14 Jan 2020

14 Jan 2020

Review status: a revised version of this preprint is currently under review for the journal CP.

Data-constrained assessment of ocean circulation changes since the middle Miocene in an Earth system model

Katherine A. Crichton1,a, Andy Ridgwell2, Daniel J. Lunt3, Alex Farnsworth3, and Paul N. Pearson1 Katherine A. Crichton et al.
  • 1School of Earth and Ocean Sciences, Cardiff University, UK
  • 2Department of Earth and Planetary Sciences, Universityof California, Riverside, CA 92521, USA
  • 3School of Geographical Sciences, University of Bristol, Bristol, UK
  • anow at: School of Geography, University of Exeter, EX4 4RJ, UK

Abstract. Since the middle Miocene, 15 Ma (million years ago), the Earth’s climate has undergone a long-term cooling trend, characterised by a reduction in sea surface temperatures by over 6 °C, with 4 to 6 °C cooling occurring in the deep ocean. The causes of this cooling are primarily thought to be linked to changes in ocean circulation due to tectonic plate movements affecting ocean seaways, together with and a drop in atmospheric greenhouse gas forcing (and attendant ice-sheet growth and feedback). In this study we assess the potential to constrain, using marine sediment proxy data, the evolving patterns of global ocean circulation and cooling of surface climate over the last 15 million years (Ma) in an Earth system model. We do this by compiling surface and benthic ocean temperature and benthic carbon-13 data in a series of seven time-slices spaced at approximately 2.5 million year intervals. We pair this with a corresponding series of seven tectonic and surface climate boundary condition reconstructions in the cGENIE (muffin release) Earth system model. In the cGENIE model, we adjust atmospheric CO2 together with the magnitude of North Pacific to North Atlantic salinity flux adjustment in a series of 2D parameter ensembles in order to match global temperature and benthic δ13C patterns in the model to the data. We identify that a relatively high CO2 equivalent forcing of 1120 ppm is required at 15 Ma in cGENIE to reproduce proxy temperature estimates in the model, noting that this CO2 forcing is dependent on cGENIEs climate sensitivity (which is as the present day) and that it incorporates the effects of all greenhouse gases. The required CO2 forcing progressively reduces throughout the subsequent six time slices delineating the observed long-term cooling trend. In order to match the evolving patterns of the proxy data, we require fundamental change in the mode of ocean circulation at 12.5 Ma with present-day-like benthic δ13C trends established by 10 Ma. We also find a general increasing strength of Atlantic overturning despite a reduction in salinity of the surface North Atlantic over the cooling period, attributable to falling intensity of the hydrological cycle and polar cooling caused by CO2-driven global cooling.

Katherine A. Crichton et al.

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Status: final response (author comments only)
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Katherine A. Crichton et al.

Model code and software

cGENIE.muffin release v0.9.8 A. Ridgwell

Katherine A. Crichton et al.


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Short summary
15 Ma (million years) ago the Earth was around 4 to 6 °C warmer than the present. We investigate the causes of global cooling over this period by comparing our model, cGENIE, to proxy data for temperature and circulation at seven time periods. We conclude that a gradual falling CO2 over the last 15 Ma drove polar cooling that, combined with changes in surface N.Atlantic salinity, resulted in the onset and strengthening of N.Atlantic overturning that today dominates global circulation.