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Ice-hot news: The cryosphere and the 1.5°C target

Ice-hot news: The cryosphere and the 1.5°C target

Every year again, the Conference of Parties takes place, an event where politicians and activists from all over the world meet for two weeks to discuss further actions concerning climate change. In the context the COP24, which started this Monday in Katowice (Poland), let’s revisit an important decision made three years ago, during the COP21 in Paris, and its consequences for the state of the cryosphere…


1.5°C target – what’s that again?

Last October, the International Panel on Climate Change (IPCC) released a special report (SR15) on the impacts of a 1.5°C global warming above pre-industrial levels. This target of 1.5°C warming was established during the 21st conference of the parties (COP21), in a document known as the Paris Agreement. In this Agreement, most countries in the World acknowledge that limiting global warming to 1.5°C warming rather than 2°C warming would significantly reduce the risks and impacts of climate change.

But wait, even though achieving this target is possible, which is not our subject today, what does it mean for our beloved cryosphere? And how does 1.5°C warming make a difference compared to the 2°C warming initially discussed during the COP21 and previous COPs?

A reason why the cryosphere is so difficult to grasp is the nonlinear behaviour of its components. What does this mean ? A good basic example is the transition between water and ice. At 99.9°C, you have water. Go down to 0.1°C and the water is colder, but this is still water. Then go down to -0.1°C and you end up with ice. The transition is very sharp and the system can be deeply affected even for a small change in temperature.

As a main conclusion, studies conducted in the context of SR15 show that, below 1.5°C of global warming, most components of the cryosphere will be slightly affected, while above that level of warming, there is more chance that the system may respond quickly to small temperature changes. In this Ice Hot News, we review the main conclusions of the SR15 concerning ice sheets, glaciers, sea ice and permafrost, answering among others the question if achieving the 1.5°C target would prevent us to trigger the potential nonlinear effects affecting some of them.

Ice sheets

The two only remaining ice sheets on Earth cover Greenland and Antarctica. If melted, the Greenland ice sheet could make the sea level rise by 7 m, while the Antarctic ice sheet could make it rise by almost 60 m. A recent review paper (Pattyn et al., 2018), not in SR15 because published very recently, shows that keeping the warming at 1.5°C rather than 2°C really makes the differences in terms of sea level rise contribution by the two ice sheets.

Greenland is a cold place, but not that cold. During the Holocene, the surface of the ice sheet always melted in summer but, in the yearly mean, the ice sheet was in equilibrium because summer melt was compensated by winter accumulation. Since the mid-1990s, Greenland’s atmosphere has warmed by about 5°C in winter and 2°C in summer. The ice sheet is thus currently losing mass from above and its surface lowers down. In the future, if the surface lowers too much, this could accelerate the mass loss because the limit altitude between snow and rainfalls may have been crossed, further accelerating the mass loss. The temperature threshold beyond which this process will occur is about 1.8°C, according to the Pattyn et al., 2018 paper.

Antarctica is a very cold continent, much colder than Greenland, but it has been losing mass since the 1990s as well. There, the source of the retreat is the temperature increase of the ocean. The ocean is in contact with the ice shelves, the seaward extensions of the ice sheet in its margins. The warmer ocean has eroded the ice shelves, making them thinner and less resistant to the ice flow coming from the interior. And if you have read the post about the marine ice sheet instability (MISI), you already know that the ice sheet can discharge a lot of ice to the ocean if the bedrock beneath the ice sheet is deeper inland than it is on the margins (called retrograde). MISI is a potential source of nonlinear acceleration of the ice sheet that, along with other nonlinear effects mentioned in the study, could trigger much larger sea level rise contribution from the Antarctic ice sheet above 2 to 2.7°C.

You can find complementary informations to the Pattyn et al., 2018 paper in SR15, sections 3.3.9, 3.5.2.5, 3.6.3.2 and in FAQ 3.1.

Glaciers crossing the transantarctic mountains, one of them ending up to Drygalski ice tongue (left side) in the Ross sea. The ice tongue is an example of those ice shelves that form as grounded ice flows toward the sea from the interior. Ice shelves are weakened by a warmer ocean, which accelerates upstream ice flow [Credit: C. Ritz, PEV/PNRA]

Glaciers

Over the whole globe, the mass of glaciers has decreased since pre-industrial times in 1850, according to Marzeion et al., 2014. At that time, climate change was a mix between human impact and natural variability of climate. Glacier response times to change in climate are typically decades, which means that a change happening, for instance, today, still has consequences on glaciers tens of years after. Today, the retreat of glaciers is thus a mixed response to natural climate variability and current anthropogenic warming. However, since 1850, the anthropogenic warming contribution to the glacier mass loss has increased from a third to more than two third over the last two decades.

Similarly to the Greenland ice sheet, glaciers are prone to undergo an acceleration of ice mass loss wherever the limit altitude where rainfall occurs more often than snowfall is higher and at the same time the glacier surface lowers. However, as opposed to ice sheets, glaciers can be found all over the world under various latitudes, temperature and snow regimes, which makes it difficult to establish a unique temperature above which all the glaciers in the world will shrink faster in a nonlinear way. There are, however, model-based global estimates of ice mass loss over the next century. The paper from Marzeion et al., 2018, shows that under 1.5-2°C of global warming, the glaciers will lose the two thirds of their current mass, and that for a 1°C warming, our current level of warming since pre-industrial times, the glacier are still committed to lose one third of their current mass. This means the actions that we take now to limit climate change won’t be seen for decades.

You can find complementary informations in SR15, sections 3.3.9, 3.6.3.2 and in FAQ 3.1.

Sea ice

As very prominently covered by media and our blog (see this post and this post), the Arctic sea-ice cover has been melting due to the increase in CO2 emissions in past decades. To understand the future evolution of climate, climate models are forced with the expected CO2 emissions for future scenarios. In summer, the results of these climate model simulations show that keeping the warming at 1.5°C instead of 2°C is essential for the Arctic sea-ice cover. While at 1.5°C warming, the Arctic Ocean will be ice-covered most of the time, at 2°C warming, there are much higher chances of a sea-ice free Arctic. In winter, however, the ice cover remains similar in both cases.

In the Antarctic, the situation is less clear. On average, there has been a slight expansion of the sea-ice cover (see this post). This is, however, not a clear trend, but is composed of different trends over the different Antarctic basins. For example, a strong decrease was observed near the Antarctic peninsula and an increase in the Amundsen Sea. The future remains even more uncertain because most climate models do not represent the Antarctic sea-ice cover well. Therefore, no robust prediction could be made for the future.

You can find all references were these results are from and more details in Section 3.3.8 of the SR15. Also, you can find the impact of sea-ice changes on society in Section 3.4.4.7.

Caption: Sea ice in the Arctic Ocean [D. Olonscheck]

Permafrost

Permafrost is ground that is frozen consecutively for two years or more. It covers large areas of the Arctic and the Antarctic and is formed or degraded in response to surface temperatures. Every summer, above-zero temperatures thaw a thin layer at the surface, and below this, we find the boundary to the permafrost. The depth to the permafrost is in semi-equilibrium with the current climate.

