General

The Action has been organized to better define the uncertainty in polar ice mass contributions to present-day global sea level rise. Global sea level has risen during the 20th C at a rate of ~1.8mm/yr, (increasing to ~2-3mm/yr during the 1990s). The contributions of the various components are not yet well resolved, although substantial progress has been made within the last few years.

Polar ice mass balance can be measured using three different approaches:

1. The mass budget method: compares ice transferred out of the system (as measured by ice velocity and thickness across the ice sheet grounding line) with that entering the system (from net accumulation measurements/models);

2. Measurement of ice elevation change over time: which together with ice density estimates allows the computation of volume change after changes in compaction and crustal uplift due to tectonic motions and Glacial Isostatic Adjustment (GIA) have been taken into account;

3. Directly determining the change in mass of the ice sheets: using time-variable gravity measurements such as from the Gravity Recovery and Climate Experiment (GRACE) twin satellites, launched in March 2002 (requires that the GIA signal is known a priori).

Of these, GRACE provides the most direct measurement of ice mass changes. Further, GRACE is presently the only technique able to deliver a continent-wide ice mass balance estimate. GRACE provides monthly estimates of Earth’s gravity field or equivalently estimates of the time variable global mass redistribution at spatial scales ≥ 300 km.

To determine ice mass balance from GRACE, other regional mass trends (atmospheric and oceanic) must be accounted for. Further, the gravity signal from GRACE will also contain a contribution from GIA. GIA is the visco-elastic response of the crust and mantle to past and present changes in ice masses. GIA models can therefore be constrained by observations of crustal motion and the changes in gravity associated with the solid Earth deformation. GIA is a dominant signal in GRACE secular mass trends in regions currently overlain by ice. Consequently, improvements in GIA models are vital to determining precise GRACE-derived ice mass changes (in order of importance: Antarctica, Greenland, and the other ice caps). For example, a published total mass trend for Antarctica from GRACE is 39±14 km3/yr but with an estimated GIA contribution of 192±79 km3/yr. In this case, the signals from present-day mass loss and GIA nearly cancel, and the GIA uncertainty prevents tight constraints on the Antarctic contribution to sea level change.

This Action seeks to improve the accuracy of current GIA models by providing new and improved observational constraints to the GIA modelling community, of which Europe is a leading centre. These new constraints are in the form of accurate and precise 3D crustal motion estimates. They will increase scientific ability to test ice load history and Earth models and then to reliably eliminate the dominant ‘noise’ signal from GRACE ice mass balance estimates.

GIA models of polar-regions are generally data poor and important new constraints are being provided by accurate, precise and geographically widespread surface velocity measurements. Whilst surface velocity estimates have been derived from geodetic data in many of the key regions, their accuracy and precision is limited by observation model errors, inter-technique biases and the accuracy of the global geodetic reference frame to which all measurements are expressed.

Individual efforts in GIA modelling and in improving ability to reliably derive surface velocities from geodetic data are already underway, and are funded by national programmes. Any advancement in more precisely quantifying the uncertainty in GIA models will only be accomplished by bringing together experts in geodesy with experts in GIA modelling. This Action seeks to develop new collaborative efforts between these groups by focusing on a specific research driven goal for a limited period. There is currently no comparable programme within Europe or elsewhere.

While the issues the Action seeks to address are global, Europe has a significant amount of international expertise in the required fields. The activity will support various international efforts, with European groups playing key roles. For instance, the Action will symbiotically align with the International Earth Rotation Service (IERS) and individual geodetic technique centres’ (Global Navigation Satellite Systems (GNSS)/ Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS)/ Very Long Baseline Interferometry (VLBI)/Satellite Laser Ranging (SLR)) efforts and the planned Global Geodetic Observing System (GGOS; http://www.ggos.org/). The Action will provide key first steps in obtaining the far-reaching goals of GGOS. With the purpose of placing optimal constraints on GIA models, the most efficient use of these existing efforts requires inter-technique collaboration tightly coupled with GIA model development.

The various observation types of interest are the i) Global Positioning System (GPS)/GNSS: receiving microwave signals used to measure ranges to Earth orbiting satellites; ii) DORIS: transmitting microwave signals used to measure range rates to Earth orbiting satellites; iii) VLBI: receiving radio signals from natural astronomical sources; iv) SLR: optical ranging to Earth orbiting satellites; v) Absolute Gravity (AG): measurement of point-wise absolute gravity (and changes with time); vi) Persistent Scatterer Interferometric Synthetic Aperture Radar (PSInSAR): changes in phase of spaceborne radar.

Currently, independently funded, specialized efforts are underway at dozens of European institutions. Inter-technique/disciplinary collaboration is missing within the community. COST is the most appropriate framework for the Action as COST would bring these individual components together, thus facilitating the next significant advance in the field. Further, GIA modelling is a highly specialised research effort and is therefore limited to a few centres worldwide. Extending Europe’s expertise in GIA modelling to more institutions is also vital to improving scientific understanding of the signal.

Current state of knowledge

The configuration of the major ice sheets during the last glacial maximum and their subsequent retreat history remain the subject of substantial uncertainty. While important advances have recently been made through new marine geophysical, relative sea level and geomorphological surveys, these measurements are limited in their geographical coverage and there remains a distinct paucity of
observations with which to constrain the maximum ice volume and deglaciation history in some key regions, particularly Antarctica (e.g., the Weddell Sea region).

