This PhD project will use complex and state-of-the-art coupled climate-ice sheet modelling and uncertainty quantification techniques to investigate what drove the slow ice sheet retreat during the last and penultimate deglaciations (23-7 and 140-120  thousand years ago) and identify the triggers of ice sheet instabilities and abrupt sea level changes. This research will improve our fundamental understanding of the Earth System and help test and improve the complex models used to project future climate and ice sheet changes.

Background

Deglaciations were periods of major climatic and environmental transition in the Earth System from glacial periods when massive ice sheets covered the Northern Hemisphere continent to interglacial periods like today with only two ice sheets (in Greenland and Antarctica). These transitions lasted about ten thousand years, during which slow changes in the Earth’s orbit and natural increases in atmospheric CO2 led to warmer summers and progressive ice sheet retreat. But such huge climatic changes did not occur smoothly. The slow warming and sea level rise were punctuated by abrupt warming and cooling events, major reorganisations of ocean circulation, and sudden collapses of ice sheets that produced fast sea level rises. For example, 14.5 thousand years ago, the N. hemisphere warmed by 2-3 degrees (10 degrees in Greenland) within decades to centuries during an event called the Bølling–Allerød warming. Around the same time, the sea level rose by a record ~ 15 m in less than 350 years during the meltwater pulse 1a. We have discovered that the rapid sea level rise was caused by a mechanism of ice sheet instability, the ”Saddle-Collapse”, triggered by the Bølling–Allerød warming (Gregoire et al., 2012, 2016), but it is challenging to simulate the correct timing and magnitude of the event with models that do not accurately represent surface mass balance and climate-ice sheet interactions. Other abrupt ice sheet changes were caused by the marine ice sheet instability (Gandy et al., 2018) that represent the largest threat for future sea level rise.

The last deglaciation (23-7 thousand years ago) is well studied and documented with a wealth of information on the climate and ice sheet changes (Ivanovic et al., 2016). High-profile hypotheses have been put forward by our research group (see list of key references) and other international colleagues about the cause of the slow and fast changes. It is, therefore, a valuable period to test and apply complex climate and ice sheet models used to study past and future changes. Furthermore, palaeoclimate records show intriguing differences between the last two deglaciations in terms of the timing and number of cooling/warming events that we do not yet understand (Menviel et al., 2019). Despite the international efforts of the PMIP, INQUA and PALSEA communities (see international networks below) and the major advances by our research group (see key references below), it remains challenging to reproduce the observed climate and ice sheet changes with complex models. This calls into question the ability of complex numerical models to simulate instabilities in the Earth system and undermines our trust in our ability to predict future tipping points in the climate.

Simulating the abrupt climate and ice sheet changes during deglaciations is challenging for the following reasons. In order to simulate deglacial abrupt climate and ice sheet changes, it is necessary to resolve processes on a wide range of temporal (daily to multi-millennial) and spatial (kilometre to global) scales involving strong interactions between the ice sheets, ocean and atmosphere. Small errors can be amplified by powerful feedback within the system, and there are large uncertainties in the models and observational datasets. Finally, abrupt events likely involved unpredictable instabilities triggered by long-term progressive climate changes.

Fortunately, huge technical advances have been made by our research group as part of two recent projects (RISICMAP19 and SMB-Gen) that have developed and applied sophisticated coupled climate-ice sheet models and new uncertainty quantification techniques to successfully simulate the last and penultimate glacial maximum (LGM: 21,000 years ago; PGM ~140,000 years ago). We have demonstrated that the LGM is a useful test for coupled Climate-Ice sheet models that can be over-tuned to reproduce the present-day (Gandy et al., 2023). Our modelling tools are now ready to be applied to simulate climate and ice sheet instabilities during deglaciations and answer exciting research questions on the drivers of deglacial climate changes and the cause of abrupt events.

