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Estimating Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR
X. Fettweis,B. Franco,M. Tedesco,J. H. van Angelen
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-3101-2012
Abstract: We report future projections of Surface Mass Balance (SMB) over the Greenland ice sheet (GrIS) obtained with the regional climate model MAR, forced by the outputs of three CMIP5 General Circulation Models (GCMs) when considering two different warming scenarios (RCP 4.5 and RCP 8.5). The GCMs selected in this study have been chosen according to their ability to simulate the current climate over Greenland. Our results indicate that in a warmer climate (i) the mass gained due to increased precipitation over GrIS does not compensate the mass lost through increased run-off; (ii) the surface melt increases non-linearly with rising temperatures due to the positive feedback between surface albedo and melt, associated with the expansion of bare ice zones which, in addition, decreases the ice sheet refreezing capacity; (iii) most of the precipitation is expected to fall as rainfall in summer, which further increases surface melt; (iv) no considerable change is expected on the length of the melting season, since heavier winter snowfall dampens the melt increase at the end of spring; (v) the increase of meltwater run-off versus temperature anomalies is dependent of the GCM-forced MAR ability to simulate the current climate; (vi) the MAR-simulated SMB changes can be approximated using the annual accumulated snowfall and summer 600 hPa temperature increase simulated by the forcing GCMs. In view of the large range in the CMIP5 future projections for the same future scenario, the GCM-based SMB approximations allow us to estimate what future projections are most likely within the CMIP5 multi-model ensemble. In 2100, the ensemble mean projects a sea level rise, resulting from a GrIS SMB decrease, estimated to be +4 ± 2 cm and +9 ± 4 cm for the RCP 4.5 and RCP 8.5 scenarios, respectively. The GrIS SMB should remain positive with respect to RCP 4.5 scenario and becomes negative around 2070 in the case of the RCP 8.5 scenario since a global warming >+3 °C is needed. However, these future projections do not consider the positive melt-elevation feedback because the ice sheet topography is fixed through the whole simulation. In this regard, the MAR simulations suggest a cumulative ice sheet thinning by 2100 of ~100–200 m in the ablation zone. This highlights the importance of coupling climate models to an ice sheet model to consider the future response of both surface processes and ice-dynamic changes, and their mutual feedbacks to rising temperatures.
Drifting snow climate of the Greenland ice sheet: a study with a regional climate model
J. T. M. Lenaerts,M. R. van den Broeke,J. H. van Angelen,E. van Meijgaard
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-1611-2012
Abstract: This paper presents the drifting snow climate of the Greenland ice sheet, using output from a high-resolution (~11 km) regional climate model (RACMO2). Because reliable direct observations of drifting snow do not exist, we evaluate the modeled near-surface climate instead, using Automatic Weather Station (AWS) observations from the K-transect and find that RACMO2 realistically simulates near-surface wind speed and relative humidity, two variables that are important for drifting snow. Integrated over the ice sheet, drifting snow sublimation (SUds) equals 24 ± 3 Gt yr 1, and is significantly larger than surface sublimation (SUs, 16 ± 2 Gt yr 1). SUds strongly varies between seasons, and is only important in winter, when surface sublimation and runoff are small. A rapid transition exists between the winter season, when snowfall and SUds are important, and the summer season, when snowmelt is significant, which increases surface snow density and thereby limits drifting snow processes. Drifting snow erosion (ERds) is only important on a regional scale. In recent decades, following decreasing wind speed and rising near-surface temperatures, SUds exhibits a negative trend (0.1 ± 0.1 Gt yr 1), which is compensated by an increase in SUs of similar magnitude.
