Monday 18 February 2013

Context and Observations of Sea level and Land Ice

Morning Chair: Simon Marsland

08:00 - 08:30	Registration and coffee/snack
08:30 - 08:40	Bruce Mapstone Welcome from Chief, CSIRO Marine and Atmospheric Research
08:40 - 08:50	Simon Marsland Welcome from organising committee and logistics
08:50 - 09:50	John Church Challenges in improving sea-level projections
09:50 - 10:20	Break
10:20 - 11:20	Mark Tamisiea The ‘static’ boundaries of sea level change
11:20 - 12:20	Robert Kopp Interpreting the noisy geological record of ancient sea level changes: What can the Quaternary tell us about ice sheet stability?
12:20 - 12:40	Catia Domingues Global and regional thermosteric sea level changes since 1970
12:40 - 14:00	Lunch

Afternoon Chair: Gokhan Dababasoglu

14:00 - 15:20	Poster Session
15:20 - 15:50	Break
15:50 - 16:10	Matt King Lower GRACE estimates of Antarctic sea-level contribution
16:10 - 16:30	Will Hobbs Can we detect long-term, global change from sparse, 135-year-old ocean data?
16:30 - 16:50	Peter Svendsen Confidence and sensitivity of sea-level reconstructions
17:00 - 20:30	Reception, BBQ and posters

Tuesday 19 February

Land Ice Theory and Models; Ocean Ice Shelf Interactions

Morning Chair: Stephanie Downes

08:00 - 08:30	Coffee/snack
08:30 - 09:30	David Holland Ice-ocean interaction observations and modeling, Greenland and Antarctica
09:30 - 09:50	Natalia Gomez A coupled ice sheet - sea level model applied to Antarctica through the last 40,000 years
09:50 - 10:20	Break
10:20 - 11:20	Patrick Heimbach Understanding the Dynamic Response of Greenland’s Marine Terminating Glaciers to Oceanic and Atmospheric Forcings
11:20 - 12:20	Phillip Jones for Wiliam Lipscomb Modeling the Antarctic ice sheet in CESM with Glimmer-CISM
12:20 - 12:40	Ben Galton-Fenzi Processes controlling ice shelf melting in East Antarctica
12:40 - 14:00	Lunch

Afternoon Chair: Matthew England

14:00 - 15:00	Eric Larour Uncertainty quantification of ice sheet mass balance projections using ISSM
15:00 - 15:20	Alberto Naveira Garabato Evidence of accelerating glacial melt in Antarctic coastal sea level rise
15:20 - 15:50	Break
15:50 - 16:50	Hartmut Hellmer Southern Ocean ice shelf melting in a warming climate
16:50 - 17:10	Jianjun Yin Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica
17:10 - 18:00	Synthesis and discussion Lead: Detlef Stammer and Jonathan Gregory

Wednesday 20 February

Ocean Model and Coupled Model Simulations

Morning Chair: Catia Domingues

08:00 - 08:30	Coffee/snack
08:30 - 09:30	Stephen Griffies Sea level simulated in a suite of forced global ocean-ice models
09:30 - 09:50	Benoit Meyssignac Anthropogenic forcing fingerprint on the tropical Pacific sea level trend pattern from the CMIP5 simulations of the 21st century
09:50 - 10:20	Break
10:20 - 11:20	Jonathan Gregory Twentieth-century global-mean sea level rise: is the whole greater than the sum of the parts?
11:20 - 12:20	Detlef Stammer Observed and simulated present-day and future regional sea level changes
12:20 - 12:40	Aimee Slangen Projecting regional sea-level changes for the 21st century
12:40 - 14:00	Lunch

Afternoon Chair: Stephen Griffies

Abstracts

Sea level change and IPCC AR5
John Church
CSIRO, Australia

Global and regional thermosteric sea level changes since 1970
Catia M. Domingues, T. Boyer, S. Good, N. White, P. Barker, J. Dunn, S. Wijffels, J. Church, N. Bindoff
Antarctic Climate and Ecosystem Cooperative Research Centre (ACE CRC), Australia

