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We have a new open-access study in the current issue of Journal of Glaciology that investigates the “cold content” of Greenland’s high-elevation firn plateau¹. Firn is the relatively low density near-surface ice-sheet layer comprised of snow being compressed into ice. Cold content is one of its quirkier properties. Of course, all firn is literally freezing – meaning below 0°C – but some firn is colder than other firn. Clearly, it takes a lot more energy to warm -30°C firn to 0°C, than it does for -1°C firn. Our study highlights at least one discernible shift in cold content – how much sensible heat energy is required to warm firn to the 0°C melting point – in response to climate change.

Figure 1 – The nine high-elevation ice-sheet sites where we assessed firn cold content in the top 20 m.

There is a strong annual cycle in firn cold content. Generally, cold content is at its maximum each April, after the firn has been cooled by winter air temperatures. Cold content then decreases through summer, as warming air temperatures and meltwater percolation pump energy into the firn, to reach a minimum each September. The magnitude of this annual cycle varies across the ice sheet, primarily as a function of the meltwater production, but also as a function of snowfall-dependent firn density. Firn density is highly sensitive to snowfall rate, and firn cold content is a function of firn density.

Figure 2 – The mean annual cycle in four-component firn cold content assessed at the nine ice-sheet sites over the 1988-2017 period. Note the relatively large latent heat release associated with meltwater at Dye-2, in comparison to other sites.

We find few discernible year-on-year trends in cold content across the highest elevation areas of the firn plateau. For example, there is perhaps a slight decrease at Summit – where we find snowfall is increasing at 24 mm/decade and air temperatures are warming at 0.29°C/decade – but statistically-significant multi-annual trends in cold content are difficult to separate from year-to-year variability. At Dye-2, however, which has the greatest melt rate of the sites that we examine, there is clear evidence of the impact of changing climate. At Dye-2, an exceptional 1-month melt event in 2012 removed ~24% of the cold content in the top 20 m of firn. It took five years for cold content to recover to the pre-2012 level.

Figure 3 – The cumulative four-component firn cold content at the nine ice-sheet sites over the 1998-2017 period. Note the sharp loss of Dye-2 cold content in 2012, and the subsequent multi-year recovery of this cold content.

The refreezing of meltwater within firn is a potential buffer against the contribution of ice-sheet melt to sea-level rise; surface melt can refreeze within porous firn instead of running off into the ocean. But refreezing meltwater requires available firn cold content. The multi-annual reset of cold content that we document at Dye-2 suggests that a single melt event can reduce firn cold content – and thus precondition firn for potentially less meltwater refreezing – for years to follow. This highlights the potential for the cold content of Greenland’s firn plateau to decrease in a non-linear fashion, as climate change pushes melt events to progressively higher elevations of the firn plateau.

¹Vandecrux, B., R. Fausto, D. van As, W. Colgan, P. Langen, K. Haubner, T. Ingeman-Nielsen, A. Heilig, C. Stevens, M. MacFerrin, M. Niwano, K. Steffen and J. Box. 2020. Firn cold content evolution at nine sites on the Greenland ice sheet between 1998 and 2017. Journal of Glaciology..

We have a new open-access study that investigates the high-elevation firn plateau of the Greenland Ice Sheet in the current issue of The Cryosphere¹. Firn is the relatively low density near-surface ice-sheet layer comprised of snow being compressed into ice. Firn is relatively porous, meaning that meltwater can percolate through it. The refreezing of meltwater within firn is a potential buffer against the ice-sheet sea-level contribution from surface melt; surface melt can refreeze within porous firn instead of running off into the ocean. Our study aims to assess how big this sea-level buffer might be, and how much sea-level buffer may have already been used.

We pull together a singularly unique dataset – 340 ice-core measurements of firn density collected over 65 years – to assess the near-surface density across the entire high-elevation firn plateau of the Greenland Ice Sheet. Many of these vertical firn density profiles were digitized and brought together for the first time from historical studies, but twenty are collected by our team and new to science. We analyze this ice-core dataset for empirical relations between firn density and accumulation or air temperature. This allows us to divide the ice sheet into three distinct firn areas, within each of which we can confidently predict the vertical profile of near-surface firn density.

