Ice-stream stability on a reverse bed slope

Marine-based ice streams whose beds deepen inland are thought to be inherently unstable. This instability is of particular concern because significant portions of the marine-based West Antarctic and Greenland ice sheets are losing mass and their retreat could contribute significantly to future sea-level rise. However, the present understanding of ice-stream stability is limited by observational records that are too short to resolve multi-decadal to millennial-scale behaviour or to validate numerical models8. Here we present a dynamic numerical simulation of Antarctic ice-stream retreat since the Last Glacial Maximum (LGM), constrained by geophysical data, whose behaviour is consistent with the geomorphological record. We find that retreat of Marguerite Bay Ice Stream following the LGM was highly nonlinear and was interrupted by stabilizations on a reverse-sloping bed, where theory predicts rapid unstable retreat. We demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed. We conclude that, as well as bed topography, ice-stream width and long-term retreat history are crucial for understanding decadal- to centennial-scale ice-stream behaviour and marine ice-sheet vulnerability

Growth and retreat of the last British–Irish Ice Sheet, 31 000 to 15 000 years ago: the BRITICE-CHRONO reconstruction

The BRITICE-CHRONO consortium of researchers undertook a dating programme to constrain the timing of advance, maximum extent and retreat of the British?Irish Ice Sheet between 31?000 and 15?000?years before present. The dating campaign across Ireland and Britain and their continental shelves, and across the North Sea included 1500?days of field investigation yielding 18?000?km of marine geophysical data, 377 cores of sea floor sediments, and geomorphological and stratigraphical information at 121 sites on land; generating 690 new geochronometric ages. These findings are reported in 28 publications including synthesis into eight transect reconstructions. Here we build ice sheet-wide reconstructions consistent with these findings and using retreat patterns and dates for the inter-transect areas. Two reconstructions are presented, a wholly empirical version and a version that combines modelling with the new empirical evidence. Palaeoglaciological maps of ice extent, thickness, velocity, and flow geometry at thousand-year timesteps are presented. The maximum ice volume of 1.8?m sea level equivalent occurred at 23?ka. A larger extent than previously defined is found and widespread advance of ice to the continental shelf break is confirmed during the last glacial. Asynchrony occurred in the timing of maximum extent and onset of retreat, ranging from 30 to 22?ka. The tipping point of deglaciation at 22?ka was triggered by ice stream retreat and saddle collapses. Analysis of retreat rates leads us to accept our hypothesis that the marine-influenced sectors collapsed rapidly. First order controls on ice-sheet demise were glacio-isostatic loading triggering retreat of marine sectors, aided by glaciological instabilities and then climate warming finished off the smaller, terrestrial ice sheet. Overprinted on this signal were second order controls arising from variations in trough topographies and with sector-scale ice geometric readjustments arising from dispositions in the geography of the landscape. These second order controls produced a stepped deglaciation. The retreat of the British?Irish Ice Sheet is now the world?s most well-constrained and a valuable data-rich environment for improving ice-sheet modelling

Društveno-ekonomski aspekti učiteljstva

U radu se prvo definira učiteljstvo kao profesija, zajednički stručni naziv – učitelj (i za učitelja, nastavnika, profesora i stručnog suradnika u školi). Zatim se govori o vrijednosti učiteljske profesije. Slijedi analiza materijalne osnove i financiranja učiteljstva, plaće učitelja i ostala materijalna prava. Potom slijedi prikaz učiteljskih udruga i učiteljske nagrade „Ivan Filipović“. Posebno se obrađuje pitanje – kakvi su učitelji potrebni novoj hrvatskoj školi. Hrvatskoj školi potrebni su sposobni, obrazovni, stručno pedagoško osposobljeni, motivirani, materijalno situirani, zadovoljni, samostalni, slobodni i demokratski učitelji. Svjetski dan učitelja prigoda je da se sjetimo svoje učiteljice-učitelja, njihovih riječi i djela koja su obilježila naše odrastanje i ostavili duboki trag u našim srcima

High Climate Model Dependency of Pliocene Antarctic Ice-Sheet Predictions

The mid-Pliocene warm period provides a natural laboratory to investigate the long-term response of the Earth’s ice-sheets and sea level in a warmer-than-present-day world. Proxy data suggest that during the warm Pliocene, portions of the Antarctic ice-sheets, including West Antarctica could have been lost. Ice-sheet modelling forced by Pliocene climate model outputs is an essential way to improve our understanding of ice-sheets during the Pliocene. However, uncertainty exists regarding the degree to which results are model-dependent. Using climatological forcing from an international climate modelling intercomparison project, we demonstrate the high dependency of Antarctic ice-sheet volume predictions on the climate model-based forcing used. In addition, the collapse of the vulnerable marine basins of Antarctica is dependent on the ice-sheet model used. These results demonstrate that great caution is required in order to avoid making unsound statements about the nature of the Pliocene Antarctic ice-sheet based on model results that do not account for structural uncertainty in both the climate and ice sheet models

West Antarctic Ice Sheet collapse – the fall and rise of a paradigm

It is now almost 30 years since John Mercer (1978) first presented the idea that climate change could eventually cause a rapid deglaciation, or “collapse”, of a large part of the West Antarctic ice sheet (WAIS), raising world sea levels by 5 metres and causing untold economic and social impacts. This idea, apparently simple and scientifically plausible, created a vision of the future, sufficiently alarming that it became a paradigm for a generation of researchers and provided an icon for the green movement. Through the 1990s, however, a lack of observational evidence for ongoing retreat in WAIS and improved understanding of the complex dynamics of ice streams meant that estimates of likelihood of collapse seemed to be diminishing. In the last few years, however, satellite studies over the apparently inaccessible Amundsen Sea sector of West Antarctica have shown clear evidence of ice sheet retreat showing all the features that might have been predicted for emergent collapse. These studies are re-invigorating the paradigm, albeit in a modified form, and debate about the future stability of WAIS. Since much of WAIS appears to be stable, it may, no longer be reasonable to suggest there is an imminent threat of a 5-m rise in sea level resulting from complete collapse of the West Antarctic ice sheet, but there is strong evidence that the Amundsen Sea embayment is changing rapidly. This area alone, contains the potential to raise sea level by around ~1.5 m, but more importantly it seems likely that it could, alter rapidly enough, to make a significant addition to the rate of sea-level rise over coming two centuries. Furthermore, a plausible connection between contemporary climate change and the fate of the ice sheet appears to be developing. The return of the paradigm presents a dilemma for policy-makers, and establishes a renewed set of priorities for the glaciological community. In particular, we must establish whether the hypothesized instability in WAIS is real, or simply an oversimplification resulting from inadequate understanding of the feedbacks that allow ice sheets to achieve equilibrium: and whether there is any likelihood that contemporary climate change could initiate collapse