A new study provides surprising details on changes in Earth's climate during the last warm period (120,000-128,000 years ago). Even though temperatures in Northern Greenland were 5-8 degrees Celsius higher than today, the thickness of the ice sheet was only a few hundred meters lower. And this despite the fact that sea level was 4-8 metres higher than today.
This indicates that the melting of the Greenland ice sheet may have contributed less than half of the total sea level rise at the time. This interglacial period (the so called "Eemian") may be a good analogue for where the Greenland ice sheet is heading today in the face of increasing greenhouse gases and warming temperatures.
These results from the North Greenland Eemian Ice Drilling (NEEM) project, led by the University of Copenhagen and with participation by the University of Bern, have now been published in "Nature."
"A thick Greenland ice sheet connected to much warmer conditions in Greenland is astounding but no reason to relax and watch what the future of human-made warming has in store for us," says Prof. Hubertus Fischer, ice core scientist at the University of Bern. "Fact is that the warming was accompanied by a sea level rise of 4-8 metres. Such a sea-level rise would be a disaster for the more than 7 billion people living on this planet today, even if it takes a couple thousand years to be reached."
The apparent good news from this study is that the Greenland ice sheet may not be as sensitive to temperature increases as previously thought. However, the bad news is that if Greenland did not disgorge larger parts of its ice into the ocean during the Eemian, then Antarctica and here especially the more climate sensitive West Antarctic Ice Sheet must be responsible for a significant part of the 4-8-metre sea level rise and may be even more sensitive to climate warming than previously thought.
The new findings also revealed temporal changes in the higher temperatures in Northern Greenland over the last interglacial period. At the beginning of the Eemian (128,000 years before today), the ice sheet in the vicinity of NEEM was 200 metres higher than today. At the same time, the temperature at the altitude of the current drill site was up to 8°C warmer.
Towards the end of the Eemian, the sheet thickness was reduced by 130 metres compared to today but the ice was still 2400 metres thick, while the temperature was still about 5°C warmer. The research team estimates that the volume of the Greenland ice sheet shrank during the Eemian by no more than 25% over 6000 years. The rate of elevation change in the early part of the Eemian was high (about 6 cm/yr) and the loss of mass from the Greenland ice sheet was likely on the same order as changes observed during the last ten years.
The concentrations of the atmospheric greenhouse gases methane and nitrous oxide in the Eemian, measured by the Bern team, were similar at that time to what is observed during the preindustrial period about 150 years ago, i.e. before the increase by human emissions started.
The strong warming in Northern Greenland during the Eemian led to frequent summer melt layers clearly recognized by a low air content and greenhouse gas concentrations, which due to the melt processes were much higher than their atmospheric value. Such melt events are very rare by comparison during the past 5000 years and require a warming during the Eemian of at least 4°C.
- D. Dahl-Jensen et al. Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 2013; 493 (7433): 489 DOI: 10.1038/nature11789
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