Apocalypse snow? … No, it’s sea ice!

Image of the Week – Apocalypse snow? … No, it’s sea ice!

Sea ice brine sampling is always great fun, but sometimes somewhat challenging !

As sea water freezes to form sea ice, salts in the water are rejected from the ice and concentrate in pockets of very salty water, which are entrapped within the sea ice. These pockets are known as “brines”.

Scientists sample these brines to measure the physical and bio-geochemical properties, such as: temperature, salinity, nutrient, water stable isotopes, Chlorophyll A, algal species, bacterial number and DNA, partial pressure of CO2, dissolved and particulate Carbon and Nitrogen, sulphur compounds, and trace metals.  All of this helps to better understand how sea ice impacts the atmosphere-ocean exchanges of climate relevant gases.

In theory, sampling such brines is very simple: you just have to drill several holes in the sea-ice ensuring that the holes don’t reach the bottom of ice and wait for half an hour. During this time, the brine pockets which are trapped in the surrounding sea ice drain under gravity into the hole. After that, you just need to sample the salty water that has appeared in the hole. Simple…

…at least it would be if they didn’t have to deal with the darkness of the Antarctic winter, blowing snow, handling water at -30°C and all while wearing trace metal clean suits on top of polar gear…hence the faces!


This photo won the jury prize of the Antarctic photo competition, organised by APECS Belgium and Netherlands as part of Antarctica Day celebrations (1st of December).

All the photos of the contest can be seen here.

Edited by Sophie Berger and Emma Smith


Jean-Louis Tison is a professor at the Université libre de Bruxelles. His activities are focused on the study of physico-chemical properties of « interface ice », be it the « ice-bedrock » (continental basal ice) , « ice-ocean » (marine ice) or « ice-atmosphere » (sea ice) interface. His work is based on numerous field expeditions and laboratory experiments, and on the development of equipments and analytical techniques dedicated to the multi-parametric study of ice: textures and fabrics, stable isotopes of oxygen and hydrogen, total gas content and gas composition, bulk salinity, major elements chemistry…

 

Image of the Week – For each tonne of CO2 emitted, Arctic sea ice shrinks by 3m² in summer

Image of the Week – For each tonne of CO2 emitted, Arctic sea ice shrinks by 3m² in summer

Declining sea ice in the Arctic is definitely one of the most iconic consequences of climate change. In a study recently published in Science, Dirk Notz and Julienne Stroeve find a linear relationship between carbon dioxide (CO2) emissions and loss of Arctic sea-ice area in summer. Our image of this week is based on these results and shows the area of September Arctic sea ice lost per inhabitant due to CO2 emissions in 2013.


What did we know about the Arctic sea ice before this study?

Since the late 1970s, sea ice has been dramatically shrinking in the Arctic, losing 3.8% of its area per decade. Sea-ice area is at its minimum in September, at the end of the melting season.

The main cause of this loss is the increase in surface temperature over the recent years (Mahlstein and Knutti, 2012), which has been more pronounced in the Arctic compared to other regions on Earth (Cohen et al., 2014). The use of statistical methods involving both observations and climate models shows that the recent warming in the Arctic can be attributed to human activity, i.e. mainly greenhouse gas emissions (Gillett et al., 2008). This suggests a direct link between human activity and Arctic sea-ice loss, which is confirmed in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC).

How exactly is sea-ice loss related to CO2 emissions ?

Notz and Stroeve (2016) relate the Arctic sea-ice decline to cumulative CO2 emissions since 1850 (i.e. the total amount of CO2 that has been emitted since 1850) for both observations and climate models. Cumulative CO2 emissions constitute a robust indicator of the recent man-made global warming (IPCC, 2014).

The two quantities are clearly linearly related (see Figure 2). From 1953 to 2015, about 3.5 million km² of Arctic sea ice have been lost in September while 1200 gigatonnes (1 Gt = 10e9 tonnes) of CO2 have been emitted to the atmosphere. This means that for each tonne of CO2 released into the atmosphere, the Arctic loses 3 m² of sea ice.

Fig 2: Monthly mean September Arctic sea-ice area against cumulative CO2 emissions since 1850 for the period 1953-2015. Grey circles and diamonds show the results from in-situ (1953-1978) and satellite (1979-2015) observations, respectively. The thick red line shows the 30-year running mean and the dotted red line represents the trend of 3 m² sea-ice area loss per tonne of CO2 emitted. [Credit: D. Notz, National Snow and Ice Data Center ]

Starting from the relationship between cumulative CO2 emissions and sea-ice area, it is then easy to attribute to each country in the world their own contribution to sea-ice loss based on their CO2 emissions per capita. The countries that stand out in the map are thus the countries emitting the most in relation to their population.

Could the Arctic be ice-free in the future?

If this relationship holds in the future (in other words, if we extend the red dotted line to zero sea-ice area in Figure 2), adding 1000 Gt of CO2 in the atmosphere would free the Arctic of sea ice in September. Since we are currently emitting about 35 Gt CO2 per year, it would take less than 30 years to have the Arctic free of sea ice in the summer (which confirms previous model studies (e.g. Massonnet et al., 2012)).

Edited by Clara Burgard and Sophie Berger

Further reading

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 – Sea Ice Floes!

Image of the Week – Sea Ice Floes!

