Abstract
Abstract. The penultimate deglaciation (PDG, ∼138–128 thousand years before present, hereafter ka) is the transition from the penultimate glacial maximum (PGM) to the Last Interglacial (LIG, ∼129–116 ka). The LIG stands out as one of the warmest interglacials of the last 800 000 years (hereafter kyr), with high-latitude temperature warmer than today and global sea level likely higher by at least 6 m. Considering the transient nature of the Earth system, the LIG climate and ice-sheet evolution were certainly influenced by the changes occurring during the penultimate deglaciation. It is thus important to investigate, with coupled atmosphere–ocean general circulation models (AOGCMs), the climate and environmental response to the large changes in boundary conditions (i.e. orbital configuration, atmospheric greenhouse gas concentrations, ice-sheet geometry and associated meltwater fluxes) occurring during the penultimate deglaciation. A deglaciation working group has recently been set up as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phase 4, with a protocol to perform transient simulations of the last deglaciation (19–11 ka; although the protocol covers 26–0 ka). Similar to the last deglaciation, the disintegration of continental ice sheets during the penultimate deglaciation led to significant changes in the oceanic circulation during Heinrich Stadial 11 (∼136–129 ka). However, the two deglaciations bear significant differences in magnitude and temporal evolution of climate and environmental changes. Here, as part of the Past Global Changes (PAGES)-PMIP working group on Quaternary interglacials (QUIGS), we propose a protocol to perform transient simulations of the penultimate deglaciation under the auspices of PMIP4. This design includes time-varying changes in orbital forcing, greenhouse gas concentrations, continental ice sheets as well as freshwater input from the disintegration of continental ice sheets. This experiment is designed for AOGCMs to assess the coupled response of the climate system to all forcings. Additional sensitivity experiments are proposed to evaluate the response to each forcing. Finally, a selection of paleo-records representing different parts of the climate system is presented, providing an appropriate benchmark for upcoming model–data comparisons across the penultimate deglaciation.
Highlights
Over the last 450 kyr, Earth’s climate has been dominated by glacial–interglacial cycles with a recurrence period of about 100 kyr. These asymmetrical cycles are characterized by long glacial periods, associated with a gradual cooling, a slow decrease in atmospheric greenhouse gas (GHG) concentrations and a progressive growth of large continental ice sheets in the Northern Hemisphere (NH), leading to a 60 to 120 m global sea-level decrease (Lisiecki and Raymo, 2005; Grant et al, 2014; Rohling et al, 2017)
Assuming the Last Glacial Maximum (LGM) is an analogue for the penultimate glacial maximum (PGM), we propose a 140 ka spin-up value and N2O transient forcing curve that starts with a 201 ppb level and linearly increases to 218.74 ppb at 134.5 ka
Using a multi-proxy approach, we investigate here the millennial-scale variability occurring during the PDG and potential changes in North Atlantic Deep Water (NADW) linked with the meltwater input of Heinrich Stadial 11 (HS11)
Summary
Over the last 450 kyr, Earth’s climate has been dominated by glacial–interglacial cycles with a recurrence period of about 100 kyr. A return to stadial conditions over Greenland, Europe and the North Atlantic occurred during the Younger Dryas (∼ 12.8–11.7 ka; Fig. 1, right) (Alley, 2000) This event likely resulted from a combination of processes (Renssen et al, 2015), possibly including a weakening of NADW formation resulting from an increase in meltwater discharge into the Arctic Ocean (Tarasov and Peltier, 2005; Murton et al, 2010; Keigwin et al, 2018) or melting of the Fennoscandian ice sheet (Muschitiello et al, 2015), and an altered atmospheric circulation due to a minimum in solar activity (Renssen et al, 2000). After a description of changes in insolation (Sect. 2), GHGs (Sect. 3), continental ice sheets (Sect. 4) and sea level (Sect. 5) occurring during the PDG, we present a framework to perform transient simulations of PDG (Sects. 6 and 7), as well as a selection of key paleo-climate and paleoenvironmental records to be used for model–data comparisons (Sect. 8)
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