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Tipping Points in Antarctic Climate Components

Periodic Reporting for period 1 - TiPACCs (Tipping Points in Antarctic Climate Components)

Reporting period: 2019-08-01 to 2021-01-31

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 destabilize the ice sheet (Tipping point 2). When this 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 in Antarctic climate components. The first 18 months of TiPACCs we specifically focused our scientific work on (1) melting at the grounding line, (2) reference ocean model FESOM, (3) a common procedure for our ice-sheet models, and (4) synthesize Last Interglacial sea surface temperature records.

(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 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. The theory shows promising results when testing the equations in a short Matlab script. However, full implementation of the new theory into the three-dimensional ocean models presents some further challenges. These will be followed-up in the next reporting period.

(2) Setting-up and validating of ocean model FESOM
The global ocean model FESOM is used as the reference ocean model in TiPACCs. At a later stage, once validated, FESOM will provide the boundary conditions for the regional ocean models MITgcm and NEMO. It is therefore pertinent to make sure FESOM is set-up in the best possible way. We have tested various grid geometries, and especially focused on finding the best vertical coordinate system. At present, the finite-element version FESOM1.4 gives satisfying results. A reference run, covering the 20th-century and a climate scenario, is performed and confirms the good model performance.

(3) Reference ice-sheet configurations
Three ice sheet models participate in TiPACCs: Elmer/Ice, PISM and Úa. We compared the different strategies adopted by each partner/model to build an initial state, and continue to work with an as common as possible set-up. We also assembled a comprehensive list of available datasets necessary to either build an initial state or to evaluate the quality of an initial state relative to observations. All three models adequately reproduce the current state of the Antarctica Ice Sheet giving high confidence that the upcoming TiPACCs perturbation experiments can be conducted starting from these initial states.

(4) Last Interglacial sea surface temperatures
Sea surface temperatures are reconstructed from marine sediment cores using proxies, which are converted to temperatures using a calibration or other statistical techniques. We have reviewed several different proxies and their respective calibrations to find those best suited for the particular environmental conditions in the Southern Ocean.
Examples of CDE activities: (1) a video emphasizing the necessity of understanding tipping points in Antarctica, and introducing TiPACCs; (2) a EGU blog on tipping points; (3) multiple Elmer/Ice courses; (4) active participation to scientific conferences such as EGU, and FRISP; (5) scientific publications.

Main results achieved:
- New theory on calculating ice shelf melt close to the grounding line
- Global ocean model adapted to the Southern Ocean
- A common initialization procedure for 3 ice-sheet models and list of necessary datasets
- Updated dataset of Last Interglacial sea surface temperatures (to be submitted July 2021)
- TiPACCs website
- TiPACCs introductory video
- Tipping points EGU blog
Examples progress beyond the state-of-the-art
(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) Our current FESOM set-up allows to explore vulnerabilities of Antarctic ice shelves in more detail than previously possible.
(3) Working with common initialization and set-up procedure of three state-of-the-art ice sheet models describing the full Antarctic Ice Sheet is novel.
(4) The Last Interglacial sea surface temperature synthesis work not only combines all relevant sedimentary proxy temperatures, but also re-calibrates these where necessary.

Expected results until the end of the project
A solid base is now laid for the “real” work: assessing the possibility and proximity of tipping points in the Southern Ocean and Antarctic Ice Sheet.
We will utilize 2 additional ocean models and assess the Southern Ocean tipping point in all 3 models. Likely the various ocean basins will indicate tipping on different time scales, perhaps even model dependent. We will investigate tipping point behavior in the ice sheet by testing the stability of the grounding line. First the current grounding lines are tested: does a small melt perturbation cause these to retreat, or are they stable at their current position? This is followed by simulations forced by a larger melt perturbation. Will this lead to a large and irreversible ice sheet retreat? Which basins are most vulnerable? We will also assess the tipping points in coupled ocean – ice sheet models (see Figure Models). Does the coupled system enhance or reduce the potential of tipping? We will further unravel the large ice sheet retreat during the LIG. Is the warming of 1-2ºC enough to have crossed the ice sheet tipping point, or are other processes at play?

Potential impacts
We will enhance our knowledge of the vulnerability of the Southern Ocean – Antarctic Ice Sheet system. This is process understanding provides crucial information for improving global sea-level rise projections. Especially the longer time scales and high-end scenarios are in dire need for updates information on the fate of the Antarctic Ice Sheet. Additionally, 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.
Numerical models involved, and how they connect.
Cross-section of the Antarctic Ice Sheet indicating TiPACCs tipping points (TP).