Thongchai Thailand

MELTING ICE SHEETS SEA LEVEL RISE

Posted on: April 3, 2021

New study links Antarctic ice-shelf melt to global sea-level rise |  Environment| All topics from climate change to conservation | DW |  12.12.2017

THIS POST IS A CRITICAL REVIEW OF AN ONLINE ARTICLE BY SKY NEWS ABOUT AGW CLIMATE CHANGE. IT SAYS THAT MELTING ICE SHEETS CAUSED SEA LEVEL RISE OF 18 METERS.

LINK TO SOURCE: https://news.sky.com/story/melting-ice-sheets-caused-global-sea-levels-to-rise-up-to-18-metres-scientists-say-12262567

Sea levels set to keep rising for centuries even if emissions targets met | Sea  level | The Guardian

PART-1: WHAT THE SOURCE ARTICLE SAYS

Melting ice sheets caused global sea levels to rise up to 18 metres, scientists say. Researchers from Durham University have learned lessons from a rapid pulse of sea-level rises in Earth’s recent past. Researchers have found the cause of an 18-metre sea level rise in Earth’s recent past. Researchers have discovered that melting ice sheets have already caused enormous sea-level rises, pushing oceans around the world higher by up to 18 metres at the end of the last ice age. The scientists at Durham University have established which ice sheet was responsible for this rapid rise, which took place at 10 times the current rate, pushing up sea levels by around 3.6 metres per century over 500 years.

New analysis of geological records show that at the end of the last ice age around 14,600 years ago, meltwater from the former North American and Eurasian ice sheets was actually driving sea level rise, rather than the oft-suspected Antarctic ice sheet. Meltwater pulse 1A (MWP1a) is the name used to describe this rapid period of sea level rise following the last ice age, when continental ice sheets were retreating and the meltwater, previously held on land, flowed into the oceans. But scientists had not been sure which ice sheet had been responsible for the enormous MWP-1A event rise, equivalent to an ice sheet twice the size of Greenland melting in only 500 years. The MWP1a discharge of freshwater disrupted ocean circulations and had enormous knock-on effects for the global climate, say the researchers, and understanding the source will enable scientists to improve the accuracy of their climate models. The results are important for our understanding of ice-ocean-climate interactions which play a significant role in shaping terrestrial weather patterns,”.

The findings are particularly timely with the Greenland ice sheet rapidly melting, contributing to a rise in sea levels and changes to global ocean circulation. Despite being identified over 30 years ago, it has been surprisingly challenging to determine which ice sheet was the major contributor to this dramatic rise in sea levels. Previously, scientists tried to work out the source of the sea-level rise based on sea-level data from the tropics, but the majority of those studies disagreed with geological records of ice sheet change. The study includes novel information from lakes around the coast of Scotland that were isolated from the ocean due to land uplift following the retreat of the British Ice Sheet, allowing us to confidently identify the meltwater sources. The research technique allows researchers to really dig into the error bars on the data and explore which ice-melt scenarios were most likely with the finding that most of the rapid sea-level rise was due to ice sheet melt across North America and Scandinavia, with a surprisingly small contribution from Antarctica. The next big question is to work out what triggered the ice melt, and what impact the massive influx of meltwater had on ocean currents in the North Atlantic. This is very much on our minds today – any disruption to the Gulf Stream, for example due to melting of the Greenland Ice Sheet, will have significant consequences for the UK climate.

meltwater-pulse

PART-2: CRITICAL COMMENTRY

As noted in a related post on the prior interglacial, the Eemian, LINK: https://tambonthongchai.com/2018/12/21/eemian/ “it is generally agreed that the Eemian, at times, was warmer than the the present by as much as 5ºC but with large fluctuations in temperature between conditions hotter than today and colder than today. A significant feature of the Eemian is sea level rise and fluctuations in sea level caused by fluctuations in temperature. Sea level rise of 3 to 6 meters are reported by some authors and 5 to 9 meters by others and is generally attributed to a complete disintegration of the West Antarctic Ice Sheet and it was likely the main source of the dramatic sea level rise found in the data. Some authors cite sudden warming of 5ºC to 10ºC and “massive surges of icebergs into the North Atlantic” as a perturbation of ocean circulation that was responsible for abrupt climate change in the Eemian

The relevance of the Eemian in the study of sea level rise in the current interglacial, the Holocene, is that the violent pulses of sea level rise in the initiation of the Holocene after the Younger Dryas usually described in terms of the disintegration of the Laurentide ice sheet into lakes and the violent stepwise discharge of these lakes into the North Atlantic described as “meltwater pulses”. One of the many unresolved issues in this scenario is that the stepwise sea level rise events were as much as 20 meters in each pulse and the amount of water available in the Laurentide discharge described above is not sufficient to explain the extent of the sea level rise.

