Glacier Bytes

     This week, in an open-access Boreas article, we describe a new Early Pleistocene peat deposit in Northwest Greenland. We discovered the deposit quite accidentally during fieldwork near Thule Air Base in September 2019. That year, there was extreme melt in North Greenland, which removed the seasonal snowpack as far as the eye. Anders Bjørk and I were working at the margin of Pingorsuit Glacier, when eagle-eyes Anders spotted some very dark, organic-rich, sediments. They were very understated tufts of peat, mostly covered in till, that were clearly being released from the melting landscape. Quick examination revealed large pieces of wood, and even a small pinecone. As this type of pine tree haven’t grown in Greenland during the Holocene, we knew this must be very old material! We opportunistically grabbed a few bags of peat to transport back to Copenhagen for Ole Bennike and colleagues to analyze.  

     It is quite tricky to provide absolute dates for Early Pleistocene formations, as they are radiocarbon dead. The published ages of these deposits ranges from 1.9±0.1 Ma at Store Koldewey to 3.4±0.5 Ma at Beaver Pond. Our just-published macrofossil analysis shows that the Pingorsuit Formation has broad similarities with the fossil assemblage observed in the Kap København Formation. We therefore suggest an age of about two-million-years old, or about the same as the Kap København Formation. The Pingorsuit flora reflects an open boreal forest ecosystem, with invertebrates typical of ponds or standing water. Some of these species are extinct during the Holocene. Ole provides the first description of an extinct waterwort species, which he has named Elatine odgaardii after the Danish naturalist Bent Odgaard. From the Pingorsuit species assemblage, we can estimate that its Early Pleistocene ecosystem likely had a mean July air temperature of >10°C. This is at least 9°C warmer than present-day, and approximately equivalent to the North Greenland climate anticipated in 2100 under the SSP5-8.5 (or “very high emissions”) climate scenario.

     Only seven organic-rich Early Pleistocene deposits have been discovered in the High Arctic. Located at ~480 m elevation, the Pingorsuit Formation is likely the only Early Pleistocene organic bed in Greenland to have remained above sea-level since its deposition. All other Early Pleistocene deposits in Greenland reflect deposition in marine conditions. This makes the Pingorsuit Formation rather unique. But it is also perhaps the most fragile Early Pleistocene deposit discovered to date. Unlike the km-scale outcrops of other Early Pleistocene marine deposits, we could only identify five small mounds with a total area of approximately 20 m². These small mounds are all located immediately adjacent to the Pingorsuit Glacier margin. This suggests they are being eroded by flowing water within years of emerging from the subglacial to the subaerial environment. Simply put, the Pingorsuit Glacier appears to be preserving a two-million-year old formation beneath it. The anticipated loss of Pingorsuit Glacier under climate change therefore provides a compelling urgency to fully assess the Early Pleistocene time capsule with the next decade or two.

Bennike, O., W. Colgan, L. Hedenäs, O. Heiri, G. Lemdahl, P. Wiberg-Larsen, S. Ribeiro, R. Pronzato, R. Manconi and A. Bjørk. 2022. An Early Pleistocene interglacial deposit at Pingorsuit, North-West Greenland. Boreas. https://doi.org/10.1111/bor.12596

We have a new open-access study out in the current volume of Journal of Geophysical Research that brings together both radar and laser altimetry measurements to assess the mass balance of the Greenland Ice Sheet between 2011 and 2020. Our assessment shows that the ice sheet lost approximately 498 Gt of ice volume, corresponding to approximately 7 mm of global sea-level equivalent during this time. The peak loss year was from April 2019 to April 2020, when the ice sheet lost 1.4 mm of global sea-level equivalent, which is equivalent to losing 15,850 tonnes of ice per second for an entire year. These values reflect only the ice sheet proper, and ignore Greenland’s peripheral glaciers. 

Over the study period, we estimate that approximately 43% of the ice loss was due to ice dynamics (i.e. more iceberg calving). Surface mass balance, or meltwater runoff, was responsible for the remaining approximately 57% of mass loss. This partition of mass loss varies tremendously between ice-sheet catchments and through time. This allows us to see the ice-sheet responding to recent climate forcing at the scale of individual seasons and catchments. Our assessment even resolves mass loss changes at the level of individual glaciers. For example, we can see that after a brief period of ice thickening during 2018 and 2019, Jakobhavn Isbræ has now returned to substantial ice thinning. 

