Forward modelling of past abrupt climate transitions

Ocean circulation plays an important role in redistributing heat around the Earth and regulating its climate. However, there remains considerable uncertainty over the interactions between ocean and climate during abrupt transitions that are known to have occurred in the past. One such example is the rapid warming and sudden change in ocean circulation associated with the start of the Bolling-Allerod around 14.6 thousand years ago. The proxy-archives used to understand these shifts (including chemical ocean circulation tracers such as d18O, d13C, radiocarbon, Pa/Th and eNd) seems to disagree at times, suggesting that we are missing fundamental knowledge for interpreting this data. In addition, it remains unknown what chain of events led to these abrupt changes; were they forced by gradual changes in atmospheric CO2? Was a critical threshold crossed in ice-sheet dynamics, atmospheric physics or ocean circulation? To what extent were the concurrent shifts in atmospheric and oceanic processes linked, or were they entirely separate? With this limited understanding, we cannot know if similar rapid transitions could occur again in the near future; our projections could be lacking crucial mechanistic understanding of climate and ocean dynamics. Great advancements have recently been made in both the fields of palaeoclimate reconstruction and climate modelling.

However, we are reaching the limits of what can be achieved by applying these specialist methods in relative isolation. I propose to make the next logical step and combine these fields of expertise in this fellowship. Using state of the art facilities, this fellowship aims to incorporate cutting-edge knowledge of ocean circulation proxies into some of the most powerful tools for understanding climate dynamics; high complexity atmosphere-ocean general circulation models, the same IPCC-class models that are used to make projections of future change. The exciting multi-proxy, multi-modal approach is designed for robustness and to explore uncertainty in both the methods employed and the results produced. With these new multidisciplinary tools, I hope to not only answer specific questions such as 'what caused the abrupt Bolling-Allerod warming?' and 'was the deglacial ocean ever in a bistable mode?', but also to equip earth system scientists with an improved, more holistic understanding of climate-ocean interactions that can be applied to a range of timescales for past, present and future scenarios. This fellowship answers a call in the field of climate science for multidisciplinarity.

The scientific community has expressed enthusiasm and excitement for the development of these tools, the expertise that I am training in and the chance to shift some of the existing paradigms of past climate-ocean dynamics. In order to achieve these goals, I have built a network of collaborators who are world leaders in their specific fields of research; Valdes and Otto-Bliesner in GCM modelling (respectively expert in the UM and CESM models), McManus in Pa/Th, Robinson in radiocarbon, Barker in d18O and d13C, Gutjahr in eNd and essentially, all collaborators have overlapping expertise. The collaborators have been involved in developing some of the hypotheses tested in this fellowship and will make world-leading facilities available to me, helping to keep the project at the cutting-edge of current understanding. Their research groups will broaden my network and establish me as a high-profile, internationally-renowned researcher with unique and highly sought after expertise; the ideal foundation for crafting my own research group.

Aims

This project focuses principally on climate-ocean interactions taking place around 21 to 12 thousand years ago.

Falling in the early last deglaciation, this encompasses the duration of several rapid climate events, for example:

• Heinrich Event 1, when the Northern Hemisphere climate was relatively cool and armadas of icebergs were released into the North Atlantic
• The abrupt Bolling warming event, when Northern Hemisphere temperatures are thought to have warmed by as much as 4-5 degrees C in just a few years-decades.
• Meltwater Pulse 1a, when sea level is thought to have risen by 12-22 m in < 350 years.

We know that these events took place, from geological records of climate, but exactly what caused them, and how they might be linked remains unknown.  The project will incorporate geochemical tracers of ocean circulation into a complex General Circulation Model (the UK Met Office’s HadCM3), to test different hypotheses for the causes and consequences of these abrupt climate events.