Evaluating model simulations of twentieth-century sea-level rise. Part II: regional sea-level changes

Twentieth-century regional sea level changes are estimated from 12 climate models from phase 5 of the Climate Model Intercomparison Project (CMIP5). The output of the CMIP5 climate model simulations was used to calculate the global and regional sea level changes associated with dynamic sea level, atmospheric loading, glacier mass changes, and ice sheet surface mass balance contributions. The contribution from groundwater depletion, reservoir storage, and dynamic ice sheet mass changes are estimated from observations as they are not simulated by climate models. All contributions are summed, including the glacial isostatic adjustment (GIA) contribution, and compared to observational estimates from 27 tide gauge records over the twentieth century (1900–2015). A general agreement is found between the simulated sea level and tide gauge records in terms of interannual to multidecadal variability over 1900–2015. But climate models tend to systematically underestimate the observed sea level trends, particularly in the first half of the twentieth century. The corrections based on attributable biases between observations and models that have been identified in Part I of this two-part paper result in an improved explanation of the spatial variability in observed sea level trends by climate models. Climate models show that the spatial variability in sea level trends observed by tide gauge records is dominated by the GIA contribution and the steric contribution over 1900–2015. Climate models also show that it is important to include all contributions to sea level changes as they cause significant local deviations; note, for example, the groundwater depletion around India, which is responsible for the low twentieth-century sea level rise in the region

Regional climate of the Larsen B embayment 1980–2014

Understanding the climate response of the Antarctic Peninsula ice sheet is vital for accurate predictions of sea-level rise. However, since climate models are typically too coarse to capture spatial variability in local scale meteorological processes, our ability to study specific sectors has been limited by the local fidelity of such models and the (often sparse) availability of observations. We show that a high-resolution (5.5 km × 5.5 km) version of a regional climate model (RACMO2.3) can reproduce observed interannual variability in the Larsen B embayment sufficiently to enable its use in investigating long-term changes in this sector. Using the model, together with automatic weather station data, we confirm previous findings that the year of the Larsen B ice shelf collapse (2001/02) was a strong melt year, but discover that total annual melt production was in fact ~30% lower than 2 years prior. While the year before collapse exhibited the lowest melting and highest snowfall during 1980–2014, the ice shelf was likely pre-conditioned for collapse by a series of strong melt years in the 1990s. Melt energy has since returned to pre-1990s levels, which likely explains the lack of further significant collapse in the region (e.g. of SCAR Inlet)

The modelled surface mass balance of the Antarctic Peninsula at 5.5 km horizontal resolution.

This study presents a high-resolution (similar to 5.5 km) estimate of surface mass balance (SMB) over the period 1979-2014 for the Antarctic Peninsula (AP), generated by the regional atmospheric climate model RACMO2.3 and a firn densification model (FDM). RACMO2.3 is used to force the FDM, which calculates processes in the snowpack, such as meltwater percolation, refreezing and runoff. We evaluate model output with 132 in situ SMB observations and discharge rates from six glacier drainage basins, and find that the model realistically simulates the strong spatial variability in precipitation, but that significant biases remain as a result of the highly complex topography of the AP. It is also clear that the observations significantly underrepresent the high-accumulation regimes, complicating a full model evaluation. The SMB map reveals large accumulation gradients, with precipitation values above 3000 mm we yr(-1) in the western AP (WAP) and below 500 mm we yr(-1) in the eastern AP (EAP), not resolved by coarser data sets such as ERA-Interim. The average AP ice-sheet-integrated SMB, including ice shelves (an area of 4.1 x 10(5) km(2)), is estimated at 351 Gt yr(-1) with an interannual variability of 58 Gt yr(-1), which is dominated by precipitation (PR) (365 +/- 57 Gt yr(-1)). The WAP (2.4 x 10(5) km(2)) SMB (276 +/- 47 Gt yr(-1)), where PR is large (276 +/- 47 Gt yr(-1)), dominates over the EAP (1.7 x 10(5) km(2)) SMB (75 +/- 11 Gt yr(-1)) and PR (84 +/- 11 Gt yr(-1)). Total sublimation is 11 +/- 2 Gt yr(-1) and meltwater runoff into the ocean is 4 +/- 4 Gt yr(-1). There are no significant trends in any of the modelled AP SMB components, except for snowmelt that shows a significant decrease over the last 36 years (-0.36 Gt yr(-2))

