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Cryospheric Sciences

For Dummies

Sea Level “For Dummies”

Sea Level “For Dummies”

Looking out over the sea on a quiet day with no wind, the word “flat” would certainly pop up in your mind to describe the sea surface. However, this serene view of a flat sea surface is far from accurate at the global scale.

The apparent simplicity behind the concept of sea level hides more complex science that we hope to explain in a simple manner in today’s “For Dummies” post, which will give you the keys to understand the important aspects of past sea change, and an ability to look into and understand how sea level is a key factor in the future.

Everyone will be familiar with news stories about current sea level rise, but it can be very confusing to understand what this means in real terms; how fast it is happening and why we should care about it anyway. So to begin with, let’s have a look at what we mean by sea level?


Sea Level – It’s all about gravity!

[Read More]

Water Masses “For Dummies”

Water Masses “For Dummies”

Polar surface water, circumpolar deep water, dense shelf water, North Atlantic deep water, Antarctic bottom water… These names pop in most discussions about the ice-ocean interaction and how this will change in a warming climate, but what do they refer to?

In our second “For Dummies” article, we shall give you a brief introduction to the concept of “water mass”, explain how to differentiate water from more water, and why you would even need to do so.


Global heat budget and the need for an ocean circulation

The global climate is driven by differences between the incoming shortwave radiation and the outgoing longwave radiation (Fig. 1):

  • In the tropics, there is a surplus of energy: the Sun brings more heat, all year-round, than what is radiated out;
  • At the poles in contrast, there is a net deficit: more energy is leaving than is coming from the Sun (who is absent in winter).

The global ocean and atmosphere circulations act to reduce this imbalance, by transporting the excess heat from the tropics to the pole. Here we will focus on the global ocean circulation only, since this post is written by an oceanographer, but similar principles also apply to atmospheric circulation.

Fig 1 :Earth’s latitudinal radiation bugdet, The tropics show a surplus of energy that compensates the Poles’ deficit[Credit: National Oceanograpy Center

Fig 1 :Earth’s latitudinal radiation bugdet, The tropics show a surplus of energy that compensates the Poles’ deficit [Credit: National Oceanograpy Center].

The global ocean circulation

In a nutshell, surface waters bring heat towards the poles where they cool down, sink to the abyss, and return towards the tropics as deep waters where they can go back to the surface..…

We talk about “the global ocean circulation” because although the Earth officially has five oceans, they are not totally separate bodies of water. In fact, the Arctic, Atlantic, Indian, Pacific and Southern oceans are interconnected, with water circulating and moving between them. How does this happen?

The global ocean circulation has two components:

  • The wind-driven circulation, fast but limited to a few hundred metres below the surface of the ocean (read more about it here for example);
  • And the thermohaline circulation (shown on Fig. 2), slower but which affects the whole depth of the ocean.

Today’s post focuses on the latter, since we will talk about water properties. The thermohaline circulation, also called density-driven circulation, depends on two water properties:

  • The temperature (‘thermo’) is mostly controlled by heat exchange with the atmosphere or the ice. Cold water has a high density.
  • The salinity (‘haline’) can be modified by evaporation, precipitation, or addition of fresh water from melted glaciers/ice sheets or rivers. Salty water has a high density.
Fig 2- The global thermohaline circulation shows warm surface currents in red, cold deep currents in blue. Deep waters form in the North Atlantic and Southern oceans. [Credit: NASA]

Fig 2- The global thermohaline circulation shows warm surface currents in red, cold deep currents in blue. Deep waters form in the North Atlantic and Southern oceans [Credit: NASA].

Roughly speaking, a water mass is any drop of the ocean within a specific range of temperature and salinity, and hence specific density. Some water masses are found at particular locations or seasons, while others can be found all around the globe, all the time. Since density sets the depth (density MUST always increase with depth), water masses will lie and travel at particular depth levels.

A quick and dirty oceanography guide

Water masses are formed.

Some are the result of the mixing of other water masses. The others start at the water surface, where they exchange gas (notably oxygen and carbon) with the atmosphere. When a water mass becomes denser than the waters below it , for example, if it is cooled by the wind or ice, it sinks to its corresponding depth within the ocean.

Fig 3- The bathymetry of the Arctic Ocean forces dense (deep) water masses to enter the region via Fram Strait whereas lighter (shallower) waters can go through the Barents Sea [Credit: adapted from IBCAO bathymetry map, Jakobsson et al., 2012 ].

