Gloomy future reflected in the past: In a global warm phase around 400,000 years ago, the thick ice sheet in north-west Greenland had disappeared for around 16,000 years. This emerges from new analysis results of a drill core from the secret military base "Camp Century". According to the researchers, a global average temperature that we are currently heading towards again as part of anthropogenic climate change led to the melting. This means there is a risk of flooding: According to the calculations, the loss of ice in Greenland had contributed to a sea level rise of at least 1.4 meters.
It looks like eternal ice: geologists also assumed for a long time that the kilometer-thick ice sheet in north-west Greenland was able to defy the warmer periods of Earth's history for millions of years. But then, about two years ago, researchers presented evidence that this was not the case. Their findings were based on examining an ice and sediment core sampled in the 1960s at the US Army's secret military base, Camp Century, located in the ice of the northern polar region. Plant remains in the sediment documented that there was once a tundra where now a kilometer-thick layer of ice lies. A rough dating already showed that the earth's surface had been exposed at some point in the last 1.1 million years.
When did the tundra spread?
When exactly and for how long, however, remained unclear. However, this information is important in order to be able to classify the event in the context of climate history. Because understanding Greenland's past is crucial to assessing the climatic conditions under which it could become critical for the preservation of the Greenland ice masses. The international research team led by Andrew Christ from the University of Vermont in Burlington has now provided a clearer picture of the ice-free period in north-west Greenland.
To do this, they subjected the material from the historical drill core to dating using so-called luminescence technology. It is based on the fact that light or darkness leads to certain changes in sediment components. Thanks to modern detection methods, this can be used as a kind of clock system to indicate when sediments were last exposed to the sun - in this specific case they were not covered by ice. In addition, the researchers subjected quartz crystals from the sediments to a special analysis technique. As they explain, certain ratios of isotopes of the elements beryllium and aluminum reveal how long the material was exposed to the sky and thus to cosmic rays.
dating with embassy
The researchers were able to show that the plant-covered tundra existed around 416,000 years ago at the site that is now thickly covered by ice. As for the duration of the ice-free period, the analysis results indicated around 16,000 years. "This is the first valid evidence that much of the Greenland ice sheet disappeared when it warmed up," says co-author Paul Bierman of the University of Vermont. The scientists say that the ice loss can now also be assigned to an already known warm phase: the so-called marine isotope stage 11 (MIS 11). It is noteworthy that previous results indicate that this interim period was marked by average temperatures only slightly higher than today.
The results thus suggest that the Greenland ice sheet may be more sensitive to human-induced climate change than previously thought: "If moderate warming over the course of MIS 11 resulted in ice loss in Greenland, then rapid, sustained and significant anthropogenic warming of the Arctic is also likely to lead to melting of the Greenland ice sheet, raising sea levels and triggering additional climate feedbacks in the coming centuries," the scientists write.
Based on model calculations, they also provide an assessment of the effects of the melting of the gigantic ice masses in Greenland 400,000 years ago. According to this, the water inflow could have contributed 1.4 meters, but perhaps even up to six meters, to the global sea level rise during the MIS 11 interglacial period.
At that time, however, there were no cities on the coasts, which are now threatened by floods. "If only parts of the Greenland ice sheet melt, sea levels will rise dramatically," said co-author Tammy Rittenour of Utah State University in Logan. "And then think of all the global population centers that are at sea level," he said.
Source: University of Vermont, professional article: Science 10.1126/science.ade4248