Michiel van den Broeke, Professor of Polar Meteorology at Utrecht University (longer bio at end of post) very kindly agreed to read William Barbat’s article “Glacier melt: How soon? How fast?” and explain what Barbat got right (and wrong) and where the science has gone in the almost 50 years since then. It’s a brilliant (imo) piece, and I hope you learn as much as I did. Please do share it, comment on it.
In the March 1980 edition of the CO2 NEWSLETTER, William Barbat reported about the threat of melting ice sheets and the rapid, multi-metre sea level rise that could ensue. Undoubtedly, Barbat had been triggered by the 1978 scientific publication of British glaciologist John Mercer (1922-1987), then employed at (what would later become) the Byrd Polar Research Centre of Ohio State University (Mercer, 1978). In his Nature article: “West Antarctic ice sheet and CO2greenhouse effect: a threat of disaster“, Mercer pointed out that the increase in CO2 concentration in the Earth’s atmosphere caused by the burning of fossil fuels would result in strong Antarctic warming, potentially leading to the disintegration of the large Ross and Filchner-Ronne ice shelves. In the absence of their buttressing effect, the West Antarctic ice sheet would collapse, raising global sea levels by several metres.
Today’s cryospheric research relies heavily on three complementary techniques: in situ observations, satellite observations and numerical models. In situ observations are often scattered in space, but to their credit have relatively long time series (typically decades in the Polar Regions), indispensable for trend detection. They moreover provide ground truth for satellites and serve to evaluate/calibrate climate and ice sheet models. Satellites, on the other hand, with their limited mission lifetime of typically 5-10 years, produce short time series, but they have the advantage of near-complete spatial coverage, filling in the spatial gaps left by the in situ observations. Numerical models, once evaluated and/or calibrated with the in situ and remotely sensed observations, can help us isolating the physical processes at work and, when they perform satisfactorily, make credible future projections.
When Mercer published his study almost 50 years ago, he had to make do with very limited observations and crude models. Although the density of in situ observations in the polar regions increased sharply after the 1957/58 International Geophysical Year (IGY, also referred to as the Third International Polar Year), observations remained very scarce notably in the ice sheet interiors. While some satellites for earth observation, notably Landsat, were available at that time, time series were less than a decade long. For Earth’s cryosphere, the satellite era started in earnest more than a decade later, with the launch of European Space Agency‘s radar-equipped ERS-1 in 1991. Finally, in the late 1970’s, climate and ice sheet models were still in their infancy; the model projections of future Antarctic warming used in Mercer’s study were from Syukuro Manabe, who in 2021 was co-awarded the Nobel prize in Physics for his pioneering contributions to climate modelling.
In spite of this, both Mercer’s 1978 Nature paper and William Barbat’s 1980 report in the CO2NEWSLETTER highlight the remarkable body of knowledge on the world’s ice sheets that had been gathered. Their reported total volume expressed in sea level rise equivalent of 66 m only deviates by 1% from today’s numbers1. Estimates of sea level stands of 6 m above present during the last interglacial (~125.000 years ago) fall well within the range of current estimates (6 to 9 m) (Dutton et al., 2015). Other remarkably accurate statements concern the approximately 50/50 partitioning of meltwater runoff and iceberg calving as sink terms in the mass balance of the Greenland ice sheet and the importance of ice shelf buttressing for grounded ice flow in Antarctica, which decades later was observationally confirmed after the sudden disintegration of Larsen B ice shelf in 2002 (Scambos et al., 2004). Mercer also correctly identified the apparent temperature threshold for the viability of Antarctic ice shelves, later corroborated by the demise of Antarctic Peninsula ice shelves after several decades of strong warming (Morris & Vaughan, 2013; Scambos et al., 2004). Also recently been confirmed is Mercer’s statement that a 5 K atmospheric warming could destabilize parts of the large Ross and Filchner-Ronne ice shelves (Van Wessem et al., 2023).
