CR
Cryospheric Sciences

permafrost

For Dummies – How do wildfires impact permafrost? [OR.. a story of ice and fire]

Fig 1: A permafrost peatland in the Northwest Territories, Canada, which was burned in 2014. Peatlands are complex, and we are just starting to understand how northern peatlands respond to fire. This picture was taken in 2015, and shows regions which remain charred a year after the fire, with the green areas representing bogs which were too wet to burn. 2014 was a record breaking year, where a total of 3.4 million hectares burned in the region! Similar large fire years have been seen in other areas of Canada, Alaska, Russia, and China in the last few decades. [Credit: Jean Holloway]

Wildfire – like the ones observed in the Northwest Territories, Canada in 2014 (Fig. 1) – is a natural part of permafrost landscapes, but fires are expected to get more frequent and severe as the climate warms. This could accelerate the degradation of permafrost, with negative consequences on the local and global scale! We have a pretty good understanding of how permafrost responds to fire today, but what should we expect as the climate warms and fire regimes change in the future?


What is permafrost anyway?

  • Permafrost is ground that is frozen for two or more consecutive years, and it typically forms in any climate where the mean annual air temperature is less than 0°C.

Fig 2: Permafrost is a mixture of ice, soil and rocks. This is a core of particularly ice-rich permafrost. [Credit: Jean Holloway]

Nerdy terminology break: when you talk about permafrost, it THAWS, it does not MELT. Melting implies phase change from solid to liquid. Permafrost isn’t just made up of ice, it also has soil and rock, which don’t melt when they go above 0°C (Fig 2). When you take a turkey breast out of the freezer it doesn’t turn into a puddle of chicken goo – it thaws! Here is a cool blog post about this issue.

Leftover Turkey Trot

Why the heck should you care about permafrost and fire?

There are three main reasons why we as a society should care about the impacts of fire on permafrost, and permafrost degradation in general. I will go into greater detail in the upcoming sections, but the take home messages are:

  1. Thawing permafrost causes ground settlement which affects local infrastructure.
  2. Post-fire changes to biogeochemistry can alter downstream water quality, which is important for local communities.
  3. Thawing permafrost releases more carbon into the atmosphere, triggering a positive-feedback loop and accelerating global climate change.

Now that I have your attention, let’s learn about some fire and ice!

How does permafrost respond to fire?

Wildfire is a natural and essential component of many permafrost landscapes, including boreal and subarctic forests, but also tundra regions. For some species in the boreal, such as black spruce (Picea mariana) and jack pine (Pinus banksiana), fire is essential because they need it in order to reproduce – they have what are called serotinous cones that open after fire and release their seeds! Fire has been a part of these permafrost landscapes for thousands of years. Changes occur immediately following the disturbance but mostly return to normal (pre-fire) levels after a few decades.

Permafrost is impacted when severe fires destroy the tree canopy and the surface organic layer (a thin layer of dead and decaying plant material, ranging from ~10-50 cm but can be much thicker!) that insulates the ground. Think of it like a blanket that keeps the heat in while you sleep, except the opposite – it keeps the ground cold. Permafrost LOVES the organic layer, because its thermal properties promote permafrost stability. For example, frozen peat has very high thermal conductivity (Fig 3). So, in the winter, it allows the cold air temperatures to penetrate deep into the ground. But… in the summer it is dries out and has low thermal conductivity, which means heat can’t get in. This helps the permafrost stay frozen!

Fig 3: Peat soil (left) and Sphagnum moss (right) favour permafrost presence due to unique thermal properties that help keep the ground cool. [Credit: Jean Holloway]

So, when fires destroy this layer it leaves the permafrost vulnerable to thaw. Further, fires destroy the tree canopy and other vegetation, which shade the ground and intercept the snow, both which protect the permafrost (Fig 4).

Fig 4: Snow trapped up in the tree canopy by coniferous trees at an unburned site near Yellowknife, Canada. This allows cold air to penetrate into the ground and protects the permafrost. [Credit: Jean Holloway]

Some other key factors that impact the permafrost following a fire include:

  • Decreased albedo (albedo is how much a surface reflects sunlight – dark charred surfaces absorb a lot of heat and make the ground warmer)
  • Changes in snow cover
  • Alterations to the surface energy balance (basically, how much energy moves in and out of the ground) and micro-climate
  • Reduction in evapotranspiration and changes in soil moisture (when there is vegetation present it moves water from the soil to the roots and up into the leaves, so when fire destroys the vegetation there is more water left in the ground)

In combination, these changes result in warmer and wetter soils, greater heat moving into the ground, and increased active layer thickness. The active layer is the top of the permafrost which freezes and thaws annually, and usually it is ~1 m deep. But after fire the active layer thickness can increase dramatically, sometimes to 3 m!

Fire changes the hydrology and biogeochemistry of permafrost landscapes

Fires can also have significant effects on the hydrology, biogeochemistry, and soil microbial communities of permafrost landscapes (for a good overview of this, check out: Tank et al., 2018). Fire changes the chemistry of the soil and water in streams and lakes. For example, we know that fire decreases soil acidity AND increases microbial activity (warmer soil temperatures after the fire = happy active microbes). Further, active layer thickening releases solutes, nutrients, and other things that were previously trapped in the frozen ground, allowing them to be transported by water. Sometimes, active layers get very thick and create thawed zones called taliks, which allow water to travel through the previously impermeable frozen ground year round! Studies have shown that aquatic ecosystems recover rapidly following fire but we don’t really know how climate warming and changing fire regimes will affect this in the future.

Lastly, fire changes the ecosystem carbon balance, because the increases in soil temperatures and active layer thickness that happen after fire make previously frozen organic matter available for decomposition by those happy microbes. The microbes eat the organic matter and release carbon into the atmosphere. This results in a positive feedback to climate change – fire thaws permafrost, releasing carbon into the atmosphere, leading to more climate change and warmer temperatures, which leads to more fire and more thawing permafrost, which releases more carbon… and so forth. This is not good! We need better regional sampling of permafrost carbon estimates to be able to predict how bad this is going to be in the future.

How does permafrost recover from fires?

