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Heat and carbon coupling reveals ocean warming due to circulation changes

Nature volume 584, pages 227–233 (2020)Cite this article

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A Publisher Correction to this article was published on 02 October 2020

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Abstract

Anthropogenic global surface warming is proportional to cumulative carbon emissions1,2,3; this relationship is partly determined by the uptake and storage of heat and carbon by the ocean4. The rates and patterns of ocean heat and carbon storage are influenced by ocean transport, such as mixing and large-scale circulation5,6,7,8,9,10. However, existing climate models do not accurately capture the observed patterns of ocean warming, with a large spread in their projections of ocean circulation and ocean heat uptake8,11. Additionally, assessing the influence of ocean circulation changes (specifically, the redistribution of heat by resolved advection) on patterns of observed and simulated ocean warming remains a challenge. Here we establish a linear relationship between the heat and carbon uptake of the ocean in response to anthropogenic emissions. This relationship is determined mainly by intrinsic parameters of the Earth system—namely, the ocean carbon buffer capacity, the radiative forcing of carbon dioxide and the carbon inventory of the ocean. We use this relationship to reveal the effect of changes in ocean circulation from carbon dioxide forcing on patterns of ocean warming in both observations and global Earth system models from the Fifth Coupled Model Intercomparison Project (CMIP5). We show that historical patterns of ocean warming are shaped by ocean heat redistribution, which CMIP5 models simulate poorly. However, we find that projected patterns of heat storage are primarily dictated by the pre-industrial ocean circulation (and small changes in unresolved ocean processes)—that is, by the patterns of added heat owing to ocean uptake of excess atmospheric heat rather than ocean warming by circulation changes. Climate models show more skill in simulating ocean heat storage by the pre-industrial circulation compared to heat redistribution, indicating that warming patterns of the ocean may become more predictable as the climate warms.

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Fig. 1: Simulated anthropogenic changes in Cant and H.
Fig. 2: Heat–carbon coupling.
Fig. 3: Error in the redistributed ocean heat storage from ESM2M.
Fig. 4: Redistribution of ocean heat storage in the upper 2,000 m.

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Data availability

The code of the GFDL ESM2M model is available from https://github.com/mom-ocean and the results from the free-circulation and fixed-circulation 1%CO2 simulations are available from https://doi.org/10.1594/PANGAEA.914424. CMIP5 model data are available from the Earth System Grid Federation data portal at https://esgf-node.llnl.gov/projects/cmip5/. The historical ocean temperature reanalysis23 from is available at http://159.226.119.60/cheng/. The estimate of historical ocean anthropogenic carbon24 is available at https://tdevries.eri.ucsb.edu/models-and-data-products/. The observed ocean heat uptake10 is available at https://laurezanna.github.io/publication/zanna-et-al-2017b/. The MITgcm code is available at http://mitgcm.org/. The GLODAPv2 synthesis product is available at https://www.nodc.noaa.gov/ocads/oceans/GLODAPv2/. The topographical data used in Figs. 1, 3, 4 and Extended Data Figs. 1, 5, 8–10 are available in MATLAB and provided by NOAA64. The CMIP5 and observational estimates of total, added and redistributed heat are available at https://laurezanna.github.io/publication/bronselaer-zanna-2020/.

Change history

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Acknowledgements

We thank M. Winton for providing the ESM2M simulations. We acknowledge the MITgcm team for making their code publicly available, the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP5, and the climate modelling groups for producing and making available the output of their models. We thank R. Keeling for comments. B.B. was supported by a NERC CASE studentship with the Met Office. L.Z. thanks NERC NE/P019218/1, NE/R000727/1, and Princeton University, AOS and GFDL for additional support during part of this work. This work made use of the facilities of HECToR and Archer.

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Authors and Affiliations

  1. Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK

    Ben Bronselaer & Laure Zanna

  2. Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA

    Ben Bronselaer

  3. Department of Geosciences, University of Arizona, Tucson, AZ, USA

    Ben Bronselaer

  4. Program in Atmospheric and Oceanic Sciences, Princeton University, Princeton, NJ, USA

    Ben Bronselaer

  5. Courant Institute of Mathematical Sciences, New York University, New York, NY, USA

    Laure Zanna

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B.B. and L.Z. conceptualized the study, analysed the data and wrote the manuscript. B.B. performed the MITgcm simulations.

