11.6 Reducing the Uncertainties in Future Estimates of Sea Level Change
It is valuable to note that the reduction in the uncertainty of estimation
of the long-term ice sheet imbalance reported in Sections
11.3.1 and 11.4 came from indirect constraints and
the synthesis of information of different types. Such syntheses offer promise
for further progress.
11.6.1 Observations of Current Rates of Global-averaged
and Regional Sea Level Change
Sections 11.3.2.1 and 11.4
reveal significant uncertainty in the analysis of 20th century sea level change.
Also, we have little knowledge of the regional pattern of sea level change.
Observational determination of such a pattern would be a powerful test of the
coupled models required for projections of globally averaged and regional sea
level rise. Requirements for reducing uncertainties include:
- A global tide gauge network (the ‘GLOSS Core Network’) (IOC, 1997)
for measuring relative change.
- A programme of measurements of vertical land movements at gauge sites by
means of GPS (Global Positioning System), DORIS Beacons and/or absolute gravity
meters (Neilan et al., 1998).
- Improved models of postglacial rebound.
- A reanalysis of the historical record, including allowing for the impact
of variable atmospheric forcing.
- A subset of mostly island tide gauge stations devoted to ongoing calibration
of altimetric sea level measurements (Mitchum, 1998).
- An ongoing high-quality (TOPEX/POSEIDON class) satellite radar altimeter
mission (Koblinsky et al., 1992) and careful control of biases within a mission
and between missions.
- Space gravity missions to estimate the absolute sea surface topography (Balmino
et al., 1996, 1999) and its temporal changes, to separate thermal expansion
from an increase in ocean mass from melting of glaciers and ice sheets (NASA,
1996, NRC, 1997) and changes in terrestrial storage.
For assessment of possible changes to the severity of storm surges, analyses
of historical storm surge data in conjunction with meteorological analyses are
needed for the world’s coastlines, including especially vulnerable regions.
11.6.2 Ocean Processes
Requirements for improved projections of ocean thermal expansion include:
- Global estimates of ocean thermal expansion through analysis of the historical
data archive of ocean observations and a programme of new observations, including
profiling floats measuring temperature and salinity and limited sets of full-depth
repeat oceanographic sections and time-series stations.
- Testing of the ability of AOGCMs to reproduce the observed three-dimensional
and time-varying patterns of ocean thermal expansion.
- An active programme of ocean and atmosphere model improvement, with a particular
focus on the representation of processes which transport heat into and within
the interior of the ocean.
11.6.3 Glaciers and Ice Caps
Requirements for improved projections of glacier contributions include (see
also Haeberli et al., 1998):
- A strategy of worldwide glacier monitoring, including the application of
remote sensing techniques (laser altimetry, aerial photography, high-resolution
satellite visible and infrared imagery e.g. from ASTER and Landsat).
- A limited number of detailed and long-term mass balance measurements on
glaciers in different climatic regions of the world, with an emphasis on winter
and summer balances in order to provide a more direct link with meteorological
observations.
- Development of energy balance and dynamical models for more detailed quantitative
analysis of glacier geometry changes with respect to mass balance and climate
change.
- Glacier inventory data to determine the distribution of glacier parameters
such as area and area-altitude relations, so that mass balance, glacier dynamics
and runoff/sea level rise models can be more realistically framed.
11.6.4 Greenland and Antarctic Ice Sheets
- Continued observations with satellite altimeters, including the upcoming
satellite laser altimeter on ICESat and the radar interferometer on CRYOSAT.
Measurements should be continued for at least 15 years (with intercalibration
between missions) to establish the climate sensitivities of the mass balance
and decadal-scale trends.
- Satellite radar altimetry and synthetic aperture radar interferometry (ERS-1,
ERS-2 and Radarsat) for detailed topography, changes in ice sheet volume and
surface velocity of the ice sheets (Mohr et al., 1998; Joughin et al., 1999),
as well as short-term variability in their flow (Joughin et al., 1996a,b)
and grounding line position (Rignot, 1998a,b,c).
- Determination of the Earth’s time-variant gravity field by the Gravity
Recovery and Climate Experiment (GRACE) satellite flown concurrently with
ICESat to provide an additional constraint on the contemporary mass imbalances
(Bentley and Wahr, 1998). This could provide estimates of sea level change
to an accuracy of ±0.35 mm/yr.
- Geological observations of sea level change during recent millennia combined
with improved postglacial rebound models and palaeoclimatic and palaeoglaciological
studies to learn what changes have occurred in the past.
- Further analysis of Earth rotational parameters in combination with sea
level measurements.
- Improved estimates of surface mass balance (including its spatial and temporal
variability) from in situ observations, accumulation rates inferred from atmospheric
moisture budgets and improved estimates of the rate of iceberg calving and
the melt-water flux.
- Improved calculation of the surface mass balance within ice sheet models
or by atmospheric models, with attention to modelling of changes in sea-ice
concentration because of the consequent effect on moisture transports and
accumulation.
- Improved understanding and modelling of the dynamics of ice sheets, ice
streams and ice shelves (requiring combined studies using glaciological, oceanographic
and satellite observations), including the physics of iceberg calving.
11.6.5 Surface and Ground Water Storage
Surface and ground water storage changes are thought to be having a significant
impact on sea level, but their contribution is very uncertain (Table
11.10, Figure 11.9), and could be either positive
or negative. They may become more important in the future, as a result of changes
related not only to climate, but also to societal decisions that are beyond
the scope of this scientific assessment. There are several general issues in
climate-related aspects:
- A more thorough investigative search of historical records could provide
addition information on ground water mining, and storage in reservoirs.
- Accurate satellite measurements of variations in the Earth’s gravity
(Herring, 1998) to detect changes in land water storage due to water-table
variations and impoundments.
- A better understanding of seepage losses beneath reservoirs and in irrigation
is required.
- A unified systems approach is needed to trace the path of water more accurately
through the atmosphere, hydrologic, and biosphere sub-systems, and to account
for various feedbacks (including the use of GCMs and improved hydrologic models).
- Satellite remote sensing offers useful technology for monitoring the global
hydrologic budget. A cumulative volume estimate for the many small reservoirs
might be possible using high-resolution radar data, targeted ground studies
and a classification of land use classes from satellite data and also of changes
in deforestation and other land-use transformations (Koster et al., 1999).
11.6.6 Summary
Sea level change involves many components of the climate system and thus requires
a broad range of research activities. A more detailed discussion of the requirements
is given in the report of the recent IGBP/GAIM Workshop on sea level changes
(Sahagian and Zerbini, 1999). We recognise that it is important to assign probabilities
to projections, but this requires a more critical and quantitative assessment
of model uncertainties than is possible at present.