The global area underlain by permafrost globally will decrease with warming, and the depth to the permafrost will increase. In a 1.5°C warmer world, permafrost extent is estimated to decrease by 21-37 % compared to today. This would, however, preserve 2 millions km2 more permafrost than in a 2°C warmer world, where 35-47 % of the current permafrost would be lost.

Permafrost stores twice as much carbon (C) as the atmosphere, and permafrost thaw with subsequent release of CO2 and CH4 thus represents a positive feedback mechanism to warming and a potential tipping point. However, according to estimates cited in the special report, the release at 1.5°C warming (0.08-0.16 Gt C per year) and at 2°C warming (0.12-0.25 Gt C per year) does not bring the system at risk of passing this tipping point before 2100. This is partly due to the energy it takes to thaw large amounts of ice and the soil as a medium for heat exchange, which results in a time lag of carbon release.
The response rates of carbon release is, however, a topic for continuous discussion, and the carbon loss to the atmosphere is irreversible, as permafrost carbon storage is a slow process, which has occurred over millennia.

Changes in albedo from increased tree growth in the tundra, which will affect the energy balance at the surface and thus ground temperature, is estimated to be gradual and not be linked to permafrost collapse as long as global warming is held under 2°C.

The above-mentioned estimates and predictions are from the IPCC special report Section 3.5.5.2, 3.5.5.3 and 3.6.3.3.

Slope failure of permafrost soil [Credit: NASA, Wikimedia Commons].

So, in summary…

In summary, what can we say? Although the 1.5°C and 2°C limits were chosen as a consensus between historical claims based on physics and a number that is easy to communicate (see this article), it seems that there are some thresholds for parts of the cryosphere exactly between the two limits. This can have consequences on longer term, e.g. sea-level rise or permanent permafrost loss. Additionally, as the cryosphere experts and lovers that we are here in the blog team, we would mourn the loss of these exceptional landscapes. We therefore strongly hope that the COP24 will bring more solution and cooperation for the future against strengthening of climate change!

Further reading

Edited by Clara Burgard and Violaine Coulon


Lionel Favier is a glaciologist and ice-sheet modeller, currently occupying a post-doctoral position at IGE in Grenoble, France. He’s also on twitter.

 

 

 

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

 

 

Clara Burgard is a PhD student at the Max Planck Institute for Meteorology in Hamburg. She investigates the evolution of sea ice in general circulation models (GCMs). There are still biases in the sea-ice representation in GCMs as they tend to underestimate the observed sea-ice retreat. She tries to understand the reasons for these biases. She tweets as @climate_clara.

 

Image of the Week – (Un)boxing the melting under the ice shelves

Image of the Week – (Un)boxing the melting under the ice shelves

The Antarctic ice sheet stores a large amount of water that could potentially add to sea level rise in a warming world (see this post and this post). It is currently losing ice, and the ice loss has been accelerating in the past decades. All this is linked to the melting of ice – not at the surface but at the base, underneath the so-called ice shelves which form the continuation of the Antarctic ice sheet over the ocean. These floating ice shelves (represented in color in our Image of the Week) are melted by ocean water from underneath. How can this process called ‘sub-shelf melting’ be included in ice-sheet models? One simple way is to divide the ice-shelf cavity into a number of ocean boxes. Let’s briefly see how it works.


How to model sub-shelf melting in ice-sheet models?

There are three main ways to do so – which way is most suitable depends on the application:

  1. The most elaborated approach is to use ocean models that resolve ocean dynamics underneath the ice shelves. However, they need a lot of computational power.

  2. As an alternative, simple parameterizations in which melting is a function of the depth of the ice-shelf base can be used. However, such parameterizations are for many applications too simple…

  3. Recently, intermediate approaches that include the basic ocean dynamics have been developed (e.g. Lazeroms et al., 2018; Pelle et al., in review). One such approach is the ocean box model (Olbers and Hellmer, 2010) that we extended for the use in an ice-sheet model. Our extension is called Potsdam Ice-shelf Cavity mOdel (PICO, Reese et al., 2018).

In the following, we take a closer look into the approach of PICO…

“Boxing” the cavity circulation

In Antarctic ice-shelf cavities (i.e. the water below the ice shelves), in general, an overturning circulation transports ocean water from the sea floor along the ice-shelf base towards the calving front (see Figure 2). It is driven by the “ice-pump” (Lewis and Perkin, 1986): ice melting near the grounding line (separation between the grounded ice sheet and the floating ice shelf) reduces the density of the ambient water. It becomes buoyant and rises along the shelf base towards the ocean. Through this process, new water from outside of the ice-shelf cavity is “pumped” along the continental shelf towards the grounding line. This leads to the typical pattern of highest melting near the deep grounding lines and lower melting towards the calving front.

 

Figure 2: Schematic showing the ocean boxes following the ice-shelf base, with the first box B1 near the grounding line, and the last box Bn at the calving front. The arrows indicate the overturning circulation. The ocean water enters the cavity from box B0 which is at depth of the continental shelf, in front of the ice shelf. [Credit: Fig. 1 of Reese et al. (2018)]

 

By dividing the ice-shelf cavity into 2 to 5 ocean boxes, the transport of the overturning circulation is simplified while the sub-shelf melt pattern is captured. The open ocean conditions are simply represented by the ocean reservoir box B0 (Figure 2). And the circulation is driven by the differences in water density between the ocean reservoir (B0 in Figure 2) and the first box near the grounding line (B1 in Figure 2). The model computes sub-shelf melting successively over the ocean boxes, starting near the grounding line.

Sub-shelf melting with PICO

Sub-shelf melting can vary a lot in-between ice shelves (Figure 1). Antarctic ice-shelf cavities can roughly be sorted into two types (Joughin et al., 2012). The first category are the cold cavities in which the ocean water is close to the freezing point and in which sub-shelf melting is generally low, about 0.1 meter per year. The second category are warm cavities which have a temperature of about 1 degree – that does not sound like much, but for an ice shelf, this feels like being in a sauna – and sub-shelf melting can easily exceed 10 meters per year. Small changes in ocean temperatures can hence have large effects on sub-shelf melting. An increase in sub-shelf melting thins the ice shelf, as for example observed in the Amundsen Sea region in West Antarctica (see this post). The ice shelves there are examples for warm cavities, and a cold cavity is, for instance, underneath the Filchner-Ronne Ice Shelf (see Figure 1 for the specific locations).

In reality, of course, things are much more complicated than simulated by our PICO model. For example, the Coriolis effect can influence ocean circulation in the cavities, sills in the bed can block access of warm water to the grounding line and so on…

Applications of PICO

To summarize, PICO is a simple and efficient modeling tool that can capture the general pattern of sub-shelf melting observed in Antarctica today. Being implemented in the Parallel Ice Sheet Model, it is openly available, so if you got excited about what it can do and want to use it yourself, you’re welcome to download it!

Further reading

Edited by David Docquier


Ronja Reese is a postdoctoral researcher at the Potsdam Institute for Climate Impact Research, Germany, in the group of Prof. Dr. Ricarda Winkelmann. She investigates ice dynamics in Antarctic with a focus on ice-ocean interactions and ice-shelf buttressing. She developed and implemented PICO together with Ricarda Winkelmann, Torsten Albrecht, Matthias Mengel and Xylar Asay-Davis. Contact Email: ronja.reese@pik-potsdam.de

Do clouds affect melting over Antarctic ice shelves?

