Intra-interglacial variability: are warmer periods climatically more unstable?

With current high-latitude warming and accelerated freshwater input into the North Atlantic expected to weaken the Atlantic Meridional Overturning Circulation (AMOC), there is a pressing need to consider the stability of AMOC under conditions of 'excess warmth'. While present monitoring does not span a sufficiently long period, past interglacials can be interrogated to gain insights into AMOC changes and attendant climate impacts.

A prominent example is the Last Interglacial (broadly equivalent to Marine Isotope sub-Stage [MIS] 5e), characterized by peak global mean temperature ~0.8degC above pre-industrial values, intense Arctic warming and global sea-level ~6-9 m above present, with 0.6-3.5 m derived from melting of the Greenland Ice Sheet (GrIS). A recent study (Tzedakis et al., 2018), has shown that the Last Interglacial was punctuated by centennial-scale cold water-mass expansions in the North Atlantic and arid events in S. Europe and suggested that Greenland ice-melt may have contributed to episodic AMOC weakening. By comparison, climate variability during our current interglacial (Holocene, MIS 1) has been relatively subdued. This raises two important questions: are periods warmer than the Holocene climatically more unstable and is this variability related to AMOC changes?

To address these questions, we propose to examine a set of 'natural experiments': cool interglacials MIS 7a-c and 7e, and warm interglacial MIS 9e and compare them with MIS 5e and 1. Studies have indicated warmer conditions off southern Greenland and extensive GrIS retreat during MIS 5e and 9e, and minimal retreat of the southern GrIS during MIS 1 and 7.

The proposed project (VARING) will document the extent of centennial-scale climate variability during these interglacials and explore its origin. We will construct a wide spatio-temporal framework from a network of sites: (1) five North Atlantic records of changes in ocean surface and deep-water conditions; (2) a long pollen sequence from central Italy with an independent chronology, based on dating of volcanic tephras, to provide evidence of the impact of North Atlantic variability on vegetation and precipitation regimes; (3) a 'rosetta stone' deep-sea sequence from the Portuguese Margin, linking marine and pollen records in the same core; (4) a North Atlantic - S. Europe stratigraphic lattice with a common chronological framework for the timing and duration of interglacial changes.

There are several possible outcomes to our study: (i) no intra-interglacial variability, which places constraints on conditions under which climate remains largely stable; (ii) surface-ocean only variability, leading to less northward oceanic heat transport without a weakening of the deep limb of the AMOC. (iii) surface- and deep-ocean changes, which implies AMOC involvement. By constraining the AMOC components, we will gain greater insight into how ocean circulation changes were occurring and their downstream effects over S. Europe.

VARING will go beyond the study of Tzedakis et al. (2018), by deriving records from multiple sites across multiple time intervals with different climate contexts to document changes and explore mechanisms. Ultimately, it will contribute to a step change in our understanding of climate variability across a range of states and will provide much needed constraints on the stability of AMOC under interglacial conditions and its linkage with S. European climate.

By documenting conditions in the North Atlantic and S. Europe during cooler and warmer interglacials, the proposed project aims to establish whether periods of excess warmth are climatically more unstable and to provide constraints on the stability of Atlantic ocean circulation.

Who could potentially benefit from this research?
Climate change is a topic that is not only a concern of the scientific community of palaeoclimate and global change scientists, but also of the public. The context, aims and findings of the proposed project will be of potential interest to students and the scientifically-minded portion of the public and media.

How might the potential beneficiaries benefit?
We will translate key climate change concepts and findings of our research into an accessible format to reach a wide audience and contribute to increased public awareness and understanding of climate science issues.

More specifically, recent studies indicate a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and suggest that continued freshening of the North Atlantic, as a result of changes in the hydrological cycle, sea-ice loss and accelerated melting of the Greenland Ice Sheet, will weaken the AMOC in the long term. According to the 5th Assessment Report of the IPCC, climate model projections suggest that "it is very likely that the AMOC will weaken over the 21st century", while "a collapse beyond the 21st century for large sustained warming cannot be excluded". Although changes could take centuries to occur, their potential climate impacts require validation. The record melting of Greenland ice in July 2019 adds an urgent dimension. There is therefore a pressing need to consider the stability of AMOC under conditions of 'excess warmth'. While present monitoring does not span a sufficiently long period, past interglacials can be interrogated to gain insights into AMOC changes and attendant climate impacts.

We propose to produce a series of short videos explaining past changes in Atlantic ocean circulation and abrupt climate variability with implications for future changes.