Beyond the instrumental record: Reconstructing Atlantic overturning over the past 7000 yrs (ReconAMOC)

The Atlantic meridional overturning circulation (AMOC) - part of the so-called 'ocean conveyor belt' - is a key component of Earth's climate system. It involves the northward transport of warm surface waters to the high latitude North Atlantic, where they cool (releasing heat to the atmosphere), sink and flow back southwards at depth. Changes in the AMOC are thought to alter global temperature and precipitation patterns, regional sea-level, and socio-economically important marine ecosystems.

There are concerns regarding the strength and stability of AMOC in the future. This is because predicted surface ocean warming and freshening could weaken the formation of dense water that helps drive the AMOC. Earlier research suggests that the AMOC may have different stable states, raising the possibility that the AMOC could rapidly switch to a weaker, or even an 'off', state, having a severe impact on global climate. IPCC models do not predict an abrupt weakening of the AMOC under typical 21st century scenarios; yet there are suggestions that current climate models may be excessively stable.

NERC and the international community have invested heavily in monitoring the AMOC, including the implementation of the RAPID array since 2004 and more recently the OSNAP array. Since observations began in 2004, AMOC has weakened at a rate ten times faster than predicted by most models. Yet the extent to which this decline can be attributed to natural multi-decadal variability is uncertain. The limited time span of the RAPID array means we are unable to gain an understanding of the nature of AMOC variability on timescales longer than interannual-to-decadal. Therefore we must turn to geological archives to reconstruct AMOC changes beyond the instrumental record. Yet there are no existing records to provide perspective on recent AMOC variability at multi-decadal and longer timescales.

Using recent, novel techniques to constrain past variability, coupled with exceptional sediment archives, ReconAMOC will constrain past AMOC variability on decadal to centennial timescales, generating records for the last 7000 years that will become benchmark constraints on AMOC behaviour. We will focus on the past 7000 years because the climate was not dramatically different to the present day, and remnant glacial ice sheets had melted away so that the major features of deep Atlantic circulation were broadly similar to modern.

ReconAMOC deploys a twin approach that utilizes (i) the characteristic subsurface temperature AMOC fingerprint, and (ii) the deep western boundary current response to AMOC change. We have verified these new paleoclimate approaches against variability in the instrumental record and demonstrated their applicability through an extensive pilot study. ReconAMOC is therefore a low risk yet ambitious project, bringing together an international team of collaborators, that will meet a long-sought and much-needed requirement of a wide range of climate scientists and modellers.

ReconAMOC will enable testing and improvement of model simulations of AMOC that help facilitate assessment of the vulnerability of the AMOC to climate change, and permit the investigation of the role of AMOC on other components of the climate system. The topics addressed by ReconAMOC are key research targets at national UK (e.g. identified strategic science themes and goals within the NERC strategy) and international (e.g. CMIP6, IMAGESII, SCOR, PAGES, IODP and NSF) levels. Specifically, the ReconAMOC proposal builds on the NERC programmes RAPID, RAPID-WATCH, and RAPID-AMOC, in which interannual to multi-decadal variability in the AMOC is a central focus, as well as NERC programme ACSIS examining interannual to decadal climate variability in the Atlantic.

Who will benefit, and how, from this research?

1. Policy makers and associated stakeholders (including environmental organisations and private corporations) involved in marine and climate strategy decisions over a range of timescales.

Future projections of climate change (derived from global climate models) are used to develop not only climate policy, but also a wide range of other long-term strategies - such as for the management of marine ecosystems and resources. It is necessary to assess whether these projections adequately capture the instability of the AMOC and the abrupt climate change that could ensue as a result of passing tipping points. At present there is a relatively limited suite of proxy reconstructions that are available for this task and there is a crucial need to expand this offering, so that a fuller understanding of the complex mechanisms and feedbacks acting throughout the climate system can be gained and predictive capabilities improved. The ReconAMOC project will provide previously unavailable information on the role that the AMOC plays in decadal to millennial scale climate change. The reconstructions produced will help validate and improve the climate models that are used to predict future climate change and its impacts (e.g. CMIP6 models). Hence, they are of direct policy relevance for national and international governmental climate change programmes, and will help provide society with the information it requires to manage the future global environment.

2. School students.

Understanding past climate changes now forms a part of the Key Stage 3 curriculum, and the expertise of university researchers can be used to help inform and educate school students and their teachers on areas of topical research. By working with schools to help educate pupils, university researchers can help enthuse a younger generation about Earth's climate system and climate change. ReconAMOC will develop teaching tools through collaboration with two schools that can then be disseminated more widely to aid the education of upper school Geographers.

3. General public understanding of science

The widespread international media coverage (TV, radio, press) of our recent publication (Thornalley et al., 2018, Nature) demonstrates that we can communicate to the lay community that high-resolution investigations into global changes in the recent past can be exciting and relevant. We will show how changes in the circulation of the Atlantic play an important role in controlling societally relevant, decadal to century time scale variability in the climate system.