Schematic showing the effects of cloud microphysics on the radiative properties of clouds for shortwave solar radiation (a & b) and longwave terrestrial radiation (c & d) [Credit: Ella Gilbert].

The Antarctic Peninsula is the ‘canary in the coalmine’ of Antarctic climate change. In the last half-century it has warmed faster than most other places on Earth, and considerable change has consequently been observed in the cryosphere, with several ice shelves collapsing in part or in full. Representing this change in models is difficult because we understand comparatively little about the effect of atmospheric processes on melting in Antarctica, especially clouds, which are the main protagonists of this Image of the Week…


The Antarctic Peninsula: a part of the southern continent that is surrounded by ice shelves, but also a place that has seen rapid and dramatic changes in the last decades. Until recently, the Antarctic Peninsula was one of the most rapidly warming regions on Earth, with annual mean surface temperatures rising by as much as 2.5°C between the 1950s and early 2000s in some places (Turner et al., 2005; 2016).

That warming has been linked to the demise of the region’s ice shelves: since 1947, more than half of the peninsula’s ice shelves have thinned, lost area, or collapsed entirely (Cook & Vaughan, 2010). Most recently, that includes Larsen C, whose area was reduced by 12% in July 2017 following a calving event where an iceberg four times the size of London broke away from the ice shelf. As a result, the ice shelf has slipped down the rankings from the 4th largest ice shelf on the continent to the 5th largest.

 

What makes ice shelves melt?

Evidence suggests that ice shelves on the peninsula are being warmed mostly from the top down by the atmosphere. This is contrary to what’s happening on other Antarctic ice shelves, like those in West Antarctica that are being eroded from beneath by the warming ocean. Atmospheric processes are much more important over peninsula ice shelves than those elsewhere on the continent.

To understand the effect of the atmosphere on melting at the top of ice shelves, we need to know how much energy is entering the surface of the ice shelf, how much is leaving, and use what’s left over to determine whether there’s residual energy available to melt the ice. That’s the general principle of the surface energy balance, and it’s called a ‘balance’ because it is usually just that – the amount of energy flowing into and out of the ice shelf averages out over the course of say, a year, to produce a net zero sum of energy left for melting. However, there are times when this balance can become either negative, leading to growth of the ice shelf, or positive, leading to ice loss via melting.

 

What affects the surface energy balance?

Many different processes influence the surface energy balance, such as weather patterns and atmospheric motion. For instance, when warm, dry air blows over an ice surface, which happens during ‘foehn‘ wind events (German readers will know this means ‘hairdryer’: a descriptive name for the phenomenon!), this can produce a surplus of energy available for melting (Grosvenor et al., 2014; King et al., 2017; Kuipers Munneke et al., 2018). If the surface temperature reaches 0°C, melting occurs.

 

What do clouds have to do with it?

Clouds also greatly influence the surface energy balance by affecting the amount of radiation that reaches the surface. The amount of incoming solar (shortwave) radiation that reaches the surface, and the amount of terrestrial (longwave) radiation that escapes is affected by what stands in the way – clouds. Of course, this obstacle is important for the surface energy balance because it affects the balance between the energy flowing into and out of the surface. However, the fine-scale characteristics of clouds (aka ‘microphysics’) produce different, often interacting and sometimes competing, effects on the surface energy balance, some of which are shown in the schematic above. Examples of these properties include:

  • Water phase (how much ice and liquid there is)
  • Number concentration (how many particles)
  • Particle size
  • Ice crystal shape

The amount of ice and liquid in a cloud can affect how much energy it absorbs, reflects and emits – for instance, the more liquid a cloud contains, the more energy it emits towards the surface, because it is thicker and tends to be warmer than a cloud with lots of ice. However, clouds made up of lots of tiny liquid droplets also tend to be brighter than ice clouds containing larger crystals, which means they reflect more incoming solar radiation back into space. This example is a typical one where different microphysical properties cause competing effects, which makes them difficult to separate from each other.

 

Radiative forcing (RF, solid bars) and Effective radiative forcing (ERF, hatched bars) of climate change during the Industrial Era (1750-2011) [Credit: adapted from IPCC Fifth Assessment Report, Figure 8.15: pp. 697].

What do we know about Antarctic clouds?

The short answer is: not that much. Clouds are the largest source of uncertainty in our estimates of global climate change (check out the huge range of error in the estimates of cloud-driven radiative forcing in the figure above, from the IPCC’s most recent report), and the science of Antarctic clouds is even more unclear because we don’t have a great deal of data to base our understanding on. To measure clouds directly, we need to fly through them – a costly and potentially dangerous exercise, especially in Antarctica.

 

Flying through a gap in cloud near Jenny Island on the approach to Rothera research station, on the Antarctic Peninsula, at the end of a data collection flight in November 2017 [Credit: Ella Gilbert].

Filling the gap

In somewhere like Antarctica where we don’t have much observational data, we have to rely on other tools. That’s where computer models can be really useful – so long as we can be confident in the results they produce. Unfortunately, that’s part of the problem. Cloud properties and their effects on the surface energy balance are complex: we know that much. But modelling those properties is even more complex, because we have to simplify things to be able to turn them into computer code.

There is hope though! Recent studies (e.g. Listowski et al., 2017) have shown that models can more realistically represent Antarctic cloud microphysics if they use more sophisticated ‘double moment’ schemes, which are able to simulate more microphysical properties. With more accurate microphysics comes better representation of the surface energy balance, and improved estimates of melt over Antarctic ice shelves.

 

Further reading

  • On the effect of foehn on wintertime melting over Larsen C:

Kuipers Munneke, P., Luckman, A. J., Bevan, S. L., Gilbert, E., Smeets, C. J. P. P., Van Den Broeke, M. R., Wang, W., Zender, C., Hubbard, B., Ashmore, D., Orr, A. King, J. C. (2018). Intense winter surface melt on an Antarctic ice shelf. Geophysical Research Letters 45, 7615–7623. doi:10.1029/2018GL077899

  • On clouds in Antarctica:

Lachlan-Cope, T. (2010). Antarctic clouds. Polar Research 29 (2), 150–158. doi:10.1111/j.1751-8369.2010.00148.x

  • On modelling cloud microphysics over the Antarctic Peninsula:

Listowski, C., & Lachlan-Cope, T. (2017). The Microphysics of Clouds over the Antarctic Peninsula – Part 2: modelling aspects within Polar WRF. Atmospheric Chemistry and Physics 17, 10195-10221. doi:10.5194/acp-17-10195-2017

Edited by Clara Burgard


Ella Gilbert is a PhD student at the British Antarctic Survey, where she uses climate modelling and observational data to understand the drivers of melt on the Larsen C ice shelf. She’s a big fan of clouds, polar science, and science communication. You can find her on Twitter @Dr_Gilbz, on her website www.larsenc.com, or the old fashioned way by email.

Image of the Week – Oh Sheet!

Image of the Week – Oh Sheet!