The present and future state of the Antarctic firn layer

Firn is the transitional product between fresh snow and glacier ice and acts as a boundary between the atmosphere and the glacier ice of the Antarctic Ice Sheet (AIS). Spatiotemporal variations in firn layer characteristics are therefore important to consider when assessing the mass balance of the AIS. In this thesis, a firn densification model, forced with a realistic climate, is used to examine contemporary (1979-2012) and future (2000-2200) variations in the Antarctic firn layer. Currently, 99% of the AIS is covered with a firn layer of 50-150 m thick. The thickest firn layers occur in the cold interior of the AIS, while thinner firn layers appear along the coastal margins, where regular melt occurs. On the remaining 1-2% of the AIS no firn layer exist; a so-called blue-ice area. Here, annual ablation, by either sublimation or melt, is larger than the annual accumulation, resulting in no long-term firn layer. The presence of a blue-ice area depends on a favorable combination of 1) ice velocity, 2) net surface ablation and 3) the mass of the existing firn layer. Next to the spatial variations, also temporal variations in firn layer characteristics exist. Due to the seasonal cycle in temperature and accumulation, the air content of the Antarctic firn layer grows in winter and shrinks in summer. As a consequence, the surface elevation of the AIS also shows cyclic behavior with a seasonal amplitude of 2.6 cm. In order to simulate the reaction of the current Antarctic firn layer on a warmer and wetter future climate, four simulations with the regional atmospheric climate model RACMO2 are performed. By forcing RACMO2 with two different global climate models (HAdCM3 and ECHAM5) and two different emission scenarios (A1B and E1), the possible spread in future climate is mimicked. The temperature increase over the AIS is similar to the global average; +1.8-3.0 K in 2100 and +2.4-5.3 K in 2200. This warmer climate leads to increased accumulation, as warmer air has a larger water vapor holding capacity. This accumulation increase outweighs the increases in both sublimation and melt, leading to a positive surface mass balance sensitivity: +98 Gt yr-1 K-1. In combination with the simulated temperature increase, this would result to a sea level drop of 73-163 mm by 2200. This is however without taking any ice dynamical response into account. Due to the future increase in snowfall, the air content of the Antarctic firn layer will increase. Roughly half of this effect is counteracted by both enhanced firn densification and a faster firn-to-ice transition at the bottom of the firn layer. Along the coast, firn air content will decrease significantly due to increasing melt. On several ice shelves at the Antarctic Peninsula, this will lead to depleted firn layers and enhanced runoff of meltwater. Averaged over the ice sheet, this decrease in firn air content is however small, resulting in an increase of the total AIS air content with 120-150 km3 yr-1, or +2.1 cm surface elevation per year

The present and future state of the Antarctic firn layer

Firn is the transitional product between fresh snow and glacier ice and acts as a boundary between the atmosphere and the glacier ice of the Antarctic Ice Sheet (AIS). Spatiotemporal variations in firn layer characteristics are therefore important to consider when assessing the mass balance of the AIS. In this thesis, a firn densification model, forced with a realistic climate, is used to examine contemporary (1979-2012) and future (2000-2200) variations in the Antarctic firn layer. Currently, 99% of the AIS is covered with a firn layer of 50-150 m thick. The thickest firn layers occur in the cold interior of the AIS, while thinner firn layers appear along the coastal margins, where regular melt occurs. On the remaining 1-2% of the AIS no firn layer exist; a so-called blue-ice area. Here, annual ablation, by either sublimation or melt, is larger than the annual accumulation, resulting in no long-term firn layer. The presence of a blue-ice area depends on a favorable combination of 1) ice velocity, 2) net surface ablation and 3) the mass of the existing firn layer. Next to the spatial variations, also temporal variations in firn layer characteristics exist. Due to the seasonal cycle in temperature and accumulation, the air content of the Antarctic firn layer grows in winter and shrinks in summer. As a consequence, the surface elevation of the AIS also shows cyclic behavior with a seasonal amplitude of 2.6 cm. In order to simulate the reaction of the current Antarctic firn layer on a warmer and wetter future climate, four simulations with the regional atmospheric climate model RACMO2 are performed. By forcing RACMO2 with two different global climate models (HAdCM3 and ECHAM5) and two different emission scenarios (A1B and E1), the possible spread in future climate is mimicked. The temperature increase over the AIS is similar to the global average; +1.8-3.0 K in 2100 and +2.4-5.3 K in 2200. This warmer climate leads to increased accumulation, as warmer air has a larger water vapor holding capacity. This accumulation increase outweighs the increases in both sublimation and melt, leading to a positive surface mass balance sensitivity: +98 Gt yr-1 K-1. In combination with the simulated temperature increase, this would result to a sea level drop of 73-163 mm by 2200. This is however without taking any ice dynamical response into account. Due to the future increase in snowfall, the air content of the Antarctic firn layer will increase. Roughly half of this effect is counteracted by both enhanced firn densification and a faster firn-to-ice transition at the bottom of the firn layer. Along the coast, firn air content will decrease significantly due to increasing melt. On several ice shelves at the Antarctic Peninsula, this will lead to depleted firn layers and enhanced runoff of meltwater. Averaged over the ice sheet, this decrease in firn air content is however small, resulting in an increase of the total AIS air content with 120-150 km3 yr-1, or +2.1 cm surface elevation per year

