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Cryospheric Sciences

climate change

Image of the week – Skiing, a myth for our grandchildren?

Image of the week – Skiing, a myth for our grandchildren?

Ski or water ski? Carnival season is typically when many drive straight to the mountains to indulge in their favorite winter sport. However, by the end of the century, models seem to predict a very different future for Carnival, with a drastic reduction in the number of snow days we get per year. This could render winter skiing something of the past, a bedtime story we tell our grandchildren at night…


Christoph Marty and colleagues investigated two Swiss regions reputed for their great skiing resorts and show that the number of snow days (defined as a day with at least 5 cm of snow on the ground) could go down to zero by 2100, if fuel emissions and economic growth continue at present-day levels, and this scenario is less dramatic than the IPCC’s most pessimistic climate change scenario (Marty et al., 2017). They show that temperature change will have the strongest influence on snow cover. Using snow depth as representative for snow volume, they predict that snow depth maxima will all be lower than today’s except for snow at elevations of 3000 m and higher. This implies that even industrially-sized stations like Avoriaz in the French Alps, with a top elevation of 2466 m, will soon suffer from very short ski seasons.

Marty et al. (2017) predict a 70% reduction in total snow volume by 2100 for the two Swiss regions, with no snow left for elevations below 500 m and only 50% snow volume left above 3000 m. Only in an intervention-type scenario where global temperatures are restricted to a warming of 2ºC since the pre-industrial period, can we expect snow reduction to be limited to 30% after the middle of the century.

Recent work by Raftery et al (2017) shows that a 2ºC warming threshold is likely beyond our reach, however limiting global temperature rise, even above the 2ºC target, could help stabilize snow volume loss over the next century. We hold our future in our hands!

Further reading/references

  • Marty, C., Schlögl, S., Bavay, M. and Lehning, M., 2017. How much can we save? Impact of different emission scenarios on future snow cover in the Alps. The Cryosphere, 11(1), p.517.
  • Raftery, A.E., Zimmer, A., Frierson, D.M., Startz, R. and Liu, P., 2017. Less than 2 C warming by 2100 unlikely. Nature Climate Change, 7(9), p.637.
  • Less snow and a shorter ski season in the Alps | EGU Press release

Edited by Sophie Berger


Marie Cavitte just finished her PhD at the University of Texas at Austin, Institute for Geophysics (USA) where she studied the paleo history of East Antarctica’s interior using airborne radar isochrone data. She is involved in the Beyond EPICA Oldest Ice European project to recover 1.5 million-year-old ice. She tweets as @mariecavitte.

Image of the Week – Arctic changes in a warming climate

Image of the Week – Arctic changes in a warming climate

The Arctic is changing rapidly and nothing indicates a slowdown of these changes in the current context. The Snow, Water, Ice and Permafrost in the Arctic (SWIPA) report published by the Arctic Monitoring and Assessment Program (AMAP) describes the present situation and the future evolution of the Arctic, the local and global implications, and mitigation and adaptation measures. The report is based on research conducted between 2010 and 2016 by an international group of over 90 scientists, experts, and members of Arctic indigenous communities. As such, the SWIPA report is an IPCC-like assessment focussing on the Arctic. Our Image of the Week summarizes the main changes currently happening in the Arctic regions.


What is happening to Arctic climate currently?

The SWIPA report confirms that the Arctic is warming much faster than the rest of the world, i.e. more than twice the global average for the past 50 years (Fig. 2). For example, Arctic surface air temperature in January 2016 was 5°C higher than the average over 1981-2010. This Arctic amplification is due to a variety of climate feedbacks, which amplify the current warming beyond the effects caused by increasing greenhouse gas concentrations alone (see the SWIPA report, Pithan & Mauritsen (2014) and this previous post for further information).

Fig.2: Anomaly of Arctic and global annual surface air temperatures relative to 1981-2010 [Credit: Fig. 2.2 of AMAP (2017), revised from NOAA (2015)].

This fast Arctic warming has led to the decline of the ice cover over both the Arctic Ocean (sea ice) and land (Greenland Ice Sheet and Arctic glaciers).