The Antarctic and Greenland ice sheets are major players in future sea level rise. Still, there is a lot about these ice sheets we do not understand. Under the umbrella of the World Climate Research Programme, the international scientific community is coming together to improve ice sheet modelling efforts to better grasp the implications of climate change for ice sheet evolution, and consequently, sea level rise…


What are ice sheets?

An ice sheet is a massive chunk of glacier ice that sits on land – covering an area greater than 50,000 square kilometres (or 1.6 times the size of Belgium) by the official definition. Currently, the only two ice sheets on Earth are in Antarctica and Greenland. Ice in ice sheets flows from inland toward the coast under gravity. Due to the geothermal heat flux, ice sheets are usually warmer at the base than on the surface. When basal melting occurs, the melted water lubricates the ice sheet and accelerates the ice flow, forming fast-flowing ice streams. When ice flows down a coastline into the ocean, it may float due to buoyancy. The floating slab of ice is called an ice shelf (see these previous posts for more on ice shelves). The boundary that separates the grounded ice and floating ice is called the grounding line.

 

Why do we care about ice sheets?

The most uncertain potential source of future sea level rise is the contribution from ice sheets. According to observations, the Greenland and Antarctic ice sheets have contributed approximately 7.5 and 4 mm of sea level rise respectively over the 1992-2011 period, and the contribution is accelerating. Knowing how the ice sheets will behave under future emission scenarios is crucial for risk assessment and policy-making (see this previous post for more on Antarctic ice sheets).

In addition to the direct impact on sea level rise, ice sheets interact with other components of the climate system. For example, ice discharge affects ocean circulation and marine biogeochemistry; changes in orography influence the atmosphere condition and circulation. In turn, the ice sheets gain mass primarily from snow fall, and lose mass through surface melting, surface sublimation, basal melting and ice discharge to the ocean, which are influenced by atmospheric and oceanographic processes. In Antarctica, the mass loss due to basal melting and iceberg calving is larger than snowfall accumulation. The Greenland Ice Sheet is also losing mass through iceberg calving and surface water runoff.

 

What’s CMIP?

Global coupled climate models are developed by different groups of scientists around the world to improve our understanding of the climate system. These models are highly complex, representing interactions between the ocean, atmosphere, land surface and cryosphere on global grids. The Coupled Model Intercomparison Project (CMIP) is a collaborative framework which provides a standard experimental protocol for the different models. The protocol includes a range of greenhouse gas emissions scenarios for future climate projections. Model output is made publicly available and forms the basis for assessments such as the Intergovernmental Panel on Climate Change (IPCC) reports. The latest phase (CMIP6) is underway now.

 

What’s ISMIP6?

Ice sheets were considered as passive elements of the climate system previously and were not explicitly included in the CMIP process. However, observations of the rapid mass loss associated with dynamic change in ice sheets highlight the need to couple ice sheets to climate models. New developments in ice sheet modelling allow previously-omitted key processes which affect ice sheet dynamics on decadal timescales, such as grounding-line migration and basal lubrication, to be simulated with higher confidence.

ISMIP6 is an international effort designed to ensure that projections from ice sheet models are compatible with the CMIP6 process, bringing together scientists from over twenty institutions (Fig. 2). It aims to improve sea level projections, exploring sea level contribution from the Greenland and Antarctic ice sheets in a changing climate and investigating interactions between ice sheets and the climate system.

 

ISMIP6 Experiments

As shown in Figure 1, the objectives of ISMIP6 rely on three distinct modeling efforts:

  1. CMIP atmosphere-ocean general circulation models (AOGCM) without an ice sheet component
  2. standalone dynamic ice sheet models (ISMs) that are driven by forcing provided by CMIP
  3. fully coupled atmosphere-ocean-ice sheet models (AOGCM-ISMs).

 

In the first phase, ISMIP6 will compare output from different ice sheet models run in ‘standalone’ or ‘offline coupled’ mode. This means that they receive forcings from the climate model components like the ocean and atmosphere without feeding back. These experiments will be used to explore the uncertainty associated with ice sheets physics, dynamics and numerical implementation. In particular, ISMIP6 is currently focused on gaining insight into the uncertainty in ice sheet evolution resulting from the choice of initialization methods (the initMIP efforts for the Greenland and Antarctic ice sheets) and understanding the response of the Antarctic ice sheet to a total loss of the ice shelves (ABUMIP).

The model output of the initMIP simulations for Greenland is now publicly available.

Fig. 2: Participants of ISMIP6 standalone ice sheet modeling.

 

ISMIP6 workshop

Regular meetings are organised to update and facilitate communication between the participants. The most recent workshop was hosted in the Netherlands during 11 – 13 September 2018. The topic of the workshop was “Developing process-based projections of the ice sheets’ contribution to future sea level.” Participants aimed to evaluate the output of the CMIP6 climate models and obtain forcing for standalone ice sheet model experiments. During the workshop, scientists made progress on establishing the experimental protocols for the ice sheet model simulations that will be discussed in the IPCC sixth assessment report.

Fig.3: Participants in the ISMIP6 workshop in Leiden, Netherlands [Credit: Heiko Goelzer]

Further reading

Edited by Lettie Roach and Clara Burgard


Sainan Sun does her postdoctoral research with Frank Pattyn at Université Libre de Bruxelles (ULB). She achieved her doctoral degree in 2014, majoring in ice sheet modeling at Beijing Normal University, Beijing, China. In her PhD study, she applied the BISICLES ice sheet model to Pine island glacier, Aurora drainage basin and Lambert-Amery drainage basin to describe the dynamical response of the Antarctic ice sheet to perturbations in boundary conditions. For the project at ULB, she aims to investigate the ice shelf features based on data acquired in Roi Baudouin ice shelf, Antarctica, and to estimate the potential instability of the Antarctic ice sheet using the f.ETISh ice sheet model. Contact Email: sainsun@ulb.ac.be

Image of the Week – The future of Antarctic ice shelves

Percent change in ice shelf melting, caused by the ocean, during the four future projections. The values are shown for all of Antarctica (written on the centre of the continent) as well as split up into eight sectors (colour-coded, written inside the circles). Figure 3 of Naughten et al., 2018 ). ©American Meteorological Society. Used with permission.

Climate change will increase ice shelf melting around Antarctica. That’s the not-very-surprising conclusion of a recent modelling study, resulting from a collaboration between Australian and German researchers. Here’s the less intuitive result: much of the projected melting is actually linked to a decrease in sea ice formation. Learn why in our Image of the Week…


Different types of Antarctic ice

Sea ice is just frozen seawater. But ice shelves (as well as ice sheets and icebergs) are originally formed of snow. Snow falls on the Antarctic continent, and over many years compacts into a system of interconnected glaciers that we call an ice sheet. These glaciers flow downhill towards the coast. If they hit the coast and keep going, floating on the ocean surface, the floating bits are called ice shelves. Sometimes the edges of ice shelves will break off and form icebergs, but they don’t really come into this story (have a look at this and this previous post if you want to read about icebergs nevertheless!).

