Global and interactively coupled climate models are important tools for projecting or even predicting the future evolution of the climate of our planet. Even though the quality and reliability of such models has increased substantially during the most recent years, large model uncertainties still exist for various climate elements, so that it is crucial to continuously evaluate them against independent observations. Changes in the distribution and availability of terrestrial water storage (TWS), which can be measured by the satellite gravimetry missions GRACE and GRACE-FO, represent an important part of the climate system in general, and the terrestrial water cycle in particular. However, the use of satellite gravity data for the evaluation of interactively coupled climate models has only very recently become feasible. Challenges arise from large model differences with respect to land water storage-related variables, from conceptual discrepancies between modeled and observed TWS, from the still rather short time series of satellite data, and from a limited spatial resolution and sensitivity of the observations.
This presentation will highlight the latest results achieved from our ongoing research on climate model evaluation based on the analysis of an ensemble of models taking part in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We will focus on long-term wetting and drying conditions in TWS, by deriving several hot spot regions of common trends in GRACE/-FO observations and regions of large model consensus. However, as the observational record currently only covers about 20 years, observed trends may still be obscured by natural climate variability. Therefore, to further attribute the wetting or drying in the identified hot spot regions to either interannual/decadal variability or anthropogenic climate change, we investigate the influence of dedicated climate modes (such as ENSO, PDO, AMO etc.) on TWS variability and trends. Furthermore, we perform a numerical model investigation with 250 years of CMIP6 TWS data to quantify the degree to which 20-year trends can be expected to represent long-term trends, and to estimate the likelihood for a mismatch of trends computed over differently long time intervals in the presence of decadal climate variability.

The ERC SYNERGY GRACEFUL project aims at characterizing time-variable processes occurring at the core-mantle boundary and in the fluid core using a synergy of satellite data of the geomagnetic field, gravity field and Earth rotation. In order to detect deep Earth signals, the satellite measurements of the gravity field should be corrected for surface water mass distributions and unrelated geophysical processes. Here, we use an ensemble of state-of-the-art models to correct the effects of the ocean-atmosphere circulation, continental water cycle and glacial isostatic adjustment in the time-variable gravity field. Over the ocean, the input of freshwater from land-based ice melting is well captured by a simple parametric model including an acceleration and a linear trend. The residual ocean mass variations are fairly accurately predicted by ocean-atmosphere circulation models, especially in the Southern Ocean, where the variance of GRACE signal is reduced by more than 80% at inter-annual time scales. However, systematic differences are observed at interannual time scales over the continents, that are larger than the differences between hydrological models or than the differences between the gravity field solutions from different centers. The amplitude of decadal trends and interannual signals is systematically underestimated in hydrological models when compared to GRACE measurements. The largest differences, characterized by decadal oscillations with amplitudes of a few centimeters of equivalent water height, are observed over the Amazon, Zambezi, Congo, Okavango and Volga basins. Significant residual mass variations are also detected at shorter time-scales, with a singularity around 6 years, also observed in the Earth rotation and the magnetic field. While the origin of such signal is unknown, part of it is still likely to be climate-related and at least partially inaccurately predicted by hydrological models. While part of the residual mass anomalies could potentially due to unrelated geophysical processes, including deep-Earth dynamics, efforts must be made to understand the systematic differences observed between GRACE measurements and hydrological models at interannual and decadal time-scales.

The world’s largest earthquakes occur at subduction zones megathrust faults, constituting a considerable hazard source. Generally, studies on the causal mechanisms of megathrust earthquakes mainly focus on tectonic stresses and strain accumulation on subduction plate interfaces. Nevertheless, the control exerted by deeper subduction processes on megathrust earthquake generation remains poorly understood. The description of deep aseismic mass fluxes along plate boundaries remains incomplete but is essential to better understand the plate subduction process in its entirety. In this study, we investigate aseismic mass variations at depth and their possible interactions with intraplate seismicity along the Chilean margin. We analyze the gravity gradients derived from the Earth’s geoid space-time variations from three datasets of GRACE geoid models over a large region surrounding the rupture zone of the Mw 8.8 2010 Maule earthquake. In all these datasets, our analysis reveals an anomalously large gravity gradient signal that progressively increases in the three months preceding the megathrust rupture. By testing diverse potential origins (errors in the data processing, shallow hydrological sources) to explain this large signal, we show that it rather finds origin from mass redistributions within the solid Earth on the continental side of the subduction zone. We discuss several possible origins for this signal within the deeper subduction system. The lateral migration of the gravity signal towards the surface suggest that the Mw 8.8 Maule earthquake may have originated from these deeper deformation propagated to the surface. This study highlights the importance of observations of the temporal variations of the Earth's gravity field from satellites, making it possible to probe slow mass variations in depth. The existence of these interactions between slow mass variations at depth detected by GRACE and interplate seismicity, opens a new field of research to better characterize and understand the dynamics of the seismic cycle at megathrusts. Information provided on processes occurring at all depths along the subduction plate boundaries over intermediate timescales, and their interactions between megathrust earthquakes is of major interest to better understand the seismic cycle as a whole.

Gravity field data are a key element of EO to aid the understanding of the tectonics and deep geological structures of the polar regions. With the limited polar coverage of GOCE (limited to 83.3 degrees N and S), airborne gravity data plays an important role both to fill the polar gaps, as well as providing higher resolution gravity field mapping than possible from space. While the Arctic has been well-covered with airborne and marine surveys since the early 2000’s, with many new surveys carried out in the last decade, Antarctica airborne coverage continues to be more sparse, with data from many older surveys often heavily biased and error prone. GOCE, supplemented with the ESA PolarGAP airborne geophysics campaign and other recent high-quality surveys, have provided new opportunities for providing enhanced gravity field models of Antarctica.

