Climate Change

28-Year Record of Greenland Ice Sheet Health

Posted by William Colgan on January 14, 2021
Climate Change, New Research, Sea Level Rise / No Comments

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.

Figure 1 – Greenland Ice Sheet mass balance estimated by IMBIE2 between 1992 and 2018. The number of independent estimates comprising each annual estimate is shown. Prior to 2003, there are only 1 or 2 independent estimates of ice-sheet health each year.

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.

Figure 2 – Comparison of our new multi-satellite radar-altimetry derived record of ice-sheet health (“Radar-VMB”) with two records estimated by the input-output method (“Colgan-IOMB” and “Mouginot-IOMB”), as well as one record estimated by satellite gravimetry (“GRACE-GMB”).

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.

Figure 3 – Our multi-satellite radar-altimetry derived map of declining ice-sheet health over the (a) the 1992-1999, (b) the 2000-2009, and (c) the 2010-2020 periods.

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 21st 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.

Tags: , , , , , , , , , , , ,

‘Cold Content’ of Greenland’s Firn Plateau

Posted by William Colgan on April 29, 2020
Climate Change, Communicating Science, New Research / No Comments

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..

Tags: , , , , , , , , , ,

Five Decades of Arctic Change

Posted by William Colgan on April 08, 2019
Climate Change, New Research / No Comments

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.

Tags: , , , , ,

Eight trillion tonnes of Arctic ice lost since 1971

Posted by William Colgan on December 20, 2018
Climate Change, New Research, Sea Level Rise / 2 Comments

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.

Tags: , , , , , , ,

Changes in Ice-Sheet Density: How and Why?

Posted by William Colgan on October 25, 2018
Climate Change, Communicating Science, New Research, Sea Level Rise / No Comments

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.

Tags: , , , , , , , ,

Q-Transect: A Hotspot of Greenland ice loss

Posted by William Colgan on June 19, 2018
Climate Change, New Research / No Comments

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.

Herm_1

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.

Herm_2

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.

HERM_3

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.

Tags: , , , , , , ,

Draining the Ice-Sheet: Crevasses vs Moulins

Posted by William Colgan on March 22, 2017
Climate Change, New Research / No Comments

We have an open-access study that explores the various ways by which meltwater drains from the Greenland ice sheet in the current volume of Journal of Glaciology1. While surface meltwater rivers and lakes are a conspicuous feature of the Greenland ice sheet, virtually all meltwater that drains from the ice-sheet margin is flowing either within or beneath the ice sheet, rather than on its surface. Where and how meltwater reaches the ice-sheet bed can have implications on ice dynamics. For example, the sharp pulses of meltwater transmitted to the bed by near-vertical conduits called moulins are believed to cause more sliding at the ice-bed interface than the subdued pulses transmitted by crevasses.

In the study, we simulated a portion of the ice sheet in West Greenland known as Paakitsoq, using a computer model. The model used the measured ice sheet topography, as well as observed locations of crevasses and moulins, to simulate how meltwater was produced and flowed across the ice sheet surface under the climate conditions of the 2009 average intensity melt season and the 2012 extreme intensity melt season. The model could also simulate catastrophic lake drainage events, known as hydrofracture events, when a surface lake becomes sufficiently deep that its pressure fractures the underlying ice and creates a new moulin.

Figure 1 – Location of the Paakitsoq study area. Black outline denotes the model domain. Blue dots denote moulin locations identified in satellite imagery. Black contours denote surface elevations. Base image is Landsat-8 from 4 August 2014.

Our simulations suggested that, during an average intensity melt season, crevasses drain almost half (47 %) of ice-sheet meltwater. The hydrofracture of surface lakes drained about 24 % of ice-sheet meltwater, the majority of which resulted from drainage into new moulins following hydrofracture events, rather than the hydrofracture events themselves. Previously existing moulins drained an additional 15 % of meltwater. (The remaining meltwater either reached the ice-sheet margin, remained stored within the model area, or drained to ice-sheet areas North or South of the model area.)