Climate models don’t typically include ice sheets, or ice shelves, or icebergs. This is due to a combination of insufficient resolution and software engineering challenges, and is one reason why future projections of sea level rise are so uncertain. However, some standalone ocean models, i.e. ocean models without a coupled atmosphere, do include ice shelves. At least, they include the little pockets of ocean beneath the ice shelves – we call them ice shelf cavities – and can simulate the melting and refreezing that happens on the undersides of ice shelves.

Modelling future ice shelf melting

We took one of these ocean/ice-shelf models and forced it with the atmospheric output of regular climate models, which periodically make projections of climate change from now until the end of this century. As forcing, we used the atmospheric output of the Australian model ACCESS 1.0 in two experiments, and the mean of the atmospheric output from 19 other climate models taking part in the Coupled Model Intercomparison Project Phase 5  (Multi-Model Mean or “MMM”) in another two experiments. Each set of experiments considered two different scenarios for future greenhouse gas emissions (“Representative Concentration Pathways” or RCPs), for a total of four simulations. Each simulation required 896 processors on the supercomputer in Canberra. By comparison, your laptop or desktop computer probably has about 4 processors. These are pretty sizable models!

In every simulation, and in every region of Antarctica, ice shelf melting increases over the 21st century. The total increase ranges from 41% to 129% depending on the emissions scenario and choice of climate model. The largest increases occur in the Amundsen Sea region, marked with red circles in our Image of the Week, which also happens to be the region exhibiting the most severe melting in recent observations. In the most extreme scenario, i.e. with the highest future greenhouse gas emissions and the more sensitive climate model, ice shelf melting in this region nearly quadruples.

Understanding the drivers of melting

So what processes are causing this melting? This is where the sea ice comes in. When sea ice forms, it spits out most of the salt from the seawater (brine rejection), leaving the remaining water saltier than before. Salty water is denser than fresh water, so it sinks. This drives a lot of vertical mixing, and the heat from warmer, deeper water is lost to the atmosphere. The ocean surrounding Antarctica is unusual in that the deep water is generally warmer than the surface water. We call this warm, deep water Circumpolar Deep Water, and it’s currently the biggest threat to the Antarctic Ice Sheet. (I say “warm” – it’s only about 1°C, so you wouldn’t want to go swimming in it, but it’s plenty warm enough to melt ice.)

In our simulations, warming winters cause a decrease in sea ice formation. This leads to less brine rejection, causing fresher surface waters, causing less vertical mixing, and the warmth of Circumpolar Deep Water is no longer lost to the atmosphere. As a result of reduced vertical mixing, ocean temperatures near the bottom of the Amundsen Sea increase and this better-preserved Circumpolar Deep Water
finds its way into ice shelf cavities, causing large increases in melting.

 

Slices through the Amundsen Sea – you’re looking at the ocean sideways, like a slice of birthday cake, so you can see the vertical structure. Temperature is shown on the top row (blue is cold, red is warm); salinity is shown on the bottom row (blue is fresh, red is salty). Conditions at the beginning of the simulation are shown in the left 2 panels, and conditions at the end of the simulation are shown in the right 2 panels. At the beginning of the simulation, notice how the warm, salty Circumpolar Deep Water rises onto the continental shelf from the north (right side of each panel), but it gets cooler and fresher as it travels south (towards the left) due to vertical mixing. At the end of the simulation, the surface water has freshened and the vertical mixing has weakened, so the warmth of the Circumpolar Deep Water is preserved. Figure 8 of Naughten et al., 2018, ©American Meteorological Society. Used with permission.

 

Going to the next level

This link between weakened sea ice formation and increased ice shelf melting has troubling implications for sea level rise. The next step is to simulate the sea level rise itself, which requires some model development. Ocean models like the one we used for this study have to assume that ice shelf geometry stays constant, so no matter how much ice shelf melting the model simulates, the ice shelves aren’t allowed to thin or collapse. Basically, this design assumes that any ocean-driven melting is exactly compensated by the flow of the upstream glacier such that ice shelf geometry remains constant.

Of course this is not a good assumption, because we’re observing ice shelves thinning all over the place, and a few have even collapsed. But removing this assumption would necessitate coupling with an ice sheet model, which presents major engineering challenges. We’re working on it – at least ten different research groups around the world – and over the next few years, fully coupled ice-sheet/ocean models should be ready to use for the most reliable sea level rise projections yet.

Further reading

Edited by Clara Burgard


Kaitlin Naughten is a postdoc at the British Antarctic Survey in Cambridge, UK. She is an ocean modeller focusing on interactions between Antarctic ice shelves, sea ice, and the Southern Ocean. Tweets as @kaitlinnaughten Website: climatesight.org

Image of the Week — Quantifying Antarctica’s ice loss

Fig. 1 Cumulative Antarctic Ice Sheet mass change since 1992. [Credit: Fig 2. from The IMBIE team (2018), reprinted with permission from Nature]

It is this time of the year, where any news outlet is full of tips on how to lose weight rapidly to  become beach-body ready. According to the media avalanche following the publication of the ice sheet mass balance inter-comparison exercise (IMBIE) team’s Nature paper, Antarctica is the biggest loser out there. In this Image of the Week, we explain how the international team managed to weight Antarctica’s ice sheet and what they found.


Estimating the Antarctic ice sheet’s mass change

There are many ways to quantify Antarctica’s mass and mass change and most of them rely on satellites. In fact, the IMBIE team notes that there are more than 150 papers published on the topic. Their paper that we highlight this week is remarkable in that it combines all the methods in order to produce just one, easy to follow, time series of Antarctica’s mass change. But what are these methods? The IMBIE team  used estimates from three types of methods:

  •  altimetry: tracking changes in elevation of the ice sheet, e.g. to detect a thinning;
  •  gravimetry: tracking changes in the gravitational pull caused by a change in mass;
  •  input-output: comparing changes in snow accumulation and solid ice discharge.

To simplify, let’s imagine that you’re trying to keep track of how much weight you’re losing/gaining. Then  altimetry would be like looking at yourself in a mirror, gravimetry would be stepping on a scale, and input-output would be counting all the calories you’re taking in and  burning out. None of these methods will tell you directly whether you have lost belly fat, but combining them will.

The actual details of each methods are rather complex and cover more pages than the core of the paper, so I invite you to read them by yourself (from page 5 onwards). But long story short, all estimates were turned into one unique time series of ice sheet mass balance (purple line on Fig. 1). Furthermore, to understand how each region of Antarctica contributed to the time series, the scientists also produced one time series per main  Antarctic region (Fig. 2): the West Antarctic Ice Sheet (green line), the East Antarctic Ice Sheet (yellow line), and the Antarctic Peninsula (red line) .

Antarctica overview map. [Credit: NASA]

Antarctica is losing ice

The results are clear: the Antarctic ice sheet as a whole is losing mass, and this mass loss is accelerating. Nearly 3000 Giga tonnes since 1992. That is 400 billion elephants in 25 years, or on average 500 elephants per second.

Most of this signal originates from West Antarctica, with a current trend of 159 Gt (22 billion elephants) per year. And most of this West Antarctic signal comes from the Amundsen Sea sector, host notably to the infamous  Pine Island  and Thwaites Glaciers.