Water masses move all around the globe…

…provided their density allows it. The vertical distribution of density in the ocean must be “stably stratified”, which means that the density increases with depth. In practice, that means that dense waters cannot climb up a shallow bathymetric feature but have to find a way around it. For example to enter the Arctic Ocean (Fig 3), a dense water mass has no choice but to go via Fram Strait, whereas a less dense one can go via the Barents Trough. Similarly, there is a depth limit of about 500 m to reach the northwestern Greenland glaciers.

Water masses retain their properties

Or rather, not all these properties change considerably with space and time. We are not talking only about temperature and salinity, but also about gas and chemical concentrations. It is then possible to track a water mass as it travels around the globe or watch its evolution with time.

You should use T-S diagrams

Visualising water properties can either be done with one graph showing how the temperature varies with depth plus another one for the salinity (multiplied by the number of locations to be observed at the same time); or all of this information can be combined on one image (as done on Fig. 4). This image is called a T-S diagram it and shows how the temperature (T) varies as a function of the salinity (S). It is customary to also draw the lines of constant density (the ‘isopycnals’, black on Fig. 3). These isopycnals give information about the types of mixing happening and the stratification, but we will talk about that in another post.

Fig 4 - an example of how to combine several profiles (top) into a T-S diagram, for one of the randomly selected Arctic historical points that I work with.[Credit: C. Heuzé]

Fig 4 – an example of how to combine several profiles (top) into a T-S diagram, for one of the randomly selected Arctic historical points that I work with [Credit: C. Heuzé].

Because each water mass occupies a very specific region of the T-S diagram (see Fig 5 for an example in the Atlantic), identifying them is relatively easy once you have plotted your data on such diagrams.

Fig 5 – example of a reference T-S diagram with the different water masses of the Atlantic Ocean. Water massed are labelled by their acronym (e.g. AABW= Antarctic Bottom Water) [Credit: after Emery and Meincke (1986)]

Why do ocean water masses matter to the cryosphere?

  • Marine ice sheet instability, and more generally basal melting, is caused by warm dense waters melting floating glaciers from below; how dense the water mass is determines whether it can even reach the glacier.
  • Sea ice formation and melting can be largely affected by water masses moving up and down, especially is those going up are warm.

But there’s a reason why we always talk about “ice-ocean” interactions: it’s not just the ocean acting on the ice, but also the ice impacting the ocean:

  • The densest water mass in the world, Antarctic Bottom Water, forms in the middle of winter if a hole in the sea-ice cover opens (that is called a polynya), suddenly exposing the relatively warm ocean to the extremely cold atmosphere. The resulting strong heat loss and the increased salinity as sea ice reforms make this water sink straight to the bottom;
  • On the other hand, deep water formation can be stopped by the cryosphere: paleorecord evidence showed that it happened in the North Atlantic due to surging ice sheet / marine ice sheet instability (so called Heinrich events) or meltwater floods (Younger Dryas);
  • Less dramatically, icebergs, ice shelves or even sea ice, can cool or freshen water masses they meet, forming “modified” water masses (for example “modified Atlantic Water”),

Each aspect of these interactions is already experiencing climate change and is much more complex than this brief overview… but that will be the topic of another post!

Further reading

 Edited by Sophie Berger and Emma Smith

Marine Ice Sheet Instability “For Dummies”

Marine Ice Sheet Instability “For Dummies”

MISI is a term that is often thrown into dicussions and papers which talk about the contribution of Antarctica to sea-level rise but what does it actually mean and why do we care about it?

MISI stands for Marine Ice Sheet Instability. In this article, we are going to attempt to explain this term to you and also show you why it is so important.


Background

The Antarctic Ice Sheet represents the largest potential source of future sea-level rise: if all its ice melted, sea level would rise by about 60 m (Vaughan et al., 2013). According to satellite observations, the Antarctic Ice Sheet has lost 1350 Gt (gigatonnes) of ice between 1992 and 2011 (1 Gt = 1000 million tonnes), equivalent to an increase in sea level of 3.75 mm or 0.00375 m (Shepherd et al., 2012). 3.75 mm does not seem a lot but imagine that this sea-level rise is evenly spread over all the oceans on Earth, i.e. over a surface of about 360 million km², leading to a total volume of about 1350 km³, i.e. 1350 Gt of water… The loss over this period is mainly due to increased ice discharge into the ocean in two rapidly changing regions: West Antarctica and the Antarctic Peninsula (Figure 1, blue and orange curves respectively).