If you’re really into satellite altimetry, we also make a rather unique cross-comparison between ICESat-2 laser measurements and CryoSat-2 radar measurements. While laser altimetry enjoys a near-complete surface scattering of the incoming laser pulse, radar altimetry has substantial volume scattering, meaning that the incoming radar pulse penetrates some depth into the ice sheet. This makes it difficult to assimilate both laser and radar altimetry measurements into a common processing pipeline. But, after applying the necessary volume-scattering correction to the radar measurements, we can assimilate both the ICESat-2 and CryoSat-2 observations into a common framework that shows good agreement (±8 cm/yr) during the common 2019/20 year.

Finally, to be consistent with an open science mandate and help the community move forward as fast as possible, we make the annual (April to April) ice-sheet maps of volume change that we assess at 1 km spatial resolution available for download at: https://datadryad.org/stash/share/gRoJh1JfpF4EA1d_Prsa_KIju9z2hJXWvXE5J1X2d8I. We hope these data will be useful for not only inter-study altimetry comparisons, but also for initializing models that calculate the elastic uplift of Greenland’s bedrock and evaluating ice flow models that simulate recent ice-sheet mass loss. 

Khan, S. A., Bamber, J. L., Rignot, E., Helm, V., Aschwanden, A., Holland, D. M., van den Broeke, M., King, M., Noël, B., Truffer, M., Humbert, A., Colgan, W., Vijay, S., and Kuipers Munneke, P. (2022). Greenland mass trends from airborne and satellite altimetry during 2011–2020. Journal of Geophysical Research: Earth Surface. 127. e2021JF006505. https://doi.org/10.1029/2021JF00650

We have a new open-access study in the current issue of Geophysical Research Letters that looks at rainfall over the Greenland Ice Sheet. In many places around the globe, rainfall is a big player in the water budget. But on the ice sheet, rainfall has traditionally been a small player in ice-sheet mass balance. In fact, virtually all of the automatic weathers stations deployed on the ice sheet today don’t even measure rainfall. These ice-sheet stations are instead optimized to measure accumulation from snowfall and ablation from melt. But, as major rainfall events are pushing higher and higher on to the ice sheet each year, that is starting to change. Today, there are a few research groups experimenting with different ice-sheet rainfall gauges.

In our new study, we simulate rainfall over the ice-sheet using a regional climate model. Specifically a “non-hydrostatic” model. This class of model is supposed to reproduce the continuum mechanics of atmospheric flow better than traditional “hydrostatic” models. The traditional hydrostatic models make some simplifying assumptions that can influence atmospheric flow and precipitation, especially across grid cells with high topographic relief. In the absence of ice-sheet rainfall observations, we compared the simulated rainfall to several weather stations operating in communities around Greenland’s coast. This comparison showed that the model could reasonably simulate the rainfall, including extreme events, that was observed at these weather stations. This gives us some confidence that the model’s rainfall physics are similarly faithful on the ice sheet.

Over the forty-year period 1981–2010, the model simulates increases in both rainfall and rainfall intensity, across the ice sheet and especially in late summer. We specifically find that total September rainfall increased by 224% over this period. The maximum intensity of September rainfall also increased by 54% during this same period. This is consistent with the expectation that the summer melt season will lengthen and intensify in a warming climate. Some of the most pronounced increases in rainfall were seen in Northwest Greenland. There, the rainfall fraction of precipitation is about twice the ice-sheet average. We speculate that this increasing trend in rainfall in Northwest Greenland may be related to a northward shift in the limit to which relatively warm and moist mid-latitude airmasses can penetrate each summer.

A large rainfall event can have a similar influence on ice dynamics as a large melt event. Once liquid meltwater enters the ice sheet, flowing in the en- and sub-glacial hydrology systems, there are myriad of ways it can make ice flow faster. Chief among these is warming and softening the ice (so the ice internally deforms and flow more easily) and pressurizing the subglacial hydrology system (so the ice-sheet slides more easily). For this reason, there is also an ice dynamic interest in big rainfall events. Some of the most pronounced increase in extreme rainfall events (i.e. >5 cm per hour) were in South Greenland. There, the ice sheet is most exposed to mid-latitude storms from the North Atlantic. There is an expectation for the intensity of storms to increase in a warming climate.