The Antarctic ice sheet has lost “400 billion elephants in 25 years”

But how is the ice disappearing? Rather, is the ice really disappearing, or is there simply less ice added to Antarctica than ice naturally removed, i.e. a change in surface mass balance? The IMBIE team studied this as well. And they found that there is no Antarctic ice sheet wide trend in surface mass balance; in other words Antarctica is shrinking because more and more ice is discharged into the ocean, not because it receives less snow from the atmosphere.

Floating ice shelf in the Halley embayment, East Antarctica [Credit: Céline Heuzé]

What is happening in East Antarctica?

Yet another issue with determining Antarctica’s weight loss is Glacial Isostatic Adjustment. In a nutshell, ice is heavy, and its weight pushes the ground down. When the ice disappears, the ground goes back up, but much more slowly than the rate of ice melting . This process has been ongoing in Scandinavia notably since the end of the last ice age 21 000 years ago, but it is also happening in East Antarctica by about 5 to 7 mm per year (more information here). Except that there are very few on site GPS measurements in Antarctica to determine how much land is rising, and the many estimations of this uplifting disagree.

So as summarised by the IMBIE team, we do not know yet what the change in ice thickness is where glacial isostatic adjustment is strong, because we are unsure how strong this adjustment is there. As a result in East Antarctica, we do not know whether there is ice loss or not, because it is unclear what the ground is doing.

What do we do now?

The IMBIE team concludes their paper with a list of required actions to improve the ice loss time series: more in-situ observations using airborne radars and GPS, and uninterrupted satellite observations (which we already insisted on earlier).

What about sea level rise, you may think. Or worse, looking at our image of the week, you see the tiny +6mm trend in 10 years and think that it is not much. No, it is not. But note that the trend is far from linear and has been actually accelerating in the last decades…

 

Reference/Further reading

The IMBIE Team, 2018. Mass balance of the Antarctic Ice Sheet from 1992-2017. Nature 558, 219–222.

Edited by Sophie Berger

Image of the Week – Super-cool colours of icebergs

Image of the Week – Super-cool colours of icebergs

It is Easter weekend! And as we do not want you to forget about our beloved cryosphere, we provide you with a picture nearly as colourful as the Easter eggs: very blue icebergs! What makes them so special? This is what this Image of the Week is about…


What are icebergs made of?

Fig.2: An iceberg with ‘scallop’ indentations [Credit: Stephen Warren].

Icebergs are chunks of ice which break off from land ice, such as glaciers or ice sheets (as you’ll know if you remember our previous post on icebergs). This means that they are mostly made up of glacial ice, which is frozen freshwater from accumulated snowfall. However, in some places where ice sheets extend to the coastline, making an ice shelf, icebergs can be made up of a different type of ice too.

 

Ice shelves can descend far down into the ocean. Seawater in contact with the ice at depth in the ocean is cooled to the freezing temperature. Because the freezing temperature decreases with decreasing pressure, if the seawater moves upwards in the ocean, it will have a temperature lower than the freezing temperature at that depth. That means it’s super-cooled – the seawater temperature is below the freezing temperature, but it hasn’t become a solid. The seawater cannot last for long in this state and freezes to the base of ice shelves as marine ice, which is seawater frozen at depth. The marine ice can help stabilize the ice shelf as it is less susceptible to fractures than glacial ice. Icebergs that calve from Antarctic ice shelves can sometimes be mixtures of glacial ice (on the top) and marine ice (on the bottom).

 

What can icebergs tell us?

Icebergs which tip over can tell us about processes that happen at the base of ice shelves. For example, scallops on the ice (the small indentations that can be seen in the second picture) can show the size of turbulent ocean eddies in the ocean at the ice shelf base. Basal cavities or channels show where oceanic melt had a large impact. Any colours visible in the iceberg can also give us information.

Fig.3: Marine ice containing organic matter, giving a greenish appearance [Credit: Stephen Warren].

Why are icebergs different colours?

Like snow (see this previous post), different types of ice appear different colours. A typical iceberg is white because it is covered with dense snow, and snowflakes reflect all wavelengths of ice equally. The albedo of snow, which is the proportion of the incident light or radiation that is reflected by a surface, is very high (nearly 1). Glacial ice is compressed snow, meaning it has fewer light-scattering air bubbles, so light can penetrate deeper than in snow, and more yellows and reds from the visible spectrum are absorbed. This results in a bubbly blue colour, with a slightly lower albedo than snow. Marine ice does not have bubbles, but light can be scattered by cracks, resulting in clear blue ice (see our Image of the Week). However, if the seawater from which the marine ice was formed contained organic matter, like algae and plankton, the resulting marine ice can have a yellowish or even green appearance (Fig. 3). If the marine ice formed near the base of an ice shelf where it meets the sea floor, it could contain sediment, giving it a dirty or black appearance.

So the colour of icebergs can tell us something about how ice was formed hundreds of metres below the ocean surface. You could even say that was super-cool…

Further reading

  • Warren, S. G., C. S. Roesler, V. I. Morgan, R. E. Brandt, I. D. Goodwin, and I. Allison (1993), Green icebergs formed by freezing of organic-rich seawater to the base of Antarctic ice shelves, J. Geophys. Res., 98(C4), 6921–6928, doi:10.1029/92JC02751.
  • Morozov, E.G., Marchenko, A.V. & Fomin, Y.V. Izv. (2015): Supercooled water near the Glacier front in Spitsbergen, Atmos. Ocean. Phys. 51(2), 203-207. https://doi.org/10.1134/S0001433815020115
  • Image of the Week – Ice Ice Bergy
  • Image of the Week – Fifty shades of snow

This post is based on a talk by Stephen Warren presented at AMOS-ICSHMO2018

Edited by Clara Burgard


Lettie Roach is a PhD student at Victoria University of Wellington and the National Institute for Water and Atmospheric Research in New Zealand. Her project is on the representation of sea ice in large-scale models, including model development, model-observation comparisons and observation of small-scale sea ice processes.  

 

Image of the Week – Geothermal heat flux in Antarctica: do we really know anything?

Spatial distributions of geothermal heat flux: (A) Pollard et al. (2005) constant values, (B) Shapiro and Ritzwoller (2004): seismic model, (C) Fox Maule et al. (2005): magnetic measurements, (D) Purucker (2013): magnetic measurements, (E) An et al. (2015): seismic model and (F) Martos et al. (2017): high resolution magnetic measurements. The color scale is truncated at 30 and 80 mW m-2. The black line locates the grounding line. Note, (B)-(F) are in order of publication from oldest to most recent. [Credit: Brice Van Liefferinge, (2018), PhD thesis]

Geothermal heat flux is the major unknown when we evaluate the temperature and the presence/absence of water at the bed of the Antarctic Ice Sheet. This information is crucial for the Beyond Epica Oldest Ice project, which aims to find a continuous ice core spanning 1.5 million years (see this previous post). A lot of work has been done* to determine geothermal heat flux under the entire Antarctic Ice Sheet, and all conclude that additional direct measurements are necessary to refine basal conditions! However direct measurements are difficult to obtain, due to the thick layer of ice that covers the bedrock. Our new image of the week goes over what we currently know about the geothermal heat flux in Antarctica and presents the five data sets that currently exist. But first, let’s see where this heat flux come from?