In the past, permafrost in many places have been stable after fire, where changes occur immediately after fire, but return to pre-fire conditions in the next several decades (see, for example, this study: Rocha et al., 2012). This happens because the vegetation undergoes what is called succession, which basically means it regrows. So that organic layer and tree canopy which is so essential for permafrost returns to normal! Factors that determine how permafrost is impacted by fire and how it will recover include landscape position, soil type, organic layer thickness, burn severity, drainage and soil moisture conditions, snow, pre-fire permafrost and vegetation conditions – it’s complicated!! As an example, a poorly drained lowland site with a thick organic layer won’t be as vulnerable to fire as a dry high site with coarse soil.

Fire causes permafrost thaw and thermokarst development

It is important to mention that when ice-rich permafrost thaws following fire we can also get the development of what is called “thermokarst” (If you want to read more about this, see: Gibson et al., 2018). Thermokarst is pitted or irregular landscapes that are formed with ice-rich permafrost thaws and settles (Fig 5). These types of landscapes with thermokarst don’t follow the same post-fire recovery patterns, likely taking much longer to recover to pre-fire conditions (if at all… we don’t really know yet!). In addition, thermokarst changes drainage, can result in forest loss, and can impact infrastructure. We have seen this type of damage after fire (and also following general permafrost thaw) all around the globe: in Alaska, Canada, China, and Russia. In addition, thermokarst can also release large amounts of carbon. However, predictive models don’t take this into account yet – exciting research to expect here in the future!!

Fig 5: Thermokarst that developed after a wildfire at a permafrost site in the Northwest Territories, Canada. You can see the water pooling, which results in even further permafrost degradation. [Credit: Jean Holloway]

Fire, permafrost, and future climate change

So permafrost is impacted by fire, but usually it has been able to recover. HOWEVER, these patterns of permafrost recovery will likely be affected by changing fire regimes. In the Alaskan and Canadian boreal, we are currently seeing more fire than ever before (Here is one of many papers that shows this: Jain et al., 2018), particularly in the western provinces and territories. As global temperatures rise everything gets drier, and there is actually more lightening, which means larger and more severe fires. These fires typically result in more of the organic layer being removed, which leads to greater permafrost thaw. Predictive models indicate that fire in tandem with climate change, will accelerate the disappearance of permafrost. Some sites may still be able to recover, but greater warming results in longer recovery periods, if at all…

And the ability to recover might currently be overestimated: modelling suggests that permafrost in poorly drained landscapes with thick organic layers, such as peatlands, tundra, and other lowland systems, will likely be stable to fire over the long-term. BUT these landscapes are often impacted by thermokarst, and right now our models don’t have the capacity to incorporate the effects of thermokarst on the system. More work needs to be done to understand this!

Further Reading


Jean Holloway is a PhD student at the University of Ottawa, in Ottawa, Canada. Her research interests surround the impacts of fire on discontinuous permafrost in the Northwest Territories, Canada. She uses a variety of techniques to investigate this, including monitoring ground temperatures, conducting annual geophysical surveys, and applying thermal modelling to predict future change. Contact e-mail: jean.holloway77@gmail.com

 

Ice-hot news: The IPCC Special Report on the Oceans and the Cryosphere under Climate Change

Ice-hot news: The IPCC Special Report on the Oceans and the Cryosphere under Climate Change

You have probably heard the name “Intergovernmental Panel on Climate Change (IPCC)” mentioned frequently over the last few years. The IPCC is the United Nations body for assessing science related to climate change and it publishes global assessment reports on this topic every 5 to 10 years. Due to the current urgency of the global climate crisis and the need for more information by decision makers, the IPCC decided to publish several smaller more “special” reports between its fifth (published in 2013/2014) and sixth (planned for 2021/2022) assessment reports. The Special Report on the Oceans and the Cryosphere in a changing Climate (short “SROCC”) was published about a month ago. In this blog post, we will give you an overview of the take-home messages about the fate of the  cryospheric elements of our planet – those parts which are frozen!


Why is there a special report about ocean and cryosphere?

Discussions about global warming are often centered around changes in air temperature and changes in places where people live. The cryosphere and the ocean are, by contrast, remote areas without dense human population. However, climate change has a high impact on the cryosphere and oceans and this, in turn, has an effect on places where people live, one prominent example of this being land loss due to sea-level rise. Less talked about but equally impactful, is thawing permafrost, leading to high carbon release to the atmosphere, and melting Arctic sea-ice cover, enhancing Arctic Amplification, which can influence climate in mid-latitudes and globally.

The Special Report on Oceans and the Cryosphere in a changing Climate (SROCC) has the aim of highlighting the links between oceans, cryosphere and sea-level rise to improve policy makers’ understanding of these key elements and of the interdependencies between them (see Fig. 2). This is particularly critical at present, as the 25th Conference of Parties (COP25), organized by the United Nations Climate Change Convention, will be happening in around one month’s time in Santiago de Chile.

Fig. 2: Summary of the changes discussed in the IPCC Special report on the Oceans (white circles) and the Cryosphere (grey circles) under Climate Change [Credit: Box 1, Figure 1 in SROCC].

The Special Report is based on peer-reviewed literature from natural sciences, social sciences and humanities. It represents the most current scientific view on the oceans and the cryosphere. But what exactly are the conclusions of this report and why is it important? This post will guide you through the key points relevant to the cryosphere: permafrost, snow cover, glaciers and ice sheers, and sea ice!

Info box: What is an RCP scenario?

In IPCC reports, the projections for the future climate evolution are conducted using scenarios for the future greenhouse gas emissions. As we do not know how our economy, agriculture, and greenhouse gas emissions will evolve in the future, scientists have developed so-called Representative Concentration Pathways (RCP). Each RCP represents one of several possible futures in economic and agricultural development, resulting in different evolutions in atmospheric greenhouse gases. The higher the concentration of atmospheric greenhouse gases, the more of the heat radiated to space by the Earth’s surface is trapped, leading to a warming of the atmosphere. RCP2.6 represents a future where we radically cut greenhouse gas emissions after a peak in 2020,  RCP4.5 a scenario, where we cut emissions after a peak in 2040, and RCP8.5 represents a scenario in which greenhouse gas levels continue to rise.