As seen in the bibliography below, a significant area of research in the paleo proxy data for these events is the resolution of this mystery, that is, where did all that extra water come from for these 20-meter pulses of sea level rise? Although many different possible sources are identified in the literature, the historical data for the Eemian and a convenient mass balance has led to the popularity of the Eemian like but stepwise disintegration of the WAIS as the source of the water.

The Durham University research presented by Sky News is not an upward revision of the late Holocene sea level rise possibilities but rather yet another paper that tries to explain the the mass imbalance issue in the pulses of sea level rise at the initiation of the Holocene about 10,000 to 15,000 years ago. It should be mentioned that the WAIS melt theory is validated only by the mass balance because it does supply the amount of water needed to explain the imbalance. There is no independent evidence the WAIS had melted at the initiation of the Holocene as it had done at the initiation of the Eemian.

meltwater-pulse

PART-3: THE RELEVANT BIBLIOGRAPHY

Clark, Peter U., et al. “Sea-level fingerprinting as a direct test for the source of global meltwater pulse IA.” Science 295.5564 (2002): 2438-2441. The ice reservoir that served as the source for the meltwater pulse IA remains enigmatic and controversial. We show that each of the melting scenarios that have been proposed for the event produces a distinct variation, or fingerprint, in the global distribution of meltwater. We compare sea-level fingerprints associated with various melting scenarios to existing sea-level records from Barbados and the Sunda Shelf and conclude that the southern Laurentide Ice Sheet could not have been the sole source of the meltwater pulse, whereas a substantial contribution from the Antarctic Ice Sheet is consistent with these records.

Bassett, S. E., et al. “Modelling Antarctic sea-level data to explore the possibility of a dominant Antarctic contribution to meltwater pulse IA.” Quaternary Science Reviews 26.17-18 (2007): 2113-2127. We compare numerical predictions of glaciation-induced sea-level change to data from 8 locations around the Antarctic coast in order to test if the available data preclude the possibility of a dominant Antarctic contribution to meltwater pulse IA (mwp-IA). Results based on a subset of 7 spherically symmetric earth viscosity models and 6 different Antarctic deglaciation histories indicate that the sea-level data do not rule out a large Antarctic source for this event. Our preliminary analysis indicates that the Weddell Sea is the most likely source region for a large (∼9 m) Antarctic contribution to mwp-IA. The Ross Sea is also plausible as a significant contributor (∼5 m) from a sea-level perspective, but glacio-geological field observations are not compatible with such a large and rapid melt from this region. Our results suggest that the Lambert Glacier component of the East Antarctic ice sheet experienced significant retreat at the time of mwp-IA, but only contributed ∼0.15 m (eustatic sea-level change). All of the ice models considered under-predicted the isostatic component of the sea-level response in the Antarctic Peninsula and the Sôya Coast region of the East Antarctic ice sheet, indicating that the maximum ice thickness in these regions is underestimated. It is therefore plausible that ice melt from these areas, the Antarctic Peninsula in particular, could have made a significant contribution to mwp-IA.

Clark, Peter U., et al. “Origin of the first global meltwater pulse following the last glacial maximum.” Paleoceanography 11.5 (1996): 563-577. Well‐dated sea level records show that the glacioeustatic rise following the last glacial maximum was characterized by two or possibly three brief intervals of rapid sea level rise separating periods with much lower rates. These very high rates of sea level rise indicate periods of exceptionally rapid deglaciation of remaining ice sheets. The Laurentide Ice Sheet is commonly targeted as the source of the first, and largest, of the meltwater pulses (mwp‐IA between ∼14,200 (12,200 14C years B.P.) and 13,700 years ago (11,700 14C years B.P.)). In all oceanic records of deglaciation of the former northern hemisphere ice sheets that we review, only those from the Gulf of Mexico and the Bermuda Rise show evidence of low δ18O values at the time of mwp‐IA, identifying the southern Laurentide Ice Sheet as a potential source for mwp‐IA. We question this source for mwp‐IA, however, because (1) ice sheet models suggest that this sector of the ice sheet contributed only a fraction (<10%) of the sea level needed for mwp‐IA, (2) melting this sector of the ice sheet at the necessary rate to explain mwp‐IA is physically implausible, and (3) ocean models predict a much stronger thermohaline response to the inferred freshwater pulse out of the Mississippi River into the North Atlantic than is recorded. This leaves the Antarctic Ice Sheet as the only other ice sheet capable of delivering enough sea level to explain mwp‐IA, but there are currently no well‐dated high‐resolution records to document this hypothesis. These conclusions suggest that reconstructions of the Laurentide Ice Sheet in the ICE‐4G model, which are constrained to match the sea level record, may be too low for time periods younger than 15,000 years ago. Furthermore, δ18O records from the Gulf of Mexico show variable fluxes of meltwater from the southern margin of the Laurentide Ice Sheet which can be traced to the opening and closing of eastward draining glacial‐lake outlets associated with surging ice sheet behavior. These variable fluxes through eastern outlets were apparently sufficient to affect formation of North Atlantic Deep Water, thus underscoring the sensitivity of this process to changes in freshwater forcing.