Thermosteric sea level (ThSL) is a major component of the global mean sea level rise observed during the late 20th century, and is projected to continue through the 21st century and beyond. At global scale, thermosteric sea-level rise is explained by the expansion in volume of the global ocean due to a net increase in ocean heat content over the past 50 years. At regional level, geographical patterns are produced in response to dynamical processes, with some areas experiencing variations above and others below the observed global mean. In this talk, we first provide an overview of the challenges in estimating ThSL for the upper 700 m of the ocean, from sparse and unevenly distributed subsurface ocean temperature data, measured by a large and changing mix of instruments. We then illustrate the impact of different instrumental bias corrections and mapping approaches on the global evolution and spatio-temporal variability of ThSL estimates.

Processes controlling ice shelf melting in East Antarctica
Benjamin K. Galton-Fenzi
Antarctic Climate & Ecosystems Cooperative Research Centre, Australia

There is a critical need to improve the ability of global ocean models to represent important coastal features and processes that control the supply of oceanic heat to ice shelves. Ice shelves in East Antarctica are showing thinning, although at lower rates than in the Western Peninsula region, sug-
gested to be caused by enhanced melting by warmer coastal oceans. However, the temperature of East Antarctic coastal seas, unlike those in the West, is primarily dependent on the presence of relatively cold and salty Dense Shelf Water (DSW). DSW is created at the sea surface due to sea ice formation
processes primarily in localised coastal leads and polynyas and is an important ingredient in the formation of Antarctic Bottom Water. Results are presented from both observations and from several regional models and a circum-Antarctic model, that are used to investigate the oceanic processes
that determine ice shelf melting. In areas of low polynya activity or under a future reduction of sea ice growth from polynyas, relatively warmer waters can instead occupy the continental shelf and increase ice shelf melting.

Evidence of accelerating glacial melt in Antarctic coastal sea level rise
Alberto Naveira Garabato
National Oceanography Center, UK

The subpolar Southern Ocean is a region of great climatic importance, hosting intense air - sea - ice interactions with far-reaching consequences for global ocean circulation and sea level. Glaciological measurements suggest that these interactions may be undergoing a profound change as a result of an accelerating glacial discharge into the Antarctic coastal seas, yet current evidence of this change is suggestive at best. In this work, we analyse the altimetric record of sea surface height (SSH) during the largely ice-free summer season to show that the subpolar Southern Ocean has experienced a pronounced, quasi-circumpolar positive trend in summertime SSH of ~1 mm / yr above the global-mean sea level rise since the early 1990s. The signal is generally amplified near the coast and in the Pacific sector, is broadly consistent in magnitude and geography with in situ observations of upper-ocean freshening in several sectors of the Antarctic shelf seas, and exhibits a magnitude that is approximately twice that implied by glaciological measurements of the accelerating glacial discharge. All in all, our analysis indicates that the widespread sea level rise in the subpolar Southern Ocean primarily reflects a halosteric response to the recent acceleration in Antarctic ice mass loss on decadal time scales, although wind forcing plays a significant role in explaining SSH variability on interannual and shorter time scales.

A coupled ice sheet - sea level model applied to Antarctica through the last 40,000 years
Natalya Gomez¹, David Pollard², Jerry X. Mitrovica¹
1 Harvard University, USA
2 Pennsylvania State University, USA

