WARMING GREENLAND ICE SHEET PASSES POINT OF NO RETURN

PART-1: WHAT THE MEDIA STORY SAYS

Greenland’s ice sheet has melted to a point of no return, according to new study
By Max Claypool and Brandon Miller, CNN [LINK]  

1. Greenland’s ice sheet has melted to a point of no return, and efforts to slow global warming will not stop it from disintegrating according to a new study by researchers at Ohio State University. The ice sheet is now in this new dynamic state, where even if we went back to a climate that was more like what we had 20 or 30 years ago, we would still be pretty quickly losing mass,” Ian Howat, co-author of the study and a professor at Ohio State University, said.
2. Greenland’s ice sheet dumps more than 280 billion metric tons of melting ice into the ocean each year, making it the greatest single contributor to global sea level rise, according to Michalea King, the lead author of the study and researcher at Ohio State University. The ice loss has been so massive in recent years, she said, that it has caused a measurable change in the gravitational field over Greenland. Ice melting in Greenland contributes more than a millimeter rise to sea level every year, and that’s likely to get worse. Sea levels are projected to rise by more than 3 feet by the end of the century, wiping away beaches and coastal properties.Coastal states like Florida, and low-lying island nations are particularly vulnerable. Just 3 feet of sea level rise could put large areas of coastline underwater. Forty percent of the US population resides in coastal areas that are vulnerable to sea level rise. There’s a lot of places, like in Florida especially, where one meter alone would cover a lot of existing land areas, and that’s exacerbated when you get storms and hurricanes and things like that, that then cause extra surge on top of a higher baseline.
3. The study also found that the ice sheet is retreating in rapid bursts, leading to a sudden and unpredictable rise in sea levels, making it difficult to prepare for the effects. The study used four decades of satellite data to measure changes in Greenland’s ice sheet. The authors found that after 2000, the ice sheet shrank so rapidly that replenishing snowfall would not keep up with the rate of melting from parts of the glacier newly exposed to warmer ocean water, even if climate change were reversed. Entire coasts of ice are retreating at once due to climate change. All 200 glaciers that make up the Greenland ice sheet have been observed retreating within the same episode.
4. Even though the retreat of the Greenland Ice sheet likely cannot be reversed, it’s just the first in a series of tipping points. If climate change continues at this rate, the rate of melting will get much worse. “We’ve passed the point of no return but there’s obviously more to come,” Howat said. “Rather than being a single tipping point in which we’ve gone from a happy ice sheet to a rapidly collapsing ice sheet, it’s more of a staircase where we’ve fallen off the first step but there’s many more steps to go down into the pit.” (translation: it is a tipping point but that does not mean we don’t take climate action because there are more tipping points down the line).

PART-2: WHAT THE CITED RESEARCH PAPER SAYS

King, M.D., Howat, I.M., Candela, S.G. et al. Dynamic ice loss from the Greenland Ice Sheet driven by sustained glacier retreat. Commun Earth Environ 1, 1 (2020). https://doi.org/10.1038/s43247-020-0001-2: [FULL TEXT]    Abstract:  The Greenland Ice Sheet is losing mass at accelerated rates in the 21st century, making it the largest single contributor to rising sea levels. Faster flow of outlet glaciers has substantially contributed to this loss, with the cause of speedup, and potential for future change, uncertain. Here we combine more than three decades of remotely sensed observational products of outlet glacier velocity, elevation, and front position changes over the full ice sheet. We compare decadal variability in discharge and calving front position and find that increased glacier discharge was due almost entirely to the retreat of glacier fronts, rather than inland ice sheet processes, with a remarkably consistent speedup of 4–5% per km of retreat across the ice sheet. We show that widespread retreat between 2000 and 2005 resulted in a step-increase in discharge and a switch to a new dynamic state of sustained mass loss that would persist even under a decline in surface melt.

