BLOG

New Research

Future of East Rongbuk Glacier

Posted by William Colgan on May 13, 2024
Climate Change, Communicating Science, New Research / No Comments

We have a new study out that looks at the fate of East Rongbuk Glacier under the current range of IPCC Shared Socioeconomic Pathways (SSPs). East Rongbuk Glacier is located on the north slope of Mount Everest, also called Chomolungma, where the climate is very cold and dry. The relatively little snowfall at East Rongbuk Glacier makes it sensitive to small increases in surface melt associated with climate change. Many different glaciological observations, including surface mass balance, ice thickness and ice temperature, have been collected at East Rongbuk Glacier by different teams working at the site over many years. This makes it a relatively data rich glacier site within high mountain Asia.

Figure 1 – East Rongbuk Glacier outlined on the north slope of Mount Everest (Chomolungma). Colors depict ice surface velocities overlaid on a satellite image of the Himalayas. Red dots denote boreholes.

In this study, we simulate East Rongbuk Glacier with an ice flow model. The surface forcing of the ice flow model is the magnitude and spatial distribution of accumulation and ablation, as well as surface temperatures. We use climate data from ten different CIMP6 ensemble members as surface forcing. The model then simulates the thickness, temperature and velocity of the glacier that fits with this surface forcing. Once we ensure that the model simulates the present-day glacier thickness, temperature and velocity, as well as recently observed ice loss, we apply the SSP-126 (low emissions), SSP-370 (middle of the road) and SSP-585 (high emissions) climate projections of the CIMP6 ensemble. The model then simulates the changing form and flow of the glacier under these future scenarios.

Although we use ten CMIP6 climate forcings, we generally focus on the pattern revealed by the ensemble mean, rather than any one particular CMIP6 climate projection. We find that under the SSP-126 scenario, East Rongbuk Glacier will likely experience maximum, or peak, meltwater runoff around 2030. About 55% of the glacier volume will remain at the end of this century. Under the SSP-370 and SSP-585 scenarios, the peak water will likely occur around 2060. Under these scenarios, less than 30% of the glacier volume will remain at the end of this century. While glacier volume less per year is decreasing by the end of the century under SSP-126, it is still decreasing quite rapidly under SSP-370 and SSP-585. This suggests that adopting SSP-126 would have a significant impact on stabilizing the volume of East Rongbuk Glacier. This may be characteristic of broadly similar glaciers in high mountain Asia.

Figure 2 – Normalized cumulative volume and area changes of East Rongbuk Glacier during 2010–2100 under three IPCC climate scenarios (SSP-126, SSP-370, and SSP-585). Dotted color lines represent the ten individual CMIP6 ensemble members, while the thick black line represents ensemble mean.

We do a variety of simulations to test the sensitivity of these projections against assumptions of the model. These sensitivity simulations highlight the importance of including ice dynamics, which change glacier geometry, when projecting meltwater runoff decades into the future. Many projections of glacier runoff in high mountain Asia do not yet include changes in ice geometry over time. While important, however, the impact of incorporating ice dynamics into glacier projections is still limited in comparison to the choice of climate forcing. Individual members of the CMIP6 climate forcing ensemble can yield very different projections. For example, a “high sensitivity” CMIP6 member suggests that East Rongbuk Glacier may disappear completely by the end of the century under SSP370, while a “low sensitivity” CMIP6 member suggests that 40% of the glacier may remain at the end of the century under SSP585. This highlights the substantial uncertainty in our present understanding of the fate of East Rongbuk Glacier.

Zhang, T., Wang, Y., Leng, W., Zhao, H., Colgan, W., Wang, C., Ding, M., Sun, W., Yang, W., Li, X., Ren, J., and Xiao, C. 2024. Projections of Peak Water Timing From the East Rongbuk Glacier, Mt. Everest, Using a Higher-Order Ice Flow Model. Earth’s Future. 12, e2024EF004545. https://doi.org/10.1029/2024EF004545

Tags: , , , , , , , ,

Geothermal Influence on Basal Ice Temperatures

Posted by William Colgan on January 28, 2024
Climate Change, Communicating Science, New Research / No Comments

