A Window to the Future: Why Greenland’s Continental Shelves Hold the Reins of Its Melting Glaciers

 By Josh Willis, Michael Wood and Ian Fenty

We were excited to learn that the Arctic Ocean Workshop will include the Sub-Arctic Oceans and the seas surrounding Greenland. Over the last two decades, researchers have established a clear connection between ocean conditions on the continental shelf and the behavior of Greenland’s more than 200 marine terminating glaciers. But there is still no comprehensive system for monitoring these changes over the long term—such as the one proposed by Straneo et al. (2019)—and it is desperately needed.

By 2050, as many as 350 million people could be affected by the rising oceans (Kulp and Strauss, 2019), and Greenland is currently the largest contributor to global sea level rise. Furthermore, there is now compelling evidence that the ocean conditions surrounding Greenland play a critical role in regulating the total ice loss (Wood et al., 2021). But despite the importance of these ocean waters in driving glacier retreat, historical observations of temperature and salinity on the continental shelf are quite sparse (see, for example, Figure 1 in the excellent blog post by Patrick Heimbach).

This picture changed dramatically in 2016, however, with the start of our NASA-funded airborne mission, Oceans Melting Greenland (OMG).  In addition to making widespread bathymetric surveys of the continental shelf and yearly surveys of the ice elevation at the edges of the glaciers, OMG collected approximately 250 temperature and salinity profiles each year, spread across the entire continental shelf (See Fenty et al, for a full overview of the mission).  OMG’s primary aim was to help establish a connection between wide-spread ocean conditions and glacier retreat. In this regard it was a success.  But it also made it clear that there is a critical need for ongoing measurements of the large-scale temperature and salinity changes on shelves surrounding the ice sheet.

On the continental shelf, warm, salty water of mostly Atlantic origin lies beneath a layer of colder, fresher water of mostly Artic origin (see Figure 1).   Because this warm water sits 100 to 200 m below the ocean surface, it is almost impossible to observe remotely.  This means that direct, in situ observations of waters on the shelf will continue to be critically important for explaining ongoing ice loss and projecting future sea level rise.

Figure 1.A cutaway diagram of a typical Greenland glacier in its fjord.  On the shelf, a layer of warm, salt water typically sits 100 to 200 meters below a layer of colder, fresher water. Most Greenland glaciers end in a vertical face like the one shown here. A plume of subsurface run off rises up the face of the glacier, pulling in the warm water and undercutting the glacier.

 Observations from OMG also show that large-scale patterns of ocean temperature change do occur on the continental shelf, and that these patterns can persist for several years.  Figure 2 shows locations of all the temperature and salinity profiles collected during a typical yearly ocean survey.  The survey is conducted by aircraft and typically takes 3-6 weeks to deploy approximately 250 expendable air-launched conductivity, temperature and depth sensors (AXCTDs).  The insets show average temperature profiles from three different survey years, over three different regions along the west coast, along with a one-standard error uncertainty bound.   The subsurface temperature maximum of approximately 2°C clearly shows the deep, warm layer.  And between 2016 and 2017, this layer cooled by almost 1°C in all three regions. This widespread cooling, likely driven by changes in the North Atlantic Oscillation, was shown to have a major impact on Greenland’s largest glacier (as ranked by discharge). After nearly two decades of thinning and retreat, Jakobshavn grew thicker for three years in a row when the cool water reached its fjord (Khazendar et al., 2019).

Figure 2. A map of the survey plan for the OMG ocean survey. Each yellow dot is the location where each temperature and salinity profile is collected (right panel). The insets show average temperature profiles in the regions shown for 2016 (blue), 2017 (green) and 2018 (red). The width of the curves is one standard error.  A 1°C cooling is visible in the warm deep layer along the entire west coast during these years.

