Periodic Reporting for period 3 - GlobalMass (Global land ice, hydrology and ocean mass trends)
Reporting period: 2019-08-01 to 2021-01-31
Globally-averaged sea level change is quantified using the global sea level budget, a ‘balance-sheet’ approach that accounts for all the factors that contribute to sea level rise. Three main factors control changes in sea level: (i) changes in ocean mass (from ice sheets, glaciers and water storage on land); (ii) changes in ocean density (largely from thermal expansion of sea water); and (iii) changes in the elevation of the ocean floor (largely from glacial isostatic adjustment, GIA, the ongoing process by which the Earth’s crust is rebounding from the last glacial maximum ~20,000 years ago).
Since the early-1990s there has been a revolution in our ability to ‘observe’ and measure each of these components through a combination of satellite and in situ approaches. However, these new datasets have varying spatial and temporal characteristics which makes them complex to combine using conventional techniques.
This complexity, coupled with their increasing size and cross-disciplinary nature, means that the different components of sea level budget have generally been tackled separately. This has led to puzzling and at times contradictory results that do not necessarily respect fundamental principles of the coupled land-ice-ocean-solid Earth system or exploit the full capabilities of the new datasets.
The GlobalMass project will, by contrast, tackle all components simultaneously at a global scale. It will do this by adopting a powerful new approach to analysing large amounts of spatio-temporal data – a Bayesian Hierarchical Model (BHM) – which allows us to estimate the most likely combination of all the components of the sea level budget along with an indication of the uncertainty of the estimates. A key advantage of the BHM approach is that it provides a rigorous way of differentiating between observations based on their varied characteristics (or ‘smoothness’) in space and time. This allows us to separate the contribution of the different geophysical processes relevant to sea level change. Additional constraints provided by prior knowledge and fundamental physical principles (such as conservation of mass) can be added to the BHM to improve this separation further still.
Thus, the overall aim of the project is to develop and use a BHM to produce simultaneous, global, statistically-rigorous estimates of all components of the sea level budget for a common time period: (i) glacial isostatic adjustment (GIA); (ii) land ice mass trends; (iii) land hydrology trends; and (iv) steric trends and sea surface height. These estimates will be data-driven (rather than the result of numerical modelling) and will combine a wide range of satellite and in-situ data. They will, for the first time, be consistent with each other and with physical constraints on the coupled system.
There are six specific objectives:
1: Develop the BHM methodology and software to undertake multivariate spatio-temporal modelling at a global scale.
2: Obtain a data-driven solution for global GIA, consistent with satellite and GPS observations.
3: Reconcile the sea level budget for 1981-2020.
4: Produce spatially-distributed land ice mass balance trends for 1992-2020, consistent with in-situ and satellite-based observations.
5: Re-evaluate twentieth century sea level rise from the tide gauge record using the BHM approach.
6: Investigate catchment-scale land hydrology trends for 2003-2020.
The initial experiments have therefore seen us add data incrementally with the main aims of testing that the framework is working correctly and exploring different experimental set-ups (e.g. different methods and datasets). The key requirement has been results that look plausible given known physical processes, and do not contain unexpected or unexplained errors. In so doing, we have achieved some notable outputs:
• Extension of a BHM approach to investigate ice mass trends for Antarctica and Greenland
• Creation of a global GPS dataset to provide a ‘clean’ signal of glacial isostatic adjustment (GIA)
• Exploration of global steric sea level trends 2005-2015
• New estimate of land ice contribution to sea level rise
• New estimate of ice sheet contribution to future sea level rise
• Investigation of global water storage stress
We have established a dedicated project website (www.globalmass.eu) and Twitter feed (@globalmassteam) and use these to provide regular updates and news. We also upload plain language summaries of all published papers (www.globalmass.eu/plain-language-summaries).
Once we are satisfied that the framework is robust and operating correctly [Objective 1], we will incrementally increase the complexity of the problem to address the other objectives. It is anticipated that a data-driven solution for global GIA [Objective 2] will be the next major milestone, as once this has been determined it becomes an input to the BHM. Once land ice [Objective 4] and hydrology trends [Objective 6] are accounted for, this will allow us to simultaneously ‘solve’ the global sea level budget for the first time [Objective 3].
Addition of tide gauge records into the BHM framework for the ~20-year period where they overlap with satellite observations of sea surface height is the final challenge. Doing so will allow us to produce a ‘tide-gauge only’ reconstruction of sea level rise for this period. This will highlight the impact of the inherent spatial and temporal sampling required when using long term tide gauge records that are skewed unavoidably towards measurements of sea level change along European and North American coastlines. Informed by this knowledge, we will then run the BHM to re-evaluate 20th century sea level rise from the tide gauge record [Objective 5].