This PhD project will use the HadCM3 climate model and state-of-the-art reconstructions of Northern Hemisphere ice sheets to investigate the causes of abrupt climate changes during the last deglaciation (21,000-11,000 years ago). By combining geological evidence, ice-sheet modelling and climate simulations, the project will determine why abrupt climate events such as the Bølling warming and Younger Dryas occurred when they did and improve our understanding of the mechanisms behind rapid climate tipping events.

Background

The last deglaciation marked Earth’s transition from an ice age to the present warm climate. Although this transition was driven by slow changes in Earth’s orbit and atmospheric greenhouse gas concentrations, the climate system responded in a highly irregular manner. Around 14,700 years ago, temperatures around the North Atlantic region rose abruptly during the Bølling warming. A little over a thousand years later, the climate rapidly returned to near-glacial conditions during the Younger Dryas before warming equally suddenly into the present Holocene. These dramatic shifts occurred within decades to centuries and were accompanied by major reorganisations of ocean circulation.

Evidence from geological records suggests that changes in the Atlantic Meridional Overturning Circulation (AMOC) played a central role in these abrupt climate transitions. Freshwater released by melting Northern Hemisphere ice sheets is thought to have influenced the strength and stability of this circulation. However, the magnitude, timing, and routing of meltwater delivery remain poorly constrained, making them among the largest uncertainties in simulations of deglacial climate change.

Recent work by the Leeds Climate-Ice group produced spontaneous abrupt climate changes in HadCM3 simulations of deglacial conditions. While this shows the model contains key mechanisms, the oscillations occur at the wrong frequency and timing compared with geological evidence. This suggests the model’s sensitivity to meltwater forcing is broadly correct, but uncertainties in ice-sheet geometry and freshwater delivery strongly influence when abrupt transitions occur.

Our group has produced some of the most advanced numerical simulations of Northern Hemisphere ice-sheet evolution during the last deglaciation, enabling us to evaluate the range of plausible ice-sheet topographies and meltwater routing evolution. Together, these developments provide a unique opportunity to identify the conditions that triggered abrupt climate change during the last deglaciation.

Aim and Objectives

The overall aim of this project is to determine how uncertainties in ice-sheet evolution and meltwater delivery influence the timing and occurrence of abrupt climate changes during the last deglaciation.

Chapter/Paper 1: Developing plausible deglacial ice-sheet and meltwater scenarios

Scenarios will be developed using geological reconstructions and outputs from state-of-the-art ice-sheet models. They will explore uncertainties in:

• Ice-sheet topography and retreat history.
• Magnitude and temporal variability of meltwater discharge.
• Timing of major drainage reorganisations and river rerouting events.

Chapter/Paper 2: Simulating abrupt climate change with HadCM3

The ice-sheet and meltwater scenarios will be tested using the HadCM3 climate model. The simulations will evaluate how different freshwater forcing histories influence:

• The timing and frequency of events.
• Changes in AMOC strength.
• Regional patterns of warming and cooling.

Chapter/Paper 3: Comparison with geological evidence and model evaluation

Model simulations will be compared with a wide range of palaeoclimate records, including temperature, ocean circulation, geochemistry and hydroclimate reconstructions. The project will identify the combinations of ice-sheet geometry and freshwater forcing that best reproduce observed changes. An alternative set of experiments may systematically vary model inputs beyond geological constraints to determine whether the model itself has incorrect sensitivity to key forcings.

Research questions

• What controls the timing and frequency of abrupt AMOC changes?
• Can realistic ice-sheet histories reproduce the observed Bølling warming and Younger Dryas?

 

Potential for high impact

Abrupt climate changes during the last deglaciation provide one of the best natural laboratories for understanding climate tipping points. By identifying the conditions that triggered the Bølling warming and Younger Dryas, this project will improve our understanding of the stability of the Atlantic overturning circulation and the role of ice-sheet meltwater in driving rapid climate change. The resulting ice-sheet and meltwater reconstructions will be valuable resources for the international palaeoclimate community, while the model evaluation framework developed by the project will help improve confidence in the next generation of climate models used to assess future climate risks.