And Jakboshavn was not alone in its reaction to these changes in ocean temperature. A comprehensive assessment of glacier retreat in Greenland was recently published by OMG investigator, Mike Wood (Wood et al., 2021). Figure 3 shows one of the central results from the work.  Using an ECCO ocean state estimate to extend the record of ocean temperature changes back to the mid-1990s, Mike and company showed that the average retreat among all of Greenland’s 226 marine terminating glaciers increased as ocean temperatures warmed, and decreased as they cooled down again.  They also found that including the full impact of ocean warming will increase current projections of sea level rise a factor of 2 or more.

Figure 3. Average glacier retreat (black) and depth-averaged ocean temperatures (red) from the ECCO ocean state estimate. The period of ocean warming coincides with faster average retreat across all of Greenland's 226 marine terminating glaciers.

This narrow strip of ocean on the continental shelf surrounding the Greenland Ice Sheet plays a key role in controlling ice loss.  But after 2021, the OMG experiment will end. And although a few key glaciers will continue to be measured along with yearly surveys in the southwest, the wide-spread measurements on the shelf will cease.  Given their importance, it seems clear to us that these regions must continue to be monitored.

And we are not alone.  The argument for sustained ocean observations has been made quite clearly for expansion of sustained observing systems that serve a variety of scientific and societal purposes (Weller et al., 2019). For Greenland ice loss this means, at the very least continuing the wide-spread collection of temperature and salinity observations around the continental shelf for decades to come, as the ice sheet continues to melt and drive sea levels higher around the globe.

A long-term system for observing these waters may look quite different than the OMG surveys of the past 5 years. The one proposed by Straneo et al. (2019) employs a wide variety of measurement systems and serves a variety of purposes. Expansion of the Argo Array of Profiling floats to cover marginal seas has long been discussed as a priority, but not yet funded in this region. OMG has tested Argo-like floats on the shelf, with some promising results that suggest floats of this type could be part of a viable solution. Gliders and other new systems may also play a key role. But regardless of the observing system we choose, the waters surrounding Greenland provide a window into the future of sea level rise, and we must not let that window close.

References

Fenty, I., Willis, J. K., Khazendar, A., DiNardo, S., Forsberg, R., Fukumori, I., et al. (2016), Oceans melting Greenland: Early results from NASA's ocean–ice mission in Greenland. Oceanography, 29(4), 72–83, https://doi.org/10.5670/oceanog.2016.100.

Khazendar, A., Fenty, I. G., Carroll, D., Gardner, A., Lee, C. M., Fukumori, I., Wang, O., Zhang, H., Seroussi, H., Moller, D., Noel, B. P. Y., Van Den Broeke, M. R., DiNardo, S., and Willis, J., (2019), Interruption of two decades of Jakobshavn Isbrae acceleration and thinning as regional ocean cools, Nat. Geosci., 12, 277–283, https://doi.org/10.1038/s41561-019-0329-3.  

Kulp, S.A., Strauss, B.H. (2019), New elevation data triple estimates of global vulnerability to sea-level rise and coastal flooding, Nat. Commun., 10, 4844 (2019), https://doi.org/10.1038/s41467-019-12808-z.

Straneo F, Sutherland DA, Stearns L, Catania G, Heimbach P, Moon T, Cape MR, Laidre KL, Barber D, Rysgaard S, Mottram R, Olsen S, Hopwood MJ and Meire L (2019), The Case for a Sustained Greenland Ice Sheet-Ocean Observing System (GrIOOS), Front. Mar. Sci., 6:138, https://doi.org/10.3389/fmars.2019.00138.

Weller RA, Baker DJ, Glackin MM, Roberts SJ, Schmitt RW, Twigg ES and Vimont DJ (2019), The Challenge of Sustaining Ocean Observations, Front. Mar. Sci., 6:105, https://doi.org/10.3389/fmars.2019.00105.

Wood, M. et al. (2021), Ocean forcing drives glacier retreat in Greenland, Science Advances, 01 Jan 2021: Vol. 7, no. 1, https://doi.org/10.1126/sciadv.aba7282.