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Heinrich-type ice-sheet surges are one of the dominant signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large amounts of ice are discharged from ice sheets into the ocean. The occurence of ice-sheet surges strongly influences the global climate evolution by altering the ocean circulation through the addition of freshwater and the atmospheric circulation through changes in ice-sheet height. The mechanisms controlling the timing and occurence of Heinrich-type ice-sheet surges remain poorly constrained to this day. Here, we use a coupled ice sheet-solid earth model to identify and quantify the importance of boundary forcing for the surge cycle length of Heinrich-type ice-sheet surges for two prominent ice streams of the Laurentide ice sheet - the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. We show that surface mass balance perturbations have the largest effect on the surge cycle length. Pertubations to the ice surface temperature and geothermal heatflux also influence the surge cycle length, but to a lesser degree. Ocean and sea-level forcing as well as different frequencies of the same forcing have a negligible effect on the surge cycle length. The simulations also highlight that only a certain parameter space exists under which ice-sheet oscillations can be maintained. Transitioning from an oscillatory state to a persistent ice streaming state, can result in an ice volume loss of up to 26% for the respective ice stream drainage basin under otherwise constant climate conditions. We show that Mackenzie ice stream is susceptible to undergoing such a transition in response to all tested positive climate perturbations. This underlines the potential of the Mackenzie region to have contributed to prominent abrupt climate change events of the last deglaciation. [ABSTRACT FROM AUTHOR]

The last deglaciation was characterized by drastic climate changes, most prominently melting ice sheets. Melting ice sheets have a significant impact on the atmospheric and oceanic circulation, due to changes in the topography and meltwater release into the ocean. In a set of transient simulations of the last deglaciation with the Max Planck Institute for Meteorology Earth System Model we explore differences in the climate response that arise from different boundary conditions and implementations suggested within the Paleoclimate Modeling Intercomparison Project ‐ Phase 4 (PMIP4) deglaciation protocol. The underlying ice‐sheet reconstruction dominates the simulated deglacial millennial‐scale climate variability in terms of timing and occurrence of observed climate events. Sensitivity experiments indicate that the location and timing of meltwater release from the ice sheets into the ocean are crucial for the ocean response. The results will allow a better interpretation of inter‐model differences that arise from different implementations proposed within the PMIP4 protocol. Plain Language Summary: The last deglaciation marked the transition between the last glacial maximum (LGM; about 21,000 years ago) and present. It was characterized by the disappearance of most northern hemispheric ice sheets (e.g., over North America and Eurasia) and a series of abrupt climate events. To investigate the ability of climate models to simulate the last deglaciation and to better understand the physical processes behind the changes, the Paleoclimate Modeling Intercomparison Project ‐ Phase 4 (PMIP4) has designed a common protocol for model simulations of the last deglaciation. This protocol comprises several choices on how to set up the model systems participating in this exercise. With a state‐of‐the‐art climate model, we explore how model simulations of the last deglaciation differ if different boundary data and methods proposed in the protocol are used. Our simulations show that the ice‐sheet boundary condition impacts the atmospheric and oceanic circulation during the LGM until about 14,000 years ago. It also determines the timing and occurrence of abrupt climate events throughout the deglaciation. These and further sensitivity experiments presented in this study will help to interpret the differences between model simulations that will be submitted for the PMIP4 deglaciation exercise. Key Points: Transient simulations of the last deglaciation with the comprehensive Max Planck Institute for Meteorology Earth System Model 1.2Glacial climate and millennial‐scale climate variability depend significantly on boundary conditions and methods proposed in Paleoclimate Modeling Intercomparison Project ‐ Phase 4 (PMIP4) deglaciation protocolEstimating uncertainties for interpretation of differences in PMIP4 model ensemble of the last deglaciation [ABSTRACT FROM AUTHOR]