An improved semi-empirical model for the densification of Antarctic firn

A firn densification model is presented that simulates steady-state Antarctic firn density profiles, as well as the temporal evolution of firn density and surface height. The model uses an improved firn densification expression that is tuned to fit depth-density observations. Liquid water processes (meltwater percolation, retention and refreezing) are also included. Two applications are presented. First, the steady-state model version is used to simulate the strong spatial variability in firn layer thickness across the Antarctic ice sheet. Second, the time-dependent model is run for 3 Antarctic locations with different climate conditions. Surface height changes are caused by a combination of accumulation, melting and firn densification processes. On all 3 locations, an upward trend of the surface during autumn, winter and spring is present, while during summer there is a more rapid lowering of the surface. Accumulation and (if present) melt introduce large inter-annual variability in surface height trends, possibly hiding ice dynamical thickening and thinning

On the formation of blue ice on Byrd Glacier, Antarctica

Blue-ice areas (BIAs) cover ⇠1% of the East Antarctic ice sheet and are visual evidence of persistent ablation. In these regions, more snow is sublimated and/or eroded than is accumulated. The physical processes driving the formation of BIAs are poorly understood. Here we combine a firndensification model with high-resolution (5.5 km) maps of surface mass balance and ice velocity to simulate the build-up and removal of a firn layer along an ice flowline passing Byrd Glacier. A BIA is formed once the complete firn layer is removed. Feedback processes, which enhance blue-ice formation through the difference in surface characteristics of snow and ice, are examined using sensitivity simulations. The presence of blue ice on Byrd Glacier is found to be mainly determined by (1) ice velocity, (2) surface mass balance and (3) the characteristics (thickness, mass) of the firn layer prior to entering the ablation area. With a moderate decrease of the surface mass balance, the location and extent of the simulated BIA on Byrd Glacier is found to be in good qualitative agreement with MODIS optical imagery

Quantifying the seasonal “breathing” of the Antarctic ice sheet

[1] One way to estimate the mass balance of an ice sheet is to convert satellite observed surface elevation changes into mass changes. In order to do so, elevation and mass changes due to firn processes must be taken into account. Here, we use a firn densification model to simulate seasonal variations in depth and mass of the Antarctic firn layer, and assess their influence on surface elevation. Forced by the seasonal cycle in temperature and accumulation, a clear seasonal cycle in average firn depth of the Antarctic ice sheet (AIS) is found with an amplitude of 0.026 m, representing a volume oscillation of 340 km3. The phase of this oscillation is rather constant across the AIS: the ice sheet volume increases in austral autumn, winter and spring and quickly decreases in austral summer. Seasonal accumulation differences are the major driver of this annual ‘breathing’, with temperature fluctuations playing a secondary role. The modeled seasonal elevation signal explains 31% of the seasonal elevation signal derived from ENVISAT radar altimetry, with both signals having similar phase

On the formation of blue ice on Byrd Glacier, Antarctica

Blue-ice areas (BIAs) cover ⇠1% of the East Antarctic ice sheet and are visual evidence of persistent ablation. In these regions, more snow is sublimated and/or eroded than is accumulated. The physical processes driving the formation of BIAs are poorly understood. Here we combine a firndensification model with high-resolution (5.5 km) maps of surface mass balance and ice velocity to simulate the build-up and removal of a firn layer along an ice flowline passing Byrd Glacier. A BIA is formed once the complete firn layer is removed. Feedback processes, which enhance blue-ice formation through the difference in surface characteristics of snow and ice, are examined using sensitivity simulations. The presence of blue ice on Byrd Glacier is found to be mainly determined by (1) ice velocity, (2) surface mass balance and (3) the characteristics (thickness, mass) of the firn layer prior to entering the ablation area. With a moderate decrease of the surface mass balance, the location and extent of the simulated BIA on Byrd Glacier is found to be in good qualitative agreement with MODIS optical imagery