We will probably hear a lot more about rainfall on the Greenland Ice Sheet in the coming years. Hopefully, once some of the technical challenges are overcome, we will start to see the systematic collection of continuous rainfall measurements on the ice sheet. Given our current climate trajectory, we will also start to see rainfall comprising a larger portion of the annual ice-sheet water budget, especially in ice-sheet basins in South Greenland. There are already several studies linking rainfall events to ice motion at lower elevations, but presumably these links will also start to be made at higher elevations. There is a lot of research to do on the topic of ice-sheet rainfall!

M. Niwano, J. E. Box, A. Wehrlé, B. Vandecrux, W. T. Colgan and J. Cappelen. 2021. Rainfall on the Greenland Ice Sheet: Present-Day Climatology From a High-Resolution Non-Hydrostatic Polar Regional Climate Model. Geophysical Research Letters. https://doi.org/10.1029/2021GL092942.

We have a new study in the current issue of Nature Communications. It looks at the stability of the North Open Water polynya over the past five millennia. The North Open Water – or ‘Pikialasorsuaq’ in Greenlandic – is a portion of northern Baffin Bay that is kept sea ice free year-round by atmospheric and oceanic currents. The North Open Water is one of the most biologically productive areas of the Arctic Ocean and has been described as a ‘regional supermarket’ because of the abundance and diversity of country foods that it supports.

In the study, we reconstruct the stability of the polynya – meaning the persistence of sea-ice free conditions over time – using two core records. The first core is an off-shore marine sediment record from beneath the heart of the polynya. The second core is an on-shore lake record from the southeast edge of the polynya, near Pituffik (Thule Air Base). The marine core was analyzed for changes in organic carbon and other indicators of biological productivity over time. The lake core was analyzed for changes in organic compounds associated with bird poop (Little Auk) over time.

We find that when the North Open Water stabilized c. 4400 years ago – meaning persistent and productive open water – Little Auk arrived at the lake. This arrival of Little Auk aligns with the onset of human settlement in Greenland inferred by previous studies. Conversely, when there was a downturn in the stability and productivity of the North Open Water between c. 2000 and 1200 years ago, we find less evidence of Little Auks. This less productive period also aligns with a previously inferred period of human abandonment of Greenland.

Looking towards the future, we suggest that the Kane Basin Ice Arch – which forms the northern border of the polynya – will become less stable under climate change. This means that the ice arch will collapse more regularly, as has been observed over the recent satellite record, allowing sea ice to flow south into the North Open Water. This influx of sea ice will likely result in a less stable and less productive polynya.

My main contribution to this study was helping to analyze the many different individual marine and lake time series to resolve common marine and lake signals to intercompare over the past five millennia. In this case, that meant writing a Monte Carlo code to account for not only the measurement uncertainty, but also the dating uncertainty, of the samples comprising each time series.

Finally, a neat aspect of this study is that it is ‘open science’. This means that in addition to the article being open access, all the underlying data and code are also freely available in this data repository. This includes the Monte Carlo time-series analysis code, which may hopefully be useful for other similar application of along-core dating uncertainty.

We have a new open-access study about Greenland Ice Sheet mass balance – or health – in the current issue of Geophysical Research Letters. In this study, we present a new 28-year record of ice-sheet mass balance. This record is relatively unique for two reasons.

Firstly, because of its length. The most recent ice-sheet mass balance inter-comparison exercise (IMBIE2) clearly highlighted how the availability of ice-sheet mass balance estimates has changed through time. During the GRACE satellite gravimetry era (2003-2017), there are usually more than twenty independent estimates of annual Greenland Ice Sheet mass balance. Prior to 2003, however, there are just two independent estimates. Our new 1992-2020 mass balance record will therefore provide especially welcomed additional insight on ice-sheet mass balance during the 1990s.

