Environment Counts | Reconstruction of the last interglacial (Eemian) climate record from Greenland ice cores
Author: Geoff Zeiss – Published At: 2013-02-02 08:58 – (1075 Reads)
There is a gap in ice-core records in Greenland corresponding to the last interglacial, called the Eemian, which extended from 130,000 to 115,000 years ago. Greenland temperatures were warmer than at present during the Eemian, making the Eemian a possible useful analogue to the future of our current climate. A 2,540-m-long ice core was drilled during 2008â€“12 through the ice at the North Greenland Eemian Ice Drilling (NEEM) site, Greenland. The top 1,419â€‰m is from the current interglacial. Below this, the ice is disturbed and folded, but it contains zones from the last interglacial, the Eemian (130â€“115â€‰thousand years before the present). This article presents a reconstruction of the Eemian record from the folded ice. Based on the climate reconstruction it is found that at the NEEM site surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8â€‰Â±â€‰4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling. Extensive surface melt occurred at the NEEM site during the Eemian. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400â€‰Â±â€‰250 meters, reaching surface elevations 122,000 years ago of 130â€‰Â±â€‰300 meters lower than the present. Model simulations based on the NEEM data suggest only a modest contribution of 2m from Greenland ice-sheet melting to sea level rise. Nature 493, 489â€“494 (24 January 2013)
A 2,540-m-long ice core was drilled during 2008â€“12 through the ice at the NEEM site, Greenland which sees an accumulation of 0.22â€‰m ice per year. The top 1,419â€‰m is from the current interglacial, the Holocene, and together with the glacial ice below it can be matched to the timescale from the NGRIP ice core down to 2,206.7â€‰m (108 thousand years before the present). Below this, the ice is disturbed and folded, but it contains zones with relatively high stable isotope values of delta oxygen-18 (ice), a proxy for condensation temperature, indicating that it stems from the Eemian. Near bedrock, low delta oxygen-18 (ice) values suggest that the ice layers are most probably from the glacial period before the Eemian.
Measurements of delta oxygen-18 (ice) have been made and air bubbles trapped within the ice have been analyzed to determine concentrations of methane (CH4) and nitrous oxide (N2O), stable isotope values delta nitrogen-15 of N2 and delta oxygen-18 of O2, and total air content. The volume of air entrapped in polar ice (air content) is related to atmospheric pressure (elevation) and temperature. If a temperature history is known, elevation changes can be inferred.
In addition, the rheology of the ice (how it flows under intense pressure), radio echo sounding (RES) images and surface temperatures and ice temperatures were used in the interpretation.
The reconstructed records of delta oxygen-18 (ice), delta oxygen-18 (atm) (reversed scale) CH4, N2O, delta nitrogen-15 and air content (reversed scale). Zone 1 (cyan) is seen to contain a fold while zone 2 (green) and zone 3 (orange) are reversed and cover identical time periods. NGRIP records (light grey) and EDML records (dark grey) are included where they are available. The CH4, N2O and air content records contain spikes from 127 to 118.3â€‰kyr bp (shaded grey).
The NEEM data shows spikes in CH4 and N2O concentrations between depths of 2,370â€‰m and 2,418â€‰m. These are ascribed to surface melting or wet surface conditions. Surface melting or percolating rain can result in production of CH4 and N2O in the firn (partially compacted snow). The spikes occur in the warmest interval. Over the past millennium very few melt layers are found in the ice core before 19957. Over the period 12â€“15 July 2012, an exceptional heat wave produced significant surface melt over 97% of the Greenland ice sheet, leaving a strong fingerprint at the NEEM site in the form of melt layers 5â€“6 cm thick at 50â€“70â€‰cm below the surface.
In a 6,000â€‰year period from 128 to 122â€‰thousand years ago, the surface elevation is estimated to have decreased from 210â€‰Â±â€‰350â€‰m above to 130â€‰Â±â€‰300â€‰m below the present surface elevation, which translates to a moderate ice thickness change of 400â€‰Â±â€‰350â€‰m. Based on this estimate, the ice thickness at NEEM decreased by an average of 7â€‰Â±â€‰4â€‰cm per year between 128 and 122â€‰ thousand years ago and stayed at this level until 117â€“114â€‰thousand years ago, long after surface melt stopped and temperatures fell below modern levels.
Although ice thickness estimates at one location on the Greenland ice sheet cannot be used to infer the overall ice-sheet changes during the last interglacial period, Greenland ice-sheet model simulations based on the NEEM data point to modest contribution (2â€‰m) to the observed 4â€“8â€‰m Eemian sea level high stand. These findings strongly imply that Antarctica must have contributed significantly to the Eemian sea level rise.
Despite the complex ice flow, the disturbed record of the deep ice in the NEEM ice core can be unambiguously reconstructed. The anatomy of the last interglacial shows that Greenland temperatures peaked after the onset of the Eemian, 126â€‰ thousand years ago, with temperatures 8â€‰Â±â€‰4â€‰Â°C warmer than the average of the recent millennium and multiple indications of summer melt. Temperatures gradually decreased during the interglacial, very probably owing to the strong local summer decreasing trend in solar radiation. The surface elevation first increased due to increased mass balance to 210â€‰Â±â€‰350â€‰m above the present at 128â€‰ thousand years ago, then decreased to 130â€‰Â±â€‰300â€‰m below the present elevation around 122â€‰ thousand years ago. The record reported here, together with recent observations of rainfall and strong surface melting in July 2012 at NEEM, suggest that conditions are conducive to the start of melt layer formation at NEEM, with the 2010â€“12 mean annual surface temperatures 1â€“2â€‰Â°C above the 1950â€“80 average.