For sea ice, not only the extent has dramatically decreased over the past decades (see Stroeve et al. 2012 and Fig. 3), but also the thickness (see Lindsay & Schweiger, 2015). Most Arctic sea ice is now first-year ice, which means that it grows in autumn-winter and melts completely during the following spring-summer. In contrast, the multiyear sea-ice cover, which is ice that has survived several summers, is rapidly disappearing.

Fig. 3: Arctic sea-ice extent in March and September from the National Snow and Ice Data Center (NSIDC) and the Ocean and Sea Ice Satellite Application Facility (OSI SAF) [Credit: Fig. 5.1 of AMAP (2017)].

In terms of land ice, the ice loss from the Greenland Ice Sheet and Arctic glaciers has been accelerating in the recent decades, contributing a third of the observed global sea-level rise. Another third comes from ocean thermal expansion, and the remainder comes from the Antarctic Ice Sheet, other glaciers around the world, and terrestrial storage (Fig. 4, see also this previous post and Chapter 13 of the last IPCC report).

Fig. 4: Global sea-level rise contribution from the Arctic components (left bar), Antarctic Ice Sheet and other glaciers (middle-left bar), terrestrial storage (middle-right bar) and ocean thermal expansion (right bar) [Credit: Fig. 9.3 of AMAP (2017)].

Besides contributing to rising sea levels, land-ice loss releases freshwater into the Arctic Ocean. Compared with the 1980-2000 average, the freshwater volume in the upper layers of the Arctic Ocean has increased by more than 11%. This could potentially affect the ocean circulation in the North Atlantic through changes in salinity (see this previous post).

Other changes currently occurring in the Arctic include the decreasing snow cover, thawing permafrost, and ecosystem modifications (e.g. occurrence of algal blooms, species migrations, changing vegetation, and coastal erosion). You can have a look at the main Arctic changes in our Image of the Week.

 

Where are we going?

The SWIPA report highlights that the warming trends in the Arctic will continue, even if drastic greenhouse gas emission cuts are achieved in the near future. For example, mean Arctic autumn and winter temperatures will increase by about 4°C in 2040 compared to the average over 1981-2005 according to model projections (Fig. 5, right panel). This corresponds to twice the increase in projected temperature for the Northern Hemisphere (Fig. 5, left panel).

Fig. 5: Autumn-winter (NDJFM) temperature changes for the Northern Hemisphere (left) and the Arctic only (right) based on 36 global climate models, relative to 1981-2005, for two emission scenarios [Credit: Fig. 2.15 of AMAP (2017)].

This Arctic amplification leads to four main impacts:

  1. The Arctic Ocean could be ice-free in summer by the late 2030s based on extrapolated observation data. This is much earlier than projected by global climate models.

  2. Permafrost extent is projected to decrease substantially during the 21st Century. This would release large amounts of methane in the atmosphere, which is a much more powerful greenhouse gas than carbon dioxide.

  3. Mean precipitation and daily precipitation extremes will increase in a warming Arctic.

  4. Global sea level will continue to rise due to melting from ice sheets and glaciers, ocean thermal expansion, and changes in terrestrial storage. However, uncertainties remain regarding the magnitude of the changes, which is linked to the different emission scenarios and the type of model used.

What are the implications?

A potential economic benefit to the loss of Arctic sea ice, especially in summer, is the creation of new shipping routes and access to untapped oil and gas resources. However, besides this short-term positive aspect of Arctic changes, many socio-economic and environmental drawbacks exist.

The number of hazards has been rising due to Arctic changes, including coastal flooding and erosion, damage to buildings, risks of avalanches and floods from rapid Arctic glacier melting, wildfires, and landslides related to thawing permafrost. Furthermore, Arctic changes (especially sea-ice loss) may also impact the climate at mid-latitudes, although many uncertainties exist regarding these possible links (see Cohen et al., 2014).

What can we do?

The SWIPA report identifies four action steps:

  1. Mitigating climate change by decreasing greenhouse gas emissions. Implementing the Paris Agreement would allow stabilizing the Arctic temperatures at 5-9°C above the 1986-2005 average in the latter half of this century. This would also reduce the associated changes identified on our Image of the Week. However, it is recognized that even if we implement the Paris Agreement, the Arctic environment of 2100 would be substantially different than that of today.

  2. Adapting to impacts caused by Arctic changes.

  3. Advancing our understanding of Arctic changes through international collaboration, exchange of knowledge between scientists and the general public, and engagement with stakeholders.

