Hätten Sie es gewusst? In der Zeit von 5000-3000 Jahren vor heute war die grönländische Eiskappe viel kleiner als heute! Das fand eine Studie der University at Buffalo heraus. Interessant: Die höchsten Atmosphären-Temperaturen der letzten 10.000 Jaren herrschten davor, nämlich vor 9000-5000 Jahren. Es dauerte also einige tausend Jahre, bis das Eis voll darauf reagierte. Auch die Ozeane zeigten eine starke Verzögerung. Ihr Temperaturhöhepunkt der Nacheiszeit ereignete sich zeitgleich zum Grönland-Eisminimum, also 5000-3000 Jahre vor heute. Im Folgenden die entsprechende Pressemitteilung der University at Buffalo vom 22. November 2013:
Greenland’s shrunken ice sheet: We’ve been here before
Clues in the Arctic fossil record suggest that 3-5,000 years ago, the ice sheet was the smallest it has been in the past 10,000 years
Think Greenland’s ice sheet is small today? It was smaller — as small as it has ever been in recent history — from 3-5,000 years ago, according to scientists who studied the ice sheet’s history using a new technique they developed for interpreting the Arctic fossil record. “What’s really interesting about this is that on land, the atmosphere was warmest between 9,000 and 5,000 years ago, maybe as late as 4,000 years ago. The oceans, on the other hand, were warmest between 5-3,000 years ago,” said Jason Briner, PhD, University at Buffalo associate professor of geology, who led the study. “What it tells us is that the ice sheets might really respond to ocean temperatures,” he said. “It’s a clue to what might happen in the future as the Earth continues to warm.”
The findings appeared online on Nov. 22 in the journal Geology. Briner’s team included Darrell Kaufman, an organic geochemist from Northern Arizona University; Ole Bennike, a clam taxonomist from the Geological Survey of Denmark and Greenland; and Matthew Kosnik, a statistician from Australia’s Macquarie University. The study is important not only for illuminating the history of Greenland’s ice sheet, but for providing geologists with an important new tool: A method of using Arctic fossils to deduce when glaciers were smaller than they are today.
Scientists have many techniques for figuring out when ice sheets were larger, but few for the opposite scenario. “Traditional approaches have a difficult time identifying when ice sheets were smaller,“ Briner said. „The outcome of our work is that we now have a tool that allows us to see how the ice sheet responded to past times that were as warm or warmer than present — times analogous to today and the near future.“ The technique the scientists developed involves dating fossils in piles of debris found at the edge of glaciers. To elaborate: Growing ice sheets are like bulldozers, pushing rocks, boulders and other detritus into heaps of rubble called moraines. Because glaciers only do this plowing when they’re getting bigger, logic dictates that rocks or fossils found in a moraine must have been scooped up at a time when the associated glacier was older and smaller. So if a moraine contains fossils from 3,000 years ago, that means the glacier was growing — and smaller than it is today — 3,000 years ago.
This is exactly what the scientists saw in Greenland: They looked at 250 ancient clams from moraines in three western regions, and discovered that most of the fossils were between 3-5,000 years old. The finding suggests that this was the period when the ice sheet’s western extent was at its smallest in recent history, Briner said. “Because we see the most shells dating to the 5-3000-year period, we think that this is when the most land was ice-free, when large layers of mud and fossils were allowed to accumulate before the glacier came and bulldozed them up,” he said.
Because radiocarbon dating is expensive, Briner and his colleagues found another way to trace the age of their fossils. Their solution was to look at the structure of amino acids — the building blocks of proteins — in the fossils of ancient clams. Amino acids come in two orientations that are mirror images of each other, known as D and L, and living organisms generally keep their amino acids in an L configuration. When organisms die, however, the amino acids begin to flip. In dead clams, for example, D forms of aspartic acid start turning to L’s. Because this shift takes place slowly over time, the ratio of D’s to L’s in a fossil is a giveaway of its age. Knowing this, Briner’s research team matched D and L ratios in 20 Arctic clamshells to their radiocarbon-dated ages to generate a scale showing which ratios corresponded with which ages. The researchers then looked at the D and L ratios of aspartic acid in the 250 Greenland clamshells to come up with the fossils’ ages. Amino acid dating is not new, but applying it to the study of glaciers could help scientists better understand the history of ice — and climate change — on Earth.
