GGOS Contributions in Understanding and Forecasting Sea-Level Rise and Variability
Leads: Tilo Schöne, Philip Woodworth, C.K. Shum, and Mark Tamisiea
This document is an attempt at an outline of topics to be undertaken as part of GGOS Theme 3 (Understanding and Forecasting Sea-Level Rise and Variability). In broad terms these topics will comprise:
- Identification of the requirements for a proper understanding of global and regional/local sea-level rise and variability especially in so far as they relate to geodetic monitoring provided by the GGOS infrastructure, and their current links to external organizations (e.g. GEO). [SL]
- Identification of the organizations or individuals who can take forward each requirement, or act as points of contact for each requirement where they are primarily the responsibility of bodies not related to GGOS. [SO]
- Identification of a preliminary set of practical (as opposed to scientific) pilot projects, which will demonstrate the viability, and the importance of geodetic measurements to mitigation of sea-level rise at a local or regional level. This identification will be followed by construction of proposals for pilot projects and their undertaking. [SP]
These topics are frequently updated if appropriate.
Sea level rise and variability has received considerable attention in recent years thanks to, for example, the various IPCC Scientific Assessments, the Wiley-Blackwell book on Understanding Sea-Level Rise and Variability (Church et al. 2010) and, not least, the prominence given to sea level in the GGOS 2020 book.
There is now a reasonable appreciation of the in-situ and space infrastructure required for furthering the long-term, continuous monitoring needed for research into global and regional sea level change. For example, Chapter 12 of Church et al. (2010) may be consulted for a thorough discussion. In that chapter, as in GGOS 2020, the importance of GGOS global infrastructure to GGOS science is emphasised, including the maintenance and development of the ITRF.
Somewhat less attention has been given to the importance of geodetic techniques to practical, rather than scientific, sea-level related applications at many local levels. For example, one may ask if there are common problems related to the long-term security of several Asian cities or island nations presently undergoing submergence which can be addressed by GGOS.
Theme 3 presents an excellent opportunity to emphasise the global, through to regional and local, importance of GGOS to a wide range of sea-level related science and practical applications. In fact, the field is so wide that there is a considerable challenge in even beginning to address the range of issues.
Objectives of GGOS 2020
Objectives for short (2011-2014) and mid-term (2013-2017) need to be a mix of scientific and practical ones. The GGOS 2020 book explains well how the GGOS and its infrastructure are largely based around science, and the difficulties introduced by the vagaries of scientific funding and by technical change. Working towards a proper understanding of sea-level rise and variability, and even its forecasting, are objectives which are matched well with GGOS’s forward looks as presented in Chapters 9 and 10 of the GGOS 2020 book.
However, in order to provide long-term resources for GGOS, it is essential that it is seen to contribute to a solution of some of today’s problems, including mitigation of the impacts of sea level rise. Consequently, there have to be practical as well as scientific objectives, on local and regional levels as well as a global one. Some sea-level related practical objectives are listed below and suggestions for others would be welcomed.
Planned Actions (Work Packages and Responsibilities)
a) Short-Term Actions (2011-2014):
Topics for attention are as follows:
Identification of the requirements for a proper understanding of global and regional sea level rise and variability especially in so far as they relate to geodetic monitoring provided by GGOS infrastructure. This will include the specification of each component of the measurement system and their accuracies needed to facilitate AM-SL-01. It is believed that this topic can be completed reasonably quickly, given the attention already devoted to this issue in recent years.
A preliminary attempt at identifying the elements for proper understanding of the sea level budget (e.g. for the 20th century or during the altimeter era) can be listed as follows:
||Global Ocean Volume (tide gauges, altimetry)
||PSMSL, Altimetry-related Services or similar initiatives (e.g., ESA CCI, NASA), Space Agencies
||Global Ocean Mass (GRACE and Continuity and follow-ons)
||Global Ocean Thermosteric sea level (XBT/MDT, Argo, Aquarius, SMOS, etc.)
||WCRP/WMO/IOC, Space Agencies
||Ice-sheet Mass Balance (Greenland, Antarctica)
||WCRP, Space Agencies
||Mountain Glaciers and Small Ice Cap Mass Balance
||WGMS, World Glacier Inventory, Space Agencies
||Global Water Balance: Terrestrial Hydrology, Aquifers, Human-Impounded Water in Dams, Permafrost
||WCRP, Space Agencies
||Vertical Motion (of the land, subglacial topography and sea floor due to, for example, glacial isostatic adjustment and tectonic motions) with respect to a well-defined ITRF
||e.g., IGS projects
For coordination with Space Agencies the support of GIAC is needed.
Prerequisites for an understanding of each element are a robust and stable ITRF (considering also centre of mass offset from geocentre motions), and its theoretical and practical relationships with different observation types, and an assumption of completion of core observing programmes by agencies. This includes, for example, GLOSS/TIGA by IOC/IGS for in situ sea-level and vertical land movement, Argo by IOC and WCRP which must be maintained as at present for the ice-free upper ocean programme but extended to ice covered areas and the deep ocean, and a large number of missions provided by space agencies including:
- X-SAR (tandem) and L-band InSAR missions,
- GRACE Continuity Mission, GRACE-Follow-on and ongoing missions for ocean, cryosphere and hydrosphere,
- CryoSat-2, ICESat-2 – ice sheets and glaciers requiring INSAR, gravity, altimetry, aircraft and in-situ monitoring (IceBridge),
- AltiKa, GFO-2, Jason-3, Sentinel-3 and ongoing radar altimetry missions,
- GNSS RO missions, and possible GNSS Reflectometry mission,
- SWOT – wide swath altimeter for coastal oceanography and higher resolution deep ocean missions,
- GOCE follow-on, and higher-resolution gradiometer missions.