A realistic simulation of the surface mass balance (SMB) is essential for simulating past and future ice-sheet changes. As most state-of-the-art Earth system models (ESMs) are not capable of realistically representing processes determining the SMB, most studies of the SMB are limited to observations and regional climate models and cover the last century and near future only. Using transient simulations with the Max Planck Institute ESM in combination with an energy balance model (EBM), we extend previous research and study changes in the SMB and equilibrium line altitude (ELA) for the Northern Hemisphere ice sheets throughout the last deglaciation. The EBM is used to calculate and downscale the SMB onto a higher spatial resolution than the native ESM grid and allows for the resolution of SMB variations due to topographic gradients not resolved by the ESM. An evaluation for historical climate conditions (1980–2010) shows that derived SMBs compare well with SMBs from regional modeling. Throughout the deglaciation, changes in insolation dominate the Greenland SMB. The increase in insolation and associated warming early in the deglaciation result in an ELA and SMB increase. The SMB increase is caused by compensating effects of melt and accumulation: the warming of the atmosphere leads to an increase in melt at low elevations along the ice-sheet margins, while it results in an increase in accumulation at higher levels as a warmer atmosphere precipitates more. After 13 ka, the increase in melt begins to dominate, and the SMB decreases. The decline in Northern Hemisphere summer insolation after 9 ka leads to an increasing SMB and decreasing ELA. Superimposed on these long-term changes are centennial-scale episodes of abrupt SMB and ELA decreases related to slowdowns of the Atlantic meridional overturning circulation (AMOC) that lead to a cooling over most of the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

The PMIP-4 protocol for modeling the last deglaciation (Ivanovic et al., 2016) allows the use of either the ICE-6G_C (Peltier et al., 2015; Argus et al., 2014) or the GLAC-1D (Tarasov et al., 2012; Tarasov and Peltier, 2002) ice sheet reconstruction. To investigate the impact of the underlying reconstructions on the climate, we have performed transient deglacial simulations with the same setup of the Max-Planck-Institute Earth System Model (MPI-ESM) only varying the ice sheet reconstruction. We have taken the ice sheet reconstructions at face value and derived the meltwater fluxes from the changes in ice sheet thickness. Our setup includes dynamic topography, bathymetry, coastlines, river routing directions, and vegetation, thus representing the full set of effects of the changing ice sheets. We show that variations in the meltwater fluxes dominate the millennial scale climate variability. However, these variations differ drastically depending on the underlying reconstructions. We present where to expect differences, and where to expect consistent behavior and point out common features that are not in accordance with proxy evidence. The results show that uncertainties in the reconstructed ice sheets dominate the simulated deglacial millennial scale climate variability. The effects on the climate of the two reconstructions will provide challenges for the comparison of the PMIP-4 transient experiments.The most dominant common feature, simulated by both ICE-6G_C and GLAC-1D, is a massive Northern Hemisphere (NH) ice sheet decay during the Bølling warming causing a freshwater pulse reaching about 0.5 Sv and thus a shutdown of the Atlantic Meridional Overturning Circulation (AMOC). The resulting AMOC collapse and NH cooling with associated Southern Hemisphere (SH) warming is a standard behavior of climate models when exposed to a strong freshwater forcing, and consistent with expectations from the basic physics. During the Younger Dryas, ICE-6G_C features ice sheet decay leading to a change in meltwater routing from the Mississippi to the St. Lawrence and Mackenzie rivers, and thus to a strong AMOC decline and NH cooling because of the higher effectiveness of freshwater hosing in this region. In contrast, GLAC-1D shows virtually no melt in this period, and the AMOC stays intact. The effects in the simulation forced with the ICE-6G_C reconstruction show the importance of correctly modeling changes in river routing. While ICE-6G_C shows an Antarctic freshwater pulse during Meltwater pulse 1B (MWP1B), causing NH warming at the end of the Younger Dryas, GLAC-1D features a northern hemisphere meltwater pulse with associated large-scale cooling during MWP1B.The simulations show that our model is capable of modeling the last deglaciation, and that the occurrence and timing of rapid climate change events is controlled by the ice sheets, making correct reconstructions crucial for modeling the chronology of the last deglacial climate variability. [ABSTRACT FROM AUTHOR]