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Satellite Observations in Support of Climate Challenges
Counting Carbon
The Big Thaw
Sea Level Rise
Water Security
Land Surface Change
Energy Resource Management

Since the beginning of high-accuracy satellite altimetry in the early 1990s, global mean sea level has been shown by both tide gauges and altimeters to be rising at a rate of just above 3 mm/year, compared to a rate of less than 2 mm/year from tide gauges over the previous century. The exact source of the accelerated rise is uncertain, but, with regard to future uncertainty, attention is being given to understanding the rate of loss of ice caps in Greenland and Antarctica. About half of the sea level rise during the first decade of the altimeter record can be attributed to thermal expansion due to a warming of the oceans; the other major contributions include the combined effects of melting glaciers and ice sheets.

Changes in the storage of water on land (such as the depletion of aquifers and increases in dams and reservoirs) remain very uncertain.

The coastal zone changed profoundly during the 20th century, primarily due to growing populations and increasing urbanisation. In 1990, 23% of the world’s population (or 1.2 billion people) lived within 100 km of the coast and no more than 100 m above sea level,with population densities about three times higher than the global average. By 2010, 20 out of 30 mega-cities will be on the coast, with many low-lying locations threatened by sea level rise. With coastal development continuing at a rapid pace, society is becoming increasingly vulnerable to sea level rise and variability – as Hurricane Katrina recently demonstrated in New Orleans. (The storm surge and high precipitation associated with hurricanes mean that they are likely to be early indicators of the effects of future sea level rise). Rising sea levels will contribute to increased storm surges and flooding, even if hurricane intensities do not increase in response to the warming of the oceans. Rising sea levels will also contribute to the erosion of the world’s sandy beaches, 70% of which have been retreating over the past century. Low-lying islands, such as coral atolls, are also vulnerable to sea level rise.

An improved understanding of sea level rise and
variability will help reduce the uncertainties
associated with future sea level projections, thus
contributing to more effective coastal planning
and management. Adaptation measures,
including enhanced building codes, restrictions
on where to build, and developing infrastructures
better able to cope with flooding, should help to
minimise the potential losses.
Sea level change 1993–2006 from satellite altimeters. (Credit: Edmisten, Univ. S. Florida)

The Third and Fourth IPCC Assessments
The Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC), published in 2001, estimated that sea level would rise between 9 and 88 cm by the end of the 21st century. The Fourth Assessment Report (AR4) released in 2007 refined this projection, with projected values between 18 and 59 cm. But this latest projection is thought to be very conservative, as it assumes a constant rate of ice flow into the oceans from Greenland and Antarctica, while there is now evidence that this rate may well be increasing.

Participants in a dedicated Workshop, held at IOC/UNESCO in Paris 6–9 June 2006, reached a consensus that the increase in the rate of global mean sea level rise towards the end of the 20th century, from less than 2 mm per year to just above 3 mm per year over the previous century, is a robust finding. However, a thorough review of current knowledge concerning all aspects of sea level rise still shows deficiencies. Sustained series of space-based and in situ observations and associated research are needed to determine uncertainties in knowledge of the contributing factors and subsequently reduce those uncertainties.

Historical and Present Sea Level Change
Beginning in 1992, global mean sea level has been observed by both tide gauges and satellite altimeters to be rising at a rate of 3.2 ± 0.4 mm/year, compared to a rate of 1.7 ± 0.3 mm/year from tide gauges over the previous century. The question is now raised as to what extent this increase in the global mean represents an actual acceleration.

Solving this question requires extending the Jason series of satellite altimeters for a second decade in order to resolve the spatial and temporal variability, as well as acceleration, in the rate of global sea level rise. These data will need to be completed by a corresponding enhancement of the Global Sea Level Observing System (GLOSS) network of approximately 300 gauges, each with high frequency sampling and real-time data availability. Gauges should be linked to absolute positioning wherever possible to enable an assessment of the coastal signatures of the open ocean patterns of sea level variability and the incidence of extreme events, as well as the calibration of satellite altimeters.
Monthly averages of global mean sea level reconstructed from tide gauges (black, 1870–2001)
and altimeters (red, 1993–2004) show an increase in the rate of sea level rise; the seasonal
cycle has been removed.