Secondly, because of its consistency. This new mass balance record has been constructed by merging radar altimetry measurements from four ESA satellites (ERS-1/2, ENVISAT, CryoSat-2 and Sentinel-3A/B) over nearly three decades into one consistent framework. While all four of these satellites use the same type of Ku-band radar altimeter, to date, their measurements have usually been analyzed independently of each other. This time, however, we use machine learning to merge the elevation changes measured by these similar-but-different satellites into a common mass balance signal through space and time. This makes our new record the only satellite altimetry record that spans the entire IMBIE period.

When we compare our new radar altimetry record of mass balance to two existing input-output records of mass balance, we find good agreement in the capture of Greenland’s high and low mass balance years. These other two multi-decade records are derived from the input-output method, in which estimated iceberg calving into the oceans is differenced from estimated surface mass balance (or net snow accumulation) over the ice sheet. While the input-output method often has limited spatial (and temporal) resolution, our radar altimetry derived record can resolve spatial variability in mass balance across the ice sheet every month since 1992.

While our new long-term record provides a new overview of the health of the Greenland ice sheet, it can also be helpful to understand the processes that influence ice-sheet health. For example, we see a sharp increase in mass balance between 2016 and 2017. When we look at this event in detail, we can attribute it to unusually high snowfall in fall 2016, especially in East Greenland, and unusually little surface melting in summer 2017, throughout the ice-sheet ablation area. We estimate that the 2017 hydrological year was likely the first year during the 21^st Century during which the ice sheet was actually in a state of true “mass balance” – or equilibrium – as opposed to mass loss.

The development of this new dataset was primarily funded by the European Space Agency (ESA), with a little help from the Programme for Monitoring of the Greenland Ice Sheet (www.promice.dk). Our multi-satellite Ku-band altimetry mass balance record is now available as tabulated data – both for the ice sheet, as well as the eight major ice-sheet drainage sectors – at https://doi.org/10.11583/DTU.13353062. Within the next two years, the ongoing Sentinel-3A/B satellite missions are clearly poised to extend Greenland’s radar altimetry record to three decades. This will allow us to start assessing ice-sheet health using the statistics of a 30-year climatology record. This keeps us excited at the prospect of updating this record in the near future. Stay tuned!

Simonsen, S., V. Barletta, W. Colgan and L. Sørensen. 2021. Greenland Ice Sheet mass balance (1992-2020) from calibrated radar altimetry. Geophysical Research Letters. L61865. doi:10.1029/2020GL091216.

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 just completed a study that assesses indicators of Arctic change since 1971. It is available open-access in the current issue of Environmental Research Letters. We assimilate nine hyper-diverse data types – air temperature, permafrost temperature, precipitation, river discharge, tundra greenness, wildfire area, snowcover duration and, of course, sea ice area and land ice loss – into standardized indices. The motivation of this study is to bring together almost a five-decade time-series of biophysical variables into a common open-data framework.

Figure 1 – Three of the nine types of Arctic indicators compiled in this study over the 1971-2017 period. These three biophysical indicators show that Arctic tundra is becoming greener in response to increasing Arctic temperature and precipitation.

If there is one climate variable to rule them all, it is air temperature. Increasing air temperature leads to an intensification of the hydrologic cycle, which is clearly evident as increases precipitation and river discharge. Increasing temperature also drives increasing land ice loss and decreasing fall sea ice area. Beyond just the physical climate system, changing air temperature is also driving changes in the biological ecosystem. There is a startlingly clear – more than 99.9 % certain – correspondence between air temperature and the greenness of Arctic-wide tundra. This means a warmer Arctic is a greener Arctic.

In addition to assessing the nine types of indicators, we also discuss some of the tremendous number of knock-on effects of these biophysical trends. These knock-on, or biophysical cascade, effects include decreased alignment of flowering and pollinating windows for plant; increased prevalence of wildfire ignition conditions; an acceleration of the CO₂ cycle with increased uptake during growing season counterbalanced by increased emissions in spring and fall; conversion between terrestrial and aquatic ecosystems; and shifting animal distribution and demographics. The Arctic biophysical system is now clearly trending away from its 20th Century state and into an unprecedented state, with biophysical implications not only within but beyond the Arctic.