An instability mechanism is predicted for marine ice sheets resting upon reversed bed slopes whereby ice-sheet thinning or rising sea level leads to irreversible retreat of the grounding line. Previous analyses of marine ice-sheets have considered the influence of a sealevel perturbation on ice-sheet stability by assuming a geographically uniform, or eustatic, change in sea level. However, gravitational, deformational and rotational effects associated with changes in the volume of grounded ice lead to markedly non-uniform spatial patterns of sea-level change. In particular, a gravitationally self-consistent sea-level theory predicts a sea-level fall in the vicinity of a shrinking ice sheet that is an order of magnitude greater amplitude than the sea-level rise that would be predicted assuming eustasy. I will highlight the stabilizing influence of local sea-level changes on marine ice sheets using an ice sheet stability theory and consider the impact of this stabilizing mechanism on the timescale of ice sheet retreat using a 1D dynamic coupled ice sheet – sea level model. I will also introduce the results of simulations in which post-glacial sea-level physics is coupled to a 3D, dynamic ice sheet-shelf model, and applied to Antarctica through the last 40,000 years. The coupled model simulates far-field and near-field sea-level change, capturing interactions due to gravitational and deformational effects of varying ice mass on the proximal ocean and grounding-line depths. Results will focus on total ice volume through time, ice distributions and sea levels at the Last Glacial Maximum and present and be compared to measurements of relative sea level and present-day uplift rates throughout Antarctica.

Twentieth-century global-mean sea level rise: is the whole greater than the sum of the parts?
Jonathan Gregory
Walker Institute and Met Office Hadley Centre, UK

Confidence in projections of global-mean sea-level rise (GMSLR) depends on an ability to account for GMSLR during the 20th century. There are contributions from ocean thermal expansion, mass loss from glaciers and ice sheets, groundwater extraction and reservoir impoundment. We have made progress towards solving the "enigma'' of 20th-century GMSLR---that is, the observed GMSLR has been found to exceed the sum of estimated contributions, especially for the earlier decades. We propose that: thermal expansion simulated by climate models may previously have been underestimated owing to their not including volcanic forcing in their control state; the rate of glacier mass loss was larger than previously estimated, and was not smaller in the first than in the second half of the century; the Greenland ice-sheet could have made a positive contribution throughout the century; groundwater depletion and reservoir impoundment, which are of opposite sign, may have been approximately equal in magnitude. We show that it is possible to reconstruct the timeseries of GMSLR from the quantified contributions, apart from a constant residual term which is small enough to be explained as a long-term contribution from the Antarctic ice-sheet. The reconstructions account for the approximate constancy of the rate of GMSLR during the 20th century, which shows small or no acceleration, despite the increasing anthropogenic forcing.  Semi-empirical methods for projecting GMSLR depend on the existence of a relationship between global climate change and the rate of GMSLR, but the implication of our closure of the budget is that such a relationship is weak or absent during the 20th century.

Dynamic sea level in a suite of CORE-forced global ocean-ice simulations
Stephen Griffies
NOAA/GFDL, USA

We analyze global and regional dynamic sea level in a suite of ~15 global ocean-ice simulations using the inter-annual CORE forcing (years 1948-2007).  Two basic questions are considered: 1/ Do CORE-forced simulations reproduce the observed thermosteric sea level rise occurring during the second-half of the 20th century? 2/ Do CORE-forced simulations reproduce the observed patterns of sea level change seen during the satellite era?

Greenland ocean - land ice / ice sheet interactions
Patrick Heimbach
MIT, USA

Southern Ocean ice shelf melting in a warming climate
Hartmut Hellmer and R. Timmermann
Alfred Wegener Institute for Polar and Marine Research, Germany

The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line through submarine glacial troughs and causing high rates of melting at deep ice-shelf bases. The interaction between ocean currents, continental bathymetry, and shelf hydrography is thus likely to influence future rates of ice loss. The evolution of basal loss in a warming climate is presented for ten Antarctic ice shelves, based on the output of two coupled ice– ocean models (BRIOS and FESOM) both forced by the IPCC-SRES E1 and A1B scenario-related atmospheric outputs of the HadCM3 and ECHAM5/MPIOM climate models. Projections of future ice shelf basal melting are similar with regard to the scenarios applied but differ substantially between the climate models used, with the HadCM3 output causing the most significant changes in continental shelf temperatures. All ice shelves face a possible increase in basal melting with the biggest changes occuring at the base of the Filchner-Ronne Ice Shelf. A redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century may lead to increased flow of warm open ocean waters into the deep southern ice-shelf cavity. Here, water temperatures can increase by more than 2^oC boosting average basal melting from 0.2 m/yr, or 82 Gt/yr, to almost 4 m/yr, or 1,600 Gt/yr. The analysis of the results suggests that the changes are caused primarily by the freshening of the shelf water masses and an increase in ocean surface stress in the southeastern Weddell Sea, both due to reduced sea ice formation and a thinning of the formerly consolidated sea-ice cover. A projected further increase of ice loss at the base of the Filchner–Ronne Ice Shelf to 2,500 Gt/yr for the year 2199 is caused by a gradual warming of the deep Weddell Sea in FESOM and does not occur in the regional BRIOS simulation.