Carlson et al. fassten im August 2014 in den Geophysical Research Letters die Entwicklung des grönländischen Eises gut zusammen. Nach Ende der letzten Eiszeit ging das Grönlandeis dramatisch zurück. Nach Beendigung des mittelholozänen Klimaoptimums legte das Eis dann wieder an Masse zu, mit einem Maximum während der Kleinen Eiszeit. Das heutige Schmelzen knabbert an diesem kürzlichen Maximum. Hier die Kurzfassung:
Earliest Holocene south Greenland ice sheet retreat within its late Holocene extent
Early Holocene summer warmth drove dramatic Greenland ice sheet (GIS) retreat. Subsequent insolation-driven cooling caused GIS margin readvance to late Holocene maxima, from which ice margins are now retreating. We use 10Be surface exposure ages from four locations between 69.4°N and 61.2°N to date when in the early Holocene south to west GIS margins retreated to within these late Holocene maximum extents. We find that this occurred at 11.1 ± 0.2 ka to 10.6 ± 0.5 ka in south Greenland, significantly earlier than previous estimates, and 6.8 ± 0.1 ka to 7.9 ± 0.1 ka in southwest to west Greenland, consistent with existing 10Be ages. At least in south Greenland, these 10Be ages likely provide a minimum constraint for when on a multicentury timescale summer temperatures after the last deglaciation warmed above late Holocene temperatures in the early Holocene. Current south Greenland ice margin retreat suggests that south Greenland may have now warmed to or above earliest Holocene summer temperatures.
Das Abschmelzen des grönländischen Inlandeises während des mittelholozänen Klimaoptimums war auch Thema einer Arbeit von Lena Håkansson und Kollegen, die im August 2014 in den Quaternary Science Reviews erschien. Die Autoren untersuchten Seen in Westgrönland. Vor 6500 Jahren war das Eis soweit abgeschmolzen, dass eine Eisausbreitung erreicht wurde, die dem heutigen Stand entspricht. Erstaunlicherweise blieb der Eisrand in den folgenden gut 1000 Jahren relativ stabil, obwohl damals Temperaturen herrschten die mehr als 2°C wärmer waren als die aktuell dort gemessenen. Vor 5400 Jahren war die Position jedoch nicht mehr zu halten, und der Eisrand zog sich 1,5 km weit ins Landinnere zurück, wobei Flächen plötzlich eisfrei wurden, die heute wieder vom Eis bedeckt sind. Während der Kleinen Eiszeit um 1750 dehnte sich das Eis dann wieder aus, wobei gegen Ende der Kleinen Eiszeit, vielleicht um 1850, das Ausdehnungmaximum erreicht wurde. Irgendwann zwischen 1750-1850 muss der Eisrand die heutige Position überschritten haben. Seit Ende der Kleinen Eiszeit schrumpft das Eis jetzt wieder, wobei schließlich die heutige Eisrandlage erreicht wurde. Hier die Kurzfassung der wichtigen Arbeit:
Slow retreat of a land based sector of the West Greenland Ice Sheet during the Holocene Thermal Maximum: evidence from threshold lakes at Paakitsoq
Records from two connected proglacial threshold lakes at Paakitsoq, west Greenland have been analyzed in order to investigate the response of a land-based ice sheet margin to Holocene climate change. The results are used to test whether or not the land terminating margin at Paakitsoq behaved synchronously with the nearby marine terminating Jakobshavn Isbræ during the Holocene. The radiocarbon dated lake sediment cores indicate that the ice margin retreated to its present position ∼6.5 ka ago and thereafter maintained a relatively stable configuration similar to the present for >1000 years, despite summer temperatures >2 °C higher than today. The lakes became non-glacial after 5.4 cal. ka BP, when the ice margin retreated behind a drainage divide situated ∼1.5 km inland of the present margin. By this time, Jakobshavn Isbræ had already reached its minimum configuration. The Paakitsoq ice margin remained >1.5 km inland of its present margin until after 240 ± 20 cal. yr BP; after this time, both Paakitsoq ice margin and Jakobshavn Isbræ reached their Holocene maxima during the later stage of the Little Ice Age. Our results suggest that the present ice margin position at Paakitsoq is relatively stable in a warming climate but after a total retreat of ∼1.5 km behind the present margin position it may become marine based and more unstable due to marine melting and calving processes.
Auch das Naturkundemuseum von Dänemark beschäftigte sich in einer Pressemitteilung vom 20. Februar 2015 mit der Eiskappenschmelze in Grönland 8000 bis 5000 Jahre vor heute. Damals war es laut Studie 2-4°C wärmer als heute und der grönländische Eisschild schrumpfte auf eine Größe ab, die deutlich kleiner war als die heutige Ausdehnung. Die Forscher berechneten, dass damals über einen Zeitraum von dreitausend Jahren das Eisvolumen pro Jahr um 100 Gigatonnen pro Jahr abnahm. Hier die Pressemitteilung im englischen Original:
