SCAR/IASC/CliC Ice Sheet Mass Balance and Sea Level (ISMASS)

The importance of estimating ice sheet mass balance

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.

How is the mass balance estimated?

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).

ISMASS Expert Group

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

Organizational contacts are: Frank Pattyn (SCAR), Gudfinna (Tolly) Adalgeirsdottir (IASC), and Edward Hanna (CliC)

Ice Sheet Melting: Estimates Still Uncertain, Experts Warn

--From the University of Lincoln Press Release (https://www.lincoln.ac.uk/news/2019/12/1582.asp)

Estimates used by climate scientists to predict the rate at which the world’s ice sheets will melt are still uncertain despite advancements in technology, new research shows.

These ice sheet estimates feed directly into projections of sea-level rise resulting from climate change. They are made by measuring how much material ice sheets are gaining or losing over time, known as mass balance, to assess their long-term health. Snowfall increases the mass of an ice sheet, while ice melting or breaking off causes it to lose mass, and the overall balance between these is crucial.

Although scientists now have a much better understanding of the melting behaviour of ice sheets than they did in previous decades, there are still significant uncertainties about their future melt rates, researchers found.

The new study, published in the scientific journal Earth Science Reviews, shows that despite recent advances in computer modelling of ice sheets in response to climate change, there are still key deficiencies in the models used to estimate the long-term health of ice sheets and related global sea-level predictions. Improving these estimates could prove vital to informing the scale of response needed to mitigate the potential impacts of climate change.  

Edward Hanna, Professor of Climate Science and Meteorology at the University of Lincoln, UK, co-ordinated the research in co-operation with a leading international group of glaciologists.

Professor Hanna said:  “The ice sheets are highly sensitive indicators of climate change, but despite significant recent improvements in data and knowledge, we still don’t understand enough about how rapidly they are likely to lose mass during and beyond the current century.

“Enhanced observations of ice sheets, mainly from satellite data fed into improved computer simulations, are vital to help refine predictions of future sea-level rise that will result from continued global warming. They are urgently needed to assist climate adaptation and impact planning across the world.”

In the last decade, the Antarctic and Greenland ice sheets have overtaken thousands of smaller glaciers as the major contributors to rising sea levels – it is thought that combined, the sheets contain enough ice to raise global sea levels by as much as 65 metres. However, while some estimates project a contribution of as much as one and a half metres from Antarctica to global sea-level rise by 2100, others suggest only a few tens of centimetres contribution.

The researchers say there is a pressing need for further research that involves enhanced satellite and ground-based observations, together with more sophisticated, interactive computer models that combine ice masses, the atmosphere, ocean and solid Earth systems.

Their study involved analysis of recent estimates of ice sheet and glacier mass balance, as well as highlighting recent advances and limitations in computer-model simulations of ice sheet change as an important basis for future work. The World Climate Research Programme, the Scientific Committee on Antarctic Research and the International Arctic Science Committee part-sponsored the research.

Professor Hanna also contributed to a recent paper in the scientific journal Nature analysing the Greenland Ice Sheet mass balance. That study, involving 96 polar scientists, showed that in the last decade, Greenland has lost ice seven times faster than in the 1990s. This tracks a high-end global warming scenario, with tens of millions more people being exposed to coastal flooding by 2100.

2018 ISMASS Paper in Nature Climate Change: 'The Greenland and Antarctic ice sheets under 1.5°C global warming'

A review of ice sheets in a 1.5°C warmer world (a new recent climate mitigation target highlighted by the Intergovernmental Panel on Climate Change) suggests that current predictions of future changes in ice-sheet mass balance are questionable due to incomplete understanding of atmospheric circulation changes around Greenland and ocean circulation around Antarctica. However, the study concludes it is important to limit global warming by 2100 to 1.5°C to maximise the chance of avoiding so-called tipping points that would dramatically accelerate mass loss.

Today, sea level rises at a pace of around 4mm per year and is accelerating. The major contributions to this rise are due to ocean expansion, melting glaciers and – becoming increasingly important – melting of the Greenland and Antarctic ice sheets. Those ice sheets are commonly considered as the ‘sleeping giants’ on Earth, as together they can potentially lead to sea-level rise by more than 70 metres over thousands of years.

Despite improved observations and computer model simulations since the IPCC’s Fifth Assessment Report (AR5) published in 2013, the review highlights some lingering key gaps in knowledge regarding climate forcing of the ice sheets and their resulting responses. On longer time scales, both ice sheets may reach tipping points above around a 1.5-2.0°C limit, leading to irreversible mass loss. For Greenland this is due to increased melting of the ice sheet, while for Antarctica this is due to marine instabilities of certain sectors of the ice sheet, in particular the West Antarctica ice sheet. This would lead to multi-metre sea level rise on centennial to millennial time scales, which has existential consequences for small island states and coastal cities across the world.

Lead author Prof. Frank Pattyn said “Limiting global atmospheric warming to 1.5°C will avoid short and long-term surprises coming from both ice sheets and significantly reduce adaptation cost when global sea-level rise is limited and not catastrophic”.

The paper “The Greenland and Antarctic ice sheets under 1.5°C global warming” is published in Nature Climate Change. This work has been carried out in the framework of ISMASS (Ice Sheet Mass Balance and Sea Level expert group supported by SCAR/CliC/IASC)

Contact:
Frank Pattyn
Laboratoire de Glaciologie, Université libre de Bruxelles
Tel : +32 485 364495
Email :

 


Terms of Reference for ISMASS