This study was developed within the Arctic Monitoring and Assessment Program (AMAP), with the ambition that high-level Arctic summary statistics are of interest to the forthcoming IPCC Sixth Assessment Report (AR6). Annual time-series of the nine types of Arctic indicators compiled for this study will soon be freely available for download at www.amap.no. For now, you can access them via this GoogleSheet.

Box, J., W. Colgan, T. Christensen, N. Schmidt, M. Lund, F.-J. Parmentier, R. Brown, U. Bhatt, E. Euskirchen, V. Romanovsky, J. Walsh, J. Overland, M. Wang, R. Corell, W. Meier, B. Wouters, S. Mernild, J. Mård, J. Pawlak and M. Olsen. 2018. Key indicators of Arctic climate change: 1971–2017. Environmental Research Letters. 14: doi:10.1088/1748-9326/aafc1b.

We have just completed a study that inventories Arctic land ice loss since 1971. It is available open-access in the current issue of Environmental Research Letters¹. While we scientists have a pretty good idea of the health — or mass balance — of glaciers and ice sheets — or land ice — since the advent of satellite altimetry in the early 1990s, there is a need for better understanding of land ice health during the pre-satellite era. Our new study estimates the annual ice loss from all glacierized regions north of 55°N between 1971 and 2017.

We use in situ data – mass balance measurements from a handful of continuously monitored glaciers – as indicators for the health of land ice in seven Arctic regions. These hard-fought in situ data are scarce, they are only measured at between 20 and 44 Arctic glaciers every year. Extrapolating these data to entire regions is statistically challenging without additional information. Fortunately, independent estimates of regional mass balance are available from satellite gravimetry during the 2003 to 2015 period. This permits calibrating in situ and satellite-derived mass balance estimates during the satellite era. This makes our pre-satellite era estimates fairly robust.

During the 41 years assessed, we estimate that approximately 8,300 Gt of Arctic land ice was lost. It is difficult to contextualize this magnitude of ice loss. The flow of Niagara Falls – which is approximately 2400 m³ per second or about 75 km³ per year – is only equivalent to about half this volume (3500 km³) over the 1971-2017 period. The total Arctic land ice loss that we document represents 23 mm of sea-level rise since 1971. Greenland is by far the largest contributor (10.6 mm sea-level equivalent), followed by Alaska (5.7 mm sea-level equivalent) and then Arctic Canada (3.2 mm sea-level equivalent).

The UN Intergovernmental Panel on Climate Change (IPCC) now highlights two periods – the “recent past” (1986-2005) and “present day” (2005-2015) – as being of special interest in climate change studies. The Arctic land ice contribution to sea-level rise that we inventory increased from 0.4 to 1.1 mm sea-level equivalent between these periods. In terms of tonnes per second (5,000 to 14,000 t/s), both the magnitude – and the increase – are staggering.

Figure 1 – The cumulative sea-level rise contribution (in mm) from land ice in seven regions of the Arctic between 1971 and 2017. Analogous estimates from satellite gravimetry (GRACE) between 2003 and 2015 shown with open symbols.

The uncertainties associated with extrapolating sparse in situ data over large areas are undeniably large. But, the reality is that climate change was already gearing up as the global satellite observation network came online. So, in the absence of satellite data that can characterize the “pre-climate change” health of Arctic land ice, we need to leverage the extremely precious pre-satellite era observations that are available in creative ways. We hope that the ice loss estimates we present will be useful comparison targets for studies that estimate pre-satellite era mass balance in other ways.

The estimates of annual land ice mass balance — or health — in seven Arctic regions produced by this study are freely available for download here. This study was developed within the Arctic Monitoring and Assessment Program (AMAP) and International Arctic Science Committee (IASC) frameworks, as a direct contribution to the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC).

Figure 2 – Annual land ice mass balance — or health — in six Arctic regions between 1971 and 2017. Individual glacier mass balance records (blue lines) are combined into a regional composite (black line). Health is expressed both as a normalized score (left axis) and in gigatonnes per year (right axis). The numbers of glaciers comprising each composite is indicated in red text.

¹Box, J., W. Colgan, B. Wouters, D. Burgess, S. O’Neel, L. Thomson and S. Mernild. 2018. Global sea-level contribution from Arctic land ice: 1971 to 2017. Environmental Research Letters.