Figure 1: Cumulative ice mass changes (left axis) and equivalent sea-level contribution (right axis) of the different Antarctic regions based on different satellite observations (ERS-1/2, Envisat, ICESat, GRACE) from 1992 to 2011 (source: adapted from Fig. 5 of Shepherd et al., 2012 ) with addition of inset: Antarctic map showing the different regions ( source )

What are the projections for the future?

Figure 2: Ice velocity of the glaciers in the Amundsen Sea Embayment, West Antarctica, using ERS-1/2 radar data in winter 1996. The grounding line (boundary between ice sheet and ice shelf) is shown for 1992, 1994, 1996, 2000 and 2011 (source: Fig. 1 of Rignot et al., 2014 ).

Figure 2: Ice velocity of the glaciers in the Amundsen Sea Embayment, West Antarctica, using ERS-1/2 radar data in winter 1996. The grounding line (boundary between ice sheet and ice shelf) is shown for 1992, 1994, 1996, 2000 and 2011 (source: Fig. 1 of Rignot et al., 2014 ).

According to model projections from the Intergovernmental Panel on Climate Change (IPCC), global mean sea level will rise by 0.26 to 0.82 m during the twenty-first century (Church et al., 2013). The contribution from the Antarctic Ice Sheet in those projections will be about 0.05 m (or 50 mm) sea-level equivalent, i.e. 10% of the global projected sea-level rise, with other contributions coming from thermal expansion (40 %), glaciers (25 %), Greenland Ice Sheet (17 %) and land water storage (8 %).

The contribution from Antarctica compared to other contributions does not seem huge, however there is a high uncertainty coming from the possible instability of the West Antarctic Ice Sheet. According to theoretical (Weertman, 1974; Schoof, 2007) and recent modeling results (e.g. Favier et al., 2014; Joughin et al., 2014), this region could be subject to marine ice sheet instability (MISI), which could lead to considerable and rapid ice discharge from Antarctica. Satellite observations show that MISI may be under way in the Amundsen Sea Embayment (Rignot et al., 2014), where some of the fastest flowing glaciers on Earth are located, e.g. Pine Island and Thwaites glaciers (Figure 2). So what exactly is MISI?

What is marine ice sheet instability (MISI)?

 

Figure 3: Antarctic map of ice sheet (blue), ice shelves (orange) and islands/ice rises (green) based on satellite data (ICESat and MODIS). The grounding line is the separation between the ice sheet and the ice shelves. Units on X and Y axes are km (source: NASA ).

Figure 3: Antarctic map of ice sheet (blue), ice shelves (orange) and islands/ice rises (green) based on satellite data (ICESat and MODIS). The grounding line is the separation between the ice sheet and the ice shelves. Units on X and Y axes are km (source: NASA ).

To understand the concept of MISI, it is important to define both ‘marine ice sheet’ and ‘grounding line’:

 

  • A marine ice sheet is an ice sheet sitting on a bedrock that is below sea level, for example the West Antarctic Ice Sheet.
  • The grounding line is the boundary between the ice sheet, sitting on land, and the floating ice shelves (Figure 3 for a view from above and Figure 4 for a side view). The position and migration of this grounding line control the stability of a marine ice sheet.

 

 

The MISI hypothesis states that when the bedrock slopes down from the coast towards the interior of the marine ice sheet, which is the case in large parts of West Antarctica, the grounding line is not stable (in the absence of back forces provided by ice shelves, see next section for more details). To explain this concept, let us take the schematic example shown in Figure 4:

  1. The grounding line is initially located on a bedrock sill (Figure 4a). This position is stable: the ice flux at the grounding line, which is the amount of ice passing through the grounding line per unit time, matches the total upstream accumulation.
  2. A perturbation is applied at the grounding line, e.g. through the incursion of warm Circumpolar Deep Water (CDW, red arrow in Figure 4) below the ice shelf as observed in the Amundsen Sea Embayment.
  3. These warm waters lead to basal melting at the grounding line, ice-shelf thinning and glacier acceleration, resulting in an inland retreat of the grounding line.
  4. The grounding line is then located on a bedrock that slopes downward inland (Figure 4b), i.e. an unstable position where the ice column at the grounding line is thicker than previously (Figure 4a). The theory shows that ice flux at the grounding line is strongly dependent on ice thickness there (Weertman, 1974; Schoof, 2007), so a thicker ice leads to a higher ice flux.
  5. Then, the grounding line is forced to retreat since the ice flux at the grounding line is higher than the upstream accumulation.
  6. This is a positive feedback and the retreat only stops once a new stable position is reached (e.g. a bedrock high), where both ice flux at the grounding line and upstream accumulation match.
Figure 4: Schematic representation of the marine ice sheet instability (MISI) with (a) an initial stable grounding-line position and (b) an unstable grounding-line position after the incursion of warm Circumpolar Deep Water (CDW) below the ice shelf (source: Fig. 3 of Hanna et al., 2013 ).