We have a new open-access study out in the current volume of The Cryosphere that looks at geothermal heat flow beneath the Greenland Ice Sheet. Geothermal heat flow is an important boundary condition for ice flow models because it influences the temperature of the ice-bed interface, which in turn influences how easily ice can deform and flow. Right now, there are about seven widely used estimates of geothermal heat flow beneath the Greenland Ice Sheet (Figure 1). These different heat flow maps come from different research groups, using different methods. It hasn’t been entirely clear what the influence of choice of heat flow map has on modeled ice flow. For example, the ensemble of Greenland ice flow projections within the Ice-Sheet Model Inter-comparison Project for CMIP6 (ISMIP6) used differing heat flow maps. Our new study simply spins up a Greenland ice flow model with all seven heat flow maps and tries to understand the resulting differences in ice thickness and velocity.

Figure 1 – The differences in the magnitude and spatial distribution of geothermal heat flow, relative to ensemble mean, in the seven Greenland geothermal heat flow maps we assessed in this study.

Although the average geothermal heat flow only varies by about ±10 mW/m2 across the seven heat flow maps, there are pronounced variations in the spatial distributions of this heat flow. This means that there can be local heat flow differences of up to ±100 mW/m2 between individual heat flow maps. When the ice sheet model is spun up in a fully transient mode with these different heat flow maps, there are lots of areas where the difference in ice thickness from ensemble mean exceeds ±150 meters (Figure 2). This is due to spatial differences in basal ice temperatures, which strongly influence the viscosity and deformation of ice. Across the seven transient spin ups, the discharge of icebergs into the ocean varies by about ±10 gigatonnes per year, which is equivalent to about ±2.5% of the total ice-sheet iceberg discharge.

Figure 2 – The differences in ice thickness after a fully transient 10,000 year spin up, relative to ensemble mean, associated with the seven Greenland geothermal heat flow maps we assessed in this study.

We also spun up the ice flow model with the seven different heat flow maps under a so-called “nudged” spin up that was a key part of ISMIP6. Unlike a fully transient spin up, a “nudged” spin up constrains modeled ice thicknesses with observed ice thicknesses; it generally ensures a very realistic geometry for the simulated ice sheet. Although the ice geometry is more-or-less constant across these “nudged” simulations, there are still pronounced differences in the magnitude and spatial distribution of ice velocity. This largely results from large differences in the extents of the frozen and thawed ice-bed areas. Depending on choice of heat flow map, between 22 and 54% of the ice-bed area is simulated as thawed (Figure 3). This has strong implications for the proportion of the ice sheet beneath which water-dependent processes, like basal sliding, can occur.

Figure 3 – The differences in temperature at the ice-bed interface after a nudged ISMIP6-style spin up, relative to pressure melting point, associated with the seven Greenland geothermal heat flow maps we assessed in this study.

So, what is the way forward after showing that the choice of heat flow map has a non-trivial impact on Greenland ice sheet simulations? Well, deciding which heat flow map is “the best” is one approach. We highlight a small, but growing, database of in situ temperature measurements against which simulated ice temperatures can be compared. There are also qualitative hints as to which heat flow map might be most appropriate. For example, does it preserve the widespread frozen basal conditions that we find in North Greenland? In terms of community ice-sheet projections, we recommend that it may be prudent to limit the direct inter-comparison of ice-sheet simulations to those using a common heat-flow map. In terms of ISMIP specifically, we suggest that future ensemble should perhaps use a range of basal geothermal forcing scenarios, similar to how it employs a range climate forcing scenarios.

Zhang, T., Colgan, W., Wansing, A., Løkkegaard, A., Leguy, G., Lipscomb, W. H., and Xiao, C.: Evaluating different geothermal heat-flow maps as basal boundary conditions during spin-up of the Greenland ice sheet, The Cryosphere, 18, 387–402, https://doi.org/10.5194/tc-18-387-2024, 2024.

