BLOG

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.

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.

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

We have a new open-access study out in the current issue of Earth Systems Science Data. In this study, we estimate the liquid water discharge – meaning meltwater and rainfall flowing into the ocean – every day since 1958 from 54,142 hydrologic basins across Greenland. About 40% of these basins are associated with glaciers or the ice sheet, and these “ice” basins accounted for ~65% of Greenland’s total liquid water discharge. On an annual basis, we estimate that Greenland’s liquid discharge varied from between ~136 km³ in 1992 and ~785 km³ in 2012. The daily discharge records and these individual basins are now available online. This dataset provides a great improvement in our understanding of when and where freshwater is entering Greenlandic fjords.

Where possible, we compared the daily discharge records of individual basins that we downscale from climate models to actual observed river discharge measurements. There are only a few continuous river gauging stations in Greenland operated by different monitoring programs and research groups. Thankfully, we could use publicly accessible observational records from nine basins (Kingigtorssuaq, Kobbefjord, Leverett, Oriartorfik, Qaanaaq, Røde Elv, Teqinngalip, Watson and Zackenberg) to assess performance of our data product. These comparisons show that the accuracy of data product varies with both basin size – or discharge volume – as well as climate model. Generally, however, the data product reproduces the magnitude and variability of observed basin discharge within a reasonable uncertainty.

Downscaling runoff from regional climate models to individual basins is clearly sensitive to errors or uncertainties in the elevation model guiding the hydrological routing. This is especially true for glacier or ice-sheet basins, which require additional assumptions about the effective water pressure within the ice. Hydrologic boundaries can shift due to slight changes in elevation or effective water pressure. We therefore ran our hydrological routing code many times to see how sensitive the location of basin outlets – meaning where water drains from ice-to-tundra or tundra-to-ocean – where to common assumptions. We found many basin outlets around the low-elevation ice-sheet ablation area can shift by more than 30 km under a range of common assumptions. This highlights the challenge of trying to balance a water budget within a given fjord. It also points to where improved knowledge of subglacial topography is most needed.

Sensitivity in assessed basin outlet location — land outlets (Left) and ice outlets (Right) — to common hydrologic routing assumptions. Ice-sheet basins likely vary with effective water pressure on both inter- and intra-annual time-scales.

A neat aspect of this study is that the source code is also made available open access. This code-sharing approach is part of the growing “open science” movement. Sharing code not only makes complex results reproducible, but also helps different research teams move forward. In this case, basin-scale runoff estimates are sensitive to the choices of both climate model and downscaling method. By making the source code available, subsequent research teams can implement precisely the same climate model and/or downscaling methods. The development of this data and code product was funded by the Danish Ministry for Climate, Energy and Supply to the Programme for Monitoring of the Greenland Ice Sheet (www.PROMICE.dk), as well as European Union’s Horizon 2020 to the INTAROS project (www.INTAROS.eu).

Mankoff, K., B. Noël, X. Fettweis, A. Ahlstrøm, W. Colgan, K. Kondo, K. Langley, S. Sugiyama, D. van As, and R. Fausto,  2020. Greenland liquid water runoff from 1958 through 2019, Earth System Science Data. 12: 2811–2841. doi:10.5194/essd-12-2811-2020.