Figure 1 – Left: Firn air content within the top 10 m (FAC10) estimated from ice-core measurements (denoted with ‘x’). The ice sheet is divided into three areas: the Dry Snow Area (DSA), the Low Accumulation Percolation Area (LAPA), and the High Accumulation Percolation Area (HAPA). Right: Change in top 10 m firn air content between 1998–2008 and 2010–2017 within Low Accumulation Percolation Area along the ice sheet’s western flank.

We find that the firn structure at the heart of the ice sheet – the highest, coldest and driest firn known as the Dry Snow Area – appears to have been stable since 1953. There is no trend in firn density within the Dry Snow Area. At lower elevations, however, we find significant changes in response to recent increases in surface melt due to climate change. The area we call the Low Accumulation Percolation Area – an elevation band of relatively low snowfall and high melt along the ice sheet’s west flank – has a marked increase in the firn densities measured pre- and post-2009. This firn density change is equivalent to a sea-level buffer loss of 1.5±1.2 mm sea-level equivalent (540±440 gigatonnes).

We compare the ice-sheet-wide firn density structure that we estimate from ice-core measurements with the firn density structure estimated from three regional climate models. The regional climate models suggest that the decrease in firn porosity initiated in the early 2000s and accelerated with post-2010 climate change. But we also find non-trivial differences between the firn porosities simulated by regional climate models, and that inferred from ice-core measurements, especially in what we call the High Accumulation Percolation Area. Here – the ice sheet’s low elevation southeast flank – modeled firn porosity can be biased the equivalent of between 3 and 7 meters of air distributed over the entire firn column depth.

Figure 2 – Left: Ice-sheet-wide firn air content within the top 10 m of firn (FAC10) simulated by three regional climate models (MAR, HIRHAM and RACMO) and derived from ice-core observations (this study) in different ice-sheet areas. Right: Same for firn air content over the entire depth of the firn column (FACtot).

This study highlights the importance of bringing together firn density measurements to document the response of ice-sheet firn – a non-trivial component of the sea-level budget – to recent climate change. The ice-sheet-wide firn porosity structure we infer from ice-core measurements can also serve as an independent evaluation target for the firn porosity structures simulated by regional climate models. This study also illustrates how new insight can be obtained from the synthesis and re-analysis of historical datasets. This emphasizes the tremendous value of open-access data within the scientific community. This work is part of the Retain project funded by the Danmarks Frie Forskningsfond (grant 4002-00234). The open-access publication is available via the hyperlink below.

¹Vandecrux, B., MacFerrin, M., Machguth, H., Colgan, W., van As, D., Heilig, A., Stevens, C., Charalampidis, C., Fausto, R., Morris, E., Mosley-Thompson, E., Koenig, L., Montgomery, L., Miège, C., Simonsen, S., Ingeman-Nielsen, T., and Box, J. 2019. Firn data compilation reveals widespread decrease of firn air content in western Greenland. The Cryosphere. 13: 845-859. https://doi.org/10.5194/tc-13-845-2019.

Fresh Research! — Have you heard that increasing surface melt is filling the near-surface pore space of the Greenland Ice Sheet? Well, we’ve put some *big* numbers on that process! ❄️
blog: https://t.co/JYpeoOiysu
article: https://t.co/vGdpnczWXB
funding: @DFF_raad @NASA pic.twitter.com/ljlAd1ga5B

— William Colgan, Ph.D. (@GlacierBytes) March 12, 2019

We investigate the high-elevation firn plateau of the Greenland Ice Sheet in a new open-access study in the current issue of Journal of Geophysical Research¹. This study pulls together singularly unique – and hard fought – ice core observations and weather station data into a super-neat firn model. This relatively porous near-surface ice-sheet layer known as firn is being increasingly scrutinized for two main reasons.