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

Bedrock Uplift from Greenland’s Peripheral Glaciers

Posted by William Colgan on January 16, 2024
Climate Change, Communicating Science, New Research / No Comments

We have a new article in the current issue of Geophysical Research Letters that looks at the influence of Greenland’s peripheral glaciers on vertical bedrock motion. Greenland’s bedrock is currently uplifting, due to both slow mantle-deformation processes associated with ice loss at the end of the Last Glacial Period, and fast elastic processes associated with ice loss today. The vertical bedrock uplift being measured in Greenland today ranges from a couple millimeters to a couple centimeters across the country. Understanding the magnitude and spatial distribution of this uplift helps us understand not only recent ice loss, but also properties of the Earth’s mantle beneath Greenland.

Figure 1 – Time series of the observed vertical land motion (VLM) at Mestersvig (MSVG) station in East Greenland. The elastic rebound associated with the Greenland Ice Sheet (GrIS), Greenland Peripheral Glaciers (GrPG) and Canadian Peripheral Glaciers (CanPG) are calculated. The post-Last Glacial Period glacioisostatic adjustment (U_GIA) is then calculated as a residual.

When folks create maps of Greenland’s present-day uplift rate, they typically use a model of changing ice-sheet geometry through time, to incorporate the effect of changing ice load on the Earth’s crust. This captures the main signal, but it ignores the cumulative effect of Greenland’s thousands of peripheral glaciers. These glaciers, which surround the ice sheet, also effect vertical bedrock motion. In this study, we also incorporate the effect of changing peripheral glacier geometry through time into uplift rates calculated at all the GNET bedrock motion sites around Greenland. In the figure above, you can see the vertical land motion budget of MSVG (Mestersvig) GNET station, which calculates post-Last Glacial Period glacioisostatic adjustment (GIA) as the residual of present-day elastic rebound.

Figure 2 – Comparison between post-Last Glacial Period glacioisostatic adjustment (GIA) that we calculate across the 58 GNET stations, compared to four widely used maps of Greenland GIA. A mismatch between the station color and the map color highlights a discrepancy between the previously calculated GIA and the GIA calculated in this study. These four previous studies used different methods, but all ignored the elastic rebound associated with peripheral glaciers.

We find that peripheral glaciers can have a disproportionately large impact on the elastic rebound of GNET sites, especially when they are located relatively far from the ice sheet. In some regions, especially in Greenland’s north and northeast, peripheral glaciers can contribute to over 20% of the total elastic response of regional GNET sites. Simply put, mapping Greenland’s present-day uplift rate with models that only incorporate the ice sheet, and not peripheral glaciers, can really underestimate the elastic rebound associated with present-day ice loss. Under estimating present-day elastic rebound can result in subsequently over estimating the post-Last Glacial Period glacioisostatic adjustment that is used to infer mantle properties.

So, perhaps the main message of this study is that although Greenland’s peripheral glaciers are quite small in comparison to the ice sheet, their recent collective ice loss can influence our understanding of Greenland’s vertical land motion in a disproportionately large way!

Open-Access Study: Berg, D., Barletta, V. R., Hassan, J., Lippert, E. Y. H., Colgan, W., Bevis, M., et al. (2024). Vertical land motion due to present-day ice loss from Greenland’s and Canada’s peripheral glaciers. Geophysical Research Letters, 51, e2023GL104851. https://doi.org/10.1029/2023GL104851

Tags: , , , , , , ,

Melting ice reveals two-million-year old peat

Posted by William Colgan on August 31, 2022
Climate Change, New Research / No Comments

     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.  

Figure 1Left: The pinecone that immediately told us the organic-rich deposit was pre-Holocene in age. Right: A dark peat tuff emerging from beneath glacier till by erosion from a supraglacial stream.

     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.

Figure 2 – Left: Pingorsuit Glacier in 1985. Right: Pingorsuit Glacier in 2019. The ice has retreated approximately 150 m during the 35 years. The Pingorsuit Formation sampled were located at the dot.

     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

Tags: , , , , , ,

Greenland Ice Sheet mass loss from combined CryoSat-2 and ICESat-2 altimetry

Posted by William Colgan on March 31, 2022
Climate Change, New Research, Sea Level Rise / No Comments

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. 

