Assessing the role of millennial-scale variability in glacial-interglacial climate change

Earth's climate varies on timescales ranging from decades to tens of millions of years. Once such mode of variability is that related to changes in the Earth's orbit around the Sun. This is known as 'orbital-timescale' variability and has characteristic timescales of tens to hundreds of thousands of years, giving rise to the well known glacial cycles of the Late Pleistocene. Superimposed on this glacial-interglacial variability is another mode of climate change, known as 'millennial-scale' climate variability (characterised by changes on a timescale of hundreds to a few thousands of years). Both of these modes of climate variability have received significant scientific enquiry because they involve major changes in global climate and yet both remain enigmatic in their underlying mechanisms. However, recent studies have suggested that these apparently separate mechanisms may in fact be intimately related. As such, improving our understanding of one should promote understanding in the other. Here we seek to investigate the potential role of millennial-scale climate variability in the wider changes associated with glacial-interglacial climate change. Specifically we will examine the effects that occur in response to abrupt changes in ocean/atmosphere circulation that may play a role in the transition from glacial to interglacial climate (such as the last deglaciation, which occurred between 20 and 10 thousand years ago).

It is thought that changes in ocean circulation and related atmospheric phenomena can give rise to dramatic temperature fluctuations such as those recorded by Greenland ice cores during the last glacial and deglacial periods. Of note is the corresponding temperature variations recorded across Antarctica, which suggest that the climate system may act like a sort of seesaw; when circulation is strong, Greenland (and north western Europe) is warm and Antarctica cools. A weakened circulation gives rise to cold conditions across Greenland while warming occurs across Antarctica. An important side effect of this so-called 'bipolar seesaw' is that atmospheric carbon dioxide appears to rise every time the circulation is in a weakened state. Of particular relevance to this proposal is the rise in carbon dioxide that occurred during the last deglaciation, which was associated with a distinct oscillation of the bipolar seesaw. Moreover, several other seesaw oscillations occurred during the last glacial period, which also gave rise to increases in carbon dioxide but did not lead to deglaciation.

We wish to find out why certain bipolar seesaw oscillations (terminal oscillations) apparently lead to deglaciation while others (non-terminal oscillations) do not. Is there anything special about these events or is their affiliation with deglaciation merely coincidence? In order to answer to this question we will combine quantitative data analysis with state-of-the-art computer models of the climate system. We will analyse climate records spanning several glacial cycles in order to provide a statistical representation of 'terminal' and 'non-terminal' oscillations of the bipolar seesaw. We will then use computer models to investigate how the seesaw operates under a variety of background conditions. Our ultimate goal is to find out what, if anything, makes terminal oscillations special. In so doing we will provide important constraints on the mechanism of deglaciation.

The beneficiaries of this research will be:

National and international policy makers on environmental and energy matters

The proposed research will be of most significant benefit to other scientists. However, it is clear that increasing our understanding of the mechanisms of abrupt climate change can and has played an important role in increasing political and public awareness of the potential risks associated with climate change in the future. We do not claim that the proposed work will lead directly to new policies but it is likely that it will contribute to the ongoing development of national and global environmental policy. For example, the most recent reports of the Intergovernmental Panel on Climate Change (IPCC) have included significant sections on paleoclimate. This has arisen due to the acknowledged need to understand how the climate system works and the ability to decipher past changes is central to that understanding. More specifically the low probability / high impact scenario of an abrupt and significant change in Atlantic Ocean circulation occurring in the near future has provoked an increased need for understanding when and how these events might occur and what influence they might have on the wider climate system. We see the proposed research feeding into the growing base of knowledge that will ultimately allow us to predict whether or not an event like this might occur and when. In this way our research will contribute to policy on a broad scale, that will be directed towards environmental stewardship and energy responsibility.

The wider public will benefit from the research both in terms of its inherent interest (the fascination aspect) and also for those with an interest in contemporary environmental (climate) issues and environmental policy.