The first reason is sea-level rise. These high regions of the Greenland ice sheet are normally preserved form intense melting, but this is changing, with more melt seen in recent years. Nevertheless, the porosity of the firn can provide a buffer against sea-level rise when meltwater refreezes within the firn instead of running off into the ocean. But exactly how much of this buffering capacity is available – and for how long – is not really understood.

The second reason is satellite altimetry. Repeat observation of ice thickness by satellite altimeter is a primary method by which ice-sheet mass balance – or overall health – is assessed. But since firn is porous, changes in elevation don’t always translate into changes in mass. For example, the firn layer can become thinner – making the ice-sheet appear thinner – when there’s actually just an increase in firn density rather than a change in mass.

Figure 1 – Locations of the four study sites on the Greenland Ice Sheet’s high-elevation firn plateau.

In this study, we were interested in teasing out the climatic controls of firn density: What makes firn porosity grow and shrink over time? So, we simulated the evolution of firn density – and therefore porosity – over time at four ice-sheet sites. These sites were carefully chosen as sites where both in-situ climate and firn measurements were available (Crawford Point, Dye-2, NASA-SE and Summit). The firn simulations used an updated version of the HIRHAM regional climate model’s firn model. At each site, we initiated simulations using firn density profiles observed from ice cores, and then ran the simulations forward in time using in-situ weather station records. We then ensured that simulated firn density also compared well with repeat firn density profiles observed again many years later. The simulations were between 11 and 15 years, depending on the data available at each site.

Figure 2 – Simulated firn density through time at the four study sites. At all sites, the relative depth of a given layer increases over time, as snowfall exceeds meltwater runoff.

A lot of recent ice-sheet research has focused on how increasing air temperatures and meltwater production are increasing firn density. And our simulations definitely confirmed that! But perhaps counterintuitively, we found that the leading driver of changes in firn density was actually year-to-year changes in amount of snowfall. Firn density decreases as snowfall increases, and vice versa. This study therefore highlights that if we want to project time-and-space variability in firn density we really need to project time-and-space variability in snowfall rates.

Figure 3 – Assessing the relative strength of four drivers of firn density change at the four study sites.

It was also satisfying to see that – given observed climate data – our simulations could reproduce the firn conditions as observed in the field. This gives confidence including this firn model in regional climate models. This finding is of course limited to the high-elevation firn plateau of the Greenland Ice Sheet, which admittedly does not experience tremendous melt. But, as the firn plateau covers over 80% of the ice-sheet area, understanding it plays a key role in tackling pressing satellite altimetry and sea-level buffering questions.

This work is part of the Retain project funded by the Danmarks Frie Forskningsfond (grant 4002-00234). The open-access publication is available via the hyperlink below.

¹Vandecrux, B., R. Fausto, P. Langen, D. van As, M. MacFerrin, W. Colgan, T. Ingeman‐Nielsen, K. Steffen, N. Jensen, M. Møller and J. Box. 2018. Drivers of firn density on the Greenland ice sheet revealed by weather station observations and modeling. Journal of Geophysical Research: Earth Surface. 123: 10.1029/2017JF004597.

We have a new open-access study in the current volume of Frontiers in Earth Science that tries to estimate snow density across the Greenland Ice Sheet¹. Snow density might seem like an unexciting topic, but it is fundamental to blending ice-sheet thinning or thickening observations with surface mass balance simulations to assess ice-sheet health. Clearly, assuming a snow density of 400 kg/m³ makes a snowfall event observed by satellite altimeter twice as massive as assuming a snow density of 200 kg/m³ (and vice versa). There are several mathematical formulations presently being used to estimate snow density. These existing approaches generally estimate snow density as a function of more accessible geographic or climatic parameters.

Figure 1 – Locations of the surface snow density measurements collected in this public dataset. Contours lines indicate elevations in meters above sea level.

In this study, we assembled a large database of snow density measurements from the Greenland Ice Sheet. These measurements were collected from a variety of scientific expeditions going back to 1954, and provide the most complete spatial coverage of the ice sheet that is presently possible. Despite running a lot of statistics on this database, we could not find a compelling proxy for snow density. Our analysis indicates that snow density cannot be reliably predicted by common geographic (i.e. elevation, latitude or longitude) or climatic (i.e. air temperature or accumulation rate) variables. As existing approaches to estimate snow density rely on these common geographic and climatic variables, this was a somewhat unexpected finding.

