Abstract
Abstract. The last interglacial period (LIG, ∼ 129–116 thousand years ago) provides the most recent case study of multimillennial polar warming above the preindustrial level and a response of the Greenland and Antarctic ice sheets to this warming, as well as a test bed for climate and ice sheet models. Past changes in Greenland ice sheet thickness and surface temperature during this period were recently derived from the North Greenland Eemian Ice Drilling (NEEM) ice core records, northwest Greenland. The NEEM paradox has emerged from an estimated large local warming above the preindustrial level (7.5 ± 1.8 °C at the deposition site 126 kyr ago without correction for any overall ice sheet altitude changes between the LIG and the preindustrial period) based on water isotopes, together with limited local ice thinning, suggesting more resilience of the real Greenland ice sheet than shown in some ice sheet models. Here, we provide an independent assessment of the average LIG Greenland surface warming using ice core air isotopic composition (δ15N) and relationships between accumulation rate and temperature. The LIG surface temperature at the upstream NEEM deposition site without ice sheet altitude correction is estimated to be warmer by +8.5 ± 2.5 °C compared to the preindustrial period. This temperature estimate is consistent with the 7.5 ± 1.8 °C warming initially determined from NEEM water isotopes but at the upper end of the preindustrial period to LIG temperature difference of +5.2 ± 2.3 °C obtained at the NGRIP (North Greenland Ice Core Project) site by the same method. Climate simulations performed with present-day ice sheet topography lead in general to a warming smaller than reconstructed, but sensitivity tests show that larger amplitudes (up to 5 °C) are produced in response to prescribed changes in sea ice extent and ice sheet topography.
Highlights
It remains challenging to understand the magnitude, timing and rate of the contributions of the Greenland and/or Antarctic ice sheets to the estimated 5 to 10 m increase in global mean sea level during the last interglacial period (LIG, 129– 116 thousand years before 1950, hereafter ka) and that of ice sheet vulnerability to multimillennial polar warming (Masson-Delmotte et al, 2013; Dutton et al, 2015)
The mean temperature is derived from borehole measurements. b For determining the δ18Oice, δ15N and d-excess values attributed to the LIG, we have taken the average of the corresponding records for North Greenland Eemian Ice Drilling (NEEM), GRIP and GISP2 between 122 and 126 ka
They imply that the mean annual firn temperature at the LIG deposition site, upstream of the current NEEM site, experienced 6–11 ◦C warming, without correcting for changes in elevation related to ice thickness change
Summary
It remains challenging to understand the magnitude, timing and rate of the contributions of the Greenland and/or Antarctic ice sheets to the estimated 5 to 10 m increase in global mean sea level during the last interglacial period (LIG, 129– 116 thousand years before 1950, hereafter ka) and that of ice sheet vulnerability to multimillennial polar warming (Masson-Delmotte et al, 2013; Dutton et al, 2015). The initial LIG temperature estimate (NEEM community members, 2013) was performed using the average Holocene δ18Oice–temperature relationship established from other central Greenland ice cores through calibration against borehole temperature at 0.5 ‰ ◦C−1 (Vinther et al, 2009) This relationship was explored in simulations using isotopically enabled atmospheric general circulation models for climate conditions warmer than the preindustrial period, either in response to increasing CO2 concentration in projections or in response to changes in orbital forcing. These models produced slopes varying from 0.3 to 0.7 ‰ ◦C−1 in Greenland, depending on changes in moisture sources driven by changes in sea ice and sea surface temperature patterns (MassonDelmotte et al, 2011; Sime et al, 2013).
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