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| Glacier mass balance
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| Definition |
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Mass balance describes the net gain or loss of snow and ice through a given year. It is usually expressed in terms of water gain or loss.
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| Rationale |
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Glacier changes are among the clearest signals found in nature that can be used to monitor ongoing trends in global warming. There is therefore a world wide focus on glacier monitoring concerned with the early detection of climate change. The central aspects connected with detection of climate change caused by anthropogenic greenhouse forcing include the secular rates of change in energy fluxes at the earth/atmosphere-interface, natural (pre-industrial) variability in these energy fluxes, possible acceleration trends of ongoing and potential future changes, and spatial patterns of observed developments. During the 20th century, glacier mass losses typically amounted to a few decimetres ice thickness per year, which is equivalent to an additional energy flux of a few Watt per square meter. Glacier shrinkage in some mountain areas such as the European Alps seems to have reached the "warm" limit of Holocene (pre-industrial) variability. Further evolution of such ice surface masses will constitute a key element of climate change detection and climate impact assessment.
In ablation areas and areas of temperate firn (which predominate at lower latitudes/altitudes and in regions with humid climatic conditions), atmospheric warming mainly causes changes in mass and geometry of glaciers. An assumed step change (d) in equilibrium line altitude (ELA = the altitude on a glacier where the annual addition (accumulation) of mass is exactly compensated by the annual disappearance (ablation) of mass) induces an immediate step change in specific mass balance (b = total mass change divided by glacier area). The resulting change in specific mass balance (db) is the product of the shift in equilibrium line altitude (dELA) and the gradient of mass balance with altitude (db/dH) as weighed by the distribution of glacier surface area with altitude (hypsometry). The hypsometry represents the local/individual or topographic part of the glacier sensitivity whereas the mass balance gradient mainly reflects the regional or climatic part (Kuhn 1990). As the mass balance gradient tends to increase with increasing humidity (Kuhn 1981), the sensitivity of glacier mass balance with respect to changes in equilibrium line altitude is generally much higher in areas with humid/maritime than with dry/continental climatic conditions (Oerlemans 1993a). Cumulative mass changes lead to ice thickness changes which, in turn, exert a positive feedback on mass balance and at the same time influence the dynamic redistribu-tion of mass by glacier flow (Oerlemans 1996).
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| Users |
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Analysts, scientists, modellers engaged in climate change studies, and regional analysts and planners (hydrology, tourism, natural hazard)
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| Assessment method |
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Geodetic/photogrammetric methods, tapeline readings.
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| Units of Measure |
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m per year
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| Frequency of measurement |
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Annually to once every 10 years.
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| Spatial resolution |
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Tiers 4 and 5
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| Accuracy/precision required |
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Dependent on application and temporal resolution. A high precision is required for detecting changes (1 to 10 m/a), 100 m for model validation (preliminary inventory data by satellite data analysis).
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| Associated measurements |
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Mass and geometry of the glacier, hydrological and meteorological data in glacierised catchments, and sea level changes.
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| Present status |
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Worldwide compilation and dissemination of standardised glacier mass balance observations is being co-ordinated by the World Glacier Monitoring Service under the auspices of the International Commission on Snow and Ice (ICSI/IAHS), the Federation of Astronomical and Geophysical Data Analysis Services (FAGS/ICSU), the Global Environment Monitoring System (GEMS/ UNEP) and the Division of Water Sciences of UNESCO. This activity is now part of the Global Climate Observing System/Global Terrestrial Observing System (GCOS/GTOS).
Two type of standardised data is being collected and distributed. One if the World Glacier Inventory (WGI) which contains data describing the spatial distribution of glaciers on a global scale. The second is the Fluctuations of Glaciers (FoG) and Mass Balance Bulletin (MBB) which contain data documenting changes in time (changes in mass, volume, area and length of glaciers). The FoG data are stored at the University of Zurich on a 'Oracle' database system, which allows fast and selective data access (at present the database is not accessible online until February 2001). WGI and MBB are both available online.
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| R and D needed |
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- Systematic application of remote-sensing products (air photos, high-resolution space imagery) should enable global coverage to be reached with respect to length/area change and inventory data.
- Analysis and worldwide intercomparisons are needed to investigate how representative the few existing long observational series are.
- Special techniques such as laser altimetry combined with kinematic GPS or gravimetry have to be used in connection with measuring large glaciers and their effects on sea level.
- The highest priority is needed on the continuation of long-term mass balance measurements and glacier length observations;
- Better use should be made on remote sensing data on glaciers in areas of difficult access;
- Baselines for long-term monitoring (surface elevation changes along representative flow lines of large glaciers in Alaska, Canada, Patagonia and the Himalayas) should be established using laser altimetry in combination with kinematic GPS;
- Mass balance programmes should be strengthened in the Southern Hemisphere.
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| Global/Regional data holders |
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- Fluctuations of Glaciers and Glacier Inventory data by the World Glacier Monitoring Service (WGMS) at http://www.geo.unizh.ch/wgms/.
- World Glacier Inventory data is mirrored to a database at the US National Snow and Ice Data Center. It can be accessed at http://nsidc.org/NOAA/wgms_inventory/.
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| Data policy |
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Open
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References |
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Haeberli,
W., Barry, R. and Cihlar, J . 2000. Glacier monitoring within the Global
Climate Observing System. Annals of Glaciology 31, 241 – 246.
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Haeberli,
W., Hoelzle, M., Suter, S. and Frauenfelder, R . 1998. Fluctuations of glaciers
1990-1995. IAHS/UNEP/UNESCO, Zürich.
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UNESCO (publ.)
1998. Into the second century of worldwide glacier monitoring prospects and
strategies. A contribution to the International Hydrological Programme (IHP)
and the Global Environmental Monitoring Systems (GEMS). by W. Haeberli, M.
Hoelzle and S. Suter (eds.). UNESCO Studies and Reports in Hydrology 56.
Paris, United Nations Educational, Scientific and Cultural Organization.
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USGS / Cascades Volcano Observatory Glossary of Selected Glacier and Related Terminology, available at: http://vulcan.wr.usgs.gov/Glossary/Glaciers/glacier_terminology.html
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