The mass balance of a glacier or ice sheet is the net balance between the mass gained by snow deposition, and the loss of mass by melting (either at the glacier surface or under the floating ice shelves or ice tongues) and calving (production of icebergs). A negative mass balance means that a glacier is losing mass, and, for grounded glaciers and ice sheets, this mass loss directly contributes to sea level rise (the melting of floating ice shelves and ice tongues does not contribute to sea level rise, because of the lower density of ice as compared to water, which determines the floating portion of the ice). This is one of the reasons why it is important, under a warming climate, to have accurate estimates of the mass balance of glaciers and ice sheets.
Past mass balance rates can be estimated from ice core data, although the proper dating of the samples is challenging. For the deeper parts of the ice core (representing the older data), the dating requires modelling the ice sheet dynamics.
For the large ice sheets of Antarctica and Greenland, the estimate of present mass balance is only possible using remote sensing (satellite or airborne) techniques, though these need to be calibrated and validated against measurements done on the glacier surface.
For predicting future mass balance, under different scenarios of climate change, it is necessary to use models of the dynamics and thermal regime of the glaciers and ice sheets. These models have to be integrated with climate models (that provide the information on accumulation and melting at the glacier surface) and oceanic models (which provide the interaction between the ice sheets and the ocean).
The Expert Group on Ice Sheet Mass Balance and Sea Level (ISMASS) is co-sponsored by the Scientific Committee on Antarctic Research, the International Arctic Science Committee (IASC), and the WCRP Climate and Cryosphere Project.The goals of ISMASS are to promote the research on the estimation of the mass balance of ice sheets and its contribution to sea level, to facilitate the coordination among the different international efforts focused on this field of research, to propose directions for future research in this area, to integrate the observations and modelling efforts, as well as the distribution and archiving of the corresponding data, to attract a new generation of scientists into this field of research, and to contribute to the diffusion, to society and policy makers, of the current scientific knowledge and the main achievements in this field of science. Further details on the goals of ISMASS can be found in the new Terms of Reference.
Current ISMASS chair: Catherine Ritz
An ice-sheet model inter-comparison splinter group meeting was held at EGU in April 2014 and at the IGS conference on Ice sheets and Sea Level rise at Chamonix, May 2014, both organized by Frank Pattyn. It was agreed to proceed with a new MISMIP (Marine Ice Sheet Model Intercomparison Project) to test ice sheet models on how to cope with grounding line retreat due to basal melting under the shelf and loss of buttressing. A preliminary website was set up: homepages.ulb.ac.be /~fpattyn/mismip+/, where details on the procedure can be found.
A workshop on ice-sheet future projections was held on Tuesday 26 August 2014, Auckland, New Zealand, linked with the SCAR Open Science Conference. The objective was to stimulate the ice-sheet community to improve methods and agree on a common framework when producing ice-sheet mass balance model projections for the next 100 years. This meeting included a report by Ryan Walker on the outputs of the "Ice sheet MIP for CMIP6" workshop that was held in July 2014 at Washington DC, supported by CliC.
A major model intercomparison initiative on West Antarctic Glacier-Ocean Models had its Kick-Off Meeting on October 27-29, 2014, at the New York University Campus in Abu Dhabi, United Arab Emirates (organized by David Holland). This is a CliC and ISMASS endorsed initiative.
Greenland Ice Sheet SMB workshop
A CliC-sponsored workshop was organised by Edward Hanna, at the University of Sheffield, UK, on 19-20 May 2015, on “Constraining uncertainty in Greenland Ice Sheet (GrIS) surface mass balance (SMB) model output and in situ validation”. The purpose of the workshop was to direct future research for reconciling the considerable differences in Greenland Ice sheet SMB between several different models that are currently widely used.
Marine Ice Sheet Model Intercomparison Project
A one-day joint SCAR/IASC/CliC ISMASS workshop will be held on August 16, 2015, before the IGS International Symposium on Contemporary Ice-Sheet Dynamics: ocean interaction, meltwater and non-linear effects. The symposium in itself touches upon many subjects that are particularly linked to ISMASS. This workshop will be seen as a preparation of IPCC AR6. We plan to use SCAR funding for this meeting, to invite scientists related to ISMASS initiatives and in particular to aid young scientists and part of the steering committee of ISMASS to attend. The ISMASS steering committee will be held in Cambridge also.