Aim and Objectives

This PhD project uses the FAMOUS-ice model (e.g. Gandy et al., 2023) to simulate and better understand the fast and slow climate and ice sheet changes that occur during Quaternary deglaciations. The simulations will continue from existing ensembles of simulations of the Last Glacial Maximum (21,000 years ago) and Penultimate Glacial Maximum (~140,000 years ago) and will follow the international experimental PIMIP4 protocols (Ivanovic et al., 2016; Menviel et al., 2019). Model results will be compared with a wealth of observational climate and ice sheet records. Advanced modelling techniques such as sensitivity analysis, perturbed physics ensembles and Gaussian process emulation to quantify uncertainties (e.g. Gandy et al., 2023), and factorial decomposition (Gregoire et al., 2015) will be at your disposal to answer exciting research questions on the climatic drivers of deglaciations and the causes of rapid climate change and sea level rise.

Examples of research questions to be addressed

The project will evolve in line with the best research on the topic. You will be fully supported to follow your interests and make the project your own. Here is an example of the direction the PhD could take, including a selection of possible research questions to answer:

Chapter/paper 1: Simulate the last deglaciation with the coupled climate-ice sheet model FAMOUS-ice.

• Can we simulate the pace and amplitude of the slow changes in climate and ice sheets?
• Are Saddle-collapse and/or marine ice sheet instabilities triggered within the simulations?
• What are the optimum parameter values to reproduce the observed climate and ice sheet changes? Do these produce good simulations of the present day?

Chapter/paper 2: Timing and amplitude of abrupt ice sheet changes.

More detailed simulations of abrupt changes will be produced with greater temporal and spatial resolution. Uncertainty quantification techniques can be used to explore the plausible timings and amplitude of unpredictable instabilities.

• Can we identify the tipping points that trigger instabilities in ice sheets?
• What factors influence the timing and amplitude of the abrupt changes simulated?
• Can we reproduce the timing and amplitude of the ice sheet and sea level changes observed?
• What is the role of ice sheet dynamics and surface mass balance in the timing and amplitude of the abrupt changes simulated?

Chapter/paper 3: Climatic drivers of the last two deglaciations.

Decompose the drivers of deglacial ice sheet changes by factor decomposition technique. The method of Gregoire et al. (2015) will be applied to the coupled climate-ice sheet model FAMOUS-ice.

• What was the relative contribution of orbital forcing and greenhouse gases to the changes in ice sheet volume during the last deglaciation?
• How do the results compare with previous studies (Abe-Ouchi et al. and Gregoire et al.) that used simpler models?
• Can we explain the differences in timing and duration of the last two deglaciations?

As you progress through your studies, you will be encouraged to drive and adapt the direction of your PhD project based on your results, intuitions, and interests with the support of your supervisors.

International Network

The candidate will have the opportunity to be a part of the following international research networks, working with scientists from around the world to generate and gain hot-off-the-press access to cutting-edge knowledge on rapid climate and sea level change:

• The PALSEA, which is co-led by the lead supervisor [https://palseagroup.weebly.com]
• The Paleoclimate Model Intercomparison Project (PMIP), and in particular the Deglaciations Working Group, which is coordinated by the supervisor [https://pmip4.lsce.ipsl.fr]
• The International Union for Quaternary Research Focus Group on deglaciations (INQUA IFG 2004F T5-0) led by the co-supervisor [https://www.inqua.org/commissions/palcom/ifg]

 

Potential for high-impact

This exciting and novel work employs recent, groundbreaking developments in our knowledge of climate-ice sheet-ocean interactions and the ability to simulate them. It can potentially correct errors in current simulations of past and future ice sheet evolution and improve our confidence in future sea-level projections. The student will develop a highly sought-after, multidisciplinary skill set, contributing towards developing an interdisciplinary field of research at the forefront of climate science. By the nature of this work and its timeliness, there is strong potential for the PhD candidate to influence the direction of international research being carried out on this theme and thus establish a world-renowned reputation for innovative science.

Training, support and research opportunities

This project affords many exciting opportunities for skills and research development, in particular:

• Joining a highly dynamic and supportive team of climate scientists in the climate-ice research cluster at the University of Leeds (led by Dr Ruza Ivanovic and Dr Lauren Gregoire). Working within the dynamic and multidisciplinary Physical Climate Change and Palaeo@Leeds research groups in the Institute for Climate and Atmospheric Studies, Earth Surface Science Institute and Priestley International Centre for Climate at the University of Leeds.
• Using state-of-the-art research facilities, including high-performance computer clusters (ARC) at the University of Leeds. Developing high-tech computer programming, model output processing and data visualisation skills with the support of the Centre of Excellence for Modelling the Atmosphere and Climate and other research scientists across the School of Earth and Environment (Leeds) who have a long track record of training highly successful PhD students with limited prior knowledge of computing.
• Collaborating with world-leading experts in climate research through the Paleoclimate Modelling Intercomparison Project (PMIP), International Union for Quaternary and PALSEA.
• Attending and presenting results at major international conferences, e.g. AGU (various venues across the USA), EGU (Vienna), PAGES, INQUA Congress and PMIP (various venues globally). Attending residential summer schools (e.g., Italy, USA, UK) and project-specific in-house workshops/courses.
• Other more generalised training through the Panorama Doctoral Training Partnership and a wide portfolio of University of Leeds training programmes.

Full support for all technical and scientific aspects of the project, including the modelling work, will be provided in-house (Leeds) and by external collaborators. With this training, the student will be well equipped to pursue their own research interests.

Entry requirements

A good first degree (1 or high 2i), Masters degree or equivalent in a physical or mathematical discipline, such as Physics, Mathematics, Oceanography, Meteorology, Climate Sciences, Earth/Environmental/Geographical Sciences, Chemistry, Engineering or Computer Sciences. Experience in computer programming (e.g., Python, Fortran, C++, MATLAB, R…) or numerical modelling is highly desirable. Candidates with either strong numerical/programming skills or a good background in climate science or glaciology would be well suited to this project.

Key reading

Gandy, N., Astfalck, L. C., Gregoire, L. J., Ivanovic, R. F., Patterson, V. L., Sherriff‐Tadano, S., . . . Rigby, R. (2023). De-Tuning Albedo Parameters in a Coupled Climate Ice Sheet Model to Simulate the North American Ice Sheet at the Last Glacial Maximum. Journal of Geophysical Research: Earth Surface, 128(8). doi:10.1029/2023jf007250

Gregoire, L. J., Payne, A. J., & Valdes, P. J. (2012). Deglacial rapid sea level rises caused by ice-sheet saddle collapses. Nature, 487(7406), 219-222. doi:10.1038/nature11257

Gregoire, L. J., Otto-Bliesner, B., Valdes, P. J., & Ivanovic, R. (2016). Abrupt Bølling warming and ice saddle collapse contributions to the Meltwater Pulse 1a rapid sea level rise. Geophysical Research Letters, 43(17), 9130-9137. doi:10.1002/2016GL070356

Ivanovic, R. F., Gregoire, L. J., Wickert, A. D., Valdes, P. J., & Burke, A. (2017). Collapse of the North American ice saddle 14,500 years ago caused widespread cooling and reduced ocean overturning circulation. Geophysical Research Letters, 44(1), 383-392. doi:10.1002/2016GL071849

Menviel, L., Capron, E., Govin, A., Dutton, A., Tarasov, L., Abe-Ouchi, A., . . . Zhang, X. (2019). The penultimate deglaciation: protocol for Paleoclimate Modelling Intercomparison Project (PMIP) phase 4 transient numerical simulations between 140 and 127 ka, version 1.0. Geoscientific Model Development, 12(8), 3649-3685. doi:10.5194/gmd-12-3649-2019

Gregoire, L. J., Valdes, P. J., & Payne, A. J. (2015). The relative contribution of orbital forcing and greenhouse gases to the North American deglaciation. Geophysical Research Letters, 42(22), 9970-9979. doi:10.1002/2015GL066005

Gandy, N., Gregoire, L. J., Ely, J. C., Cornford, S. L., Clark, C. D., & Hodgson, D. M. (n.d.). Collapse of the last Eurasian Ice Sheet in the North Sea modulated by combined processes of ice flow, surface melt, and marine ice sheet instabilities. Journal of Geophysical Research: Earth Surface. doi:10.1029/2020jf005755

Wickert, A. D., Williams, C., Gregoire, L. J., Callaghan, K. L., Ivanovic, R. F., Valdes, P. J., . . . Jennings, C. E. (2023). Marine-Calibrated Chronology of Southern Laurentide Ice Sheet Advance and Retreat: ∼2,000-Year Cycles Paced by Meltwater–Climate Feedback. Geophysical Research Letters, 50(10). doi:10.1029/2022GL100391