Permafrost

Permafrost – ground, which is soil or rock frozen for more than two years in a row – contains one third of global near-surface carbon stocks, and thawing of parts of the permafrost-affected area can render this carbon more available for microorganisms, thus causing emissions of stored carbon as the greenhouse gases CO2 and CH4. This additional source of greenhouse gases could cause more surface warming, thus more permafrost thaw, thus more warming and so on (Mu et al., 2017; Sun et al., 2018a).

Because ice is solid, but water drains away, permafrost thaw can cause changes in the surface (subsidence) in Northern land areas, thus damaging infrastructure and buildings. Thaw can further cause landslides or increase rockfall rates, thus risking mountain accidents and the safety of local communities (PERMOS, 2016).

The IPCC reports that permafrost temperatures have increased by 0.29°C ± 0.12°C from 2007 to 2016 on average for polar and high mountain areas. Sparse long-term data series in heterogeneous alpine environments is the largest challenge faced when quantifying temperature trends in alpine permafrost, which makes up about 28 % of global permafrost, but estimates show a 0.19°C ± 0.05 °C warming per decade in alpine permafrost on average.

Arctic long time series are sparse, but estimates show that from 2007 to 2016, the coldest permafrost areas have warmed by 0.39 °C ± 0.15 °C, whereas the ‘warmer’ permafrost close to thaw has warmed on average by 0.2 °C ± 0.10 °C – more energy has been used for melting of ice.

Fig. 3: Left: Trend in annual average temperatures in high-mountain regions divided into geographical regions and ground material [Credit: Figure 2.5 in the SROCC. Right: Projections for global permafrost area following emission scenarios RCP 2.6, RCP4.5 and RCP8.5 [Credit: Figure 3.10 in the SROCC].

The Special Report concludes that the frequency and intensity of wildfires has increased, removing the organic topsoil and degrading permafrost faster than historically. Read more about fire in the Arctic in this post.

The amount of projected permafrost thaw by 2100 is directly related to the RCP scenario followed by humanity (see explanatory box on RCPs). Overall, permafrost occurrence is projected to be reduced by 8 to 40% for the low-emission scenario RCP 2.6 and 49 to 89 % for the high-emission scenario RCP8.5. While the Arctic permafrost within the first three meters of the surface is projected to have decreased by 2 to 66 % in RCP2.6 and by 30 to 99 % in RCP8.5, the permafrost area of the Tibetan plateau is projected to decrease by 22 % in 2100 under RCP2.6 by 64 % in RCP8.5. In conclusion, if we follow the high-emission scenario RCP8.5, tens to hundreds of gigatons of carbon will be released to the atmosphere by 2100.

The reason for the large intervals of projections are mainly that “translating” air temperature to a ground temperature, hence permafrost presence or absence, is a difficult task due to the impact of surface properties, such as snow depth and vegetation, and ground properties, such as ground ice and carbon content, on energy propagation into and out of the soil surface. IPCC projections are based on a comparison of many models for a more robust estimate – and they simulate precipitation and vegetation patterns differently, thus impacting the simulated permafrost area.

For communities in permafrost regions, for whom 70 % of local infrastructure will be at risk of damage due to permafrost thaw by 2050, short- and long term adaptation and mitigation measures need to complement each other. Measures need to be developed with the use of scientific and local knowledge from Northern communities, and local ecosystem monitoring can be a key data source for this.

You can find full details about permafrost changes in the SROCC Section 2.2.4 and 3.4.

Snow cover

Snow cover is a critical element of the cryosphere, firstly because it absorbs surface runoff from glacier and ice sheet surfaces, and secondly because it has a high albedo, meaning that it reflects more incoming solar radiation than the darker surfaces they cover, such as ground, trees, and ice. Snow covers the terrestrial Arctic (north of 60 ºN) for up to nine months each year, and influences the surface energy budget (through its reflectivity), freshwater budget (through water storage and release), ground thermal regime (through insulation) and ecosystem interactions.

Since the beginning of the satellite era in 1981, spring snow cover extent has declined by over 13% per decade. Snow cover duration has also declined, both in spring (by up to 3.9 days per decade in certain regions) and autumn (by up to 1.4 days per decade in certain regions). Maximum snow depth has been declining, though trends are uncertain because of sparse observations and large spatial variability. These reductions are very likely driven by surface temperature rise in the Arctic. Warming-induced snow cover reduction creates a self-reinforcing cycle where the surface is darker than when it is snow covered and therefore reflects less incoming solar radiation, leading to a warmer surface and even more melting.

Fig. 4: Observed changes in June snow cover extent anomalies, and projected change in June snow cover under low (RCP2.6, blue), medium (RCP4.5) and high (RCP8.5) greenhouse gas emissions scenarios [Credit: Figure 3.10 in the SROCC]

Arctic snow cover duration is projected to decrease (later autumn onset in and earlier spring melting), as a result of continued Arctic-wide warming. However, trends differ between model scenarios. For example, Arctic snow cover duration stabilizes by the end of the century under RCP4.5, whereas under RCP8.5 it declines to -25% (compared to the period 1986 – 2005). In Greenland, snow cover is projected to retreat to higher, flatter areas of the ice sheet. The rate at which this could occur is currently not well reproduced in climate models, mostly due to uncertainty in the way snow processes are included in these models.

Snow cover reductions are already being felt negatively in the Arctic, especially by communities reliant on snow cover for food sources, drinking water, and livelihoods, such as reindeer herding.

You can find full details of the estimates and predictions for ice sheets in SROCC, section 3.4 and in FAQ 3.1.

Sea ice

Sea ice is frozen sea water and displaces as much water as is produced when it melts. Melting sea ice therefore does not play a role for sea-level rise. It is, however, an important element for climate as it reflects large amounts of incoming sunlight back to space (similar to snow); it provides habitat for bacteria, plants and animals below, in, and above the ice; and it influences weather in mid-latitudes, the region between roughly 30 and 60 degrees of latitude, home to over 50% of the human population.