Drifting snow climate of the Greenland ice sheet: a study with a regional climate model
J. T. M. Lenaerts,M. R. van den Broeke,J. H. van Angelen,E. van Meijgaard
The Cryosphere , 2012, DOI: 10.5194/tc-6-891-2012
Abstract: This paper presents the drifting snow climate of the Greenland ice sheet, using output from a high-resolution (～11 km) regional climate model. Because reliable direct observations of drifting snow do not exist, we evaluate the modeled near-surface climate instead, using automatic weather station (AWS) observations from the K-transect and find that RACMO2 realistically simulates near-surface wind speed and relative humidity, two variables that are important for drifting snow. Integrated over the ice sheet, drifting snow sublimation (SUds) equals 24 ± 3 Gt yr 1, and is significantly larger than surface sublimation (SUs, 16 ± 2 Gt yr 1). SUds strongly varies between seasons, and is only important in winter, when surface sublimation and runoff are small. A rapid transition exists between the winter season, when snowfall and SUds are important, and the summer season, when snowmelt is significant, which increases surface snow density and thereby limits drifting snow processes. Drifting snow erosion (ERds) is only important on a regional scale. In recent decades, following decreasing wind speed and rising near-surface temperatures, SUds exhibits a negative trend (0.1 ± 0.1 Gt yr 1), which is compensated by an increase in SUs of similar magnitude.
Brief Communication "Expansion of meltwater lakes on the Greenland ice sheet"
I. M. Howat,S. de la Pe？a,J. H. van Angelen,J. T. M. Lenaerts
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-4447-2012
Abstract: Forty years of satellite imagery reveal that meltwater lakes on the margin of the Greenland Ice Sheet have expanded substantially inland to higher elevations with warming. These lakes are important because they provide a mechanism for bringing water to the ice bed, warming the ice and causing sliding. Inland expansion of lakes could accelerate ice flow by bringing water to previously frozen bed, potentially increasing future rates of mass loss. Increasing lake elevations in West Greenland closely follow the rise of the mass balance equilibrium line, suggesting no physical limit on lake expansion there. This is not included in ice sheet models.
Brief Communication "Expansion of meltwater lakes on the Greenland Ice Sheet"
I. M. Howat,S. de la Pe？a,J. H. van Angelen,J. T. M. Lenaerts
The Cryosphere , 2013, DOI: 10.5194/tc-7-201-2013
Abstract: Forty years of satellite imagery reveal that meltwater lakes on the margin of the Greenland Ice Sheet have expanded substantially inland to higher elevations with warming. These lakes are important because they provide a mechanism for bringing water to the ice bed, causing sliding. Inland expansion of lakes could accelerate ice flow by bringing water to previously frozen bed, potentially increasing future rates of mass loss. Increasing lake elevations closely follow the rise of the mass balance equilibrium line over much of the ice sheet, suggesting no physical limit on lake expansion. Data are not yet available to detect a corresponding change in ice flow, and the potential effects of lake expansion on ice sheet dynamics are not included in ice sheet models.
Sensitivity of Greenland Ice Sheet surface mass balance to surface albedo parameterization: a study with a regional climate model
J. H. van Angelen,J. T. M. Lenaerts,S. Lhermitte,X. Fettweis
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-1531-2012
Abstract: We present a sensitivity study of the surface mass balance (SMB) of the Greenland Ice Sheet, as modeled using a regional atmospheric climate model, to various parameter settings in the albedo parameterization. The snow albedo parameterization uses grain size as a prognostic variable and further depends on cloud cover, solar zenith angle and black carbon concentration. For the control experiment the overestimation of absorbed shortwave radiation (+6 %) at the K-transect (West Greenland) for the period 2004–2009 is considerably reduced compared to the previous density-dependent albedo parameterization (+22 %). To simulate realistic snow albedo values, a small concentration of black carbon is needed. A background ice albedo field derived from MODIS imagery improves the agreement between the modeled and observed SMB gradient along the K-transect. The effect of enhanced retention and refreezing is a decrease of the albedo due to an increase in snow grain size. As a secondary effect of refreezing the snowpack is heated, enhancing melt and further lowering the albedo. Especially in a warmer climate this process is important, since it reduces the refreezing potential of the firn layer covering the Greenland Ice Sheet.