Figure 4: Schematic representation of the marine ice sheet instability (MISI) with (a) an initial stable grounding-line position and (b) an unstable grounding-line position after the incursion of warm Circumpolar Deep Water (CDW) below the ice shelf (source: Fig. 3 of Hanna et al., 2013 ).

  • In summary, the MISI hypothesis describes the condition where a marine ice sheet is unstable due to being grounded below sea level on land that is sloping downward from the coast to the interior of the ice sheet.
  • This configuration leads to potential rapid retreat of the grounding line and speed up of ice flow from the interior of the continent into the oceans.

Is there evidence that MISI is happening right now?

 

Figure 5: Buttressing provided by Larsen C ice shelf, Antarctic Peninsula, based on a model simulation (Elmer/Ice). Buttressing values range between 0 (no buttressing) and 1 (high buttressing). The red contour shows the buttressing=0.3 isoline. Observed ice velocity is also shown (source: Fig. 2 of Fürst et al., 2016 ).

Figure 5: Buttressing provided by Larsen C ice shelf, Antarctic Peninsula, based on a model simulation (Elmer/Ice). Buttressing values range between 0 (no buttressing) and 1 (high buttressing). The red contour shows the buttressing=0.3 isoline. Observed ice velocity is also shown (source: Fig. 2 of Fürst et al., 2016 ).

In reality, the grounding line is often stabilized by an ice shelf that is laterally confined by side walls (see Figure 5, where Bawden and Gipps ice rises confine Larsen C ice shelf) or by an ice shelf that has a contact with a locally grounded feature (Figure 6). Both cases transmit a back force towards the ice sheet, the ‘buttressing effect’, which stabilizes the grounding line (Goldberg et al., 2009; Gudmundsson, 2013) even if the configuration is unstable, i.e. in the case of a grounding line located on a bedrock sloping down towards the interior (Figure 4b).

 

However, in the last two decades, the grounding lines of the glaciers in the Amundsen Sea Embayment (Pine Island and Thwaites glaciers for example) retreated with rates of 1 to 2 km per year, in regions of bedrock sloping down towards the ice sheet interior (Rignot et al., 2014). The trigger of these grounding-line retreats is the incursion of warm CDW penetrating deeply into cavities below the ice shelves (Jacobs et al., 2011), carrying important amounts of heat that melt the base of ice shelves (Figure 4). Increased basal melt rates have led to ice-shelf thinning, which has reduced the ice-shelf buttressing effect and increased ice discharge. All of this has led to grounding-line retreat. The exact cause of CDW changes is not clearly known but these incursions are probably linked to changes in local wind stress (Steig et al., 2012) rather than an actual warming of CDW.

 

 

Figure 6: Schematic representation of ice-shelf buttressing by a local pinning point (source: courtesy of R. Drews ).

Figure 6: Schematic representation of ice-shelf buttressing by a local pinning point (source: courtesy of R. Drews ).

There is currently no major obstacle to these grounding line retreats. Therefore, the Amundsen Sea Embayment is probably experiencing MISI and glaciers will continue to retreat if no stabilization is reached. This sector of West Antarctica contains enough ice to raise global sea level by 1.2 m.

 

What can we do about it?

MISI is probably ongoing in some parts of Antarctica and sea level could rise more than previously estimated if the grounding lines of the glaciers in the Amundsen Sea Embayment continue to retreat so fast. This could have catastrophic impacts on populations living close to the coasts, for example more frequent flooding of coastal cities, enhanced coastal erosion or changes in water quality.

Thus, it is important to continue monitoring the changes happening in Antarctica, and particularly in West Antarctica. This will allow us to better understand and project future sea-level rise from this region, as well as better adapt the cities of tomorrow.

Edited by Clara Burgard and Emma Smith


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.