In the context of climate change, sea ice is also an especially good element of the cryosphere  to illustrate the effects of global warming. The Arctic September sea-ice area is directly linked to cumulative CO2 emissions (see this previous post) and therefore changes in sea ice directly reflect, in a very visible way, the path of climate change.

Fig. 5: Observed and modelled historical changes in the Arctic September sea ice extent since 1950 and projected future changes under low (RCP2.6, blue) and high (RCP8.5, red) greenhouse gas emissions scenarios [Credit: Figure SPM1 in the SROCC].

The sea-ice review in SROCC is similar to the Special Report on 1.5°C published end of 2018 (see this previous post). Arctic sea-ice loss was observed in both area and thickness in the past decades and is projected to continue through mid-century. The rate of the loss depends on the amount of warming. In the Antarctic, there is low confidence in the projected sea-ice evolution.

More details about the sea-ice evolution under climate change can be found in Section 3.2 of SROCC.

Polar ice sheets and glaciers

The two polar ice sheets and the world’s glaciers are a fundamental element of the world’s cryosphere. Glaciers and ice sheets cover about 10% of the Earth’s surface and contain around 70% of the world’s fresh water. They regulate the global climate system by interacting with the ocean and atmosphere and provide valuable fresh water resources to much of our planet.

The overwhelming consensus is that the polar regions are losing ice, and that the rate of ice loss has increased. In Greenland, ice loss is occurring through a combination of enhanced surface melting and runoff, and increasing dynamic thinning (ice loss as a result of accelerated ice flow). Greenland has lost around 247 Gt of ice every year between 2012 and 2016. In Antarctica, incursion of warm ocean waters is driving rapid, accelerating ice loss from the West Antarctic Ice Sheet. The mass loss signal from the West Antarctic Ice Sheet (-122 ± 10 Gt yr -1 from 2003 – 2013) is dominated by the increasing ice loss from outlet glaciers in the Amundsen Sea Embayment. On the Antarctic Peninsula, the majority of marine-terminating outlet glaciers are retreating. The pattern of change on the East Antarctic Ice Sheet is more ambiguous and complex, with regional gains and losses and no clear overall mass trend. Increased surface melt intensity and duration on both ice sheets has led to a self-reinforcing cycle, where, like for snow, the melt of the ice sheets uncovers dark surfaces, which absorb more incoming solar radiation than ice, and therefore heat up and lead to further melt. This reduces the capacity of snow and firn to store runoff.

Fig. 6: Ice sheet from above [Credit: Matt Palmer on Unsplash].

Continued ice loss from Greenland and Antarctica is projected to alter the polar regions. While glaciers have been the dominant contributor to global sea-level rise, this will be replaced by ice sheets as they continue to lose mass into and beyond the 21st century.

However, a number of processes that could potentially lead to rapid ice loss in Antarctica are poorly understood, particularly their timescale and future rate. Therefore, large uncertainty remains in projecting future changes to these polar regions, especially beyond this century. Portions of Antarctica resting on bedrock below sea level could be vulnerable to self-sustaining feedbacks such as the Marine Ice Sheet Instability (MISI, see this previous post). It is uncertain from current observational data whether irreversible retreat is underway.

Ice shelves and their interactions with surface meltwater will play a key role in the response of Antarctica to future warming. Freshwater input to the oceans, from icebergs and ice shelves melting, is freshening global water mass currents and circulation, and this may be accelerating. This could inhibit the formation of important oceanic water masses such as the Atlantic Meridional Overturning Circulation and Antarctic Bottom Water. Surface runoff and basal melting from both ice sheets are enhancing the input of dissolved nutrients into fjords and the ocean such as iron, which has been linked to enhanced primary productivity.

The report stresses that low emissions scenarios will potentially limit the rate and magnitude of future changes to the cryosphere. For example, polar glaciers are projected to lose much less mass between 2015 and 2100 under RCP 2.6 compared with RCP 8.5. Ice loss from polar ice sheets and glaciers presents a growing challenge to polar governance, and requires more coordinated efforts to build long-term resilience. In the Arctic, mitigating the effects of climate change can benefit strongly from community-led adaptation.

You can find full details of the estimates and predictions for ice sheets in SROCC, section 3.3 and in FAQ 3.1.

In summary

As a summary of our summary, and if you are interested in the ocean topics as well, have a look at this useful infographic by John Lang!

Here, you can see the most important points for the cryosphere (click on the link below or the picture to have a larger view):

Fig. 7: Parts of a summarizing infographic about SROCC [Credit: John Lang].

Further reading

Edited by Emma Smith


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 postdoctoral researcher at the Max Planck Institute for Meteorology in Hamburg. She investigates new methods to compare sea-ice as simulated by climate models, sea-ice as observed by satellites, and real sea ice. She tweets as @climate_clara.

 

 

 

Jenny Arthur is a PhD student at Durham University, UK. She investigates supraglacial lakes in Antarctica using remote sensing, and is especially interested in their seasonal evolution and interaction with ice dynamics. She tweets as @AntarcticJenny.

Cryo-Comm – Degrading Terrains

Cryo-Comm – Degrading Terrains

 
Beneath dusted peaks of mountain dew
A dense and rigid backcloth skulks,
Worn down and compacted with
Fractured decades of aged powder;
Trodden into rocky outcrops
To lie barrenly against
This frozen, ancient soil.
Subtle shifts of these forgotten rocks
Ripple across subterranean sediments,
Dislodging once-stable foundations
That now cascade like an ocean;
Echoing across the fragile firmament
To loudly denounce their buried past.
Beneath the jutting shadows
Of glaring, metallic stations
We bore artificial holes,
Treading carefully
As we silently caress exposed skin;
Mapping the resistance to our touch
Like goose bumps
Rising to the surface
On a withered, sun-kissed limb.
Charting out these imperfections
Reveals the unevenness
Of our approach,
As broken consequences
Reverberate beneath our feet;
An unheard shot across the bow.

This poem is inspired by recent research, which has found that there has been a clear degradation in mountain permafrost across the central Alps over the past two decades.