What determines geothermal heat flux and how can we estimate it?

Heat flux measured at the surface of the Earth has two sources: (i) primordial heat remaining from when the Earth formed and (ii) contemporary-sourced heat coming from radioactive isotopes present in the mantle and the crust. This heat, concentrated in the Earth’s centre, can propagate to the surface through both conduction in the solid earth (inner core and crust) and convection in the liquid-viscous earth (outer core, lower and upper mantles). The net heat flux to reach the surface of the crust and penetrate the overlying ice is what we refer to as the ‘geothermal heat flux’. Wherever the crust is thinner, convection in the mantle can transfer heat more efficiently to the surface. In those locations, the net geothermal heat flux is higher, and vice versa. At mid-ocean ridges and in active volcanic areas, the heat can be delivered almost directly to the surface by advection (i.e. by the movement of magma), therefore leading to a higher net surface geothermal heat flux (think of Iceland, where the shallow crust allows them to take advantage of geothermal heat flux directly).

As a result, we know that the geology determines the magnitude of the geothermal heat flux and the geology is not homogeneous underneath the Antarctic Ice Sheet:  West Antarctica and East Antarctica are significantly distinct in their crustal rock formation processes and ages.

Nowadays, five independent global geothermal heat flux data sets exist: Shapiro and Ritzwoller, (2004); Fox Maule et al., (2005); Purucker, (2013); An et al., (2015); Martos et al., (2017) (see image of the week). All geothermal heat flux data sets compiled and currently used have been inferred from the properties of the crust and the upper mantle, as geology dictates the magnitude of geothermal heat flux spatially. Let’s see together how the estimation of geothermal heat flux has evolved over the years….

Using constant values (Panel A)

The simplest method, which consists in using a constant value of geothermal heat flux over the entire continent, was common at first and is still sometimes used (e.g. sensitivity tests and model intercomparison projects) as it facilitates model inter-comparisons. Pollard et al. (2005), in panel A, used bands of constant geothermal heat flux values (70, 60, 55 and 41 mW m-2), with geothermal heat flux decreasing from West Antarctica to East Antarctica, consistent with the known geology.

2004, a seismic model (Panel B)

Shapiro and Ritzwoller (2004) are the first to propose a geothermal heat flux distribution map based on seismic methods, and not strictly on rock composition. They extrapolate the geothermal heat flux from a global seismic model of the crust and the upper mantle which is an analysis of seismicity all over the world. Regions of the globe are grouped by their similarity in seismic structure. Assuming that a certain magnitude of seismicity represents a certain geothermal heat flux value, they assign geothermal heat flux value to regions where geothermal heat flux cannot be directly measured by using geothermal heat flux data from regions of similar seismicity. The geothermal heat flux spatial distribution obtained, with values up to 80 mW m-2 in West Antarctica and 48 mW m-2 in East Antarctica, agrees with that of Pollard et al. (2005). However, errors associated with this method are quite large, reaching 50% of the geothermal heat flux value.

 

2005, magnetic measurements (Panel C)

A year later, Fox Maule et al. (2005) derive a geothermal heat flux map based on satellite magnetic measurements and a thermal model. The objective is to determine the depth to the Curie temperature, the temperature at which a material loses its permanent magnetic properties. They set the Curie temperature to 580 °C, while the temperature at the ice-bedrock interface is set at 0 °C. Satellite magnetic measurements allow the calculation of the depth of each of these boundaries. The geothermal heat flux is then obtained using a thermal model of the crust between the depth of the two boundary temperatures. This method also has a large associated error, 60% of the geothermal heat flux value for the East Antarctic interior.

2013, reanalysis of magnetic measurements (Panel D)

In 2013, Purucker updates the Fox Maule et al. (2005) geothermal heat flux map with new magnetic data. The spatial geothermal heat flux pattern obtained still retains the characteristic pattern of low values in West Antarctica and high values in East Antarctica, but predicts lower absolute values for East Antarctica and around the West Antarctic coast.

2015, new seismic model (Panel E)

More recently, An et al. (2015) derive a new geothermal heat flux distribution based on seismic velocities. The method is similar to that used by Shapiro and Ritzwoller (2004). They analyse the Earth’s mantle properties using a new 3D crustal shear velocity model to calculate crustal temperatures and the surface geothermal heat flux. However, their spatial distribution of geothermal heat flux differs quite a bit from the other data sets, particularly in East Antarctica where geothermal heat flux values differ by 10 mW m-2 from those of Shapiro and Ritzwoller (2004). An et al. (2015) find very low geothermal heat flux values at the domes, which is good news for the search of Oldest Ice, but rather high overall values for East Antarctica compared to the other data sets. They explain that the model is invalid for geothermal heat flux values exceeding 90 mW m-2. But such high values should only impact young crust areas, mainly West Antarctica and therefore the variability observed in East Antarctica cannot be explained.

2017, high resolution magnetic measurements (Panel F)

In 2017, Martos et al. provide a high resolution geothermal heat flux map based on the spectral analysis of airborne magnetic data. They use a compilation of all existing airborne magnetic data to determine the depth to the Curie temperature and infer the geothermal heat flux using a thermal model. Their continent-wide spatial distribution of geothermal heat flux obtained agrees with previous studies, but they show higher overall magnitudes of geothermal heat flux including East Antarctica. They report an error of 10 mW m-2 which is interestingly smaller than for the other data sets. However, their data set does not take into account point measurements of geothermal heat flux. The same year, Goodge (2017) calculates an average geothermal heat flux value of 48 mW m-2 for East Antarctica with a standard deviation of 13.6 mW m from the analysis of clasts in the region between Dome A and the Ross Sea. A geothermal heat flux value of 48 mW m-2 is consistent with the mean value of the data sets described above.

All in all

To sum up, although all geothermal heat flux data sets agree on continent scales (with higher values under the West Antarctic ice sheet and lower values under East Antarctica), there is a lot of variability in the predicted geothermal heat flux from one data set to the next on smaller scales. A lot of work remains to be done …

* (e.g. Shapiro and Ritzwoller, 2004; Fox Maule et al., 2005; Purucker, 2013; An et al., 2015; Fisher et al., 2015; Parrenin et al., 2017; Seroussi et al., 2017; Martos et al., 2017; Goodge, 2017)

References

Van Liefferinge, B., Pattyn, F., Cavitte, M. G. P., Karlsson, N. B., Young, D. A., Sutter, J., and Eisen, O.: Promising Oldest Ice sites in East Antarctica based on thermodynamical modelling, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-276, in review, 2018.

Van Liefferinge, B. Thermal state uncertainty assessment of glaciers and ice sheets: Detecting promising Oldest Ice sites in Antarctica, PhD thesis, Université libre de Bruxelles, Brussels, 2018.

Edited by Sophie Berger


Brice Van Liefferinge  has just earned his PhD at the Laboratoire de Glaciology, Universite Libre de Bruxelles, Belgium. His research focuses on the basal conditions of the ice sheets. He tweets as @bvlieffe.

