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Abstract. The penultimate deglaciation (PDG, ∼138–128 thousand years before present, hereafter ka) is the transition fromthe 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 changesoccurring during the penultimate deglaciation.It is thus importantto investigate, with coupled atmosphere–ocean general circulation models (AOGCMs),the climate and environmental response to the large changesin 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 protocolto 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 changesin 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 deglaciationunder 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 ofcontinental ice sheets.This experiment is designed for AOGCMs to assessthe 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 appropriatebenchmark for upcoming model–data comparisons across the penultimate deglaciation.
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Simulating Marine Isotope Stage 7 with a coupled climate–ice sheet model
Abstract. It is widely accepted that orbital variations areresponsible for the generation of glacial cycles during the latePleistocene. However, the relative contributions of the orbital forcingcompared to CO2 variations and other feedback mechanisms causing thewaxing and waning of ice sheets have not been fully understood. Testingtheories of ice ages beyond statistical inferences, requires numericalmodeling experiments that capture key features of glacial transitions. Here,we focus on the glacial buildup from Marine Isotope Stage (MIS) 7 to 6covering the period from 240 to 170 ka (ka: thousand years before present). Thistransition from interglacial to glacial conditions includes one of thefastest Pleistocene glaciation–deglaciation events, which occurred during MIS 7e–7d–7c (236–218 ka). Using a newly developed three-dimensional coupledatmosphere–ocean–vegetation–ice sheet model (LOVECLIP), we simulate thetransient evolution of Northern Hemisphere and Southern Hemisphere ice sheets duringthe MIS 7–6 period in response to orbital and greenhouse gas forcing. For arange of model parameters, the simulations capture the evolution of globalice volume well within the range of reconstructions. Over the MIS 7–6period, it is demonstrated that glacial inceptions are more sensitive toorbital variations, whereas terminations from deep glacial conditions needboth orbital and greenhouse gas forcings to work in unison. For someparameter values, the coupled model also exhibits a critical North Americanice sheet configuration, beyond which a stationary-wave–ice-sheettopography feedback can trigger an unabated and unrealistic ice sheetgrowth. The strong parameter sensitivity found in this study originates fromthe fact that delicate mass imbalances, as well as errors, are integratedduring a transient simulation for thousands of years. This poses a generalchallenge for transient coupled climate–ice sheet modeling, with suchcoupled paleo-simulations providing opportunities to constrain suchparameters.
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- Award ID(s):
- 1903197
- PAR ID:
- 10232854
- Date Published:
- Journal Name:
- Climate of the Past
- Volume:
- 16
- Issue:
- 6
- ISSN:
- 1814-9332
- Page Range / eLocation ID:
- 2183 to 2201
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Proxy evidences suggest abrupt southward displacements of the intertropical convergence zone (ITCZ) during Heinrich Stadial 1 (HS1) and Younger Dryas (YD) against a long‐term trend of northward ITCZ migration from Last Glacial Maximum to modern climate. Climate model simulations reveal that the abrupt ITCZ changes in HS1 and YD are mainly driven by ice‐sheet‐induced meltwater while the long‐term ITCZ trend primarily results from orbital variations, rising atmospheric greenhouse gases and ice‐sheet retreats during the last deglaciation. Atmospheric energetics analysis elucidates two important processes driven by meltwater—less net radiation entering the top‐of‐atmosphere (TOA) in the Northern Hemisphere than the Southern Hemisphere and a reduced global cross‐equatorial oceanic heat transport from the compensation between Atlantic and Indo‐Pacific heat transports—induce the southward ITCZ shift during HS1. Ice sheet extent changes also create a large interhemispheric TOA radiation asymmetry during HS1, which, however, is not via the surface albedo feedback.more » « less
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Abstract. We examine results from two transient modeling experiments that simulate the Last Interglacial period (LIG) using the state-of-the-art Community Earth System Model (CESM2), with a focus on climate and ocean changes relevant to the possible collapse of the Antarctic ice sheet. The experiments simulate the early millennia of the LIG warm period using orbital forcing, greenhouse gas concentrations, and vegetation appropriate for 127 ka. In the first case (127ka), no other changes are made; in the second case (127kaFW), we include a 0.2 Sv freshwater forcing in the North Atlantic. Both are compared with a pre-industrial control simulation (piControl). In the 127ka simulation, the global average temperature is only marginally warmer (0.004 °C) than in the piControl. When freshwater forcing is added (127kaFW), there is surface cooling in the Northern Hemisphere (NH) and warming in the Southern Hemisphere (SH), consistent with the bipolar seesaw effect. Near the Antarctic ice sheet, the 127ka simulation generates notable ocean warming (up to 0.4 °C) at depths below 200 m compared to the piControl. In contrast, the addition of freshwater in the North Atlantic in the 127kaFW run results in a multi-century subsurface ocean cooling that rebounds slowly over multiple millennia near the Antarctic ice sheet. These results have implications for the thermal forcing (and thereby mass balance) of the Antarctic ice sheet. We explore the physical processes that lead to this result and discuss implications for climate forcing of Antarctic ice sheet mass loss during the LIG.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/cp-16-2183-2020