Permafrost is permanently frozen ground, consisting of rock or soil that has remained at or below 0ºC for at least two years. In Europe, Arctic permafrost is found only in the northernmost parts of Scandinavia, meaning that mountain permafrost is the dominating permafrost across the continent. This permafrost influences the evolution of mountain landscapes and can severely affect both human infrastructure and safety, with permafrost warming (or thawing) affecting the potential for natural hazards, such as rock falls. Mountain permafrost is also sensitive to climate change and has been affected by a significant warming trend across the European Alps over the last two decades. However, as this permafrost occurs across a large variety of very different locations, any warming trends are far from uniform, and in order to better understand these changes more accurate monitoring is required.

The researchers in this study made use of Electrical Resistivity Tomography (or ERT), a technique which involves inducing a current in the ground, and then measuring the extent to which the flow of this current is resisted (see this previous post). Any resistivity will depend on what the subsurface structure looks like, and it can be used to map geologic variations such as fracture zones, variations in soil structure, and the presence of permafrost. By using long-term ERT monitoring across a network of permanent sites in the central Alps, this research has demonstrated that there has been a noticeable degradation in mountain permafrost over the past 19 years across this region. The researchers have also demonstrated that a degradation is detectable across shorter timescales, a finding which is significant for better understanding the impacts of climate change across the region.

Further reading

The poem and blog post was written by Dr Sam Illingworth, and also features on his website here.

Edited by Jenny Turton


Dr Sam Illingworth is a senior lecturer in science communication at Manchester Metropolitan University, UK. His research focuses on how science can be used to empower society through creative tools and products. To read more of Sam’s poetry and blogs, and to listen to more podcasts, visit https://www.samillingworth.com/. He tweets from @samillingworth.

The hidden part of the cryosphere – Ice in caves

The hidden part of the cryosphere – Ice in caves

The cryosphere can be found in various places in many forms and shapes… in the atmosphere, on land and sea. A lesser known part of the cryosphere is hidden deep in the dark, in the cold-karstic areas of the planet: Ice caves! The ongoing climate change affecting ice all over the world is now rapidly melting these hidden ice masses as well. We therefore need to hurry up and try to collect as much information as we can before all will melt away…


The big melting

The ice masses around the globe, in ice sheets, sea ice, and mountain glaciers, have been melting away in past decades (see this previous post). The reduction of the cryosphere, both in terms of area and mass, has particularly been visible in the European Alps over the last 30 years. On the one hand, large and small Alpine glaciers decline, fragment and even disappear, and this trend has accelerated since the mid 1980s. Mountain glaciers are therefore considered to be sensitive indicators for climate variability. On the other hand, the warming climate is also acting on permafrost degradation, mostly affecting the stability of rock-slopes and cliffs.

What makes the international scientific community worry at the moment is how fast this abrupt glacial reduction is occurring globally. However, not all the natural environments respond in the same way to sudden changes in the climate system! Fortunately for us scientists, there are physical environments and ecological niches more resilient to external perturbations. This aspect has sometimes allowed the preservation of environments and information in the Earth’s climatic history that would have been otherwise destroyed.

Caves are resilient

Among the most resilient natural environments there are caves, “protected” by the rocky mass within which they were formed. In the mountains, high-altitude karst cavities can contain huge deposits of ice representing a lesser known part of the cryosphere. Speleologists face such ice in caves both as a joy and a damnation: fascinating by their beautiful shapes and morphologies, they also see it as an unwieldy presence that prevents explorations of still unknown voids in the alpine karstic systems.

Fig. 2: An ice deposit in a cave of the southeastern Alps [Credit: Renato R. Colucci].

 

But ice in caves is not just something beautiful (but isn’t it? Look at Fig. 2!). It rather represents a precious natural archive, sometimes with high temporal resolution, able to tell the climate history of large part of the Holocene (the last 11700 years of the Earth’s history). The permanent ice deposits, i.e. the ice staying longer than just a winter season, often defined in a colorful way as “fossil ice” by speleologists, is what counts the most. As it typically gets older than 2 years, which is one threshold for the general definition of permafrost, this phenomenon is part of the mountain permafrost… right or wrong, ice in caves is ground ice!

Fig. 3: Huge entrance of a cave opening in the Dachstein limestones of the Canin-Kanin massif, southeastern Alps [Credit: Renato R. Colucci].

 

Generally in the Alps such ice deposits lie in caves having their opening at altitudes above 1,000 m (Fig. 3), but locally even lower. The formation of these unique environments depends on a combination of geomorphological and climatic characteristics, which allow for accumulation and preservation of ice also in places where this would be very unlikely.

Now, although the caves are resilient environments, ice melting due to climate change is rapidly increasing there as well. This is why it is important to save as much information as possible from the remaining ice, before it is definitely lost!

The C3 project – Cave’s Cryosphere and Climate

The C3-Cave’s Cryosphere and Climate project is under the scientific guidance of the National Research Council (CNR) of Italy, and precisely the climate and paleoclimate research group of ISMAR Trieste. It aims to monitor and study ice deposits in caves. Such ice deposits store several information related to the paleoclimate, the biology, the chemistry and ecology of these environments.

Fig. 4: Drilling ice cores with the aim to extract the CCC layer from this ice body in a cave of the southeastern Alps [Credit: Arianna Peron].

The project started in 2016, following the discovery of a coarse cryogenic calcite deposit (CCCcoarse) in an ice layer (in-situ) in a cave of the Canin-Kanin massif, in the Julian Alps, located between Italy and Slovenia. This finding, representing the first evidence of CCC in the southern Alps, provides an important opportunity to understand the processes associated to the formation of these particular calcite crystals (Fig. 4). Previously, the CCC (Fig. 5) was only found on the floor in caves where ice had already melted away. What makes it interesting is the fact that it is possible to date these crystals using the isotopic ratio of some trace elements in radioactive materials, typically Uran and Thorium.

Fig. 5: Millimetric crystals of coarse cryogenic calcite found in-situ in the southern Alps [Credit: Renato R. Colucci].