Figure 1 – Annual mass loss, partitioned into meltwater runoff (SMB) and iceberg calving (Dynamic) components, across the eight major ice-sheet sectors during 2011-2020. Central map shows the average ice volume change over 2011-2020 resolved from both CryoSat-2 and ICESat-2 altimetry measurements. Individual glaciers indicated: Jakobshavn Isbræ (JI), Helheim Glacier, Kangerlussuaq Glacier, Nioghalvfjerdsfjorden Glacier, the Zachariae Isstrøm, Storstrømmen Glacier, Petermann Glacier, Humboldt Glacier, and Northeast Greenland Ice Stream.

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.

Figure 2 – Ice-sheet volume change over the April 2019 to April 2020 period from ICESat-2 laser altimetry (A), CryoSat-2 radar altimery (B) and their difference (C). This year of peak mass loss, during which time the ice sheet was well-sampled by both altimeters, saw a record ~498 Gt of ice loss from the ice sheet.

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

Tags: , , , , , , , ,

Greenland Bedrock Uplift and Iceberg Discharge

Posted by William Colgan on August 22, 2021
New Research / No Comments

We have a new open-access study linking bedrock uplift and iceberg discharge at three major Greenland outlet glaciers in the last issue of Geophysical Research Letters. We look at recent changes in observed uplift rates and ice discharges at Jakobshavn, Kangerlussuaq and Helheim Glaciers. The idea of the study was to explore what we thought was a rather straightforward relation between uplift and discharge – uplift rates are relatively high when discharge rates are relatively high (and vice versa) – and see if there as any predictive power in this relation.  

The uplift rates are observed at GNet GPS stations and the ice discharges are observed by satellite-derived ice velocity combined with knowledge of ice thickness. When we analyzed these records, we found that the uplift-discharge relation is indeed very statistically strong, but – rather counterintuitively – at two of the glaciers it was bedrock uplift that serves as a good predictor for ice discharge. Simply put, rather than changes in bedrock uplift lagging changes in ice discharge, we instead found that changes in ice discharge lag changes in bedrock uplift. Clearly, surface mass balance is the primary and instantaneous driver of elastic bedrock uplift; bedrock uplift increases immediately after a big melt and runoff event. We are effectively showing that the associated ice discharge response is lagged.

Figure 1 (a) Predicted detrended dynamic ice loss from past GNet GPS data at Jakobshavn Glacier (blue curve) and satellite-observed ice discharge (black curve). (c) Same as (a) but for Helheim Glacier. (d) Cumulative dynamic records instead of detrended records. (f) Same as (d) but for Helheim Glacier. Note the differing offsets between records at Jakobshavn and Helheim Glaciers.

At Jakobshavn Glacier, changes in ice discharge appear to lag changes in bedrock uplift by almost one year (0.87 years). Simply put, if there is a big melt and uplift event in August, the ice discharge response will peak the following June. If we trust this relation, recent uplift observations at Jakobshavn Glacier suggest that ice discharge will return to pre-2018 levels by the end of 2021. This would mark a clear end to a three-year period of relatively low ice discharge and ice-sheet thickening in the lower reaches of the ice stream over the 2016-2018 melt seasons. At Helheim Glacier, by contrast, there was no significant lead or lag; changes in uplift rate seem completely coincident with changes in ice discharge. Simply put, peak uplift and ice dischrage tends to be simultaneous.

Figure 2 Locations of the KAGA G-Net station at Jakobshavn Glacier (left) and the HEL2 G-Net station at Helheim Glacier. The relation between bedrock uplift and ice discharge is dependent on many local factors like geology, ice configuration, and glacier hydrology.

You can speculate that this uplift-discharge relation changes from glacier to glacier around Greenland due on local differences in bedrock geology and glacier dynamics or hydrology. Reflecting, for example, the elastic modulus of the bedrock or the reservoir time of englacial hydrology of each glacier. The sensitivity of this relation – meaning how many mm/yr uplift per Gt/yr mass loss – also varies from GPS station to GPS station based on the local ice configuration and distance of the GPS station to the center of ice loss. These relations are therefore only valid over local scales.

Overall, however, it does seem possible to use the GNet stations to develop local relations between bedrock uplift and ice discharge on a glacier-by-glacier basis all the way around Greenland. This would be very helpful for using GPS stations to reconstruct detailed records of local ice loss prior to the 2016 onset of weekly satellite monitoring of ice discharge. Exploring this uplift-discharge relation at more GNet stations may also help us understand exactly why sub-annual changes in ice discharge appear to be lagging changes in vertical bedrock motion at some glaciers. Any new clues about processes that regulate Greenland’s ice discharge into the ocean are always valuable!