Introduction:  The Greenland Ice Sheet (GrIS) has been losing mass for several decades1 due to both increased surface meltwater runoff and ablation of marine-terminating outlet glaciers via calving and submarine melting, termed ice discharge. Total GrIS mass loss over the 1992–2018 period was due to approximately equal contributions from both terms1 but with greater contribution from increased melt runoff after 2000, when mass losses accelerated2,3,4. Estimates of ice sheet discharge over multiple decades and at annual, or finer, resolution provide insight into the ice sheet’s response to long-term climate forcing and ongoing change4,5. Seasonal and interannual variability in ice sheet dynamics are challenging to resolve prior to the year 2000 due to temporal and spatial data gaps. Here, we apply the rigorous methodology in ref. 3 to gain improved constraints on estimates of ice sheet discharge over three decades, including the period leading up to the onset of rapid glacier retreat and acceleration. Rates of Greenland glacier retreat have accelerated6 and previous work has identified relationships between glacier speed and retreat7 and glacier area8 for smaller subsets of Greenland glaciers. We also then combine these data with high-resolution observations of time-varying calving front position changes and perform a GrIS-wide analysis of how these two variables relate on individual, regional, and ice sheet-wide spatial scales over the multi-decadal record. We discuss the timing of changes in retreat, thinning, and acceleration across the ice sheet, quantify the sensitivity of ice discharge to retreat, and describe the roles of both long-term changes in ice dynamics and surface mass balance in preconditioning regions of the ice sheet for rapid retreat, thinning, and accelerated discharge. We find that GrIS-wide discharge is now ~14% greater than the rate observed during 1985–1999, following an observed step-increase during the early 2000’s. Widespread glacier retreat explains nearly all (>90%) of the observed multi-decadal variability in discharge, with a observed increase in discharge of 4–5% per every weighted mean kilometer of retreat. We find that this sensitivity is proportionally consistent across different regions of the ice sheet, despite highly variable long-term trends in discharge. Following the step-increase in discharge, GrIS-wide totals have remained relatively stable at rates near 495–500 Gt yr−1, reflecting an increase that was sufficient to effectively shift the ice sheet to a state of persistent mass loss.

Results:  Long-term changes in ice discharge and comparison with other studies. We find a step-increase in decadal-scale ice discharge (Fig. 1a), with a ~60 Gt yr−1, or 14%, increase between 1985–1999 and 2007–2018 means. After reaching a temporally local maximum in 2005, annual D then temporarily decreased for 3 years. Following the temporary decline, discharge accelerated again at a slower pace of 2 Gt yr−2 during 2008–2018, reaching a peak annual value of 502 ± 9 Gt yr−1 in 2017 and 2018, or 17% above the 1980’s average. The increase in mean annual D since 2008 has been mostly due to a steady increase in seasonal minimum values increasing with a trend of 3 Gt yr−2 since 2007, indicating greater wintertime velocities relative to summertime maxima, most evident in the northwest (Supplementary Fig. 1) and in the most recent 3 years of the central west. The seasonal amplitude in D has also changed, increasing by nearly 50%, from a 1985–1990 average of 17 ± 6 to 25 ± 6 Gt yr−1 for 2000–2018. To account for the uncertainty in D due to this temporal gap in ice thickness observations, we estimate D assuming the end member-cases of (1) all thickness change occurring in the first year, which maximizes the impact of thinning at the start of the period, and (2) all thinning occurring in the last year, which minimizes the impact until ~2000. We find that during the 1985–1999 period, estimates of D can vary by an average 13 Gt yr−1 (Fig. 1a) depending on when thinning occurred between temporally sparse elevation data (AeroDEM, ~1985 and ASTER, nominally ~2000), described in more details in the Methods section.

PART-3: CRITICAL COMMENTARY

Although Greenland’s summer ice melt and melt ponds in July and August can be understood as an atmospheric phenomenon, the Iceland hotspot under Greenland and other related geological features of this region of the Arctic require that the extreme instability of the ice sheet described in the paper must be understood in terms of geological features under the Greenland ice sheet specifically with respect to the Iceland Hot Spot rather than exclusively in terms of the atmosphere above it. Details of these geological features of the Greenland-Iceland region of the Arctic may be found in the bibliography provided below. See also:  https://tambonthongchai.com/2019/07/01/arctic/  