The strongest financial and logistic support to the project is given by the Alpine Society of the Julian Alps through its speleological group, the E. Boegan Cave Commission. In addition to the CNR and other Italian institutions such as the University of Trieste, University of Bologna, Insubria University in Varese, Milano Bicocca University and the Natural Park of the Julian Prealps, the project involves research institutes and universities from Germany (Institute of Physics of Heidelberg University), Switzerland (Paul Scherrer Institut; Swiss Institute for Speleology and Karst Studies), Austria (Innsbruck University; Palynology and Archaeobotany Research Group), and Slovenia (Geological survey of Slovenia).

Many activities and several results already unveiled few of the secrets hidden in such environments: the realization of the first thermo-fluido-dynamic model in an ice cave, the development of innovative techniques for studying the mass balance of the ice, the study of the thermal characteristics of the rock and therefore of the permafrost and the active layer, the development of innovative and multidisciplinary methods of ice dating.

But there is little time to do all, and we must exploit it to the fullest!

Further reading

Edited by Clara Burgard


Renato R. Colucci works in the climate and paleoclimate research group of ISMAR-CNR, Department of Earth System Sciences and Environmental Technology. He is also adjunct Professor of glaciology at the University of Trieste (Italy). During his PhD he honed his skills in glacial and periglacial geomorphology at UNIS (University Center in Svalbard). His research centers around the interactions between cryosphere (glaciers, permafrost, ice caves) and the climate, spanning from the end of the Last Glacial Maximum to the present days.

Image of the Week – Permafrost features disappearing from subarctic peatlands

Image of the Week – Permafrost features disappearing from subarctic peatlands

Some of the most remarkable, marginal features of permafrost – palsas – are degrading and disappearing metre by metre from North European peatlands, and are driven close to extinction by the climate change.


What are these permafrost features?

A palsa is a peat mound with an icy core, which stays frozen throughout summer due to the insulating property of dry peat. These mounds can rise up to 10 metres above the surface of surrounding mire (wet terrain dominated by peat-forming vegetation), and they may occur as just a single palsa, group of palsas or as an extensive, but not very high (ca. 1 to 2 m) peat “platform”. The occurrence of palsas is limited by such factors as: low mean annual air temperature (< 0 °C), low annual precipitation (< 500 mm) and at least 40–50 cm thickness of peat layer, which is needed to sufficiently insulate the core during summer (Seppälä, 2011).

The established theory on palsas formation (Seppälä, 2011) is the following:

  1. The formation of a palsa begins when a part of mire freezes deeper in a windblown area with thinner snow cover, which normally protects the ground below from freezing temperatures.

  2. If the frozen peat doesn’t melt completely during summer, an ice lens forms inside the peat layer resulting in uplifting of the mire surface in this area.

  3. In the following winters, the snow is even more likely to be windblown from the mound, which again fosters deeper penetration of frost and formation of new ice lenses.

  4. As soon as a part of mire rises above the water level, the vegetation starts to change and the peat dries out, which contributes to the survival of the ice core during summers.

 

Breaking of the surface and erosion is a natural “step” for mature palsas, when the permafrost has reached the mineral ground below the peat. The melting of a palsa is a form of thermokarst, i.e. thawing of ice-rich permafrost (see this post for more details about thermokarst).

Block-erosion of peat on ridge-type palsa in Nierivuoma mire in Enontekiö, Finland [Credit: Mariana Verdonen].

Palsa, peat hummock or permafrost plateau?

The terminology used when speaking about these permafrost mounds varies, usually according to the continent the research was conducted on or the background of the authors. The term “palsa” comes from Lapland, and was used by Sami and northern Finns to refer to “hummock rising out of a bog with a core of ice” (Seppälä, 1972). In Fennoscandia, this term is used commonly for all main types: ridges, mounds and plateau palsas, whereas in North America the more common terms are either ‘peat or permafrost plateau’ or ‘wooded palsa’ depending on the shape and vegetation cover of the feature (Luoto et al, 2004).

Degrading permafrost of Fennoscandia

More often than not, one may encounter a desolate sight in North European palsa mires: most of the permafrost mounds are degrading by block erosion and/or melting away as a result of thawing of their frozen core. The vegetation that once was growing on hummocks above the wet mire surface, is now dead black in shallow thermokarst ponds surrounding palsas here and there. Although, in some places the conditions may still be favorable for new palsas to form, the general picture is devastating. Palsas are disappearing in most of their area of existence, and it is happening fast.

Thawing palsas of Nierivuoma captured from drone in July 2018. This peatland sprawls across ~7 km2 and is the largest palsa mire in Finland [Credit: Timo Kumpula].

Why should we care?

As climatic change is likely to increase winter and summer precipitation, and is already notable in rising mean annual air temperatures, palsas are predicted to disappear in Fennoscandia almost completely by the end of the 21st century (Fronzek et al, 2010).

It is noteworthy, that the palsa mire is the only mire and bog habitat that is listed as “critically endangered” in the 2016 European Red List of Habitats. While some other cold climate ecosystems may shift to higher latitudes and altitudes, palsa mires seem to be restricted from developing in higher areas, especially because of the required peat layer thickness (Luoto et al, 2004).

If just the loss of this diverse ecosystem type is not alarming by itself, there are couple of issues that I want to highlight:

  • Thawing of the perennially frozen peat changes the carbon fluxes of palsa mires as carbon previously trapped by permafrost becomes available for decay. As the area of dry peat surface decreases, more carbon is released into the atmosphere in the form of more effective greenhouse gas methane (CH4) instead of carbon dioxide (CO2). Recently, also the effects of permafrost thaw on the emissions of nitrous oxide (N2O), which is a strong greenhouse gas, have gained more attention (Marushchak et al, 2011).

  • The heterogeneity formed by variety of mire surfaces, thermokartst ponds and dry palsa mounds creates favorable conditions for species richness in these subarctic environments. In particular, the number and density of bird species seems to be high in the zone of palsa mires compared to more southern mire zones in Fennoscandia, even though no species have been reported to be exclusive to palsa mires (Luoto et al, 2004). This relationship, as well as overall significance of palsa mires for biodiversity is still poorly understood, however.