Inevitably, these early reports also have flaws and large uncertainties, which the authors frankly admit. Lacking direct observations, and realising that around 1980 mass changes of both ice sheets were significantly smaller than they are today (IMBIE, 2018, 2020), not much could be said about the magnitude of mass loss of the ice sheets, let alone the processes that caused them. It would take the launch in 2002 of the satellite pair of the Gravity Anomaly and Climate Experiment (GRACE) before mass loss from both ice sheets was convincingly demonstrated (Velicogna & Wahr, 2005; Velicogna, 2006). GRACE also showed that the recent mass loss in Antarctica is concentrated in the Amundsen and Bellingshausen Seas sectors, and is associated with ice shelf thinning owing to increased ocean melting at their base, rather than weakened buttressing of the Ross and Filchner-Ronne ice shelves. Making projections based on scanty information proved even harder. Mercer’s assumption that CO2 concentration in the atmosphere would double in 50 years was too pessimistic: atmospheric CO2 levels increased by 26%, from 337 to 426 parts per million, between 1979 and 2025. As a result, Antarctic warming remains far from the values reported in his paper.
This begs the question: if we were in Mercer’s shoes today, would we do much better in projecting the future of the Earth’s big ice sheets? Based on the latest IPCC report (IPCC, 2021), my take is that the uncertainties are still surprisingly large and not so dissimilar to what they were in 1978. Since then, our knowledge and technical (observational, modelling) capabilities have of course expanded tremendously, but we have also identified numerous new unknowns. The net result is that future ice sheet mass change and associated sea level rise remain highly uncertain, and that we still may be in for unpleasant surprises from nonlinear processes leading to tipping points that are currently not or poorly understood. Given the complex interactions between atmosphere, ocean and ice sheets that straddle several orders of magnitude in temporal and spatial scales, it is clear that this deep uncertainty will not be resolved anytime soon. It thus seems fitting to conclude with the statement made by Mercer in his 1978 paper, which still firmly stands: “…despite the crudities and inadequacies of present techniques for modelling the climatic effects of increasing atmospheric CO2content and the resultant doubts […], we cannot afford to let the atmosphere carry out the experiment before taking action because if the results confirm the prognosis, and we should know one way or the other by the end of the century, it will be too late to remedy the situation…”.
Bibliography
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Mercer, J. H. (1978). West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster. Nature, 271(5643), 321–325. https://doi.org/https://doi.org/10.1038/271321a0
Morris, E. M., & Vaughan, D. G. (2013). Spatial and Temporal Variation of Surface Temperature on the Antarctic Peninsula And The Limit of Viability of Ice Shelves. In Antarctic Peninsula Climate Variability: Historical and Paleoenvironmental Perspectives (pp. 61-68). https://doi.org/10.1029/AR079p0061
Scambos, T. A., Bohlander, J. A., Shuman, C. A., & Skvarca, P. (2004). Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters, 31(18). https://doi.org/10.1029/2004gl020670
Van Wessem, J. M., Van den Broeke, M. R., Wouters, B., & Lhermitte, S. (2023). Variable temperature thresholds of melt pond formation on Antarctic ice shelves. Nature Climate Change. https://doi.org/10.1038/s41558-022-01577-1
Velicogna, I., & Wahr, J. (2005). Greenland mass balance from GRACE. Geophysical Research Letters, 32(18). https://doi.org/10.1029/2005gl023955
Velicogna, I. a. J. W. (2006). Measurements of Time-Variable Gravity Show Mass Loss in Antarctica. Science, 311(5768), 1754-1756. https://doi.org/DOI: 10.1126/science.1123785
Footnotes
1 Combining radar flight lines of ice thickness with mass conservation provide us with accurate estimates of the sea level equivalent volumes of the ice sheets of Greenland (7.4 m) and Antarctica (57.8 m), (Morlighem et al., 2017; Morlighem et al., 2019).
Michiel van den Broeke (Rotterdam, 1968) has been Professor of Polar Meteorology at Utrecht University since 2008, where he studies the interaction between the climate and the large ice sheets of Antarctica and Greenland. Between 2016 and 2022, Michiel served as Scientific Director of the Institute for Marine and Atmospheric Research Utrecht (IMAU), where around 90 people work on developing a fundamental understanding of all components of the climate system.
