Is Greenland really melting?

Sampriti Roy, University of Calcutta

“When the ocean speaks, the Greenland Ice Sheet listens,” said Oceans Melting Greenland’s Principal Investigator Josh Willis.

The image above presents a finding that is definitely a step forward in terms of research and rather worrying in terms of the current climate crisis. The image captured by Operational Land Imager (OLI) on Landsat 8, shows the condition of the Sverdrup Glacier on September 21, 2020. The lines seen indicate a worrying trend that has been seen for quite some time now- the retreating position of the glacier front since 2000. 

Sverdrup Glacier in northwest Greenland is one of the many coastal glaciers flowing deep into fjords. But Sverdrup has another feature that it is being noted for at the moment- it represents a large class of glaciers in Greenland that are undergoing a rapid retreat due to warm ocean water.

According to Michael Wood at NASA’s Jet Propulsion Laboratory (JPL), the Sverdrup’s retreat and ice loss were triggered by warm water. The Sverdrup glacier now faces the problem of an unstable configuration and is predicted to continue to retreat for many years no matter what the ocean does.

So how is the warming effect on glaciers studied?

In 2015, NASA had launched the Earth Venture Suborbital mission “Oceans Melting Greenland” (OMG) in order to begin annual measurements of:

• Salinity and ocean temperature with airborne and ship-based expendable conductivity-temperature-depth (CTD) sensors (2015–2019);
• Coastal bathymetry on the continental shelf and in fjords from multibeam echo sounding (ship-based) (2015–2019) and airborne gravity (2016).
• Thinning of glaciers from airborne interferometric surface topographic surveys (2016–2019);

Scientists with the Oceans Melting Greenland mission have been studying the Greenland glaciers that are retreating due to oceans. In a study led by Michael Wood, it was shown that when discussing the melting of glaciers, the depth of fjords is an important factor. This is because glaciers in deep fjords come into contact with ‘warmer’ ocean water than do the glaciers in shallow fjords. In the former condition, hastening of undercutting is seen- a process in which a layer of warm, saltwater at the bottom of a fjord melts at the base of a glacier, which causes the ice above to break apart.

Some observations:

In the study by Wood et al., there were 226 glaciers surveyed. 

• Among the glaciers studied, 74 were in deep fjords (DW glaciers) – something that was said to account for nearly half of the total ice loss from Greenland between the years 1992 and 2017. These were the glaciers that experienced the most undercutting. 
• Deep warm water (DW) glaciers: 

• Controlled 47% of the grounded ice loss and 49% of the total ice sheet mass loss.
• Have played a dominant role in Greenland’s ice dynamics changes and in turn, on the ice sheet mass loss as a whole.

• The study has demonstrated that the very large and abrupt warming of∼1.9°C of subsurface ocean waters around Greenland in 1998–2007 relative to 1992–1997 had caused a widespread mass elimination at the front of marine-terminating glaciers. This had forced them into a state of retreat.
• Although a pause was noticed in ocean warming between 2008 and 2017, the glaciers still continued to retreat at an accelerated rate. Why? This was due to the fact that they had already experienced such severe undercutting in the previous decade that they still continued to retreat. The authors posit that the constant dynamic thinning at glacier margins, particularly after the year 2000, led to an unstable configuration of the glaciers, which could not reach a new equilibrium state.
• The findings of the paper suggest that climate models may underestimate ice loss experienced by the glaciers by at least a factor of 2 if the undercutting experienced by them due to a warm ocean is not accounted for.

Judging from the availability of several evidences- from published research to Al Gore’s famous 2006 documentary, among others- we can say that the state of Greenland’s melting glaciers are common to the public for quite some time now. The significance of the study in discussion with respect to the others, as put forth by Eric Rignot, (OMG Deputy Principal Investigator), was that though it has been known for over a decade that warmer oceans play a major role in the evolution of Greenland glaciers, this study has presented for the first time the quantification of the undercutting effect as well as overriding impact of the same on the retreat of the glaciers over the last 20 years.

The significance of the paper also lies in another factor, as put forward by OMG principal Investigator Josh Willis (of JPL). It is indeed discoveries like this one, regarding the influence of the ocean on the 74 fjords, that will eventually aid us in predicting how fast ice will shrink in the future- a vital piece of information for this generation as well as the next.

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Source:

• NASA Earth Observatory image by Joshua Stevens, using Landsat data from the U.S. Geological Survey and data courtesy of Wood, M. et al. (2021). Story by Kathryn Hansen and Ian O’Neill
• Wood, M., Rignot, E., Fenty, I., An, L., Bjørk, A., van den Broeke, M., …& Zhang, H. (2021). Ocean forcing drives glacier retreat in Greenland. Science Advances, 7(1), eaba7282.DOI: http://10.1126/sciadv.aba7282

Source for Image: https://earthobservatory.nasa.gov/images/147776/undercutting-sverdrup-glacier

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