Earth’s climate cooled markedly during the late Miocene from 12 to 5 million years ago, with far-reaching consequences for global ecosystems. However, the driving forces of these changes remain controversial. A major obstacle to progress is the uncertainty over the role played by greenhouse gas radiative forcing. Here we present boron isotope compositions for planktic foraminifera, which record carbon dioxide change for the interval of most rapid cooling, the late Miocene cooling event between 7 and 5 Ma. Our record suggests that CO2 declined by some 100 ppm over this two-million-year-long interval to a minimum at approximately 5.9 Ma. Having accounted for non-CO2 greenhouse gasses and slow climate feedbacks, we estimate global mean surface temperature change for a doubling of CO2—equilibrium climate sensitivity—to be 3.9 °C (1.8–6.7 °C at 95% confidence) on the basis of comparison of our record of radiative forcing from CO2 with a record of global mean surface temperature change. We conclude that changes in CO2 and climate were closely coupled during the latest Miocene and that equilibrium climate sensitivity was within range of estimates for the late Pleistocene, other intervals of the Cenozoic and the twenty-first century as presented by the Intergovernmental Panel on Climate Change.
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The experimental data are available at https://doi.org/10.5258/SOTON/D2258.
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We are grateful to the staff at the Bremen core repository and the ΙODP for providing sample material. We also thank M. Spencer, B. Hambach, J. A. Milton, M. Cooper, and A. Michalik as well as the rest of the ‘B-team’ for laboratory assistance. This work was supported by Natural Environment Research Council grants NE/P011381/1 to G.L.F., P.A.W. and T.B.C. and NE/L002531/1 to R.M.B., the Leverhulme Trust through support to T.B.C. and the Royal Society (Challenge Grant to P.A.W. and A.J.C.) and Wolfson Merit awards to P.A.W. and G.L.F.
The authors declare no competing interests.
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a) Map of mean annual modern ΔCO2 (parts per million, ppm) between surface ocean and the atmosphere with sample site ODP 926 (3° 43’16.49” N and 42° 54’47.83” W, and water depth of 3598 m). Data from ref. 60. b) Map of the sites used to generate the ΔSST stack with mean annual modern SST from Levitus World Ocean Atlas database. See supplementary Table 3 for locations and depths of ΔSST sample sites. Figures constructed and data visualised in Ocean Data View138.
Data collected from ODP 926, ODP 1000 (red square), ODP 761 (red diamond) and ODP 872 (red triangle). Error bars represent 2sd.
Extended Data Fig. 3 Sensitivity of atmospheric carbon dioxide and ΔFCO2 estimates to δ11B for seawater.
Reconstructed CO2 (left axis, blue lines) and ΔFCO2 (right axis, orange lines) for time intervals before (6.5 Ma), during (5.9 Ma) and after (5.0 Ma) Late Miocene temperature minimum using modern DIC (Top) at ODP 926 (2027 (±250) µmol kg−1, ref. 108) and Neogene DIC reconstructions (Bottom, ref. 10).
Alkenone derived ΔSST records used in ΔSST stack. Recalibrated using BAYSPLINE method from ref. 43.
Each ΔSST record is sequentially removed from the stack to ensure that no one record has undue weight on the stack.
a) Map of the sites used to generate the ΔGMAST stack with mean annual modern SST from ref. 60 for the Late Miocene and last 100kyr stacks. See Supplementary Table 3 for locations of ΔSST sample sites and references. Figures constructed and data visualised in Ocean Data View138 with modern SST data from Levitis Ocean Atlas Data base. b) ΔGMAST stacked record using the same sites as used in the late Miocene Stack for the last 100 kyr (dark blue error band, 68% of Monte Carlo simulations; light blue error band, 95% of Monte Carlo simulations) and from ref. 51 (dark red error band, 1sd; light red error band, 2sd). Change in LGM ΔGMAST for recent proxy compilation (black dashed line)132.
(Top) Forced alignment between ΔFCO2 (orange) and ΔGMAST stack (blue). Light blue/orange and dark blue/orange represent 95% and 68% uncertainty as calculated in Monte Carlo simulations described in methods for ΔFCO2 and ΔGMAST respectively. (Bottom) SIMEX regression91 of aligned ΔFCO2 and ΔGMAST. Error bars represent 2sd error. Black line represents SIMEX regression. Dashed blue line represent error on SIMEX regression.
Supplementary Figs. 1–9.
Reports information relevant to site location, sampling resolution and references to sources of original temperature measurements for late Miocene SST stack.
Data for stable isotopes (δ11B, δ13C and δ18O), elemental ratios (Al/Ca, Mg/Ca and B/Ca) and pH and CO2 estimates based on δ11B.
Compilation of key equilibrium climate sensitivity studies.
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Brown, R.M., Chalk, T.B., Crocker, A.J. et al. Late Miocene cooling coupled to carbon dioxide with Pleistocene-like climate sensitivity. Nat. Geosci. 15, 664–670 (2022). https://doi.org/10.1038/s41561-022-00982-7