As part of the ESA 4D-Antarctica project, a new compilation of GOCE/GRACE EO data, as well as airborne, surface, and marine altimetry data has been developed, and presented both as grids of gravity anomalies, gravity gradients and geoid. The new compilation gives a much improved basis for understanding the solid earth structures below the Antarctic ice sheet, and highlights also the outstanding quality of GOCE data. The new 4D compilation also highlights the need for more geophysical data, especially in East Antarctica.

Airborne gravity surveys in the coastal regions are important also for climate change applications, as gravity represents more or less the only practical method for mapping large-scale bathymetry under the ice shelves, and aid in the radar measurement of glacier thickness for large, deep outlet glaciers. A new initiative – RINGS – have recently been adopted as an Action Group of SCAR, in order to prepare plans for an international complete geophysical and glaciological airborne mapping of the entire Antarctica grounding line zone, in an internationally coordinated multi-aircraft, multi-sensor project. Experience from the ESA PolarGap project, and the improved sensors of modern state-of-the-art airborne geophysics, shows that even highly challenging logistics projects can be carried out with success by true international cooperation, and we ask the community to support the RINGS initiative, not just for gravity, but also for magnetic, radar, and lidar data, useful for a broad range of sciences, as well as validation of future satellites.

Satellite radar interferometry (InSAR) is a powerful technique for monitoring deformation phenomena. While deformation phenomena occur in a three-dimensional (3D) world, one of the limitations of the InSAR phase observations is that they are only sensitive to the projection of the 3D displacement vector onto the radar line-of-sight (LoS) direction. To uniquely estimate the three displacement components, we would require at least three sets of spatiotemporally coinciding independent (STCI) LoS observations, i.e., scatterers on an object that is not subject to internal deformations, observed at the same time. More importantly, the system of equations needs to have a full-rank coefficient matrix. Unfortunately, in most practical situations at most two STCI LoS observations are available, resulting in an underdetermined system with an infinite amount of possible solutions.

With only one LoS observation, the solution space of the inverse problem is a plane orthogonal to the LoS displacement vector, i.e., the null-space. With two LoS observations, the solution space is a null-line, the intersection of two null-planes. The local orientation of the null-line is dependent on the location on earth, and the characteristics of the satellite orbits. The null-line provides information on the type of displacement product that can be derived from the LoS observations. Therefore, it is a key metric for every InSAR study.

We developed Null-space Orientation (NO), an open-source tool that computes the orientation and geometry of the local null space, given the location on Earth and the orbits of the available satellite missions. The software leverages the Delft Radar Modelling and performance Analysis (DRaMA) software library to compute the acquisition geometry for a given location from the two-line element set of an arbitrary SAR mission. In combination, the No-DRaMA tool provides suggestions on the displacement products that can be derived from the available LoS observations.

There are many different possibilities for the information product. For cases with two LoS observations, the orientation of the null-line defines which displacement product can be derived. Since the satellites are not sensitive to the displacements into the direction of the null-line it is always possible to give unbiased displacement estimates in the row space, i.e., the plane orthogonal to the null-line. However, often the orientation of this plane does not represent the displacement directions of interest. For such cases, we may project the LoS observations onto a plane which represents the vertical and a particular horizontal direction, whereas it should be noted that the ‘projected product’ is not the same as e.g. ‘the vertical displacement’.

A valuable aspect of No-DRaMA is that it can be used prior to an InSAR study to assess what satellite mission, or combination of missions, should be used to deliver a deformation product with highest value depending on the user needs. With the orbits of the different missions, different deformation directions can be estimated, which will result in different deformation products. When those deformation products are compared a well-considered choice on the mission can be made.

Shallow volcanic reservoirs that can produce observable surface displacement are the shallowest expression of an extensive, interconnected trans-crustal magmatic system. In the Galápagos, the majority of deformation sources are found in the upper 5 km of the crust. These sources change volume in response to local influx of magma, but also pressure changes within the underlying magmatic system. Here, we propose that correlated ground displacement patterns can provide an insight into the degree of interconnectivity between neighbouring volcanic systems. If neighbouring volcanoes have interconnected magmatic systems, then their shallow reservoirs may change volume, and produce ground displacement simultaneously. We assess the extent of inter-volcanic connectivity between Galápagos volcanoes, using Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) to measure correlated ground displacement. The widespread volcanic unrest, as well as their remote location, make the Galápagos a prime target for InSAR based studies. Previous authors have postulated about these volcanic connections, on the basis of seismicity, displacement measurements, and composition of erupted material between Fernandina, Alcedo, Darwin and Wolf from 2006–2010. Here, we test this idea using Sentinel-1 imagery from 2016 to the present.

We construct time series of displacement, covering five periods of significant volcanic activity. Coincident changes in ground displacement, at various volcanoes, occur during at least five periods from 2016 to 2021. We test the significance of these episodes of apparently related displacements using temporal Independent Component Analysis and correlation analysis. We demonstrate that the same components of ground motion are active at several volcanoes, and show that this is not a measurement artefact by using both descending and ascending imagery, assessing the impact of both choice of reference pixel and correction of atmospheric artefacts.

We find that static stress changes caused by eruptions at Sierra Negra, Fernandina and Cerro Azul are almost all below the levels expected to influence volcanic activity at neighbouring centres. Our preferred explanation is that these correlated periods of local volcanic displacement are a response to connections in a deep, interconnected magmatic system.