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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

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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.

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