Can we detect long-term, global change from sparse, 135-year old ocean data?
Will Hobbs¹ and J Willis²
1 IMAS, Australia
2 Caltech/NASA Jet Propulsion Laboratory, USA

Since almost all the climate system's heat capacity resides in the global ocean, observed long-term changes in ocean heat content (OHC) are invaluable for estimating the plant's radiative imbalance. Several sudies produce such estimates from in situ observations, but generally these estimates are restricted to the late 20th century onwards, prior to which there was little global observation. Recent studies have compared modern Argo-based estimates of global ocean temperature with estimates from the 1873-1876 HMS Challenger expedition, the first global-scale survey of the subsurface oceans, and report a significant temperature difference between Challenger and Argo periods. In this work, using simulations from the CMIP5 suite of earth system models, we ask firstly whether this temperature difference between two relatively short time periods can be attributed to an anthropogenic warming over the entire global ocean, and secondly how well this difference represents a truly global change, We demonstrate that it is extremely unlikely that the temperature difference along the Challenge cruise track could be caused by natural variability alone. Furhtermore, the Challenger data provides a reasonable proxy for the true 135-year global temperature difference.

Ice-ocean interaction observations and modeling, Greenland and Antarctica
David Holland
Courant Institute, USA

A review of ice-ocean interaction studies, spanning observational and modeling, covering both Greenland and Antarctica is presented. From an observational point of view the past decade has seen a considerable leap forward in observational studies in Greenlandic fjords (e.g. Ilulissat in Greenland and Pine Island in Antarctica to mention just two representative locations). Surprising results emerge from such studies, partially suggesting that still more observational research is yet required in order to build a more definitive view of the mechanisms involved in ice-ocean interaction. Modeling studies have also progressed, giving some insight into the circulation involved in ice-ocean interaction and at the same time suggest a forward pathway for both future strategies for observational campaigns and coupled models.

Modeling the Antarctic ice sheet in CESM with Glimmer-CISM
Phil Jones (on behalf of William Lipscomb)
LANL, USA

We will describe the Community Ice Sheet Model (CISM) and the coupling of this ice sheet model to the climate system within the Community Earth System Model (CESM) for the purpose of performing projections of future sea level rise in climate change scenarios.  The Glimmer-CISM model will be presented with an emphasis on methods for coupling this model with both the land surface and the ocean components.  We have implemented a new boundary scheme for simulating ocean flow under ice shelves within a climate system model.  Early results from large-scale simulations around the Antarctic continent will be shown.

Lower GRACE estimates of Antarctic sea-level contribution
M. King^1,2, R. Bingham¹, P. Moore¹, P. Whitehouse³, M. Bentley³, G. Milne⁴
1 School of Civil Engineering and Geoscience, UK
2 School of Geography and Environmental Studies, University of Tasmania, Australia
3 Department of Geography, Durham University, UK
4 Department of Earth Sciences, University of Ottawa, Canada

