Longterm ice sheet–climate interactions under ant

Authors: Miren Vizcaíno Uwe Mikolajewicz Matthias Gröger Ernst MaierReimer Guy Schurgers Arne M E Winguth

Publish Date: 2008/01/30

Volume: 31, Issue: 6, Pages: 665-690

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Abstract

Several multicentury and multimillennia simulations have been performed with a complex Earth System Model ESM for different anthropogenic climate change scenarios in order to study the longterm evolution of sea level and the impact of ice sheet changes on the climate system The core of the ESM is a coupled coarseresolution Atmosphere–Ocean General Circulation Model AOGCM Ocean biogeochemistry land vegetation and ice sheets are included as components of the ESM The Greenland Ice Sheet GrIS decays in all simulations while the Antarctic ice sheet contributes negatively to sea level rise due to enhanced storage of water caused by larger snowfall rates Freshwater flux increases from Greenland are one order of magnitude smaller than total freshwater flux increases into the North Atlantic basin the sum of the contribution from changes in precipitation evaporation runoff and Greenland meltwater and do not play an important role in changes in the strength of the North Atlantic Meridional Overturning Circulation NAMOC The regional climate change associated with weakening/collapse of the NAMOC drastically reduces the decay rate of the GrIS The dynamical changes due to GrIS topography modification driven by mass balance changes act first as a negative feedback for the decay of the ice sheet but accelerate the decay at a later stage The increase of surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long term Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced to 3/4 of the original size From then the reduction in the albedo of Greenland strongly accelerates the decay of the ice sheetThe melting of glaciers and the thermal expansion of the oceans associated with anthropogenic climate change are expected to produce substantial sea level changes during the next few centuries Meehl et al 2007 The continentalsize glaciers on Earth the Greenland and Antarctic ice sheets GrIS and AIS store a volume of water equivalent to a sea level rise of 72 and 611 m respectively Church et al 2001 While half of the current ablation in the GrIS is due to surface melting almost all ablation happens as calving at the margin of the AIS due to the very low atmospheric temperatures Alley et al 2005 and Gregory et al 2004 showed that for a uniform warming of more than 3 K the GrIS would begin to decay disappearing in between 1000 and several thousand years depending on the magnitude of the warmingIn addition to producing changes in sea level future modifications of the ice sheets could produce changes in the climate system via changes in ocean circulation surface albedo and/or atmospheric circulation patterns Such modifications of climate could potentially affect the mass balance of the ice sheets and therefore play an important role in their own evolution There are a few examples of modelling studies in the literature performed with coupled ice sheet–climate models with the aim of identifying and quantifying these feedbacks between ice sheets and climate for both the cases of past and future climate For the studies of past climates Intermediate Complexity Models EMICs for an overview see Petoukhov et al 2005 have been used as tool due to the long time scales and the limitations imposed by computational resources For future climate projections some studies have focused exclusively on the feedbacks between ice sheets and ocean Huybrechts et al 2002 Fichefet et al 2003 while others have investigated the feedbacks between ice sheets and atmosphere as well Ridley et al 2005 Driesschaert et al 2007 Only the studies of Ridley et al 2005 with HadCM3 and Mikolajewicz et al 2007b were performed with General Circulation Models GCMs both for the ocean and atmosphere and with full coupling between the three components ice sheet ocean and atmosphere This type of tool enables the investigation of the feedbacks between ice sheets and both the circulations of atmosphere and ocean Feedback studies using models of intermediate complexity may allow the estimate of some of the relevant feedbacks but are in general limited by the strong simplifications of atmospheric physics and dynamics and their potential impact on some of the simulated feedbacksThe albedofeedback is considered to have played a key role in the growth of ice sheets during past glacial inceptions Kageyama et al 2004 Calov et al 2005 Manabe and Broccoli 1985 found that the albedo and topographic effects of ice sheets alone explain much of the northern hemisphere cooling identified in paleoclimatic records of the last glacial maximum In Ridley et al 2005 albedo changes act as a negative feedback for the decay of the GrIS under a constant 4× CO2 scenario The evolution of convective cells with rising warm air over the areas becoming icefree and descending air over the ice sheet originates the transport of cold air from the interior of the ice sheet into the ablation zones at the margins of the ice sheet reducing surface meltingThe heightfeedback ie the modification of surface temperature due to changes in elevation can play an important role in the processes of growth and decay of ice sheets for instance at glacial inception Gallee et al 1992 Wang and Mysak 2002 or in the future development of the GrIS In Huybrechts and de Wolde 1999 a substantial contribution of this feedback to the decay rate of the GrIS is shown These authors also investigated the effect on the velocity field of changes in topography driven by changes in the surface mass balance They found a deceleration of the decay rate during a first phase which turned into acceleration at a later stageThe freshening of ocean waters at deep convection sites by meltwater from the ice sheets could potentially modify the strength of the NAMOC Several authors have investigated this issue for past climates and future climate projections The discharge of meltwater from the northern hemisphere ice sheets has been proposed as the triggering mechanism for abrupt climate change in the past at the time of the Younger Dryas Broecker et al 1988 MaierReimer and Mikolajewicz 1989 Fichefet et al 2003 found a substantial weakening of the NAMOC in response to increased meltwater fluxes from the GrIS in a twentyfirst century simulation Driesschaert et al 2007 investigated the response of the climate system to different greenhouse gas scenarios with a threedimensional 3D EMIC including a dynamic ice sheet component They found a noticeable weakening of the NAMOC due to freshwater fluxes from the GrIS only under the forcing of the most extreme scenarioA reduction in northward ocean heat transport due to a collapsed or weakened NAMOC potentially modifies the climate of the North Atlantic region eg Stouffer et al 2006 Fig 4 This regional climate change could produce a reduction of meltwater fluxes from the ice sheets in the region acting as a mechanism stabilising the NAMOC This negative feedback between ice sheet melting and NAMOC changes could modify the time scales of recovery of the NAMOC after strong meltwater fluxes pulses both in past and future climates This mechanism has not been received much attention in the literature yet In the greenhouse gas simulation of Swingedouw et al 2006 with a coupledocean atmosphere model and a simple landice melting parameterisation the freshwater forcing from ice sheet melting increases nearly linearly from 0 to 02 Sv in 140 years of 1 per year increase of atmospheric CO2 The NAMOC weakens from 10 to 5 Sv with a substantial contribution of ice sheet melting to this weakening The negative atmospheric temperature anomaly associated with this decline has its maximum far from Greenland over the Barents Sea being its absolute value less than 2 K over Greenland The mechanism suggested above does not seem relevant in this study since ice sheet melting does not appear to decrease substantially following the weakening of the NAMOC Similarly in the 2× CO2 simulation of Swingedouw et al 2007 the impact on the Greenland climate of the reduced ocean heat transport accompanying the decline of the NAMOC does not prevent landice melting from causing the shutdown of the NAMOCChanges in the topography of the ice sheets can modify the general circulation of the atmosphere Modelling studies of past climates indicate that the path of the jet stream is highly dependent on the topography of the Laurentide ice sheet Manabe and Broccoli 1985 Cook and Held 1988 The changes in the atmosphere that the absence of the GrIS would cause in the case of its complete deglaciation have been investigated in studies where the ice sheet is absent either from a preindustrial basic state eg Lunt et al 2004 Toniazzo et al 2004 Junge et al 2005 or from a humanperturbed climate Ridley et al 2005 These studies found a regional impact of the absence of the GrIS with maximal warming in Greenland due to drastic reduction of surface albedo and topographic height and substantial changes in the path of the storm track and in the mean circulation of the atmosphere in the northern high latitudes

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