Image of the Week – A Hole-y Occurrence, the reappearance of the Weddell Polynya

Image of the Week – A Hole-y Occurrence, the reappearance of the Weddell Polynya

During both the austral winters of 2016 and 2017, a famous feature of the Antarctic sea-ice cover was observed once again, 40 years after its first observed occurrence: the Weddell Polynya! The sea-ice cover exhibited a huge hole (of around 2600 km2 up to 80,000 km2 at its peak!), as shown on our Image of the Week. What makes this event so unique and special?


Why does the Weddell Polynya form?

The Weddell Polynya is an open ocean polynya (a large hole in the sea ice, see this previous post), observed in the Weddell Sea (see Fig.2). It was first observed in the 1970s but then did not form for a very long time, until 2016 and 2017…

 

Fig. 2: Map of the sea ice distribution around Antarctica on 25th of September 2017, derived from satellite data. The red circle marks the actual Weddell Polynya [Credit: Modified from meereisportal.de]

In the Southern Ocean, warm saline water masses underlie cold, fresh surface water masses. The upper cold fresh layer acts like a lid, insulating the warmer deep waters from the cold atmosphere. While coastal polynyas (see this previous post) are caused by coastal winds, open ocean polynyas are more mysteriously formed as it is not as clear what causes the warm deep water to be mixed upwards. In the case of the Weddell polynya, it forms above an underwater mountain range, the Maud Rise. This ridge is an obstacle to the water flow and can therefore enhance vertical mixing of the deeper warm saline water masses. The warm water that reaches the surface melts any overlying sea ice, and large amounts of heat is lost from the ocean surface to the atmosphere (see Fig. 3).

 

Fig. 3: Schematic of polynya formation. The Weddell polynya is an open ocean polynya [Credit: National Snow and Ice Data Center].

 

Why do we care about the Weddell Polynya?

Overturning and mixing of the water column in the Weddell Polynya forms cold, dense Antarctic Bottom Water, releasing heat stored in the ocean to the atmosphere in the process. Antarctic Bottom Water is formed in the Southern Ocean (predominantly in the Ross and Weddell Seas) and flows northwards, forming the lower branch of the overturning circulation which transports heat from the equator to the poles (see Fig. 4). Antarctic Bottom Water also carries oxygen to the rest of the Earth’s deep oceans. The absence of the Weddell polynya could reduce the formation rate of Antarctic Bottom water, which could weaken the lower branch of the overturning circulation.

Fig.4: Schematic of the overturning (thermohaline) circulation. Deep water formation sites are marked by yellow ovals. Modified from: Rahmstorf, 2002 [©Springer Nature. Used with permission.]

How often does the Weddell Polynya form?

The last time the Weddell Polynya was observed was during the austral winters of 1974 to 1976 (see Fig. 5). It was then absent for nearly 40 years (!) up until austral winter 2016. In a modelling study, de Lavergne et al. 2014 suggested that the Weddell Polynya used to be more common before anthropogenic CO2 emissions started rising at a fast pace. The increased surface freshwater input from melting glaciers and ice sheets, and increased precipitation (as climate change increases the hydrological cycle) have freshened the surface ocean. This freshwater acts again as a lid on top of the warm deeper waters, preventing open ocean convection, reducing the production of Antarctic Bottom Water.

Fig. 5: Color-coded sea ice concentration maps derived from passive microwave satellite data in the Weddell Sea region from the 1970s. The Weddell Polynya is the extensive area of open water (in blue) [Credit: Gordon et al., 2007, ©American Meteorological Society. Used with permission.].

The reappearance of the Weddell Polynya over the past two winters despite the increased surface freshwater input suggests that other natural sources of variability may be currently masking this predicted trend towards less open ocean deep convection. Latif et al. 2013 put forward a theory describing centennial scale variability of Weddell Sea open ocean deep convection, as seen in climate models. In this theory, there are two modes of operation, one where there is no open ocean convection and the Weddell Polynya is not present. In this situation, sea surface temperatures are cold and the deep ocean is warm, and there is relatively large amount of sea ice. The heat at depth increases with time, as it is insulated by the sea ice and freshwater lid. Then, eventually, the deep water becomes warm enough that the stratification is decreased sufficiently so that open water convection begins again, forming the Weddell Polynya. This process continues until the heat reservoir depletes and surface freshwater forcing switches off the deep convection. Models show that the timescale of this variability is set by the stratification, and models with stronger stratification tend to vary on longer timescale, as the heat needs to build up more in order to overcome the stratification.

 

In the end, the Weddell Polynya is still surrounded by some mystery… Only the next decades will bring us more insight into the true reasons for the appearance and disappearance of the Weddell Polynya…

 

Further reading

Edited by Clara Burgard


Rebecca Frew is a PhD student at the University of Reading (UK). She investigates the importance of feedbacks between the sea ice, atmosphere and ocean for the Antarctic sea ice cover using a hierarchy of climate models. In particular, she is looking at the how the importance of different feedbacks may vary between different regions of the Southern Ocean.
Contact: r.frew@pgr.reading.ac.uk

Image of the Week – Vibrating Ice Shelf!

Image of the Week – Vibrating Ice Shelf!

If you listen carefully to the Ekström ice shelf in Antarctica, a strange sound can be heard! The sound of a vibrating truck sending sounds waves into the ice. These sound waves are used to “look” through the ice and create a seismic profile of what lies beneath the ice surface. Read on to find out how the technique works and for a special Cryosphere Christmas message!


What are we doing with this vibrating truck on an ice shelf?

In early December a team from the Alfred Wegener Institute (AWI) made a science traverse of the Ekström ice shelf, near the German Neumayer III Station. Their aim was to make a seismic survey of the area. The seismic source (sound source) used to make this survey was a vibrating truck, known as a Vibroseis source (Fig. 2).

Fig. 2: The Vibroseis truck. It is attached to a “poly-sled” so that it can be easily towed across the ice shelf. The vibrating plate can be seen suspended below the centre of the truck. [Credit: Judith Neunhaeuserer]

It has a round metal plate, which is lowered onto the ice-shelf surface and vibrates at a range of frequencies, sending sound waves into the ice. When the snow is soft the plate often sinks a little, leaving a rather strange “footprint” in the snow (Fig. 3).

Fig. 3: The “footprint” of the Vibroseis truck plate in the snow [Credit: Olaf Eisen].

The sound waves generated travel through the ice shelf, through the water underneath and into the rock and sediment of the sea floor, they are reflected back off these different layer and these reflections are recorded back on the ice surface by a string of recording instruments – geophones (Fig 1). There are sixty geophones in a long string, a snow streamer, which can be towed behind the truck as it moves from location to location. By analyzing how long it takes the sound waves to travel from the source to the geophones an “image” of the structures beneath the ice can be made. For example, you can see a reflection from the bottom of the ice shelf and from the sea floor as well as different layers of rock and sediment beneath the sea floor. This allows the team to look into the geological and glaciological history of the area, as well as understand current glaciology and oceanographic processes!

 

As it happens, the team from AWI consists of your very own EGU Cryosphere Division President, Olaf Eisen and ECS Rep, Emma Smith! As this is the last post before Christmas, we wanted to wish you a merry Christmas from Antarctica!

Merry Christmas! As you can see the weather is beautiful here! [Credit: Jan-Marcus Nasse]

Edited by Sophie Berger