References

 

Edited by Clara Burgard


Mariana Verdonen is an Early Stage Researcher at the University of Eastern Finland. She focuses on optical, multi-temporal and multiscale remote sensing of environmental changes in Arctic and Subarctic areas. Mariana’s scientific interests are generally in geomorphology, permafrost-landscape dynamics and remote sensing of the Cryosphere. She tweets as @MarianaVerdonen. Contact Email: mariana.verdonen@uef.fi

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 — Biscuits in the Permafrost

Fig. 1: A network of low-centred ice-wedge polygons (5 to 20 m in diameter) in Adventdalen, Svalbard [Credit: Ben Giles/Matobo Ltd]

In Svalbard, the snow melts to reveal a mysterious honeycomb network of irregular shapes (fig. 1). These shapes may look as though they have been created by a rogue baker with an unusual set of biscuit cutters, but they are in fact distinctive permafrost landforms known as ice-wedge polygons, and they play an important role in the global climate.


Ice-wedge polygons: Nature’s biscuit-cutter

In winter, cracks form when plummeting air temperatures cause the ground to cool and contract. O’Neill and Christiansen (2018) used miniature accelerometers to detect this cracking, and found that it causes tiny earthquakes, with large magnitude accelerations (from 5 g to at least 100 g (where g = normal gravity)!). Water fills the cracks when snow melts. When the temperature drops, the water refreezes and expands, widening the cracks. Over successive winters, the low tensile strength of the ice compared to the surrounding sediment means that cracking tends to reoccur in the ice. As the cycle of cracking, infilling, and refreezing continues over centuries to millennia, ice wedges develop.

Subsurface ice wedge growth causes small changes in the ground surface microtopography. There are linear depressions, known as troughs, above the ice wedges (fig. 2). Adjacent to the troughs, the soil is pushed up into raised rims. From these raised rims, the elevation drops off into the polygon centre, forming low-centred polygons (fig. 2a).

Shaping Arctic landscapes

Permafrost in the Northern hemisphere is warming due to increasing air temperatures (Romanovsky et al. (2010). As air temperatures rise, the active layer (the ground that thaws each summer and refreezes in winter) deepens.

As permafrost with a high ice content thaws out, the ice melts and the ground subsides. On the other hand, permafrost containing no ice does not experience subsidence. Consequently, permafrost thaw can cause differential subsidence in ice-wedge polygon networks. This re-arranges the surface microtopography: ice wedges melt, the rims collapse into the troughs, and the polygons become flat-centred and then eventually high-centred (fig. 2b and c; Lara et al. (2015)). Wedge ice is ~20 % of the uppermost permafrost volume, and so this degradation could have a big impact on the shape of Arctic landscapes.

Are ice wedge polygons climate amplifiers?

Fig. 2: Schematic diagrams of polygon types and features [Credit: Wainwright et al. (2015)].

The transition from low-centred to high-centred ice-wedge polygons affects water distribution across the polygonal ground. The rims of low-centred polygons tend to block water drainage, whereas the troughs facilitate relatively fast and effective drainage of water from the polygonal networks (Liljedahl et al., 2012). So, during summer, the centres of low-centred polygons are frequently flooded with stagnant water, whereas the central mounds of high centred polygons are well drained (and good to sit on at lunchtime!). The contrast in hydrology influences vegetation, surface energy transfer, and biogeochemistry, in turn influencing carbon cycling and the release of greenhouse gases into the atmosphere.

High-centred polygons can have increased carbon dioxide emissions compared to low-centred polygons, on account of their lower soil moisture, reduced cover of green vascular vegetation and the well-drained soil (Wainwright et al., 2015). On the other hand, once plant growth during peak growing season is accounted for, this can actually cause a net drawdown of carbon dioxide in high-centred polygons (Lara et al., 2015). In contrast, there is general agreement that low-centred polygons are associated with high summer methane flux (Lara et al., 2015; Sachs et al., 2010; Wainwright et al., 2015). This is due to multiple interacting environmental factors. Firstly, low centred polygons have a higher temperature, which increases methane production rates. Secondly, they also have moister soil, which decreases the consumption of methane, owing to the lower oxygen availability. Thirdly, the low-centred polygons often have more vascular plants that help transport the methane away from its production site and up into the atmosphere. Lastly, the low-centred polygons had higher concentrations of aqueous total organic carbon, which provides a good food source for methanogens.

Outlook

As the climate warms, ice wedge polygons will increasingly degrade. The challenge now is to figure out whether the transition from low-centred to high-centred polygons will enhance or mitigate climate warming. This depends on the balance between the uptake and release of methane and carbon dioxide, as well as the rate of transition from high- to low-centred polygons.

Further Reading

Lara, M.J., et al. (2015), Polygonal tundra geomorphological change in response to warming alters future CO2 and CH4 flux on the Barrow Peninsula. Global Change Biology, 21(4), 1634-1651

Liljedahl, A.K., et al. (2016), Pan-Arctic ice-wedge degradation in warming permafrost and its influence on tundra hydrology. Nature Geoscience, 9, 312-316.

Wainwright, H.M., et al. (2015), Identifying multiscale zonation and assessing the relative importance of polygons geomorphology on carbon fluxes in an Arctic tundra ecosystem. Journal of Geophysical Research: Biogeosciences, 707-723.

On permafrost instability: Image of the Week – When the dirty cryosphere destabilizes! | EGU Cryosphere Blog

On polygons in wetlands: Polygon ponds at sunset | Geolog

Edited by Joe Cook and Sophie Berger


Eleanor Jones is a NERC PhD student on the EU-JPI LowPerm project based at the University of Sheffield and the University Centre in Svalbard. She is investigating the biogeochemistry of ice-wedge polygon wetlands in Svalbard. She tweets as @ElouJones. Contact Email: eljones3@sheffield.ac.uk

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 – Powering up the ground in the search for ice

Electric Resistivity Tomography profile of the north-facing slope of the Rohrbachstein in canton Bern, Switzerland [Credit: University of Fribourg, Switzerland].