Thermal Expansion
Current estimates of thermal expansion account for approximately half of the change observed in global mean sea level rise over the first decade of the satellite altimeter record, but only about a quarter of the change during the previous half century. However, it is still necessary to ascertain the extent to which this reflects under-sampling of ocean temperature data versus a manifestation of enhanced climate change in the last decade.

The recent completion of the Argo array of 3,000 profiling floats will help obtain broad-scale, Monthly averages of global mean sea level reconstructed from tide gauges (black, 1870–2001) and altimeters (red, 1993–2004) show an increase in the rate of sea level rise; the seasonal cycle has been removed. upper ocean (from surface to 2000 m depth) observations of the temperature and salinity fields.
The Argo system is still in need of being sustained in the long term, and of having its capability extended in order to enable the collection of similar observations under the sea ice.

As noted in the previous case study, terrestrial glaciers and the Greenland and Antarctic ice sheets have the potential to raise global sea level many metres. Most of the world’s terrestrial glaciers are shrinking.
During the last decade, they have been melting at about twice the rate of the past several decades.
On the polar ice sheets, there is observational evidence of accelerating flow from outlet glaciers, both in southern Greenland and in critical locations in Antarctica.
Once launched, the CryoSat-2 radar altimeter satellite – complemented by aircraft altimetry – and appropriate follow-on missions will be very useful to survey changes in the surface topography of the ice sheets. The Gravity Recovery and Climate Experiment (GRACE) satellites and appropriate follow-on missions will also help infer changes in the mass of the glaciers and ice sheets.
Continued access to satellite Interferometric Synthetic Aperture Radar (InSAR) data will assist in the measurement of flow rates in glaciers and ice sheets, particularly over near-coastal regions of Greenland and Antarctica.

Terrestrial Water Storage
The IPCC Assessment Reports noted that the largest uncertainties in contributions to sea level rise are associated with terrestrial water storage.
To significantly reduce these uncertainties requires a combination of satellite observations, which provide finer resolution, broader coverage and longer duration, and appropriate in situ data.

Increases in dam and reservoir storage can reduce the rate of sea level rise.

GRACE data can be utilised to observe changes in land water storage. After its launch in 2009, ESA’s Soil Moisture and Ocean Salinity (SMOS) spacecraft will help observe changes in soil moisture. Current (Jason and Jason-2, Envisat, and GFO) and future (SARAL/Altika, HY-2, Jason-3, Sentinel-3) satellite altimeters will help observe river, lake and reservoir levels along the satellite ground tracks. An advanced wide-swath altimeter is needed to observe the two-dimensional surface water levels on land and their changes in space and time.

Geodetic Observing Systems
The development and implementation of geodetic techniques has enabled a revolution in the Earth sciences, providing the fundamental reference frame critical for the collection of all satellite observations and many others made in situ that address sea level rise and variability. However, to take advantage of those capabilities, they must be reliable and consistent over the long term (i.e. decades). While these techniques collectively define the International Terrestrial Reference Frame (ITRF) being brought together through the efforts of the Global Geodetic Observing System (GGOS), they are at the same time losing support and degrading in capability.

Integrating existing geodetic capabilities can provide an improved reference frame.

Updating and integrating complementary geodetic capabilities (SLR, VLBI, DORIS and GPS) into a reliable and consistent global geodetic ground and space network (co-locating them where possible) is now needed.
This has to be complemented by installing GPS positioning at all appropriate GLOSS tide gauge stations to determine changes in global and regional sea level, as well as developing an integrated geodetic modelling capability that can be combined with Earth science models.

Once launched, the observations of Earth’s time-invariant gravity field from ESA’s Gravity field and steady-state Ocean Circulation Explorer (GOCE) will be utilised to determine the precise geoid, thereby enabling an estimation of the absolute ocean circulation for constraining climate models, as well as an improvement in understanding geophysical processes related to sea level.