Figure 2 – Snow density (0 to 10 cm depth) plotted against: (a) measurement year, (b) site latitude, (c) site longitude, (d) site elevation, (e) mean annual air temperature, and (f) accumulation rate.

Our study therefore recommends that the average measured density of 315 ± 44 kg/m³ (± standard deviation) is the most statistically defensible assumption for snow density. This recommendation of a constant, or zero-order approximation, differs from previous studies that have recommended estimating snow density as a second-order polynomial function of near-surface ice-sheet temperature. We show that these previous approaches may systematically overestimate snow density by 17 to 19 %. This is partially due to their mathematical formulations, but mainly due to previously considering measurement depths of up to 1 m as characteristic of “snow density”. As density increases with depth in the relatively porous near-surface layers of the ice sheet, we are instead careful to only include density measurements to a depth of 10 cm.

Figure 3 – Snow density (0-10 cm depth) versus mean annual air temperature. Solid line indicates the regression of this study, while the dotted and dashed lines indicate previously published temperature-dependent formulations for estimating snow density.

We hope that the approach of estimating snow density that we are proposing, which is mathematically less complex but statistically more robust, will be useful to researchers working with both surface mass balance simulations and satellite altimetry observations, as well as researchers modelling process-level studies of snow compaction and meltwater percolation in the near-surface ice-sheet layers. This study was supported by the Danish Research Council and the Programme for Monitoring of the Greenland Ice Sheet. Our database of 254 snow density measurements is freely available in the supplementary material of the study.

¹Fausto R., J. Box, B. Vandecrux, D. van As, K. Steffen, M. MacFerrin, H. Machguth, W. Colgan, L. Koenig, D. McGrath, C. Charalampidis and R. Braithwaite. 2018. A Snow Density Dataset for Improving Surface Boundary Conditions in Greenland Ice Sheet Firn Modeling. Frontiers in Earth Science 6:51. doi:10.3389/feart.2018.00051.

Here’s a plain-written intro to our paper on Greenland Ice Sheet snow density: https://t.co/eKmcnALonB
The unexpected upshot? Snow density does not actually appear to vary w/ geographic or climatic variables! ❄️ pic.twitter.com/ILwFkTZRdS

— William Colgan (@GlacierBytes) May 7, 2018

We have a new study out this month in Frontiers in Earth Science¹ that describes using an old-school hydrogeology method on the Greenland Ice Sheet. We used pump-testing, which has been conventionally used to measure soil permeability for groundwater flow, to infer the permeability of ice-sheet firn to meltwater flow. We wanted to quantitatively measure how massive ice layers formed by refreezing meltwater in the near-surface ice sheet firn could inhibit meltwater flow in subsequent years.

Figure 1 – The low tech and low cost pump-testing device used to infer firn permeability on the Greenland Ice Sheet. A vacuum is applied at depth in the sealed vacuum borehole and the resulting pressure response is measured in the sealed monitoring borehole.

In conventional pump-testing, water is pumped out of a borehole at a controlled rate, and the groundwater level response, or drawdown is observed in a monitoring borehole located some distance away. We did something similar in the ice-sheet firn, pumping air out of a vacuum borehole and measuring the air pressure response is a sealed monitoring borehole about one meter away. We did pump tests at six ice sheet sites that had varying degrees of massive ice layers in the near-surface firn.

We found that vertical permeability between firn layers was generally much lower than horizontal permeability within a firn layer, and that vertical permeability decreased with increasing ice content. At the lowest elevation site, where meltwater production and refreezing is most prevalent, we drilled into an exceptionally massive ice layer the pump borehole was able to maintain an effective vacuum. In other words, thick massive ice layers are indeed impermeable to fluids. That was a little surprising!