We present a new estimate of the contribution of the Antarctic Ice Sheet to sea-level rise during the GRACE era. We correct the GRACE data for the ongoing effects of Glacial Isostatic Adjustment (GIA) by employing a new model that has been developed using a numerical ice-sheet model constrained by glaciological and geological data, and Earth viscosity models that optimise the fit to relative sea-level data and GPS observations of present-day uplift. Error bars provided with the GIA model, which reflect uncertainty in both the Earth model and ice history, enable us to place bounds on the contribution of present-day ice-mass change to the observed GRACE signal. We seek to reproduce the magnitude and spatial distribution of the GIA-corrected GRACE rates using a forward model approach considering 26 continental basins plus others outside the grounded ice sheet. throughout Antarctica during the last decade that is required to. We pay particular attention to the sensitivity of the derived spatial patterns of change due to leakage of signal between basins. Due to a reduced GIA correction compared to older models, we obtain substantially smaller rates of ice mass loss, and hence sea level rise, than previous studies.

Interpreting the noisy geological record of ancient sea level changes: What can the Quaternary tell us about ice sheet stability?
R. Kopp
Rutgers University, USA

Sea level rise - driven in part directly by changes in ocean temperature and in part by melting land ice - figures prominently among the effects of a warming climate. Melt dynamics are, however, complicated and challenging to project using forward models. The geological record of past sea level changes provides a complementary source of information about ice sheet stability. Yet this record is composed of proxies that are uncertain in their meaning, uncertain in their ages, and reflect sea level as seen through the filter of a range of physical process that cause local sea level change to deviate, and sometimes even differ in sign, from changes in mean global sealevel. In this talk, I will discuss the challenges of inferring past sea level and ice sheet changes from geological observations, while taking into account both uncertainties and our understanding of the relevant physics. I will also explore what inferred sea level during past warm periods may be able to tell us about the stability of ice sheets in the coming centuries

Uncertainty quantification of ice sheet mass balance projections using ISSM
Eric Larour
JPL, USA

Anthropogenic forcing fingerprint on the tropical Pacific sea level trend pattern from the CMIP5 simulations of the 21st century
Benoit Meyssignac¹, D. Salas y Melia² and A. Cazenave¹
1 LEGOS/CNES, France
2 CNRM, France

In a recent study, Meyssignac et al. (2012) investigated how the spatial trend patterns in the tropical Pacific sea level –as revealed by satellite altimetry- evolved in space and time during the last decades and centuries. For that purpose, they analysed data from past 2-D sea level reconstructions (over 1950-2009) as well as model outputs from multi-centennial control runs and 20th century runs derived from an ensemble of 8 CMIP3 coupled climate models. They showed that the sea level trend patterns, computed over successive 17-year windows (i.e., corresponding to the satellite altimetry operating period at the time of the study) using past 2-D sea level reconstruction data, fluctuated with time following a low frequency modulation of ENSO (period between 25 and 30 years). A similar behaviour was found in multi-centennial control runs of the coupled climate models with constant external forcing (i.e. with no anthropogenic emissions, no solar/volcanic variability). In the 20th century model runs that include all external forcings, the tropical Pacific sea level trend pattern appeared to have fluctuated at a slightly higher frequency. But the difference with the control runs appeared too small to be significant, suggesting that tropical Pacific sea level trend pattern reported by satellite altimetry over the last ~2 decades essentially reflects the internal variability of the climate system. In this presentation, we go a step further and analyse CMIP5 simulations for the 21st century forced by specified GHG concentrations consistent with high anthropogenic emissions scenario (RCP8.5 runs). The objective is to determine how the observed tropical Pacific sea level trend pattern fluctuations evolve in such a high GHG concentration scenario, and to eventually detect the anthropogenic forcing fingerprint. We apply a similar analysis as in Meyssignac et al. (2012) with multi-centennial control runs and RCP8.5 runs from an ensemble of 21 CMIP5 coupled climate models. The results provide a first estimation of the minimum level of anthropogenic forcing that allows a detection of its impact (above the large signal generated by the internal climate variability) on the tropical Pacific sea level trend pattern.

