TiPACCs used ocean and ice sheet numerical models to investigate the two tipping points. Our main project results are:
(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 shows importance of geometry and winds
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 switching from a ‘cold’ to a ‘warm’ ocean regime.
(3) Realistic state-of-the-art ocean modelling show abrupt transitions
We performed many simulations with our three ocean models (FESOM1.4 NEMO and MITgcm), investigating the possibility of crossing the ocean tipping point. All three models show abrupt transitions under future climate forcing. However, the response depends on the ocean model, region, timescale, and climate forcing applied. These shifts seem to be reversible, albeit not instantly, and can therefore rather be considered as abrupt transitions than a tipping point.
(4) Ice-flow models show future ice sheet tipping points
The collaborating ice-flow model teams ensured a coherent design and set-up of short-perturbation experiments for the three models (PISM, Elmer/Ice, Úa). Thorough stability analyses suggest that the Antarctic grounding lines are stable in their current configuration, and that the ongoing ice retreat is not due to a crossed tipping point (or Marine Ice Sheet Instability, MISI) – yet. Our work also suggests that grounding lines will become unstable, leading to irreversible ice loss, under sustained and/or enhanced climate warming. The exact timing of tipping point crossing depends on the strength and duration of the climate perturbation, and on initial model state and model parameters.
(5) Ice sheet tipping points crossed in the geological past
We evaluated proxy methods and applied this knowledge to update sea-surface and deep water temperatures in the Southern Ocean on glacial-interglacial time scales. Ice sheet model simulations (PISM) suggest that large scale tipping points have been crossed over the last 400,000 years, which caused sea level fluctuations in the order of several meters.
(6) Fully coupled ocean-ice flow models confirm ocean regime shifts and ice sheet tipping points
We developed three coupled ocean-ice flow models (see Fig. Numerical models). Simulations applying these coupled models show that evolving ice-shelf cavities affect the stability of the ice sheet grounding lines in a complex way, where changes in sub shelf geometry can both amplify and suppress melt rates and circulation in the cavity, impacting the timing of ice sheet tipping.
CDE activities aimed to transfer TiPACCs results to the general public, scientific community, and policy makers included: (1) four videos introducing TiPACCs, explaining the tipping points, and highlighting our main conclusions; (2) active participation at >142 scientific conferences; (3) 53 peer-reviewed high-impact scientific publications; (4) (co-)organizing >8 research-to-policy events (e.g. EU science2policy event; Tipping Points event; stakeholder event in Bremerhaven; Antarctic policy event in Brussels); (5) Virtual Antarctica interactive map; (6) training of >12 Early Career Researchers; (7) teacher’s materials on Antarctic tipping points in 4 languages; (8) “Tipping Points in Antarctica” Policy Brief (see Fig. policy brief).