Hansen, K., Truffer, M., Aschwanden, A., Mankoff, K., Bevis, M., Humbert, A., van den Broeke, M., Noel, B., Bjørk, A., Colgan, W., Kjær, K., Adhikari, S., Barletta, V., and S. Khan. (2021). Estimating ice discharge at Greenland’s three largest outlet glaciers using local bedrock uplift. Geophysical Research Letters, 48, e2021GL094252. https://doi.org/10.1029/2021GL094252

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

Rainfall on the Greenland Ice Sheet

Posted by William Colgan on August 04, 2021
Climate Change, New Research / 2 Comments

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.

Figure 1 – Average annual rainfall (left) and trend in annual rainfall (right) over the 1980 to 2019 period. Evaluation weather stations identified by WMO (World Meteorological Organization) numbers: Aasiaat (04220), Sisimiut (04230), Nuuk (04250), Narsarsuaq (34270), Danmarkshavn (34320), and Ittoqqortoormiit (34339).

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.

Figure 2 – September maximum hourly rainfall rates over the ice sheet and each sector. For the entire ice sheet (GrIS), as well as southeast (SE), south (S), and southwest (SW) sectors, linear trends are indicated together (dashed lines).

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.

Tags: , , , , , , ,

Instability of the North Water Polynya

Posted by William Colgan on July 27, 2021
Climate Change, New Research / No Comments

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.

Figure 1 – The location of the North Open Water polynya and core sites. Inset images show examples of June sea ice conditions within (top) and without (bottom) the Kane Basin Ice Arch.

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.

Figure 2 – Conceptual model of polynya stability and human migrations over the past five millennia.

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.

Figure 3 – Measurements of ‘heavy nitrogen’ (N15) in the lake core, which is a proxy for Little Auk presence. Top: Measurement and dating uncertainty for each sample. Bottom: Monte Carlo probabilities of changing N15 through time.

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.

Tags: , , , , , ,

New View on Geothermal Heat Flow in Greenland and Antarctica

Posted by William Colgan on January 15, 2021
New Research / No Comments

We have a new open-access study about geothermal heat flow beneath the Greenland and Antarctic ice sheets in the Journal of Geophysical Research: Earth Surface. 

Presently, there’s a lot of uncertainty about the magnitude and pattern of geothermal heat flow beneath both ice sheets. That’s because it has only been sampled at a handful of widely spaced deep ice cores (Figure 1). While the average value of geothermal heat flow is relatively small, getting it right is essential for ice-flow models. If you run a computer simulation of an ice sheet with a severe over- or under-estimation of the geothermal heat flow, you can easily end up generating an ice sheet that is either too warm or too cold. Ice flow is very sensitive to temperature – particularly near the bed – so geothermal heat flow is a critical variable for simulating the form and flow of Earth’s ice sheets.

Figure 1 – Peering into the drill trench at the NEEM deep ice core site. NEEM is one of only six deep ice core sites in the Greenland Ice Sheet interior where geothermal heat flow has been measured to date.

Our study took a fresh look at changes in geothermal heat flow across space, but not those due to the subtle variations in Earth’s crust and mantle properties over tens of km. Instead, we examined the effect of the ice-sheet bed’s topographic relief on geothermal heat flow to generate the first comprehensive snapshot of changes in geothermal heat flow at scales of hundreds of meters due to that relief. It’s been known for over a century that geothermal heat flow is greater in valleys and smaller on ridges. Basically, if the heat escaping Earth’s interior is looking for the quickest way to radiate into the atmosphere, a deeply incised valley provides the fastest exit. This effect is readily observable from the fact that geotherms – surfaces of constant temperature – are packed more closely together beneath valleys, indicating a stronger temperature gradient there (and hence heat flow) in comparison to ridges.