Figure 2 – Inferred horizontal (kr) and vertical (kz) firn permeability values at five ice-sheet sites. Horizontal blue lines indicate the depths of ice layers at each site. Vertical cyan and magenta shading represents inferred permeability limits.

While it may sound esoteric, the permeability of near-surface firn is an increasingly visible topic in ice-sheet research. Studies have shown that firn can act to either buffer sea level rise by absorbing meltwater², or enhance sea level rise by forming impermeable refrozen ice layers³. As climate change increases meltwater production within the historical accumulation zone of the ice sheet, a greater area of ice-sheet hydrology will be influenced by refrozen ice layers. In future, higher vacuum pressures and repeated measurements should allow firn permeability to be measured over larger scales to improve our understanding of changing firn permeability.

For now, the proof-of-concept pump-testing device is relatively low tech and low cost. Aside from air-pressure sensors and a data logger, it was constructed by items you could find at your local hardware store; plastic PVC pipes channeling the power of a shop vacuum. Development of the firn pump-testing device was initiated by a University of Colorado Dean’s Graduate Student Research Grant to highly innovative lead-author Aleah Sommers, and it was deployed in collaboration with the FirnCover project during the 2016 field campaign.

Figure 3 – Max Stevens and Aleah Sommers preparing to insert the pressure sensor and seal into the monitoring borehole at Saddle, Greenland, in May 2016.

¹Sommers et al. 2017. Inferring Firn Permeability from Pneumatic Testing: A Case Study on the Greenland Ice Sheet. Frontiers in Earth Science. 5: 20.

²Harper et al. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.

³Machguth et al. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390-393.

Intro to our new study inferring #Greenland firn permeability from pump-testing. Hydrogeology meets the ice sheet!https://t.co/3aPjIlMixd pic.twitter.com/lK76Nfa4gA

— William Colgan (@GlacierBytes) March 6, 2017

We have a new open-access study in the current volume of Annals of Glaciology that tracks the fate of meltwater in the relatively porous near-surface firn of the Greenland Ice Sheet using temperature sensors¹ (available here). One of the main goals of this study was to understand what fraction of the meltwater produced at the ice sheet surface percolates vertically into the firn and locally refreezes, rather than leaving the ice sheet as runoff and contributing to sea level rise. The total retention capacity of all of Greenland’s firn could be a non-trivial buffer against sea level rise².

For this particular study, we deployed firn temperature sensors at depths of up to 15 m at KAN_U. The sensors were automated to record data throughout the year, between our spring sites visits. KAN_U is located at 1840 m elevation in Southwest Greenland in the lower accumulation area. While KAN_U traditionally receives more mass from snowfall than it loses from melt, our study focused on the “extreme” 2012 melt season, which was the first year since records began that there was more meltwater runoff than snowfall at the site.

Figure 1 – Lead author Charalampos Charalampidis drilling a borehole on the Greenland Ice Sheet near KAN_U during the 2013 spring field campaign.

As refreezing meltwater releases a tremendous amount of latent energy, the location of refreezing meltwater within the firn can be inferred from temperature anomalies. We assessed temperature anomalies by comparing our observed firn temperatures against modeled firn temperatures, whereby the modeled temperatures only accounted for heat exchanged with the ice sheet surface via diffusion, not latent heat release. This allowed us to identify depths where firn temperatures were warmer than expected.

Figure 2 – Automated observations of firn temperatures in the top 10 m of firn at KAN_U over four years. There is a strong annual cycle in near-surface firn temperatures.

We found that despite 2012 being an extreme melt year, meltwater percolation and refreezing only occurred to 2.5 m depth during the melt season. It was only after the end of the melt season that some meltwater managed to percolate and refreeze in discrete bands at 5.5 and 8.5 m depth. This inference of relatively inefficient vertical meltwater percolation during the melt season appears to support the idea that thick and impermeable ice lenses that had previously formed within the firn during 2010 were inhibiting the percolation of 2012 meltwater³.