PART-4: THE RELEVANT BIBLIOGRAPHY

1. Martos, Yasmina M., et al. “Geothermal heat flux reveals the Iceland hotspot track underneath Greenland.” Geophysical research letters 45.16 (2018): 8214-8222. ABSTRACT: Curie depths beneath Greenland are revealed by spectral analysis of data from the World Digital Magnetic Anomaly Map 2. A thermal model of the lithosphere then provides a corresponding geothermal heat flux map. This new map exhibits significantly higher frequency but lower amplitude variation than earlier heat flux maps and provides an important boundary condition for numerical ice‐sheet models and interpretation of borehole temperature profiles. In addition, it reveals new geologically significant features. Notably, we identify a prominent quasi‐linear elevated geothermal heat flux anomaly running northwest–southeast across Greenland. We interpret this feature to be the relic of the passage of the Iceland hotspot from 80 to 50 Ma. The expected partial melting of the lithosphere and magmatic underplating or intrusion into the lower crust is compatible with models of observed satellite gravity data and recent seismic observations. Our geological interpretation has potentially significant implications for the geodynamic evolution of Greenland.  IN PLAIN LANGUAGE: Heat escaping from the Earth’s interior provides important clues about areas of geology and geodynamics. In addition, where a region is covered by an ice sheet, such as Greenland, variations in the heat supplied from the Earth’s interior can potentially influence how the ice flows, and hence its future changes. Unfortunately, in ice covered regions direct measurements of heat flow are limited to sparse boreholes, meaning this important quantity is poorly understood. In this study we used variations in the Earth’s magnetic field to map out the variations in the amount of heat being supplied to the base of the Greenland Ice Sheet from the Earth’s interior. Ice sheet models incorporating these new and improved results will help better constrain future predictions of ice sheet evolution. Overall, the new map not only shows less extreme variations than previous studies, but also reveals a previously unseen band of warmer than expected rock stretching northwest to southeast across Greenland. This band, together with lithospheric models derived from gravity data, is interpreted to be the scar left as the Greenland tectonic plate moved over a region of hot upwelling mantle (the material beneath the tectonic plates), which now underlies Iceland. [LINK] 
2. Rezvanbehbahani, Soroush, et al. “Predicting the geothermal heat flux in Greenland: A machine learning approach.” Geophysical Research Letters 44.24 (2017): 12-271.  ABSTRACT:  Geothermal heat flux (GHF) is a crucial boundary condition for making accurate predictions of ice sheet mass loss, yet it is poorly known in Greenland due to inaccessibility of the bedrock. Here we use a machine learning algorithm on a large collection of relevant geologic features and global GHF measurements and produce a GHF map of Greenland that we argue is within ∼15% accuracy. The main features of our predicted GHF map include a large region with high GHF in central‐north Greenland surrounding the NorthGRIP ice core site, and hot spots in the Jakobshavn Isbræ catchment, upstream of Petermann Gletscher, and near the terminus of Nioghalvfjerdsfjorden glacier. Our model also captures the trajectory of Greenland movement over the Icelandic plume by predicting a stripe of elevated GHF in central‐east Greenland. Finally, we show that our model can produce substantially more accurate predictions if additional measurements of GHF in Greenland are provided.  IN PLAIN LANGUAGE:The heat generated at the interior regions of Earth (geothermal heat flux, GHF) can be high enough to melt the bottom layers of ice sheets, decrease friction between ice and bedrock, and increase ice discharge to the ocean. This heat, however, cannot be directly measured in ice sheets because the bedrock is inaccessible. Here we present a novel approach to estimate this heat. We combine all the available geologic, tectonic, and GHF data that are available on all continents. We then establish a complex relationship between GHF and all the geologic‐tectonic features using machine learning techniques and then predict the GHF for the Greenland Ice Sheet. We utilize all information from available ice cores and bedrock boreholes to improve the GHF prediction in Greenland. Thus, the new GHF map honors tectonic settings, regional geology, and measurements from ice cores and can be used as an important input parameter to numerical ice sheet models that aim at lowering the uncertainties of future sea level rise predictions. [FULL TEXT]
3. Alley, R. B., et al. “Possible role for tectonics in the evolving stability of the Greenland Ice Sheet.” Journal of Geophysical Research: Earth Surface 124.1 (2019): 97-115. ABSTRACT: The history of the Greenland Ice Sheet has been influenced by the geodynamic response to ice sheet fluctuations, and this interaction may help explain past deglaciations under modest climate forcing. We hypothesize that when the Iceland hot spot passed beneath north‐central Greenland, it thinned the lithosphere and left anomalous heat likely with partially melted rock; however, it did not break through the crust to supply voluminous flood basalts. Subsequent Plio‐Pleistocene glacial‐interglacial cycles caused large and rapidly migrating stresses, driving dike formation and other processes that shifted melted rock toward the surface. The resulting increase in surface geothermal flux favored a thinner, faster‐responding ice sheet that was more prone to deglaciation. If this hypothesis of control through changes in geothermal flux is correct, then the long‐term (105 to 106 years) trend now is toward lower geothermal flux, but with higher‐frequency (≤104 to 105 years) oscillations linked to glacial‐interglacial cycles. Whether the geothermal flux is increasing or decreasing now is not known but is of societal relevance due to its possible impact on ice flow. We infer that projections of the future of the ice sheet and its effect on sea level must integrate geologic and geophysical data as well as glaciological, atmospheric, oceanic, and paleoclimatic information.  IN PLAIN LANGUAGE: The behavior of the Greenland Ice Sheet and its effect on future sea level depends on its geologic history as well as on greenhouse warming. The Iceland hot spot passed beneath Greenland millions of years ago, and left hot, possibly melted rock deep beneath the island. Since then, growth and shrinkage of the ice sheet have changed stresses in the rocks beneath. These stress changes may have shifted the melted rock upward, perhaps all the way to the base of the ice sheet, probably in pulses tied to times of rapid ice sheet change. This would have changed the heat flow from the Earth into the base of the ice, which affects how easily the ice sheet grows and shrinks. The future of the ice sheet depends primarily on how much the climate warms, but better understanding of the interactions between the ice and the rocks beneath will allow better predictions of ice sheet changes. [FULL TEXT]