The polar regions are covered by a thin sheet of sea ice – frozen water that forms out of the same ocean water it floats on. Often, portrayals of Earth’s sea ice cover show it as a great, white, sheet. Looking more closely, however reveals the sea ice cover to be a varied and jumbled collection of floating pieces of ice, known as floes. The distribution and size of these floes is vitally important for understanding how the sea ice will interact with its environment in the future. [Read More]

Image of the Week – What an ice hole!

Image of the Week – What an ice hole!

Over the summer, I got excited… the Weddell Polynya was seemingly re-opening! ”The what?” asked my new colleagues. So today, after brief mentions in past posts, it is time to explain what a polynya is.


Put it simply, a polynya, from the Russian word for “ice hole”, is a hole in the sea-ice cover. That means that in the middle of winter, the sea ice locally and naturally opens and reveals the ocean.

There are two types of polynyas

  • coastal polynyas, also known as latent heat polynyas, open because strong winds push the sea ice away from the coast.
    The ocean being way warmer than the winter-polar night atmosphere, there is a strong heat loss to the atmosphere. New sea ice also forms,  rejecting brine (salt) and forming a very cold and salty surface water layer, which is so dense that it sinks to the bottom of the ocean. This type of polynya can close back when the wind stops.
  • open ocean polynyas, sometimes called sensible heat polynyas, open because the sea ice is locally melted by the ocean. In normal conditions, a cold and fresh layer of water sits above a comparatively warm and salty layer. But mixing can occur which would bring this warm water up, directly in contact with the sea ice, which then melts. Similar to the coastal one, once the polynya has open heat loss and sea ice processes form dense water that will sink. But in this case, the sinking sustains the polynya: it further destabilises the water column, so more warm water has to be mixed up, which prevents sea ice from reforming…
What a polynya looks like, from MODIS satellite: (https://modis.gsfc.nasa.gov/)

What a polynya looks like, from MODIS satellite: (https://modis.gsfc.nasa.gov/) [Credit: David Fuglestad for Wikimedia Commons]

Some polynyas worth mentioning

  • the North Water Polynya, between Greenland and Canada in Baffin Bay, is the largest in the Arctic with 85 000 km2 (Dunbar 1969) and was officially discovered as early as 1616 by William Baffin. In fact, Inuit communities have lived in its vicinity for thousands of years (e.g. Riewe 1991), since this hole in the ice is extremely rich in marine life (e.g. Stirling, 1980).
  • Hell Gate Polynya, in the Canadian archipelago which owes its name to a dramatic event…  but this is a story for later as today we would like to leave you,  reader, with a positive impression about polynyas!
  • the Weddell Polynya, in the Weddell Sea, was discovered as we started monitoring sea ice by satellites in the 1970s. It was a huge open ocean polynya, reaching 200-300 000 km2 and lasting three winters (Carsey 1980), and it is so famous because it has not re-opened since. Although this year, the signs are here… it may happen again! It is also my personal favourite because I spent my PhD studying its representation in climate models, which wrongly simulate its opening every winter, for reasons that are still not totally clear…

Polynyas are a fascinating feature of the cryosphere, not least because they occur in the middle of winter in harsh environments and cannot be instrumented easily. They are a key spot where the ocean, the ice and the atmosphere interact directly. Their opening has a large range of consequences from plankton bloom to deep water formation. And we still struggle to represent them in models, so there is lots of work to do for early career scientists!

References and further reading

  • Carsey, F. D (1980). “Microwave observation of the Weddell Polynya.” Monthly Weather Review 108.12: 2032-2044.
  • Dunbar, M (1969). “The geographical position of the North Water”. Arctic. 22: 438–441. doi:10.14430/arctic3235
  • Riewe, R (1991). “Inuit use of the sea ice.” Arctic and Alpine Research 1:3-10. doi:10.2307/1551431
  • Smith Jr, W. O., and D. Barber, eds (2007). “Polynyas: Windows to the world”. Vol. 74. Elsevier.
    Stirling, I. A. N. (1980). “The biological importance of polynyas in the Canadian Arctic.” Arctic: 303-315, http://www.jstor.org/stable/40509029

Edited by Sophie Berger and Emma Smith

Image of the Week – Goodness gracious, great balls of ice!

Image of the Week – Goodness gracious, great balls of ice!

At first glance our image of the week may look like an ordinary stoney beach…but if you look more closely you will see that this beach is not, in fact, covered in stones or pebbles but balls of ice! We have written posts about many different weird and wonderful ice formations and phenomena (e.g. hair ice or ice tsunamis) here at the EGU Cryosphere blog and here is another one to add to the list – ice balls!


During the northern hemisphere winter these naturally formed balls of ice have been found on several Arctic shores; as well as Estonia there have been reports of them in RussiaNorth America and Northern Germany. There are even photos of “ball ice” in the Great Lakes from a 1966 book of aerial photography published by the University of Michigan. However, they are still a rare occurrence, surprising and delighting onlookers when they appear.

How do they form and why are they not seen more often?

These ice balls are thought to form from ice slush, which is amalgamated by turbulent water to form rough lumpy ice masses – similar to the way you would roll a small snow ball into a much larger one to form a snow man. The ice masses are then rounded into the smooth spherical shapes you see in our image of the week by wave action rolling them around in shallow water near the shore (see video below). This is much the same way as pebbles on a beach are smoothed and rounded – it just happens a lot faster with ice balls than solid pebbles!

It seems that the right combination of wind strength, wind direction, sea temperature and coast line shape are needed to form these features and then bring them on to the shore. For all of these things to occur at the same time is rare and special!