Our study assessing Arctic glacier loss since 1971 is out today.
I've written about *billions* of tonnes of ice loss before, but this is my first time writing about sea-level rise from *trillions* of tonnes. 🌊🙄
blog: https://t.co/GP880GMNpl
study: https://t.co/KSUXDTtBpS pic.twitter.com/Rz95RmIg1t

— William Colgan, Ph.D. (@GlacierBytes) December 21, 2018

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 are introducing a rich trove of ice-sheet surface mass balance measurements in an open-access study in the current issue of Journal of Geophysical Research¹. The Qagssimiut Lobe is among the most southern ice lobes of the Greenland Ice Sheet. The Q-transect – which runs up the heart of the Qagssimiut Lobe – has been home to automatic weather stations recording ice and climate measurements since 2000. In this study, we have compiled sixteen years of annual surface mass balance measurements and also added three hard-fought years of winter snow accumulation measurements. These data – spanning 300 to 1150 m elevation – now form an exceedingly unique record of ice-sheet health.

Figure 1. The Qagssimiut Lobe in South Greenland. Measurement locations are denoted with white dots. The Sermilik Glacier catchment is delineated with a black line. The ice-sheet margin is delineated with a white line. The background image was acquired by the ESA Sentinel-2 satellite on 28 August 2016 and clearly illustrates the bare ice area below equilibrium line altitude.

These comprehensive in situ measurements allowed us to evaluate the accuracy of the surface mass balance simulated by climate models. TO do this, we stacked our measurements against comparable simulations from three leading regional climate models (HIRHAM5, MAR and RACMO2). The climate models generally did well, but were never bang-on the measurements. One climate model consistently simulated more negative surface mass balances and lower equilibrium line altitudes than we measured. The other two model usually did the opposite, implying the ice sheet was healthier than in reality. These biases appear to stem from differences in simulated winter snow accumulation – which can vary by 200 % at low elevations – between models.

Figure 2. Elevation profiles of measured and simulated winter snow accumulation in (a) 2013/2014, (b) 2014/2015, and (c) 2016/2017. Shaded areas indicate uncertainty ranges. In (c), black lines illustrate the comparison of the model mean for 2000/2001 to 2015/2016 with the 2016/2017 observations.

Combining our knowledge of surface mass balance over the Qagssimiut Lobe with independent observations of iceberg calving rate at Sermilik Glacier – the main tidewater draining Qagssimiut Lobe – allowed us to calculate a total mass balance. We found that the relatively small Sermilik Glacier catchment is now losing up to 2.7 Gt of ice per year. That is a rather astounding – 20 times greater than the ice sheet average – the Sermilik Glacier catchment represents only about 0.03 % of ice-sheet area but is contributing about 0.61 % of ice-sheet mass loss. Its extreme southern location clearly makes Sermilik Glacier a hotspot of ice-sheet mass loss. Its rate of ice loss is more characteristic of lower latitude Andean glaciers than the vast majority of Greenland.

Figure 3. Left: Estimated total mass balance of Sermilik Glacier catchment between 2001 and 2012 in Gt/yr (uncertainty denoted by spread). Right: The Sermilik Glacier catchment overlaid on an ice velocity map derived from the ESA Sentinel-1 satellite. Thin lines indicate adjacent ice flow lines.

We hope that this study will be useful to climate modelers, as they further improve the accuracy with which their models simulate ice-sheet surface mass balance. We also hope that highlighting the Q-transect as a hotspot for both ice loss and in situ data availability will help inform future measurement campaigns seeking to improve our understanding of the physical processes influencing surface mass balance. All measurements of surface mass balance and winter snow accumulation are freely available in the study’s online material.

¹Hermann, M., J. Box, R. Fausto, W. Colgan, P. Langen, R. Mottram, J. Wuite, B. Noel, M. van den Broeke and D. van As. 2018. Application of PROMICE Q-transect in situ accumulation and ablation measurements (2000-2017) to constrain mass balance at the southern tip of the Greenland ice sheet. Journal of Geophysical Research. 123: 10.1029/2017JF004408.