Surface Mass Loading
The main mass loads considered here are the great ice sheets, which covered large areas during the last glacial maximum. The Earth is still responding to the removal of those loads through subsequent melting. In addition, changes in the present ice sheets, glaciers, ice caps and terrestrial storage result in ongoing changes in surface loading. Uncertainties in models of Glacial Isostatic Adjustment (GIA) caused by uncertainties in modelled vertical land movements affect sea level measurements by tide gauges. These uncertainties also impact satellite altimeter measurements of sea level and measurements of changes in surface loads (including sea level and ice sheet mass balance) made by temporal gravity missions such as GRACE. Other changes in mass loads include those associated with tectonic activity, such as earthquakes, as well as local extraction of water and hydrocarbons.

A warmer ocean may contribute to more intense hurricanes.
Measurements from tide gauges equipped with a capability for absolute positioning, together with observations from satellite altimetry, gravity, GPS, and other datasets, can improve models of past ice sheet loading and glacial isostatic adjustment that are used to estimate sea level change.

Extreme Events and Impacts on Society
Global sea level rise will have a pervasive impact by raising the mean water level, on top of which must be added the combined effect of high tides, surface waves, storm surge, and flooding rivers. This will make the incidence of flooding to a given level more frequent, i.e. a 100-year coastal flooding event may become a 10-year event at some locations. Unless such change is taken into account, design criteria for existing coastal structures can become out of date and lead to catastrophic flooding such as that experienced in New Orleans with Hurricane Katrina. Moreover, the possibility that severe weather events may become more frequent and/or intense with our changing climate will only make matters worse. There is a need to convert current knowledge of sea level rise into easily understood information that can be used by coastal planners and engineers, emergency managers, insurers and the public at large.

Requirements for a Space-based Observing System for Sea Level Rise
Improving our understanding of sea level rise and variability, as well as reducing the associated uncertainties, depends critically on the availability of adequate observations. The WCRP workshop ( helped develop international scientific consensus for those observational requirements needed to address rising sea level and its variability. 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. This access would include real-time, high-frequency sea level data from the 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 access to data archives — retrieving and making accessible historical, paper-based sea level records, especially those extending over long periods and in the Southern Hemisphere. Moreover, comparable 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.

The existing systems that should be sustained include those observing sea level. This includes the Jason series of satellite altimeters, as well as completion of the GLOSS network of approximately 300 gauges (each with high-frequency sampling, real-time reporting, and geodetic positioning). In order to estimate the change in sea level due to steric effects (thermal expansion and salinity-density compensation of sea water), the Argo array – which achieved global coverage of the ice-free oceans with 3,000 profiling floats in November 2007 – needs to be sustained. In order to estimate the change in sea level due to changes in ocean mass due to melting ice caps and glaciers and changes in terrestrial water storage, observations of the time-varying gravity field from GRACE need to be sustained.

Other existing and future systems to be sustained are those required to observe changes in ice sheet and glacier topography and thickness – satellites utilising radar (e.g. Envisat, GFO, Cryosat-2 and Sentinel-3) and laser (ICESat) altimeters, complemented by aircraft and in situ observations. All of these measurements require that the International Terrestrial Reference Frame (ITRF), which integrates the geodetic components – SLR, VLBI, DORIS – and GNSS (GPS, together with GLONASS and Galileo), must be made more robust and stable than is currently the case. Finally, observations of the time-invariant gravity field from GOCE and other stand-alone missions are needed to determine the precise geoid.

The Jason-2 satellite, a critical element of the CEOS Ocean Surface Topography Constellation, was launched in June 2008.

New and improved observing systems which need to be developed include those directed at changes in the ocean volume. Based on experience 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 topography. Access to InSAR data and ongoing InSAR missions is 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 changes associated with the oceanic mesoscale field, coastal variability, and marine geoid/bathymetry; surface water levels on land and their changes in space and time; and surface topography of glaciers and ice sheets.

In order to provide the necessary resources and coordination and ensure that the required continuity of observations will be available, CEOS has established an Ocean Surface Topography Virtual Constellation team, comprising those countries and agencies engaged in the provision and planning of the necessary instruments and spacecraft.

Further Information
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