Maintaining the relatively sensitive automatic weather station needed to accurately measure firn temperatures and surface energy fluxes in the relatively harsh ice sheet environment was no easy task. It took a number of scientists and funding agencies, which are listed in the acknowledgement section of the paper, to make this study possible. The KAN_U weather station continues to report real-time climate data via the Programme for Monitoring of the Greenland Ice Sheet (PROMICE) data portal: www.promice.dk.

Figure 3 – A: Location of Kangerlussuaq Upper Station (KAN_U) on the Greenland Ice Sheet. B: A PROMICE climate station deployed to measure firn temperatures and surface energy budget.

¹Charalampidis, C., D. van As, W. Colgan, R. Fausto, M. MacFerrin and H. Machguth. 2016. Thermal tracing of retained meltwater in the lower accumulation area of the Southwestern Greenland ice sheet. Annals of Glaciology. doi:10.1017/aog.2016.2.

²Harper, J., N. Humphrey, W. Pfeffer, J. Brown and X. Fettweis. 2012. Greenland ice-sheet contribution to sea-level rise buffered by meltwater storage in firn. Nature. 491: 240-243.

³Machguth, H., M. MacFerrin, D. van As, J. Box, C. Charalampidis, W. Colgan, R. Fausto, H. Meijer, E. Mosley-Thompson and R. van de Wal. 2016. Greenland meltwater storage in firn limited by near-surface ice formation. Nature Climate Change. 6: 390–393.

Intro to our new #Greenland firn temp. study inferring suppressed melt percolation in 2012: https://t.co/yDRXRzmjaG pic.twitter.com/75EeukYk5v

— William Colgan (@GlacierBytes) May 19, 2016

Our Arctic Circle Traverse 2016 (“ACT16”) campaign is getting underway next month, and one look at the expedition map and it seems like we’ve outgrown our name! The ACT expedition series began in 2004, as snowmobile traverses roughly aligned with the Arctic Circle (66 °N) in support of the NASA Program for Arctic Regional Climate Assessment (PARCA). Since the 2013 initiation of the NASA FirnCover program, however, there has been a strong motivation to simultaneously sample more remote sites on the ice sheet. Firn compaction rate, the key process that FirnCover seeks to measure and model, is sensitive to both air temperature and snowfall rate. That means firn compaction rates vary with latitude and elevation, so when the FirnCover team goes to Greenland, we try to sample the ice sheet from North-South and low-high. That makes for a lot of travel!

Figure 1 – Logistics behind our Arctic Circle Traverse 2016 (ACT16) expedition route. Red denotes US Air National Guard flights. Purple denotes NSF charter flights. Green denotes commercial flights. Blue denotes snowmobile traverses.

This April the ACT16 team will gather in Schenectady, NY to hitch a ride to Kangerlussuaq, GL with the US Air National Guard. After a pause in Kanger, the 109^th Airlift Wing will deliver us to their Camp Raven skiway near Dye-2 in the ice sheet interior. Once in the ice sheet interior, the ACT16 team will fission into two groups, with a base group staying at Dye-2 for detailed firn measurements, and a traverse group snowmobiling to firn instrumentation sites along the Arctic Circle. Afterwards, our two groups will join up and catch an NSF charter flight off the ice to Kanger for some brief decompression. Then a subset of the ACT16 team will fly north to Summit and the NEGIS deep coring site for more firn instrumentation and measurements. Eventually we’ll make our way back to Kanger and head home on commercial flights via Iceland. With military and NSF charter flights, temperamental snowmobiles, and a mix of commercial airlines, the logistics for this five week field season are pretty intense!

Figure 2 – A ski-equipped C-130 from the 109th Airlift Wing of the US Air National Guard taxiing on the Camp Raven skiway near Dye-2 during ACT13.

I’m most excited to visit NEGIS, not because I think it will be any more (or less) spectacular than any other location in the ice sheet interior, but simply because I haven’t been there before. A new dot on the map is always cause for delight. This field season, however, I will be keeping track of my personal carbon footprint, and I expect the charter flight to NEGIS and back is going to figure prominently in that calculation.

This post is cross-posted on the FirnCover blog.