In an earlier post, we talked briefly about below-ground ice and the consequences of its disappearing. However, to estimate the consequences of disappearing ground ice, one has to know that there actually is ice in the area of study. How much ice is there – and where is it? As the name suggests, below-ground ice is not so easy to spot with the naked eye. Using geophysical methods, however, it is possible to obtain a good idea of the presence and whereabouts of ground ice, and of frozen ground, in an area of interest.


Looking for ice

Before starting a geophysical survey, which requires instrumentation and time, you might want to take a look at your area of interest and estimate, whether ice presence is even an option. The first indicator is temperature, which has to be in the favor of permafrost presence. Other indicators for presence are surface features such as mounds that could be caused by considerable frost heave, lobes perpendicular to the slope and front angles exceeding the critical angle of repose. They can indicate that ice has had an influence on the geomorphology in the area.

If you suspect ground ice in your area of interest, and you want to confirm or rule out your suspicion as well as investigate the extent of the ice, you might consider doing a geophysical survey. There are a few useful inherent properties of ice that make it possible to distinguish it from rock, air or water. These properties will determine the choice of geophysical methods to use. This week, we will illustrate two methods which, when combined, can be useful tools for determining ground ice presence or absence. The test subject is an area of suspected frozen ground just below 3000 m altitude – the Rohrbachstein in canton Bern, Switzerland.

Electrical resistivity tomography

In an electrical resistivity tomography (ERT) survey, we measure the potential difference (ΔU) of a material, over a given distance, when applied with a certain current strength (I). From the fact that resistance is computed by dividing U by I, the electrical resistivity of the material can be estimated. The resistivity can be seen as the reciprocal of the material’s electrical conductivity and is measured in mΩ. Practically, an array of electrodes are placed in the ground with a certain spacing and a certain length of the profile. The spacing and length of the profile determine the resolution and penetration depth. All electrodes are then connected with a cable to each other and to the instrument, which works as both a voltmeter and a source of current. Then, systematic measurements of potential difference can be conducted throughout the whole profile.

Water has an electrical resistivity of 10-100 mΩ, whereas ground ice has a resistivity of 103 to 106 mΩ. This makes this method practical for distinguishing liquid from frozen water in permafrost areas. The resistivity of rock is between 102 to 105 mΩ, and the resistivity of sediment depends on the mixture of rock, water, ice and air. Air has an extremely high resistivity, which should be easy to point out, but since below-ground material is mostly a mixture of all the mentioned components, things are very often more blurry. What one actually looks for in the measurements is areas of higher, lower and in-between electrical resistivity values. An example of such a case is displayed in our Image of the Week.

Our Image of the Week shows the resistivity profile of a slope at just below 3000 m altitude in the Bernese Alps, Switzerland. For comparison, the same slope is shown in a normal photo in Fig. 2 (not to scale). Blue colours mark high resistivities, red mark low, and green mark somewhere in between. From this profile, we might conclude that the upper layers of the lower slope are moist and underlain by bedrock (red and green, respectively, whereas the upper slope seems to be moist below an area of high resistivity (red below green-blue). Additionally, there is a significant feature of high resistivity in the middle of the slope. This slope could contain ice in those blue areas. However, the high resistivities could also be caused by air volume in this blocky site. To be certain, we can use an additional method.

Fig. 2: Photo of the north-facing slope of the Rohrbachstein in canton Bern, Switzerland. The photo was taken facing east and shows the upper part of the slope analyzed with ERT and seismic refraction, but is not to scale compared with the Image of the Week and Fig.4 [Credit: Laura Helene Rasmussen].

Seismic refraction analysis

To distinguish air from ice, we can do a survey of the subsurface using seismic refraction analysis. Seismic refraction surveys use the fact that the speed (in ms-1) of sound wave propagation is different through different materials. The speed is estimated by placing geophones in a profile line and creating a sound wave by hitting the ground with a sledgehammer in between them (Fig. 3). The geophones detect the sound wave from this hammer blow one by one as it travels through the subsurface, and the time it takes for each geophone to receive the signal is noted. This allows us to calculate the seismic (sound) velocity from the distance and travel time. Different layers in the subsurface with different properties, and thus different seismic velocities, will cause the sound wave arriving at their surface to be refracted with different delay compared to the direct wave (which travels straight from the hammer to each geophone), and that fact can reveal properties of below-ground material.

Fig. 3: Hammer-swinging doing a seismic refraction profile [Credit: Hanne Hendricks].

The advantage of this method for ground ice studies is that ice has a seismic velocity of about 3000 ms-1, whereas sound waves move through air with only 330 ms-1. Thus, a rough profile of that same slope from our Image of the Week and Fig. 2 using seismic refraction geophysics looks like Fig. 4.

In this profile, red colours denote high seismic velocities and blue colours are very low seismic velocities. The high-resistivity feature in the middle of the ERT profile at about 3-4 m depth, which could contain air or ice, would cause red-purple colours (high velocities) if the feature contained ice, and blue colours (low velocities), if it was air volume. As seen from Fig. 4, colours at depths are reddish and certainly not blue, which makes it likely that the ERT feature at 3-4 m depth is actually an ice body. The high-resistivity area in the surface layers of the upper profile, however, corresponds to the blue colours in this seismic refraction profile, and with high resistivity, but low seismic velocity, this area is most likely air volume and not ground ice.

Fig. 4: Seismic refraction profile of the north-facing slope of the Rohrbachstein in canton Bern, Switzerland [Credit: University of Fribourg, Switzerland].

The method depends on the setting

Ground ice does, obviously, come in different forms in different environments, and so the methodological considerations when using geophysical techniques vary in different settings. In this case, we look for ice in a blocky slope. That type of setting presents challenges such as contact problems between sensors and the ground, which can impede the measurements. That issue would not worry a scientist mapping ground ice in a moist Arctic lowland site. The lowland scientist might, however, have to consider resolution issues or salt content in her soil solution when evaluating the results. Perhaps she wants to combine with yet other methods such as drilling permafrost cores for detailed information on ice- and sediment type. As non-destructive methods, covering relatively large spatial areas without having to get a drill rig to the high mountains or a remote Arctic area, however, geophysics can be a good option for ground ice detection.

Further reading

Edited by Clara Burgard and Emma Smith


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.

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