  4. Raising public awareness by sharing information about Arctic changes.

Further reading

Edited by Scott Watson and Clara Burgard


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

Image of the Week – Understanding Antarctic Sea Ice Expansion

Fig. 1: Average monthly Antarctic sea ice extent time series in black, with the small increasing trend in blue. [Credit: NSIDC]

Sea ice is an extremely sensitive indicator of climate change. Arctic sea ice has been dubbed ‘the canary in the coal mine’, due to the observed steady decline in the summer sea ice extent in response to global warming over recent decades (see this and this previous posts). However, the story has not been mirrored at the other pole. As shown in our image of the week (blue line in Fig. 1), Antarctic sea ice has actually been expanding slightly overall!


The net expansion is the result of opposing regional trends

The small increasing trend in Antarctic sea-ice extent is the sum of opposing regional trends (click here for definitions of area, concentration and extent). Sea ice in the Weddell and Ross seas has expanded whereas in the Amundsen and Bellingshausen (A-B) seas the sea-ice cover has diminished (Holland 2014). The size of these trends varies with the seasons (Fig. 2). There are no significant trends in ice concentration – the fraction of a chosen area/grid box that is sea ice covered — if you look at (Southern hemisphere) winter values, however we do see trends when looking at a time series of summer values. The differences in trends between seasons suggests interactions with atmosphere and ocean (feedbacks) that amplify (in the spring) and dampen (in the autumn) changes in the ice cover, creating this seasonality. Some of this variability can be explained by changes in the winds (Holland and Kwok, 2012). But the complexity of the trends can’t be explained by one single change in forcing (e.g. winds, snowfall or temperature) or a single process (e.g. ice albedo feedback acting in the spring/summer).

 

Fig. 2: Seasonal trend in ice concentration. Maximum trends are seen in summer. Large increases are seen in the Weddell and Ross seas, and decreases in the Amundsen and Bellingshausen (A-B) seas. [Credit: Fig 2 from Holland (2014). , reprinted with permission by Wiley and Sons].

Why hasn’t Antarctic sea ice extent been decreasing?

There is no clear consensus on this. In short, we don’t really know… It is not as intuitive as the ‘warmer climate results in less ice’ narrative for the Arctic. We only have a time series of Antarctic sea ice extent from 1979 (the start of satellite observations). We therefore can’t be sure what role natural variability is having on decadal and longer timescales, i.e. if this is just natural ups and downs or an “unusual” trend related to climate change. Another difficulty is that we don’t have a reliable time series of sea ice volume as we have difficulties in getting reliable sea ice thickness measurements, because of the thick snow covering on sea ice in the Southern Ocean. For example, it could be that the ice is becoming thinner although the sea-ice area has increased.

There are important processes and/or feedbacks between sea ice and ocean or between sea ice and atmosphere that we are missing from our models

Currently, global climate models are poor at reproducing the observed Antarctic sea ice changes (Turner et al. 2013). Models simulate a decrease in the overall sea ice extent, instead of the observed increase. They also fail to reproduce the correct spatial variations, as shown in Fig. 2. This makes it very hard to make predictions about future changes in Antarctic sea ice from model results, and implies that there are important processes and/or feedbacks between sea ice and ocean or between sea ice and atmosphere that we are missing from our models, and therefore our understanding of the Southern Ocean climate system is incomplete.

 

However, there are some suggestions as to processes that could explain some of the observed Antarctic sea ice variability. The largely fall into two main categories: natural variability and anthropogenic changes.

 

1.Natural Variability

Natural variability refers to the repeating oscillations and patterns we see in the climate system. Some of these repeating patterns can be correlated with increases/decreases in Antarctic sea ice. In particular El Nino Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) have been linked to Antarctic sea ice changes. The SAM is a measure of the difference in pressure between 40°S and 65°S, a positive SAM indicates a stronger difference in pressure, driving stronger westerly winds around Antarctica, increasing the thermal isolation of Antarctica. Stronger westerlies are associated with cooler sea surface temperatures and expansion of the sea ice cover on short  timescales (seasons to years).

