The Full Picture

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Additional existing systems to be sustained are those

to observe changes in ice sheet and glacier topography

and thickness – satellites utilizing radar (e.g. Envisat,

GFO and Sentinel-3) and laser (ICESat and, once

launched, CryoSat-2) altimeters, complemented by

aircraft and in situ observations. All of these measure-

ments require that the International Terrestrial Reference

Frame (ITRF), which integrates the geodetic components

– SLR, VLBI, DORIS, and GNSS (GPS, together with

GLONASS and Galileo once launched), must be made

more robust and stable. Finally, observations of the time-

invariant gravity field from GOCE, once launched, and

other stand-alone missions are needed to determine the

precise geoid.

New and improved observing systems which need to be

developed include those directed at changes in the ocean

volume, specifically extending the Argo-type capability to

enable the collection of similar observations under the sea

ice, as well as the design and implementation of an effort

to obtain observations for the deep ocean. Based on expe-

rience gained with radar and laser satellite altimeters, the

development of a suitable follow-on capability is needed

to improve observations of ice sheet and glacier topogra-

phy. Access to InSAR data and ongoing InSAR missions

are needed to observe flow rates in glaciers and ice sheets.

Finally, the development of an advanced wide-swath

altimeter is needed to observe sea level associated with the

sea-level rise. However, the current suite of ice sheet models does not

adequately represent many of these processes and thus projections of

ice sheet contributions to both 21st century and longer-term sea-level

rise may be underestimated.

Concern that the sea-level projections may be biased low has been

reinforced by a comparison showing that since 1990, observed sea

level has been rising more rapidly than the central range of the IPCC

projections. It is now at the very upper end of those projections.

To address these uncertainties, 163 scientists from 29 countries

attended the Workshop on Understanding Sea-level Rise and Variability,

hosted by the Intergovernmental Oceanographic Commission of

UNESCO in Paris (6-9 June, 2006). The workshop was organized by

the World Climate Research Programme (WCRP) to bring together all

relevant scientific expertise with a view to identifying the uncertain-

ties associated with past and future sea-level rise and variability, as well

as the research and observational activities needed for narrowing these

uncertainties.

The Workshop was also conducted in support of the

Global Earth Observation System of Systems (GEOSS) 10-Year

Implementation Plan.

As such, it helped develop international and

interdisciplinary scientific consensus for those observational require-

ments needed to address sea-level rise and its variability, especially

GEOSS activities focused on climate and hazards.

Reducing uncertainties

Improving our understanding of sea-level rise and variability, as well

as reducing the associated uncertainties, critically depends on the avail-

ability of adequate observations, as proposed under GEOSS. These

requirements include sustaining existing systematic observations, as

well as the development of new and improved observing systems.

An overarching observational requirement is the need for an open

data policy, together with timely, unrestricted access for all. Using the

Argo and Jason policies

as a guide, this access would include real-

time, high-frequency sea-level data from the Global Sea Level

Observing System (GLOSS) tide gauges and co-located GPS stations,

as well as data from satellite missions and in situ observing systems.

Further requirements include the need for appropriate data archaeol-

ogy – retrieving and making accessible historical, paper-based sea-level

records, especially those extending over long periods and in the

Southern Hemisphere. It’s important to note that an immediate prior-

ity for paper records is electronic scanning and making them available

for subsequent digitization. Moreover, satellite observations need to be

as continuous as possible, with overlap between successive missions.

There also needs to be a corresponding collection of appropriate in

situ observations for calibration and validation. In general, ongoing

satellite and in situ observing systems should adhere to the Global

Climate Observing System (GCOS) observing principles.

The existing systems that should be sustained include those observ-

ing sea level – the Jason series of satellite altimeters, as well as

completing the GLOSS network of approximately 300 gauges (each

with high-frequency sampling, real-time reporting, and geodetic posi-

tioning). Together these satellite and in situ observations enable the

measurement of both absolute and relative changes in sea level. In

order to estimate the change in sea level by ocean thermal expansion,

the Argo array of profiling floats needs to be completed and sustained

to observe the upper-ocean in ice-free areas. To estimate the contri-

bution to changes in sea level from melting ice caps and glaciers and

changes in terrestrial water storage, observations of the time-varying

gravity field from the Gravity Recovery and Climate Experiment

(GRACE) need to be sustained.

The Gold Coast, Australia. Coastal development is continuing

around the world

Photo: Bruce Miller

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