Liu, J. Paul, and John D. Milliman. “Reconsidering melt-water pulses 1A and 1B: global impacts of rapid sea-level rise.” Journal of Ocean University of China 3.2 (2004): 183-190. Re-evaluation of the post-glacial sea level derived from the Barbados coral-reef borings suggests slightly revised depth ranges and timing of melt-water pulses MWP-1A (96–76m, 14.3–14.0ka cal BP) and IB (58–45m, 11.5–11.2ka cal BP), respectively. Ages of non-reef sea-level indicators from the Sunda Shelf, the East China Sea and Yellow Sea for these two intervals are unreliable because of the well-documented radiocarbon (14C) plateau, but their vertical clustering corresponds closely with MWP-1A and IB depth ranges. Close correlation of the revised sea-level curve with Greenland ice-core data suggests that the 14C plateau may be related to oceanographic-atmospheric changes due to rapid sea-level rise, fresh-water input, and impaired ocean circulation. MWP-1A appears to have occurred at the end of Bølling Warm Transition, suggesting that the rapid sea-level rise may have resulted from lateral heat transport from low to high-latitude regions and subsequent abrupt ice-sheet collapses in both North America-Europe and Antarctica. An around 70 mm a−1 transgression during MWP-1A may have increased freshwater discharge to the North Atlantic by as much as an order of magnitude, thereby disturbing thermohaline circulation and initiating the Older Dry as global cooling.

Bard, Edouard, Bruno Hamelin, and Doriane Delanghe-Sabatier. “Deglacial meltwater pulse 1B and Younger Dryas sea levels revisited with boreholes at Tahiti.” Science 327.5970 (2010): 1235-1237. Reconstructing sea-level changes during the last deglaciation provides a way of understanding the ice dynamics that can perturb large continental ice sheets. The resolution of the few sea-level records covering the critical time interval between 14,000 and 9,000 calendar years before the present is still insufficient to draw conclusions about sea-level changes associated with the Younger Dryas cold event and the meltwater pulse 1B (MWP-1B). We used the uranium-thorium method to date shallow-living corals from three new cores drilled onshore in the Tahiti barrier reef. No significant discontinuity can be detected in the sea-level rise during the MWP-1B period. The new Tahiti sea-level record shows that the sea-level rise slowed down during the Younger Dryas before accelerating again during the Holocene.

Brendryen, Jo, et al. “Eurasian Ice Sheet collapse was a major source of Meltwater Pulse 1A 14,600 years ago.” Nature Geoscience 13.5 (2020): 363-368. Rapid sea-level rise caused by the collapse of large ice sheets is a threat to human societies. In the last deglacial period, the rate of global sea-level rise peaked at more than 4 cm yr−1 during Meltwater Pulse 1A, which coincided with the Bølling warming event some 14,650 years ago. However, the sources of the meltwater have proven elusive, and the contribution from Eurasian ice sheets has been considered negligible. Here, we present a regional carbon-14 calibration curve for the Norwegian Sea and recalibrate marine 14C dates linked to the Eurasian Ice Sheet retreat. We find that marine-based sectors of the Eurasian Ice Sheet collapsed at the Bølling transition and lost an ice volume of 4.5–7.9 m sea-level equivalents (SLE) over 500 years. During peak melting, 3.3–6.7 m SLE of ice was lost, potentially explaining up to half of Meltwater Pulse 1A. A mean meltwater flux of 0.2 Sv over 300 years was injected into the Norwegian Sea and the Arctic Ocean at a time when proxy evidence suggests vigorous Atlantic meridional overturning circulation. Our reconstruction shows that massive marine-based ice sheets can collapse in as little as 300–500 years.