The SAM has been in a mostly positive phase since the mid-1990s, so is believed may have something to do with some of the small increase in sea ice extent we have seen. However, variability on longer time scales (decades or longer) could also explain some of the small increase, but this is tricky to assess without a longer observational time series.

 

2. Anthropogenic Changes

The main two human-induced changes on the Antarctic climate system are the ozone hole and increased melting of the Antarctic ice sheet.

  • Ozone hole
    The ozone hole causes the westerly winds to strengthen, making the sea ice cover expand. However it is more complicated than this, as the impact on the sea ice may depend on what timescale we look at. Over longer timescales (years to decades) the initial response may be outweighed by an increase in ocean upwelling (due to the stronger winds). This brings warm water from below the cold surface layer up to the surface, melting the sea ice from below, eventually resulting in a net sea ice area decrease in response to the ozone hole. See Ferreira et al. (2015) for details.
  • Increased melting of the Antarctic ice sheet
    This could also play a role in the observed sea ice expansion, by increasing the ocean stratification. This results in a cooler and fresher surface layer, favouring the growth of sea ice (Bintanja et al. 2015).

 

It is very tricky to distinguish what is natural variability, what is human induced, or a complicated combination of two.

 

It is very tricky to distinguish what is natural variability, what is human induced, or a complicated combination of two. This means we don’t really know whether the observed large decrease in Antarctic sea ice extent seen in 2016/2017 (read more about it here) is just an anomaly or the start of a decreasing trend. So, in summary Antarctic sea ice is confusing, and we still can’t claim to completely understand observed variability. But this makes it interesting and means there is still a wealth of secrets left to be discovered about Antarctic sea ice!

 

Further reading

 

Edited by Clara Burgard et Sophie Berger


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 – Ice Stupas: a solution for Himalayan water shortage?

Image of the Week – Ice Stupas: a solution for Himalayan water shortage?

As the world searches for practical innovations that can mitigate the impact of climate change, traditional methods of environmental management can offer inspiration. In Hindu Kush and Karakoram region, local people have been growing, or grafting, glaciers for at least 100 years. Legend has it that artificial glaciers were grown in mountain passes as early as the twelfth century to block the advance of Genghis Khan and the Mongols!


 

What exactly are we talking about?

People in the Himalayas need water for the irrigation of their crops. Naturally, they get this water during the melting period of local glaciers. Glacial melt, however, is insufficient to satisfy the demands in early spring (April-June). Artificial ice structures can increase the availability of water for crop irrigation during this period. They are grown during the winter season preventing the water to waste away into the ocean. The Ice Stupa project is bringing these practices back from the realm of folklore for the everyday use of mountain farmers again.

 

How it works

An artificial glacier is built following a simple technique. Water is piped away from high altitude reservoirs (glacial lakes or streams) in winter. Further downstream, the water is allowed to “leave” the pipe. Due to gravity, the pressure that has built up on the way forces the water to leave the pipe as a water fountain. In contact with subzero temperatures, the water fountain freezes, building a huge cone of ice. In its final form, this artificial glacier looks like a traditional buddhist building, hence the name “stupa“. In the following video, you can get a better visual idea of the process!

 

 

An ice stupa is needed for crop irrigation. The water contained in the stupa should therefore also be released during the right time of the year. To this purpose, it is also designed to conserve water in ice form as long into the summer as possible. It can then, as it melts, provide irrigation to the fields until the real glacial melt waters are sufficient in June. Since these ice cones extend vertically upwards towards the sun, they receive less of the sun’s energy per unit of volume of water stored. Hence, they will take much longer to melt compared to an artificial glacier of the same volume formed horizontally on a flat surface.

 

Further reading

Edited by Clara Burgard


Suryanarayanan Balasubramanian is a mathematician who has been managing the Ice Stupa Project for the past three years. He studies the life cycle of Ice Stupas through field measurements and dynamic models. He is currently developing the project in Peru, Switzerland, Nepal and India. Contact Email: gayashiva91@gmail.com

Image of the Week — Climate change and disappearing ice

The first week of the Climate Change summit in Bonn (COP 23  for those in the know) has been marked by Syria’s decision to sign the Paris Accord, the international agreement that aims at tackling climate change. This decision means that the United States would become the only country outside the agreement if it were to complete the withdrawal process vowed by President Trump.