Observations of ice-ocean interactions in Greenland and Antarctica and impact on mass balance
Eric Rignot
University of California, Irvine, USA

Southern Ocean water masses and sea level from models and observations
Bernadette Sloyan
CSIRO, Australia

Observed and simulated present-day and future regional sea level change
Detlef Stammer¹, M. Carson¹, A. Köhl¹, A. Slangen², C.A. Katsman³, R.S.W. van de Wal² and L.L.A.Vermeersen^4;5
1 Center for Earth System Research and Sustainability (CEN), University of Hamburg, Germany
2 Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, The Netherlands
3 Royal Netherlands Meteorological Institute (KNMI), The Netherlands
4 Delft Climate Institute, Faculty of Aerospace Engineering, TU Delft, The Netherlands
5 Royal Netherlands Institute for Sea Research (NIOZ), The Netherlands

Sea level is changing not only on global average, but also shows pronounced regional changes. Those regional changes can vary on a broad range of timescales, and in some regions can even lead to a reversal of long-term global mean sea level trends. The underlying causes are associated with dynamic variations in the ocean circulation as part of climate modes of variability and with an isostatic adjustment of Earth’s crust to past and ongoing changes in polar ice masses and continental water storage. The talk will focus on present day and future regional sea level changes resulting from changes of the ocean circulation and/or changes in the heat and freshwater content. Present-day regional sea level changes appear to be caused primarily by natural climate variability. However, the imprint of anthropogenic effects on regional sea level will grow with time as climate change progresses, and toward the end of the twenty-first century, recent CMPI5 results suggest that regional sea level patterns will be a superposition of climate variability modes and natural and anthropogenically induced static sea level patterns.

Condence and sensitivity of sea-level reconstructions
P. Svendsen
National Space Institute, Technical University of Denmark, Denmark

For the last two decades, satellite altimetry has provided a near-global view of spatial and temporal patterns in sea surface height (SSH). When combined with records from tide gauges, a historical reconstruction of sea level can be obtained; while tide gauge records span up to 200 years back, their combined quality for reconstruction purposes is limited by the sparsity of their geographical distribution and other factors. We examine both a traditional EOF analysis of sea surface height, and another method known as minimum/maximum autocorrelation factors (MAF), which takes into account the spatial nature of the data fields. We examine the sensitivity of a reconstruction with respect to the length of calibration time series, and the spatial distribution of tide gauges or other proxy data. In addition, we consider the eect of isolating certain physical phenomena (e.g. ENSO) and annual signals and modelling these outside the reconstruction. The implementation is currently based on data from compound satellite datasets (i.e., two decades of altimetry), and the Simple Ocean Data Assimilation (SODA) model, an existing reconstruction, where a calibration period can be easily extracted and our model's basic performance can be relatively easily assessed. This means that we will consider only the last 50-60 years of sea level data. This is a preliminary analysis to pave the way for an improved reconstruction in the Arctic area, a major focus of my PhD project.

The “Static” Boundaries of Sea Level Change
Mark Tamisiea
NOC, UK

As mass moves between the ice sheets and the ocean, the resulting loading causes both crustal deformation and changes to the Earth's gravity field, the two bounding surfaces of sea level. These “static” changes, also termed self-attraction and loading (SAL), introduce both local and global effects. SAL produces large-scale patterns in the resulting sea level change and causes apparent differences in regional measurements derived from tide gauges and altimetry. Near marine-based ice sheets that are losing mass, the resulting crustal uplift can impact the grounding line evolution. Variations in continental water storage can generate SAL effects that bias coastal sea level measurements compared to the global average. Even the sea level changes from thermal expansion, which drives mass onto the continental shelves, are amplified by SAL effects. In addition to the static changes caused by present-day motion of mass, the Earth and ocean are also still responding to past changes of the ice sheets, termed glacial isostatic adjustment (GIA). In this talk, I will review the different aspects of static sea level change, how different measurements are affected, and implications for future projections and understanding.

