Periodic Reporting for period 2 - TiPACCs (Tipping Points in Antarctic Climate Components)
Période du rapport: 2021-02-01 au 2022-07-31
Ongoing sea-level rise threatens human lives, settlements and infrastructure worldwide. Understanding the processes causing sea-level rise is of crucial importance for society. Melting of the Antarctic Ice Sheet is a major contributor to sea-level rise. If the ice sheet were to become unstable and suddenly lose more mass, this has immense implications for coastal communities worldwide. Unfortunately, there are processes that could cause ‘tipping points’ to be crossed. The Antarctic continent is surrounded by cold waters. However, observations show that relatively warm waters can find their way below Antarctic ice shelves. This can tip the Antarctic continental shelf seas from a ‘cold’ to a ‘warm’ state (Tipping Point 1). Warmer waters melt the shelves from below. Ice shelves support the inland ice sheet by buffering the ice outflow. When ice shelves thin - or even collapse - reduced buttressing can destabilise the ice sheet (Tipping point 2). When this second tipping point is crossed, the enhanced flow of grounded ice leads to sea-level rise. In TiPACCs we investigate these processes and the two mentioned tipping points (see also Fig. cross-section of the Antarctic Ice Sheet), with the overall objective to assess the likelihood of large and abrupt near-future changes in the contribution of the Antarctic Ice Sheet to global sea level.
We use ocean and ice sheet numerical models to investigate the two tipping points. The first 3 years we specifically focused our scientific work on:
(1) New theory on melting at the grounding line
Basal melting close to the grounding lines of the ice shelves play a major role for ice sheet stability, but cannot be simulated easily with current ocean models due to grid resolution and stability issues. We developed a simple new theory for the ocean circulation and melt very close to the grounding line, inspired by flows in fjords.
(2) Idealized ocean modelling
After a thorough literature evaluation, we created an idealized Southern Ocean configuration (using NEMO). The simulations highlight the importance of the geometry of the basin (i.e. if it contains ridges), which together with the strength and direction of the winds impact the ocean currents and stratification. The structure (stratification) of the ocean is crucially important to avoid crossing the Ocean Tipping point.
(3) Realistic state-of-the-art ocean modelling
We tested various grid geometries and vertical coordinate systems in our reference model FESOM. A reference run with FESOM1.4 confirms the good model performance.
We performed many simulations with both FESOM1.4 and NEMO, investigating the possibility of crossing the Ocean Tipping point. Interestingly, the models identify different ocean basins to be most sensitive to ocean tipping. This might be due to model (parameter/grid) differences. Further evaluation of the exact mechanisms and conditions for tipping is ongoing.
(4) Designing, setting-up and running ice-flow models
The collaborating ice-flow model teams ensured a coherent design and set-up of short-perturbation experiments for the three models. Initial stability simulations suggest that the present-day Antarctic Ice Sheet is likely positioned in a stable regime, when only considering its present geometry and grounding line position, but basically ignoring climate forcing. However, including climate forcing, we cannot rule out that the West Antarctic Ice Sheet become unstable and retreats.
(5) Past sea-surface temperatures and ice sheet simulations
We evaluated proxy methods and applied this knowledge to update sea-surface temperatures in the Southern Ocean for the last 200,000 years. We design ice sheet model simulations (PISM), focusing on understanding the (likely) retreated Antarctic Ice Sheet during the Last Interglacial. Model set-up, boundary conditions and climate forcing are not trivial, and work is ongoing.
(6) Developing, comparing, and setting-up coupled ocean-ice flow models.
We further developed 3 coupled ocean-ice flow models. We discussed and evaluated their initialization procedures. All are now ready for coupled ocean-ice simulations in the next phases of the TiPACCs project.
Examples of CDE activities: (1) two videos introducing TiPACCs and explaining Ocean Tipping Point; (2) EGU blog on tipping points; (3) multiple Elmer/Ice courses; (4) active participation to scientific conferences; (5) peer-reviewed high-impact scientific publications; (6) co-organizing policy events (i.e. EU science2policy event; Tipping Points event; COP26side event); (7) training of Early Career Researchers; (8) scientific communication training TiPACCs consortium.
Examples of main results:
- New theory on ice shelf melt close to grounding line
- New understanding on impact of basin geometry and wind on ocean stratification
- Global ocean model adapted to the Southern Ocean (FESOM)
- Understanding that ocean basins have different (model dependent) Ocean Tipping points
- Common initialization procedure for 3 ice-sheet models
- Understanding on the stability of the Antarctic Ice Sheet, under small melt perturbation
- Updated synthesis of past Southern Ocean sea-surface temperatures
- Coupled ocean-ice flow models
- Website with news, videos, publications and more
- Transferring TiPACCs results to general public, scientific community and policy makers
(1) New theory on melting at the grounding line
Basal melting close to the grounding lines of the ice shelves play a major role for ice sheet stability, but cannot be simulated easily with current ocean models due to grid resolution and stability issues. We developed a simple new theory for the ocean circulation and melt very close to the grounding line, inspired by flows in fjords.
(2) Idealized ocean modelling
After a thorough literature evaluation, we created an idealized Southern Ocean configuration (using NEMO). The simulations highlight the importance of the geometry of the basin (i.e. if it contains ridges), which together with the strength and direction of the winds impact the ocean currents and stratification. The structure (stratification) of the ocean is crucially important to avoid crossing the Ocean Tipping point.
(3) Realistic state-of-the-art ocean modelling
We tested various grid geometries and vertical coordinate systems in our reference model FESOM. A reference run with FESOM1.4 confirms the good model performance.