Sensitivity of Greenland Ice Sheet surface mass balance to surface albedo parameterization: a study with a regional climate model
J. H. van Angelen,J. T. M. Lenaerts,S. Lhermitte,X. Fettweis
The Cryosphere , 2012, DOI: 10.5194/tc-6-1175-2012
Abstract: We present a sensitivity study of the surface mass balance (SMB) of the Greenland Ice Sheet, as modeled using a regional atmospheric climate model, to various parameter settings in the albedo scheme. The snow albedo scheme uses grain size as a prognostic variable and further depends on cloud cover, solar zenith angle and black carbon concentration. For the control experiment the overestimation of absorbed shortwave radiation (+6%) at the K-transect (west Greenland) for the period 2004–2009 is considerably reduced compared to the previous density-dependent albedo scheme (+22%). To simulate realistic snow albedo values, a small concentration of black carbon is needed, which has strongest impact on melt in the accumulation area. A background ice albedo field derived from MODIS imagery improves the agreement between the modeled and observed SMB gradient along the K-transect. The effect of enhanced meltwater retention and refreezing is a decrease of the albedo due to an increase in snow grain size. As a secondary effect of refreezing the snowpack is heated, enhancing melt and further lowering the albedo. Especially in a warmer climate this process is important, since it reduces the refreezing potential of the firn layer that covers the Greenland Ice Sheet.
Liberate konserwatisme? H.G. Stoker en die ideologie van apartheid: 'n Verkenning
J. H. van Wyk
Koers : Bulletin for Christian Scholarship , 1994, DOI: 10.4102/koers.v59i3&4.700
Abstract: In this article the intriguing question is discussed how it could be possible that a Christian philosopher o f the stature of H. G. Stoker could legitimise a policy of apartheid. It is argued that each ideology has a blinding effect on its supporters; thus making it almost impossible to take a critical stance towards it. It has been attempted to explain in more detail that some key features of the Biblical message were overlooked and tinder-developed in the philosophy of Stoker.
Die politieke dimensie in die romankuns van André P. Brink: 'n Etiese evaluering
J. H. van Wyk
Koers : Bulletin for Christian Scholarship , 1986, DOI: 10.4102/koers.v51i1.924
Abstract: The author justifies writing about the political dimension in the novels of André P. Brink on the basis that committed literature also embraces the political dimension as one of its m any dimensions. He thus proposes to deal not with whether but how this dimension manifests its e lf in the literature. A rising line of political involvement is detected in the late st novels - and this tendency can be defined in terms of "in situation alized violence ". An analysis of a number of novels, including Kennis van die aand, Houd-den-bek and Muur van die Pes underscores his views, and reveals, too, a view that the church is caught up in pietistic immobility, and that the novels abound in Existentia list, Marxist and even at times Calvin is ethical motifs. The political dimension is then evaluated in terms of Calvin is views and the Calvin is tradition - especially in view of the concept of the justifiability of violence and in surreciton.
Nywerheidsontwikkeling in Suid-Afrika *)
H. J. Van Eck
Koers : Bulletin for Christian Scholarship , 1963, DOI: 10.4102/koers.v30i11.1516
Abstract: U het seker al baie keer die inleidingswoorde van ’n spreker gehoor dat dit vir hom 'n groot eer en ’n voorreg is om sy deurlugtige gehoor toe te spreek. U aanvaar dit waarskynlik as ’n konvensionele hoflikheid wat die spreker en sy gehoor ’n paar sekondes kans gee om mekaar te leer ken. Ek wil u egter die versekering gee dat ek dit werklik waardeer het toe ek byna ’n jaar gelede die uitnodiging ontvang het om u vandag toe te spreek. Ek het al die verskonings beskikbaar gehad om te bedank weens veeleisende werksaamhede, wat werklik waar is, en het nog boonop die deurslaggewende verskoning gehad dat vandag een van die min dae was wat ek by my familie tuis kon deurbring en eintlik moes deurbring, want dit is vandag ook my verjaarsdag. Ek het egter gevoel dat as ’n man ’n verjaarsdag wil vier nadat by alreeds die amptelike aftree- ouderdom bereik het, hy dit nie beter kan doen as in die geselskap van honderde jong manne en vroue wat bymekaargekom bet om hulle grade te ontvang en wat op die drumpel van hulle produktiewe lewe staan nie. My vrou en ek vier dus sommer my verjaarsdag terwyl ons saam met u die toekenning van u grade vier.
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