In this context, it has become a tradition for this blog to use the  United Nations climate talks as an excuse to remind us all of some basic facts about climate change and its effect on the part we are most interested in here: the cryosphere! This year we have decided to showcase a few compelling animations, as we say “a picture is sometimes worth a thousand words”…


Arctic sea ice volume

Daily Arctic sea ice volume is estimated by the PIOMAS reconstruction from 1979-present [Credit: Ed Hawkins]

The volume of Arctic sea ice has declined over the last 4 decades and reached a record low in September 2012. Shrinking sea ice has major consequences on the climate system: by decreasing the albedo of the Arctic surface, by affecting the global ocean circulation, etc.

More information about Arctic sea ice:

Land ice losses in Antarctica and Greenland

Change in land ice mass since 2002 (Right: Greenland, Left: Antarctica). Data is measured by NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites. [Credit: Zack Labe]

Both the Antarctic and Greenland ice sheets have been losing ice since 2002, contributing to global sea-level rise (see previous post about sea level) .  An ice loss of 100 Gt raises the  sea level by ~0.28 mm (see explanations  here).

More information about ice loss from the ice sheets:

 

The cause: CO2 emissions and global warming

Finally we could not close this post without showing  how the concentration of carbon dioxide have evolved  over the same period and how this has led to global warming.

CO₂ concentration and global mean temperature 1958 – present. [Credit:Kevin Pluck]

More information about CO2 and temperature change

  • Global Temperature | NASA: Climate Change and Global Warming
  • Carbon dioxide | NASA: Climate Change and Global Warming

More visualisation resources

Visualisation resources | Climate lab

 

Edited by Clara Burgard

Image of the Week — High altitudes slow down Antarctica’s warming

Elevations in Antarctica. The average altitude is about 2,500m. [Credit: subset of Fig 5 from Helm et al (2014)]

When it comes to climate change, the Arctic and the Antarctic are poles apart. At the north of the planet, temperatures are increasing twice as fast as in the rest of the globe, while warming in Antarctica has been milder. A recent study published in Earth System Dynamics shows that the high elevation of Antarctica might help explain why the two poles are warming at different speeds.


The Arctic vs the Antarctic

At and around the North Pole, in the Arctic, the ice is mostly frozen ocean water, also known as sea ice, which is only a few meters thick. In the Antarctic, however, the situation is very different: the ice forms not just over sea, but over a continental land mass with rugged terrain and high mountains. The average height in Antarctica is about 2,500 metres, with some mountains rising as high as 4,900 metres.

A flat Antarctica would warm faster

Mount Jackson in the Antarctic Peninsula reaches an altitude of 3,184 m  [Credit: euphro via Flickr]

Marc Salzmann, a scientist working at the University of Leipzig in Germany, decided to use a computer model to find out what would happen if the elevation in Antarctica was more like in the Arctic. He discovered that, if Antarctica were flat, there would be more warm air flowing from the equator to the poles, which would make the Antarctic warm faster.

As Antarctica warms and ice melts, it is actually getting flatter as time goes by, even if very slowly. As such, over the next few centuries or thousands of years, we could expect warming in the region to speed up.

Reference/further reading

planet_pressThis is modified version of a “planet press” article written by Bárbara Ferreira and originally published on 18 May 2017 on the EGU website
(a Serbian version is also available, why not considering adding a new language to the list? 🙂 )

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

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

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

Not all glaciers behave alike

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

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

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

A kinematic wave of thinning

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

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

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

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

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

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

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

Further reading

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

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

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


Simulating the climate with a regional model

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

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

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

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

From 11 km to 1 km : downscaling RACMO2.3

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

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

 

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

Endangered peripheral ice caps

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

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

 

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

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

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

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

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

What about the Greenland ice sheet?

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

Data availability

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

Further reading

Edited by Sophie Berger


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

Image of The Week – Ice Flows!

Image of The Week – Ice Flows!

Portraying ice sheets and shelves to the general public can be tricky. They are in remote locations, meaning the majority of people will never have seen them. They also change over timescales that are often hard to represent without showing dramatic images of more unusual events such as the collapse of the Larsen B Ice Shelf.  However, an app launched in the summer at the SCAR (Scientific Committee for Antarctic Research) Open Science Conference in Kuala Lumpur set out to change this through a game. Developed by Anne Le Brocq from the University of Exeter, this game is aptly named – Ice Flows!


