Periodic Reporting for period 3 - TiPACCs (Tipping Points in Antarctic Climate Components)
Période du rapport: 2022-08-01 au 2024-01-31
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 second tipping point is crossed, the enhanced flow of grounded ice leads to sea-level rise. In TiPACCs we investigated 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.
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).
(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).
Progress and key results
All the new knowledge generated through TiPACCs provides scientific progress beyond the state-of-the-art. Perhaps our most important key results are that both our ocean models and observations consistently point to the potential of abrupt transitions from ‘cold’ to ‘warm’ ocean basins, and that the Antarctic groundling lines have not crossed a tipping point yet, but will be in the future under sustained and/or enhanced climate warming. We have made enormous technical progress in coupling ice sheet and ocean models. Conducting coupled ice+ocean simulations confirm that the abrupt ocean changes and ice sheet tipping behaviour are robust features. Their interactions create a complexity that needs to be further analysed for individual ocean and ice basins.
Potential impacts
We enhanced knowledge of the Southern Ocean – Antarctic Ice Sheet vulnerability. Our scientific publications, and transfer of gained knowledge, support major international scientific assessments such as the IPCC, COPs and the Global Tipping Points Report both directly and indirectly. Our model development, and climate and ice sheet simulations following future climate scenarios, help with increasing confidence in the most uncertain part of sea level rise projections: the potential for an abrupt mass loss of the Antarctic Ice Sheet. Our coupled ocean – ice sheet simulations suggest possible rates of sea level rise in the order of 2-3 mm/yr from Antarctica, after a regime shift to a warm ocean state.
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 helped shape the next generation of European climate scientists.
Our communication of our scientific results in various formats provide European decision and policy makers with scientifically sound and easily accessible information, paving ways for future knowledge priorities and climate mitigation and adaptation strategies.
All the new knowledge generated through TiPACCs provides scientific progress beyond the state-of-the-art. Perhaps our most important key results are that both our ocean models and observations consistently point to the potential of abrupt transitions from ‘cold’ to ‘warm’ ocean basins, and that the Antarctic groundling lines have not crossed a tipping point yet, but will be in the future under sustained and/or enhanced climate warming. We have made enormous technical progress in coupling ice sheet and ocean models. Conducting coupled ice+ocean simulations confirm that the abrupt ocean changes and ice sheet tipping behaviour are robust features. Their interactions create a complexity that needs to be further analysed for individual ocean and ice basins.
Potential impacts
We enhanced knowledge of the Southern Ocean – Antarctic Ice Sheet vulnerability. Our scientific publications, and transfer of gained knowledge, support major international scientific assessments such as the IPCC, COPs and the Global Tipping Points Report both directly and indirectly. Our model development, and climate and ice sheet simulations following future climate scenarios, help with increasing confidence in the most uncertain part of sea level rise projections: the potential for an abrupt mass loss of the Antarctic Ice Sheet. Our coupled ocean – ice sheet simulations suggest possible rates of sea level rise in the order of 2-3 mm/yr from Antarctica, after a regime shift to a warm ocean state.
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 helped shape the next generation of European climate scientists.
Our communication of our scientific results in various formats provide European decision and policy makers with scientifically sound and easily accessible information, paving ways for future knowledge priorities and climate mitigation and adaptation strategies.