Abstract
Abstract. The sensitivity of interglacial Greenland temperature to orbital and CO2 forcing is investigated using the NorthGRIP ice core data and coupled ocean-atmosphere IPSL-CM4 model simulations. These simulations were conducted in response to different interglacial orbital configurations, and to increased CO2 concentrations. These different forcings cause very distinct simulated seasonal and latitudinal temperature and water cycle changes, limiting the analogies between the last interglacial and future climate. However, the IPSL-CM4 model shows similar magnitudes of Arctic summer warming and climate feedbacks in response to 2 × CO2 and orbital forcing of the last interglacial period (126 000 years ago). The IPSL-CM4 model produces a remarkably linear relationship between TOA incoming summer solar radiation and simulated changes in summer and annual mean central Greenland temperature. This contrasts with the stable isotope record from the Greenland ice cores, showing a multi-millennial lagged response to summer insolation. During the early part of interglacials, the observed lags may be explained by ice sheet-ocean feedbacks linked with changes in ice sheet elevation and the impact of meltwater on ocean circulation, as investigated with sensitivity studies. A quantitative comparison between ice core data and climate simulations requires stability of the stable isotope – temperature relationship to be explored. Atmospheric simulations including water stable isotopes have been conducted with the LMDZiso model under different boundary conditions. This set of simulations allows calculation of a temporal Greenland isotope-temperature slope (0.3–0.4‰ per °C) during warmer-than-present Arctic climates, in response to increased CO2, increased ocean temperature and orbital forcing. This temporal slope appears half as large as the modern spatial gradient and is consistent with other ice core estimates. It may, however, be model-dependent, as indicated by preliminary comparison with other models. This suggests that further simulations and detailed inter-model comparisons are also likely to be of benefit. Comparisons with Greenland ice core stable isotope data reveals that IPSL-CM4/LMDZiso simulations strongly underestimate the amplitude of the ice core signal during the last interglacial, which could reach +8–10 °C at fixed-elevation. While the model-data mismatch may result from missing positive feedbacks (e.g. vegetation), it could also be explained by a reduced elevation of the central Greenland ice sheet surface by 300–400 m.
Highlights
Greenland ice cores, such as the longest NorthGRIP record, spanning the last 123 000 years (NorthGRIP-communitymembers, 2004), offer continuous and quantitative archives of past local climate variability at orbital time scales (e.g. Vinther et al, 2009) as well as the evidence for abrupt climate events (e.g. Capron et al, 2010a)
Continuous records of water stable isotopes (δ18O or δD) have been measured along several deep Greenland ice cores; the longest record published so far was obtained from the NorthGRIP ice core (NorthGRIP-community-members, 2004) (Fig. 1)
Based on the shape of north Atlantic SST records synchronized on the EDC3 age scale (Masson-Delmotte et al, 2010a), one may assume that the isotopic values of the deepest part of the NorthGRIP ice core may be representative of a multi-millennial temperature plateau
Summary
Greenland ice cores, such as the longest NorthGRIP record, spanning the last 123 000 years (NorthGRIP-communitymembers, 2004), offer continuous and quantitative archives of past local climate variability at orbital time scales (e.g. Vinther et al, 2009) as well as the evidence for abrupt climate events (e.g. Capron et al, 2010a). The last interglacial period, about 130–120 thousand years before present (ka), is proposed to be a good analogue for future climate change driven by anthropogenic greenhouse gas emissions (Clark and Huybers, 2009; Otto-Bliesner et al, 2006; Sime et al, 2009; Turney and Jones, 2010), especially in the Arctic In this manuscript, we address the following questions:. For direct model-data comparisons, we use the sea surface conditions (sea surface temperature, SST and sea ice) from the coupled climate model to drive its atmospheric component equipped with the explicit modeling of precipitation isotopic composition (LMDZiso) This allows the stability of the isotope-temperature change through time and the mechanisms that can alter this relationship to be explored.
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