Measurements of ongoing changes in Earth’s gravity field and shape due to GIA are also valuable for constraining models of ice history. GIA is an ongoing three-dimensional visco-elastic deformation of the solid Earth due to ice melting at the end of the last ice age. The ice history is linked to these measurements using an Earth model, which is also not known to a sufficiently high degree of accuracy. Rates of deformation are typically largest in the vertical component, with velocities ranging up to >15 mm/yr. Horizontal velocities are typically <5 mm/yr. However, a comparison of GIA models for Antarctica reveals the models to be discrepant. In addition, comparing different models shows variation greater than 100% in many locations. Ongoing additions of new geophysical data sets result in incremental improvements in GIA model accuracy, but the uncertainty is still substantial.

Indeed, the differences between contemporary GIA models have recently come under renewed focus as a result of the Gravity Recovery and Climate Experiment (GRACE) satellite mission. GIA dominates the gravity signal measured by GRACE in many parts of the globe. On the other hand, the presence of real ice mass or hydrological changes that also can not be modelled sufficiently accurately, indicate that GRACE data alone cannot unambiguously determine GIA. Additional independent data are required. In this sense, geodetic observations have great potential for making a significant contribution to understanding the problem, but only if the geodetic analysis is tightly coupled with the GIA modelling.

For reasons of cost and convenience, most geodetic observations of GIA use data from GPS receivers. However, determining coordinate time series and velocities depends on (among other things) the availability of an accurate global reference frame. This current reference frame is based on an optimal combination of GPS solutions with VLBI, SLR and DORIS data. The accuracy of the
GIA measurements by GPS therefore depends on both the accuracy and precision of the GPS measurements at the site and the accuracy of the derived global reference frame to which the GIA measurements refer.

Well-known limitations exist in the current analysis performed by the technique analysis centres (ACs). The most recent realization of the global International Terrestrial Reference System, the International Terrestrial Reference Frame 2005 (ITRF2005) has dramatically highlighted the ongoing problems with inter-technique biases, meaning that the various techniques are not yet easily combined. Estimates of local site vertical velocities are particularly sensitive to the choices made at the reference frame computation stage. These misfits have provoked intense techniquespecific study. Thus, further work is required at the technique level as well as at the combination level (reference frame level) before geodetic data sets may be fully exploited for the purpose of placing new constraints on GIA models.

Reasons for the Action

There will be three fundamental results of this Action:

1. A reliable quantification of the present-day contribution of the largest ice masses to global sea level rise.

2. Improvement in the global reference frame

3. Enhanced ability of present and future time-variable gravity missions to determine present day mass change, including (but not only) ice mass change

The first fundamental result, i.e., a reliable quantification of the contribution of the large ice masses to global sea level rise, will be used by an untold number of scientists and policy makers. Understanding the interaction between the cryosphere, hydrosphere, and atmosphere at global scales is imperative for improving predictions of how climate change will progress and how it will affect humanity. Significant headway into understanding the complex interaction of these systems has been hampered by the lack of global data sets.

The GRACE gravity mission is providing estimates of the time variable redistribution of surface masses, namely continental water and ice. Estimates of the present day ice melt from GRACE rely heavily on GIA models, which in many regions are insufficiently known. Incorporating the soon to be available data sets inspired by the International Polar Year (IPY; 2007-9) will allow better estimates of the present day ice melt. New GPS sites have recently been installed in key locations and more are being installed during November 2007-February 2008 as part of the IPY POLENET project (http://www.polenet.org/). The timing of the Action is therefore right to take advantage of these new data.

Defining the reference frame is a pressing issue in geodesy as evidenced by the numerous workshops and symposia dedicated to this subject. In addition, the maintenance of a stable, accurate and global reference frame is crucial for all Earth observations and many practical applications. This maintenance task is the greatest priority of the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) (http://www.ggos.org/).

There is significant likelihood of a European and a US time variable gravity mission which will extend the mass change time series from GRACE well into the future. In addition, Cryosat-2, an ESA mission to precisely monitor changes in the altitude of the polar ice sheets, is due for launch in 2009. The data from these satellites alone cannot provide estimates of present day ice change without reliable estimates of GIA. This Action will provide those estimates, thus adding significant value to future satellite missions.

Based on the above arguments, the Action is aimed at scientific, but also societal, advances.

The development of a well-calibrated GIA model is a complex process that requires collaboration between researchers with diverse expertise. A range of data types are relevant to the problem (e.g. present-day crustal motion, paleo sea-level histories, morphological indicators of past ice extent) and so collaboration between geodesists, Quaternary geologists and GIA modellers is necessary to ensure that the data employed in the modelling have been critically assessed and, where necessary, corrected for other non-GIA processes (such as tectonic motion). Input from glaciologists is important to ensure that only glaciologically realistic ice histories are explored in seeking a fit to the data, thus reducing the non-uniqueness of the problem. This non-uniqueness can be reduced further by also considering a variety of information from studies that constrain the structure and properties of the solid Earth (e.g. seismology, rock/mineral deformation). These teams of experts exist and currently maybe two to three may collaborate to develop a model of GIA. This Action will allow for the collaboration of all the components together which is clearly necessary to make a significant advance. Without a high level of collaboration between the various experts, such as provided by this Action, the potential of geodetic data sets will never be fully realized.

Complementarity with other research programmes

International collaborative efforts are underway under the (unfunded) banner of the IGS Tide Gauge (TIGA) project, investigating vertical rates of motion of tide gauges in order to provide absolute sea level change rates. This Action is allied to TIGA with regard the vertical component of motion, but extends it to three dimensions and further involves the essential collaboration of GIA modellers. The International Polar Year (IPY) 2007-9 is an international collaborative ‘year’ of research on polar- regions. As mentioned, many of the new data sets to be analysed in this Action will come via IPY international projects. The collaborative and open data access spirit of IPY puts it in close allegiance with COST Actions.

                                                                Further information may be found in the Action MoU.