Kienast, Markus, et al. “Synchroneity of meltwater pulse 1a and the Bølling warming: new evidence from the South China Sea.” Geology 31.1 (2003): 67-70. A twofold decrease in long-chain n-alcane (n-nonacosane) concentrations in a downcore record from the northern South China Sea indicates a rapid drop in the supply of terrigenous organic matter to the open South China Sea during the last deglaciation, paralleled by an equally rapid increase in sea-surface temperatures, corresponding with the Bølling warming at 14.7 ka. The sudden drop in terrigenous organic matter delivery to this marginal basin is interpreted to reflect a short-term response of local rivers to rapid sea-level rise, strongly implying that the Bølling warming and the onset of meltwater pulse (MWP) 1a are synchronous. This phase relation contrasts with the widely cited onset of this MWP 1a ca. 14 ka, and implies that previous studies postulating a weakening of deep-water formation in the North Atlantic due to massive meltwater discharge during MWP 1a need to be reevaluated. (blogger’s note: the Bolling warming was a sudden and brief warming during the initial stage of the last deglaciation).

Carlson, Anders E. “Geochemical constraints on the Laurentide Ice Sheet contribution to meltwater pulse 1A.” Quaternary Science Reviews 28.17-18 (2009): 1625-1630. Planktonic and benthic δ18O records adjacent to the runoff outlets of the Laurentide Ice Sheet (LIS) indicate that the LIS contributed to the abrupt ∼20 m rise in sea level ∼14.6 ka, Meltwater Pulse 1A (MWP-1A). However, the magnitude of the LIS contribution still remains unresolved. Here, I use a freshwater runoff–ocean mixing model to calculate the LIS meltwater required to explain the decreases in planktonic and benthic δ18O observed during MWP-1A at the southern, eastern and northern runoff outlets of the LIS. Maximum LIS contributions in equivalent sea level rise for a 500-year long MWP-1A are 2.7 m discharged into the Gulf of Mexico as a combined hyperpycnal and hypopycnal flow, 2.1 m discharged into the North Atlantic, and 0.5 m into the Arctic Ocean, for a total LIS contribution of ≤5.3 m. A LIS contribution of <30% to MWP-1A supports the hypothesis that a significant component of this MWP was sourced from the Antarctic Ice Sheet.

Bassett, Sophie E., et al. “Ice sheet and solid earth influences on far-field sea-level histories.” Science 309.5736 (2005): 925-928. Previous predictions of sea-level change subsequent to the last glacial maximum show significant, systematic discrepancies between observations at Tahiti, Huon Peninsula, and Sunda Shelf during Lateglacial time (∼14,000 to 9000 calibrated years before the present). We demonstrate that a model of glacial isostatic adjustment characterized by both a high-viscosity lower mantle (4 × 1022 Pa s) and a large contribution from the Antarctic ice sheet to meltwater pulse IA (15-meters eustatic equivalent) resolves these discrepancies. This result supports arguments that an early and rapid Antarctic deglaciation contributed to a sequence of climatic events that ended the most recent glacial period of the current ice age.

Ocean temperature thresholds for Last Interglacial West Antarctic Ice Sheet  collapse - Sutter - 2016 - Geophysical Research Letters - Wiley Online  Library

FOOTNOTE

As described by Professor Carl Wunsch in a related post, “From one point of view, scientific communities without adequate data have a distinct advantage because they can construct interesting and exciting stories and rationalizations with little or no risk of observational refutation. Colorful, sometimes charismatic, characters come to dominate the field, constructing their interpretations of a few intriguing, but indefinite observations that appeal to their followers, and which eventually emerge as “textbook truths.” The following characteristics are ascribed to one particularly notoriously data-poor field. (1) Tremendous self-confidence and a sense of entitlement and of belonging to an elite community of experts, (2) An unusually monolithic community, with a strong sense of consensus, whether driven by the evidence or not, and an unusual uniformity of views on open questions. These views seem related to the existence of a hierarchical structure in which the ideas of a few leaders dictate the viewpoint, strategy, and direction of the field. (3) In some cases a sense of identification with the group, akin to identification with a religious faith or political platform. (4) A strong sense of the boundary between that group of experts and the rest of the world. (5) A disregard for and disinterest in the ideas, opinions, and work of experts who are not part of the group, and a preference for talking only with other members of the community. (6) A tendency to interpret evidence optimistically, to believe exaggerated or incorrect statements of results and to disregard the possibility that the theory might be wrong. This is coupled with a tendency to believe results are true because they are widely believed, even if one has not checked (or even seen) the proof oneself. (7) A lack of appreciation for the extent to which a research program ought to involve risk.”

LINK: https://tambonthongchai.com/2018/07/23/carlwunsch2010/

carlwunsch

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