Identification of the organizations or individuals who can take forward each requirement or act as points of contact for each requirement where they are primarily the responsibility of bodies not related to GGOS. This action will commence following AS-SL-01.
A preliminary attempt at identifying such potential linkages is shown above for AS-SL-01 in square brackets. However, many of these linkages are within GGOS itself or are not too distant from it (e.g. as Services formerly within FAGS and now within the World Data System).
For example, global sea-level studies require:
- Sea surface height from altimeters [IAS if established or ESA/NASA initiatives]
- Tide gauge measurements with collocated GNSS [PSMSL and IGS/TIGA]
- Global geoid (GOCE and gravimetric measurements) [IGFS]
- Mass changes in the ocean (GRACE and follow-on missions) [A variation of IGFS, or a new service]
- Steric sea level over global ocean (XBT/CBT/Argo/Aquarius/SMOS)
- Mass measurements in glaciers/ice-caps, ice-sheets, permafrost, land water storage (radar and laser altimetry, SAR/InSAR, GRACE, passive/optical microwave and in situ measurements) [WGMS]
- ITRF including geocentre motion and accounting for GIA [IERS, and IGS, ILRS, IDS, IVS]
while local coastal sea level studies require:
- Tide gauge and GNSS [as above]
- Coastal altimetry (e.g. ESA COASTALT and CNES PISTACH projects)
- InSAR for short spatial-scale vertical land movement [see below]
- Changes in coastal morphology.
The InSAR/geodetic imaging activity listed above is an example of an area of work where there is little expertise globally so far and requires particular focus. This activity has not developed to the extent that SAR images can be used routinely and worldwide to test the representativeness of vertical land movements from GNSS at tide gauges.
Identification of a preliminary set of practical (as opposed to scientific) demonstration projects which will make clear the importance of geodetic measurements to mitigation of sea-level rise at a local level. Successful projects should demonstrate the added-value of utilizing the GGOS infrastructure and IAG Services.
Possible projects could include:
a. Forecasting of sea-level rise to be expected in medium term (e.g. less than 30 years) at major cities and population centres for input to coastal defense planning. Such projects (as argued effectively by H-P Plag at earlier GGOS meetings) would be of considerably greater utility to planners than the 2100 projections provided by IPCC Assessments, for example, a timescale of 30 years being long enough for many civil engineering schemes to be undertaken.
It is felt that such projects could benefit from previous experience with the Thames Estuary 2100 study for London and similar studies in the Netherlands etc. Potential study areas might include Manila, Bangkok, Shanghai, Djakarta and other large Asian cities for which submergence is a major consideration, or countries such as Bangladesh or Egypt which include large deltas where high rates of sea-level rise are a natural phenomenon to be lived with.
b. A more general project would be the construction of present and future sea level rise hazard maps (extreme level risks) on multiple temporal and spatial scales for input to coastal vulnerability assessment.
b) Mid-Term Actions (2013-2017):
Description of mid-term actions:
Construction of proposals for the pilot projects given above and their subsequent undertaking.
c) Longer-Term Actions:
It is clear that a long-term objective and action is the proper forecasting of global and regional coastal sea-level rise to the end of 21st century.
However, it is equally clear that GGOS cannot undertake this task alone, or even lead it. Consequently, the way forward for GGOS Theme 3 must be an interdisciplinary one, particularly with regard to interpretation of data sets and in prediction. Nevertheless, the successful completion of a number of short and mid-term GGOS projects (as identified above and many others) should enable the long-term objective to be achieved.
Topics which are not directly accessible by GGOS include sea-level relevant modelling of various kinds. Such models for global studies include AOGCMs, ocean process study models, hydrological and land use, and glaciological models, all coupled in principle. Global geophysical models include those of glacial isostatic adjustment (GIA, 1D and 3D), those derived from fingerprint studies (which rely on the availability of continental mass changes listed above), and of tectonics. Such models for regional and local studies include high-resolution coastal tide and surge models.
The responsible institutions and responsible persons for each item above will evolve with time as the Theme develops and different resources will be required at different times.
Schedule and Milestones
The most important items at present are:
- Make the Call for Participation for a set of Demonstration Projects (see AS-SP-03 above) more public. Address Individuals and research groups. Form a core group.
- Prepare a Web Site entry at the GGOS Home Page/Portal
- Prepare presentations for upcoming meetings
Interactions / Relations to the Other GGOS Action Plans
Theme 3 interacts with many other aspects of GGOS. For example, sea level studies clearly need access to the best possible data sets as described in Plag and Pearlman (2009) and Chapter 12 of Church et al. (2010). The Theme interacts with the Working Groups on Infrastructure, Satellite and Space Missions, Earth System Modelling and Data and Information Systems, as well as with the GGOS Bureau on Networks and Communication. In addition, the goals of Theme 3 needs alignment with GGOS Theme 1 (Unified Height Systems).
Church, J.A., P. L. Woodworth, T. Aarup, and W.S. Wilson, 2010: Understanding sea-level rise and variability, Wiley-Blackwell, London. ISBN 978-1-4443-3451-7 (hardback) 978-1-4443-3452-4 (paperback). 428pp.
Plag, H-P. and M. Pearlman, (eds.) 2009: Global Geodetic Observing System: Meeting the requirements of a global society on a changing planet in 2020. Springer Geoscience: Berlin. 332pp.