The game in a nutshell!

Ice Flows is a game that allows the player to control various variables of an ice shelf (floating portion of an ice sheet) environment, such as ocean temperature and snowfall, and see the changes that these cause. For example, increasing the amount of snowfall increases the ice thickness but increasing the ocean temperature causes thinning of the ice shelf. The aim of the game is to help penguins feed by altering the variables to create ice shelf conditions which give them access to the ocean. Although the game is based around penguins, importantly, it is changing the ice shelf environment that the player controls, this allows a player to investigate how changing environmental conditions affect the ice. Our Image of the week shows a still from the game, where the player has created ice conditions which allow the penguins to dive down and catch fish.

What is the educational message?

The polar regions are constantly changing and assigning these changes to either natural cycles or anthropogenic (human induced) climate change can be tricky. Ice shelves tend to only hit the news when large changes happen, such as the recent development of the Larsen C rift which is thought to be unrelated to the warming climate of the region but may still have catastrophic consequences for the ice shelf. Understanding that changes like these can sometimes be part of a natural process can seem conflicting with the many stories about changes caused by warming. That’s why ice flows is a great way to demonstrate the ways in which ice shelves can change and the various factors that can lead to these changes. And the bonus chance to do this with penguins is never going to be a bad thing!

The game allows players to visualise the transformation of ice sheet to ice shelf to iceberg. This is an especially important educational point given the confusing ways that various types of ice can be portrayed by the media; reports, even if factually correct, will often jump from sea ice to ice shelves and back (see this example). It is also common for reports to cloud the climate change narrative by connecting processes thought to be due to natural causes (such as the Larsen C rift) to a warming climate (such as this piece). This confusion is something I often see reflected in people’s understanding of the cryosphere. In my own outreach work I start by explicitly explaining the difference between ice shelves and sea ice (my work is based on ice shelves). Even so, I can usually guarantee that many people will ask me questions about sea ice at the end of my talk.

Xue Long the Snow Dragon Penguin [Credit: Ice Flows game ]

Despite the messages that it is trying to convey, the app doesn’t come across as pushing the educational side too much. There is plenty of information available but the game also has genuinely fun elements. For example, you can earn rewards and save these to upgrade your penguins to some extravagant characters (my favourite has to be Xue Long – the snow dragon penguin!) Although the focus may be drawn towards catching the fish for the penguins while you’re actually playing, it would be hard for anyone to play the game and walk away without gaining an understanding of the basic structure of an ice shelf and how various changing environmental factors can affect it.

Developing the game…

The game was developed by Anne Le Broq in collaboration with games developers Inhouse Visual and Questionable Quality, using funding from the Natural Environment Research Council. Of course, many scientific researchers were also involved to ensure that the game was as scientifically accurate as possible whilst still remaining fun to play.

A key challenge in developing the game was modelling the ice flow. In order to be used in the app, the ice flow model needed to represent scientific understanding as well as being reactive enough to allow the game to be playable. This required some compromise, as one of the scientists involved in the development, Steph Cornford (CPOM, University of Bristol), explains on the CPOM Blog:

On one hand, we wanted the model to reflect contemporary understanding well enough for students to learn about ice sheets, ice shelves, and Antarctica in particular. On the other, the game had to be playable, so that any calculations needed to be carried out quickly enough that the animation appeared smooth, and changing any of the parameters (for example, the accumulation rate) had to lead to a new steady state within seconds, to make the link between cause and effect clear.

— Steph Cornford

The resulting model works really well, creating a fun, challenging and educational game! See for yourself by downloading the free to play game from your app store, or online at www.iceflowsgame.com!

Further reading

  • Find out more about the game on the University of Exeter website or visit the game’s own website here.
  • You can read in more detail about Steph’s modelling here.

Edited by Emma Smith


Sammie Buzzard has recently submitted her PhD thesis where she has developed a model of ice shelf surface melt, focusing on the Larsen C Ice Shelf. She is based at the Centre for Polar Observation and Modelling within the University of Reading’s Department of Meteorology. She blogs about her work and PhD life in general at https://iceandicing.wordpress.com/ and tweets as @treacherousbuzz.