While our simulations suggest that crevasses now drain more meltwater from the ice sheet at Paakitsoq (47 %) than previously existing and newly hydrofractured moulins together (39 %), our 2012 extreme intensity melt season simulation suggests that this ratio may change. The proportion of meltwater drainage via moulins increases, and the proportion of drainage via crevasses decreases, in the 2012 extreme intensity melt season simulation, which may be characteristic of a warmer future climate. This increase in both relative and absolute moulin drainage under warmer conditions is due to an increase in moulins created by hydrofracture events, as meltwater production moves to higher elevations.

Figure 2 – Partitioning surface meltwater drainage into eight categories under the average melt (2009; R1) and extreme melt (2012; R11) simulations. “Lake” is the volume remaining in lakes at season end. “Remaining Flow” is the volume in transit at season end. “Lateral Outflow” is the volume that drains through North and South model domain boundaries. “Ice Margin” is the volume that reaches the ice-sheet edge. The volumes captured in “Crevasses” and “Moulins” are denoted. The volumes drained by lake hydrofracture induced surface-to-bed connections are “Lake Hydrofracture Lake” (LHL), while the subsequent drainage into the new surface-to-bed connection is “Lake Hydrofracture Moulin” (LHM).

Overall, our study provided insight on the space-time variation of pathways by which the vast majority of ice-sheet meltwater descends to the ice-sheet bed prior to reaching the ice-sheet margin. A better understanding of how meltwater travels through the ice-sheet can help improve scientific understanding of not only the ice-sheet mass loss caused by runoff, but also the implications of increasing meltwater production on the mass loss caused ice dynamics. Our simulations also suggest there is value in ice-sheet wide mapping of surface hydrology features like crevasses, rivers, lakes, and moulins, as computer models can use this knowledge to improve simulations of meltwater routing.

1Koziol, C., N. Arnold, A. Pope and W. Colgan. 2017. Quantifying supraglacial meltwater pathways in the Paakitsoq region, West Greenland. Journal of Glaciology. 1-13. doi:10.1017/jog.2017.5

Tags: , , , , , ,

Suppressed Melt Percolation in Greenland Firn

Posted by William Colgan on May 19, 2016
Climate Change, New Research / No Comments

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.

Fieldwork

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.

Babis_thermistor

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.

KAN_U_location

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.

Tags: , , , , , , ,

Greenland Ice Sheet Melt-Albedo Feedback

Posted by William Colgan on December 01, 2015
Climate Change, New Research / No Comments

We have a new study in the current issue of The Cryosphere that looks at the surface energy budget at a site on the Greenland Ice Sheet, and particularly the energy available for meltwater production, over a five-year period spanning the 2010 and 2012 exceptional melt years1. While both the summers of 2010 and 2012 were exceptionally warm, only 2012 resulted in a negative mass balance. In fact, 2012 was the first year since records began that there was more meltwater runoff than snowfall at the site (KAN_U at 1840 m elevation in Southwest Greenland).

In the study we describe how the 2010 exceptional melt year appears to have preconditioned the near-surface layers of the ice sheet to dramatically strengthen the melt-albedo feedback in the subsequent 2012 exceptional melt year. Essentially, we suggest that near-surface ice lenses created by refreezing meltwater in the 2010 melt season made the ice sheet surface transition more readily from relatively high albedo light snow to relatively low albedo dark ice in the 2012 melt season. The substantially darker 2012 ice sheet surface absorbed more solar energy, and therefore caused more melt per ray of sunshine, than in 2010. We estimate that this melt-albedo feedback resulted in approximately 58 % more solar energy absorbed, and available for melt, in 2012 than in 2010.

While 2010 and 2012 were exceptional melt seasons in the context of the past thirty years, they are likely to have foreshadowed the upcoming thirty years. As Greenland climate is now rapidly warming, summer melt intensity no longer oscillates around its long term mean, and instead previously exceptional events are becoming normal. We therefore speculate that under persistent climate change, the firn at the KAN_U site will likely become saturated with refrozen ice lenses, which will enhance the melt-albedo feedback and perhaps even inhibit the downward percolation of meltwater. Ultimately, this will accelerate the transition of the contemporary lower accumulation area underlain by firn into an ablation area underlain by superimposed ice.

