Our Common Future Under Climate Change

International Scientific Conference 7-10 JULY 2015 Paris, France

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Tuesday 7 July - 16:30-18:00 UNESCO Fontenoy - ROOM IX

1107 - Sea level rise and ice sheets

Parallel Session

Chair(s): M. Siegert (Imperial College London, London, United Kingdom), S. Charbit (CEA Saclay, Gif-sur-Yvette, France)

Convener(s): T. Payne (University of Bristol, Bristol, United Kingdom), C. Ritz (UGA/CNRS, Saint Martin d'Hères cedex, France), D. Blankenship (University of Texas at Austin, Austin, TX, United States of America)

16:30

Present-day sea level rise

A. Cazenave (CNRS-CNES, Toulouse, France)

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Present-day sea level rise

A. Cazenave (1)
(1) CNES, LEGOS, Toulouse, France

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We review most recent progress realized in measuring global mean sea level (GMSL)  over recent decades and satellite altimetry era (starting in 1993), as well as in understanding the causes of the observed rise (i.e., ocean thermal expansion, land ice loss and terrestrial water storage change). The IPCC 5th Assessment Report revisited the sea level budget for the last few decades and came to rather good agreement between observed GMSL rise and sum of climate and non-climate contributions. Over the satellite altimetry era, GMSL rise (of 3.2 mm/yr) is reasonably well explained by ocean thermal expansion (contributing ~ 37%) and land ice loss from glaciers and ice sheets (contributing ~ 55%).  Terrestrial water storage change (mostly due to ground water pumping) is supposed to explain the remaining, but this component is quite uncertain. For about 10 years, new observing systems from space and in situ (e.g., GRACE space gravimetry, Argo profiling floats) allow improved estimates of the various contributions to sea level rise, in particular direct estimate of ocean mass change. These observations indicate that over 2005-2013, ocean thermal expansion rose less rapidly than during the 1990s (with a contribution of only 28%) while the ocean mass increase now explains ~ 66% of the GMSL rise. This mostly results from ice mass loss acceleration from the ice sheets. This improved sea level budget approach allows us to put constraints on the contribution of the deep ocean (not seen by Argo) and its role in the current ‘hiatus’.

 

16:50

Measuring Earth's polar ice sheets from space

A. Shepherd (University of Leeds, Leeds, United Kingdom), E. Ivins, (Caltech, Pasadena, United States of America)

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Measuring Earth's polar ice sheets from space

A. Shepherd (1) ; E. Ivins, (2)
(1) University of Leeds, School of earth and environment, Leeds, United Kingdom; (2) Caltech, Pasadena, United States of America

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Earth’s ice responds rapidly to climate forcing, and satellite observations have become an essential tool with which to measure and understand these changes. Although considerable progress has been made in quantifying recent fluctuations in ice sheet mass balance, significant challenges remain in attempting to predict future ice sheet losses. This is because past changes have been too small to reliably judge the performance of the ice sheet models upon which predictions are currently based. In consequence, there is a continued need for satellite-based observations of the polar ice sheets - both to inform the development of ice sheet models, and to deliver synoptic assessments of their contribution to contemporary sea level rise.

There are three popular satellite-based techniques for estimating the sea level contribution due losses from the polar ice sheets; satellite altimetry can measure changes in ice sheet volume; satellite interferometry can measure outlet glacier discharge, and satellite gravimetry can measure fluctuations in ice sheet mass. Although all three approaches are technically sound, in the past there has been poor agreement between independent assessments. The Ice Sheet Mass Balance Inter-Comparison Exercise (IMBIE) was established in 2011 as a joint initiative of the European Space Agency (ESA) and the US National Aeronautics and Space Administration (NASA) as an attempt to resolve the apparent disagreement between geodetic estimates of ice sheet mass balance. Within IMBIE, estimates of ice sheet mass balance are developed from all three geodetic techniques using a common spatial and temporal reference frame and a common appreciation of the contributions due to external signals. The project brings together the laboratories and space agencies that have been instrumental in developing independent estimates of ice sheet mass balance to date.