We performed many simulations with both FESOM1.4 and NEMO, investigating the possibility of crossing the Ocean Tipping point. Interestingly, the models identify different ocean basins to be most sensitive to ocean tipping. This might be due to model (parameter/grid) differences. Further evaluation of the exact mechanisms and conditions for tipping is ongoing.
(4) Designing, setting-up and running ice-flow models
The collaborating ice-flow model teams ensured a coherent design and set-up of short-perturbation experiments for the three models. Initial stability simulations suggest that the present-day Antarctic Ice Sheet is likely positioned in a stable regime, when only considering its present geometry and grounding line position, but basically ignoring climate forcing. However, including climate forcing, we cannot rule out that the West Antarctic Ice Sheet become unstable and retreats.
(5) Past sea-surface temperatures and ice sheet simulations
We evaluated proxy methods and applied this knowledge to update sea-surface temperatures in the Southern Ocean for the last 200,000 years. We design ice sheet model simulations (PISM), focusing on understanding the (likely) retreated Antarctic Ice Sheet during the Last Interglacial. Model set-up, boundary conditions and climate forcing are not trivial, and work is ongoing.
(6) Developing, comparing, and setting-up coupled ocean-ice flow models.
We further developed 3 coupled ocean-ice flow models. We discussed and evaluated their initialization procedures. All are now ready for coupled ocean-ice simulations in the next phases of the TiPACCs project.
Examples of CDE activities: (1) two videos introducing TiPACCs and explaining Ocean Tipping Point; (2) EGU blog on tipping points; (3) multiple Elmer/Ice courses; (4) active participation to scientific conferences; (5) peer-reviewed high-impact scientific publications; (6) co-organizing policy events (i.e. EU science2policy event; Tipping Points event; COP26side event); (7) training of Early Career Researchers; (8) scientific communication training TiPACCs consortium.
Examples of main results:
- New theory on ice shelf melt close to grounding line
- New understanding on impact of basin geometry and wind on ocean stratification
- Global ocean model adapted to the Southern Ocean (FESOM)
- Understanding that ocean basins have different (model dependent) Ocean Tipping points
- Common initialization procedure for 3 ice-sheet models
- Understanding on the stability of the Antarctic Ice Sheet, under small melt perturbation
- Updated synthesis of past Southern Ocean sea-surface temperatures
- Coupled ocean-ice flow models
- Website with news, videos, publications and more
- Transferring TiPACCs results to general public, scientific community and policy makers
Examples progress
(1) The new theory on melt close to the grounding line is the first that can capture ocean circulation and melt in this crucial region
(2) Understanding the impact of basin geometry (&wind shifts) on ocean stability
(3) Assessing ocean basins sensitivity to tipping using multiple global/circumpolar ocean models
(4) Common initialization and set-up procedure for 3 state-of-the-art Antarctic Ice Sheet models allows a novel analysis of its stability
(5) Sea-surface temperature synthesis provides new benchmark
(6) Realistically coupled ocean-ice flow models!
Expected results
Besides the results described above, we expect to improve understanding on exactly what mechanism or forcing drives tipping in the various ocean basins. We aim to establish an early warning system, such that we know when ocean tipping is on its way.
We will continue our analyses of the stability of the grounding lines of the Antarctic Ice Sheet, applying larger melt perturbations. Will this lead to a large and irreversible ice sheet retreat? Which basins are most vulnerable?
We will assess the tipping points in coupled ocean – ice sheet models. Does the coupled system enhance or reduce the potential of tipping?
Potential impacts
We will enhance knowledge of the Southern Ocean – Antarctic Ice Sheet vulnerability. This is process understanding provides crucial information for improving global sea-level rise projections, especially for high-end scenarios and longer time scales. If our work shows that the ice sheet is in fact vulnerable, and if we are near to crossing these tipping points, this might even affect sea-level projections on shorter time scales.
Besides this direct societal impact, the tight collaboration between ocean and ice sheet modelers from different European institutes, strengthens European-based excellent science. With the large group of Early Career Researchers involved in the project, we furthermore build the next generation of climate scientists.
(1) The new theory on melt close to the grounding line is the first that can capture ocean circulation and melt in this crucial region
(2) Understanding the impact of basin geometry (&wind shifts) on ocean stability
(3) Assessing ocean basins sensitivity to tipping using multiple global/circumpolar ocean models
(4) Common initialization and set-up procedure for 3 state-of-the-art Antarctic Ice Sheet models allows a novel analysis of its stability
(5) Sea-surface temperature synthesis provides new benchmark
(6) Realistically coupled ocean-ice flow models!
Expected results
Besides the results described above, we expect to improve understanding on exactly what mechanism or forcing drives tipping in the various ocean basins. We aim to establish an early warning system, such that we know when ocean tipping is on its way.
We will continue our analyses of the stability of the grounding lines of the Antarctic Ice Sheet, applying larger melt perturbations. Will this lead to a large and irreversible ice sheet retreat? Which basins are most vulnerable?
We will assess the tipping points in coupled ocean – ice sheet models. Does the coupled system enhance or reduce the potential of tipping?
Potential impacts
We will enhance knowledge of the Southern Ocean – Antarctic Ice Sheet vulnerability. This is process understanding provides crucial information for improving global sea-level rise projections, especially for high-end scenarios and longer time scales. If our work shows that the ice sheet is in fact vulnerable, and if we are near to crossing these tipping points, this might even affect sea-level projections on shorter time scales.
Besides this direct societal impact, the tight collaboration between ocean and ice sheet modelers from different European institutes, strengthens European-based excellent science. With the large group of Early Career Researchers involved in the project, we furthermore build the next generation of climate scientists.