Maintaining the relatively sensitive automatic weather station needed to accurately measure 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.

2010_2012_Fluxes

Figure 1 – Monthly mean energy fluxes observed at KAN_U: shortwave (ES), longwave (EL), sensible heat (EH), evaporative (EE), geothermal (EG), precipitation (EP) and melt (EM). The melt flux was calculated as a residual.

KAN_U_location

Figure 2 – A: Location of Kangerlussuaq Upper Station (KAN_U) on the Greenland Ice Sheet. B: The PROMICE climate station deployed to measure surface energy budget.

1Charalampidis, C., D. van As, J. Box, M. van den Broeke, W. Colgan, S. Doyle, A. Hubbard, M. MacFerrin, H. Machguth and C. Smeets. 2015. Changing surface–atmosphere energy exchange and refreezing capacity of the lower accumulation area, West Greenland. The Cryosphere. 9: 2163-2181.

Tags: , , , , , , ,

New Book: Iluliaq – Isbjerge – Icebergs

Posted by William Colgan on September 22, 2015
Climate Change, Communicating Science, Glaciers and Society / No Comments

I was very pleased to have the opportunity to write a preface for Iluliaq – Isbjerge – Icebergs, which contains 100+ pages of watercolours and photographs depicting diverse icebergs around Greenland, along with accompanying Danish/English narration about the iceberg lifecycle (ISBN 978-87-93366-34-3 | available here). I am very supportive of projects like this, which seek to bridge the arts-sciences chasm. It was actually science-editing the iceberg factoids in this book that compelled me to start providing mass loss rates in equivalent tonnes per second in my subsequent publications. I now find saying that Greenland is losing 262 gigatonnes of ice per year, is more abstract than saying it is losing 8300 tonnes per second. Evidently, my perspective was shifted by this delightful project! Below I provide the preface in full.

iluliaq

Preface for Iluliaq – Isbjerge – Icebergs:

“While an individual iceberg is ephemeral, icebergs are a ubiquitous feature of Greenland’s landscape. The shifting nature of icebergs, a constantly drifting and capsizing population, makes them challenging to observe. As they are partway through the transition from glacier ice into ocean water, icebergs are somewhat peripheral to both glaciology/geology and oceanography. Despite these intrinsic difficulties in their study, however, icebergs have never been more important to society than today. Due to climate change, Greenland’s glaciers are now flowing faster than a century ago. The resulting increase in Greenland’s iceberg production is now raising global sea level by 2 cm each decade.

In contrast to the iconic climate change indicators of diminishing sea ice area and glacier volume, there are now more icebergs being produced than a century ago. This provides a very strong motivation to understand the iceberg lifecycle. This lifecycle begins with a thunderous calving at genesis, followed by years of slow drifting and reduction, and quietly ends when the last ice melts into water. In this book, Pernille Kløvedal Nørgaard, Martin von Bülow and Ole Søndergaard provide visually compelling insights on selected aspects of this lifecycle.

By ensuring they not only communicate the natural majesty, but also climatic importance, of Greenland’s icebergs, the authors are helping icebergs assume a rightful place in contemporary public consciousness. The sense of humility evoked by the icebergs depicted here will be familiar to Arctic enthusiasts. These photos and watercolours represent multiple expeditions and extensive travels around Greenland. Similar to documentarians and artists who have accompanied polar expeditions since the Victorian Era, the authors have intentionally sought out a harsh environment, and invited confrontation with adverse conditions, to encapsulate a unique feature of Earth that most people could otherwise never appreciate. Society benefits from such hardy souls, whose passion for nature allows bleak and inaccessible landscapes to be transmitted into our civilized homes.”

Tags: , , , , , ,