We created a simple statistical model to estimate this topographic influence on geothermal heat flow. This model essentially uses a digital elevation model of the bedrock topography to assess local topographic relief and then converts this local relief into a fractional correction for geothermal heat flow. It produces a positive correction – an increase – for valleys, and vice versa for ridges. Our approach is admittedly simple and empirical – a literal “first-order” approximation – but it seems to reliably reproduce the topographic variability in geothermal heat flow in all the settings for which we could find previous studies. So, we applied this statistical model to digital elevation models for Greenland and Antarctica. This revealed much more detail in a geothermal heat flow map than we are used to seeing.

Figure 2 – Left: An existing regional geothermal heat flow model (Martos2017). Right: Regional geothermal heat flow corrected for local topographic relief.

Across both Greenland and Antarctica, we see patterns of increased geothermal heat flow within deeply incised glacier valleys and decreased geothermal heat flow along ridges and mountains. In many regions, most notably the Antarctic Peninsula (Figure 2) and Central East Greenland (Figure 3), we find that local topography routinely modifies regional geothermal heat flow by more than ~50%.

Figure 3 – Left: An existing regional geothermal heat flow model (Martos2018). Right: Regional geothermal heat flow corrected for local topographic relief.

In Greenland, we estimate that there are ~100 outlet glaciers that are both sufficiently narrow and deeply incised to more than double local geothermal heat flow relative to that of the regional average value. The model also suggests that – especially deep within the interior of the Greenland Ice Sheet – local geothermal heat flow may be sufficiently suppressed along prominent subglacial ridges to cause subglacial water to refreeze. (At least, in ice-sheet areas where the ice-bed interface is near the freezing point.) 

The topographic correction for geothermal heat flow that we model is only as a good as the topographic relief that we derive from subglacial digital elevation models. Generally, in areas where subglacial topography is best resolved, the effect of the topography on the local geothermal heat flow is greater than uncertainty in the underlying regional geothermal heat flow model (Figure 4). There are still large swaths of the ice sheets where subglacial topography remains poorly resolved. In these areas, our model is not tremendously useful; the existing uncertainties between regional geothermal heat flow models are still larger than any local topographic correction that we can estimate.

Figure 4 – Maps of Antarctica and Greenland identifying areas, illustrated in red, where the influence of local topographic relief on geothermal heat flow is at least as important as the choice of geothermal heat flow model. As subglacial topography is still poorly resolved in both ice sheet interiors. These red areas may be expected to expand with improved mapping of subglacial topography.

The topographic corrections for geothermal heat flow in Greenland and Antarctica that we have calculated are now available as dimensionless fields in NetCDF format, with grids that are the same resolution as BedMachine, for each region via the PROMICE data portal (www.promice.dk). This means that they can be anonymously downloaded and applied to any regional geothermal heat flow model of the user’s choice. We hope that these topographic corrections for geothermal heat flow will be adopted into ice-flow models to improve both present-day ice-sheet simulations, as well as our understanding of the role of geothermal heat flow in the feedback between ice flow and topography on geologic timescales.

It has certainly been a long and winding road to this publication – the original draft of this article was first submitted in June 2019 – and we are grateful for Noah Finnegan (University of California Santa Cruz) and Olga Sergienko (Princeton University) for serving as editors to four very helpful peer-reviewers. Interdisciplinary projects can clearly provide a bumpier ride than staying in your own lane, but – in this instance – the journey seems to have taken us to a very different view of geothermal heat flow in Greenland and Antarctica.

Development of this data product was funded by the award “HOTROD: Prototype for Rapid Sampling of Ice-Sheet Basal Temperatures” provided by the Experiment Programme of the Villum Foundation. Improved understanding of the spatial variability in subglacial geothermal heat flow helps us optimize drill site selection and analyze ice temperature measurements. This data product was also supported by the Danish Ministry for Climate, Energy and Supplies through the Programme for Monitoring of the Greenland Ice Sheet (PROMICE).

Colgan, W., J. MacGregor, K. Mankoff, R. Haagenson, H. Rajaram, Y. Martos, M. Morlighem, M. Fahnestock and K. Kjeldsen. 2021. Topographic Correction of Geothermal Heat Flux in Greenland and Antarctica. Journal of Geophysical Research. 125: e2020JF005598. doi:10.1029/2020JF005598.

Tags: , , , , , , , , ,

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: , , , , , , , , , , , ,