In 2012, the first IMBIE assessment was delivered. It included 19-years of satellite radar altimeter data, 5-years of satellite laser altimeter data, 18 years of satellite radar interferometer data, 7-years of satellite gravimetry data, 32-years of surface mass balance model predictions, and predictions from a suite of post glacial rebound models. Since then, further changes have occurred in Antarctica and Greenland, and the record of satellite observations has been extended through continued operation of existing missions, such as GRACE, and through the availability of measurements from new sensors, including CryoSat-2 and Sentinel-1a.

This presentation will review the latest satellite measurements of Antarctica and Greenland, and will outline progress towards a second ice sheet mass balance inter-comparison exercise.

17:10

Polar ice sheets and sea-level rise: threats and uncertainties

G. Durand (LGGE / CNRS, St Martin d'Hères, France)

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Polar ice sheets and sea-level rise: threats and uncertainties

G. Durand (1)
(1) LGGE / CNRS, St Martin d'Hères, France

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During the last 20 years, the Greenland and Antarctic ice sheets have been loosing mass with an increasing rate. They now significantly contribute to sea-level rise and will most probably continue in the foreseeable future. Their potential total contribution is tremendous as ice sheets contain tens of meters of sea level equivalent. However, the rate of their coming outflow remains speculative. In particular both ice sheets may exhibit some instability processes that could be initiated once oceanic or atmospheric perturbations exceed a given threshold. Once engaged in such instabilities, ice sheet mass loss is self-entertained and large regions may collapse. This lecture will describe the essential processes at the origin of the observed ice sheets’ imbalance, our current knowledge on potential tipping points and highlight the main reasons of the uncertainties in the projection of ice sheets’ contribution to sea level. 

17:30

Projections of sea level change

J. Gregory (NCAS, University of Reading and Met Office Hadley Centre, Reading, United Kingdom)

Abstract details
Projections of sea level change

J. Gregory (1)
(1) NCAS, University of Reading and Met Office Hadley Centre, Reading, United Kingdom

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It is very likely that the rate of global mean sea level rise (GMSLR) during the 21st century will exceed the rate during 1971-2010, due to increases in ocean warming and loss of mass from glaciers and ice sheets. Ocean thermal expansion is the largest contributor to projections of GMSLR during the 21st century. For a given scenario, there is a substantial spread in climate model projections of thermal expansion, and in the geographical pattern of sea level change due to ocean density and circulation change. Larger uncertainty in projections of GMSLR comes from the land-ice contributions, especially ice-sheet dynamical change. These contributions also influence regional sea-level change, through their effect on gravity and the solid Earth. GMSLR by 2100 is likely to be in the range 0.28-0.61 m above the 1986-2005 mean under a scenario of strong mitigation (RCP2.6), and 0.52-0.98 m under a scenario of high emissions (RCP8.5). Unlike surface temperature change, GMSLR depends on the pathway of CO2 emissions, not only on the total; earlier emissions of the same total lead to greater GMSLR. By the end of the century, the rate of GMSLR under RCP2.6 could stabilise at rates similar to those of the early 21st century, while under RCP8.5 it could approach the average rates that occurred during the last deglaciation. It is very likely that regional sea level rise will be positive over about 95% of the world ocean, and about 70% of the global coastlines are projected to experience a relative sea level change within 20% of the global mean. Based on current understanding, only the collapse of marine-based sectors of the Antarctic ice sheet, if initiated, could cause GMSLR above the likely ranges during this century, but GMSLR will continue for many subsequent centuries, because of the long timescales of ice-sheet change and deep-ocean warming, and could be partly irreversible.

17:50

Discussion and Q&A session

M. Siegert (Imperial College London, London, United Kingdom), A. Shepherd (University of Leeds, Leeds, United Kingdom), G. Durand (LGGE / CNRS, St Martin d'Hères, France)

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Discussion and Q&A session
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