Model representation of Southern Ocean bottom water mass formation and circulation
S. M. Downes¹, A. Gnanadesikan², S. M. Grffies³ and J. L. Sarmiento⁴
1 Research School of Earth Sciences, The Australian National University, Australia
2 Department of Earth and Planetary Sciences, Johns Hopkins University, USA
3 NOAA/Geophysical Fluid Dynamics Laboratory, USA
4 Program in Atmospheric and Oceanic Sciences, Princeton University, USA

The formation and circulation of Antarctic bottom waters is a key process in the distribution of heat in the abyssal ocean. However, associated ice and ocean model parameterizations create a biased representation of bottom waters. The current state of representation of bottom waters in coarse resolution models is analyzed using fully-coupled general circulation climate models and an assimilated solution. In a density framework, it is shown that primary model-model differences for deep and bottom waters (that is, the lower limb of the overturning circulation) are linked with different surface buoyancy fluxes, ocean density field and diapycnal mixing. Possible suggestions for physical circulation processes requiring attention in the modeling community will also be put forward.

Projecting regional sea-level changes for the 21st century
Aimee B.A. Slangen¹, M. Carson², C.A. Katsman³, R.S.W. van de Wal¹, A. Köhl², L.L.A.Vermeersen^4;5 and D. Stammer²
1 Institute for Marine and Atmospheric research Utrecht (IMAU), Utrecht University, The Netherlands
2 Center for Earth System Research and Sustainability (CEN), University of Hamburg, Germany
3 Royal Netherlands Meteorological Institute (KNMI), The Netherlands
4 Delft Climate Institute, Faculty of Aerospace Engineering, TU Delft, The Netherlands
5 Royal Netherlands Institute for Sea Research (NIOZ), The Netherlands

Sea-level rise is one of the most important consequences of a warming climate, affecting many densely populated coastal communities. Obtaining local information on sea-level change is therefore essential for adequate coastal management. Increased scientific understanding of the contributing processes now allows us to go from global mean projections to regional patterns. We will present the latest regional sea-level projections for the 21st century based on the new CMIP5 climate model ensemble, for a moderate (RCP4.5) and a warm (RCP8.5) climate change scenario. Compared to previous estimates, we now include more processes that influence regional sea-level changes, and also show the uncertainty estimates for these processes. Processes considered are the gravitational effects resulting from land ice changes and groundwater depletion, the projected ocean density variations and associated changes in ocean dynamics, the changes in atmospheric pressure loading, and the contribution of glacial isostatic adjustment. In total, the two climate change scenarios yield global mean changes of 0.52±0.18 m and 0.71±0.25 m respectively. Regionally however, values of up to 30% above the global mean are projected in the equatorial and subtropical regions and around Australia and Southern Africa. We also find values of 50% below the global mean in for instance the Arctic Ocean. In addition we now provide a more rigorous error estimate for all the contributions.

Different magnitudes of projected subsurface ocean warming around Greenland and Antarctica
Jianjun Yin¹, Jonathan Overpeck¹, Stephen Griffies², Aixue Hu³, Joellen Russell¹ and Ronald Stouffer²
1 Department of Geosciences, University of Arizona
2 GFDL
3 NCAR

The observed acceleration of outlet glaciers and ice flows in Greenland and Antarctica is closely linked to ocean warming, especially in the subsurface layer. Accurate projections of ice-sheet dynamics and global sea-level rise therefore require information of future ocean warming in the vicinity of the large ice sheets. Here we use a set of 19 state-of-the-art climate models to quantify this ocean warming in the next two centuries. We find that in response to a mid-range increase in atmospheric greenhouse-gas concentrations, the subsurface oceans surrounding the two polar ice sheets at depths of 200-500 m warm substantially compared with the observed changes thus far. Model projections suggest that over the course of the twenty-first century, the maximum ocean warming around Greenland will be almost double the global mean, with a magnitude of 1.7-2.0°C. By contrast, ocean warming around Antarctica will be only about half as large as global mean warming, with a magnitude of 0.5-0.6°C. A more detailed evaluation indicates that ocean warming is controlled by different mechanisms around Greenland and Antarctica. We conclude that projected subsurface ocean warming could drive significant increases in ice-mass loss, and heighten the risk of future large sea-level rise.