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Correspondence to Ben Bronselaer.

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Extended data figures and tables

Extended Data Fig. 1 Physical changes in ESM2M experiments.

a, b, Using the free-circulation model, the fixed-layer depth of the pre- industrial control mean (a) and the change in mixed-layer depth (relative to the control) (b), for years 61–80 of the 1%CO2 free circulation. c, Years 61–80 of the 1%CO2 fixed-circulation simulation. In the fixed-circulation experiment, north of 30° N in the North Atlantic Ocean, there is an average reduction in mixed-layer depth of 15%, where mixed layer depths were deeper than 200 m in the control mean. d–f, As in a–c, but for the meridional overturning streamfunction, where 1 Sv = 106 m3 s−1. g–i, As in a–c, but for the barotropic streamfunction.

Extended Data Fig. 2 The heat–carbon coupling parameter, α.

a, α, obtained in ESM2M using α(t) = Ĥ(t)/Ĉant(t) (pink), and as estimated using equation (14) for \(\tilde{\alpha }\) (grey line). The green line shows the magnitude of the first term of the right-hand side of equation (14), and the blue line shows the second term of the right-hand side of equation (14), which is related to the temperature-feedback term. The grey shading shows the error in the estimate that is due to the assumption of linearity and to various other processes and feedback mechanisms ignored, such as land carbon uptake. The negative values of α and \(\tilde{\alpha }\) at the start of the experiment are due to natural variability when the external forcing is small (giving a low signal-to-noise ratio). b, α(t) = Ĥ(t)/Ĉant(t), relative to the years 1870–1890 in CMIP5 historical/RCP8.5 simulations, with the mean given by the thick pink line and the range given by the pale yellow area. Thin lines show individual models (ESM2M is shown in green for reference) and dashed lines show the models that simulate ocean heat loss in the mid-twentieth century that are excluded from the mean and range. The blue solid line shows the observations (using ref. 10 for heat) and its blue shading shows the 66% confidence interval from observations.

Extended Data Fig. 3 ESM2M zonal-mean ocean redistributed warming.

a, b, The year 61–80 zonal-mean ocean redistributed heat, here expressed as the zonal-mean temperature change, and calculated using the diagnosed fields from the ESM2M fixed-circulation experiment (a), and using the expression Had(y, z, t) = α(t)Cant(y, z, t) (b). The parameters (y, z, t) indicate the zonal-mean quantities instead of vertical integrals, and α(t) is calculated globally as before, except here divided by volume, density and specific heat capacity so that values are given in terms of temperature instead of heat capacity. c, The difference between a and b; the black lines on the z = 0 axis show the mean sea-ice extent.

Extended Data Fig. 4 1951–2011 zonal-mean ocean redistributed warming.

Ocean redistributed heat Had(y, z, t) = α(t)Cant(y, z, t) in terms of temperature instead of than heat. Changes are shown as the linear trends over the indicated periods multiplied by the length of the period. a, b, The results from observational estimates for the Atlantic basin (a) and the Pacific basin (b). c, d, Changes in the CMIP5 historical RCP8.5 ensemble. The black horizontal line indicates the 2,000 m depth level above which we show the observations. The hatching indicates where the changes are not significant at the 66% uncertainty level. For the observations, the uncertainty is from the estimates of H and Cant, whereas for the CMIP5 models the uncertainty is from the mean of the ensemble due to inter-model spread.

Extended Data Fig. 5 CMIP5 ocean redistributed heat.

a, Average 1951–2011 Hr for the three CMIP5 models (CNRM-CM5, GFDL ESM2M and GFDL ES2MG) with a value of Hr that best match the observational reconstruction. b, Average 2011–2060 Hr for the same models as in a. The hatching indicates where the changes are not significant at the 66% uncertainty level; the uncertainty is from the mean of the ensemble due to inter-model spread.

Extended Data Fig. 6 Latitudinal profiles of ocean carbon and CFCs.

a, ESM2M zonal-mean contribution of DIC (green), DIC+ALK (blue) and DIC+ALK+T (red) towards the change in ocean surface \({p}_{{{\rm{CO}}}_{2}}\). Changes in \({p}_{{{\rm{CO}}}_{2}}\) (\(\varDelta {p}_{{{\rm{CO}}}_{2}}\)), are roughly equal to \((\partial {p}_{{{\rm{CO}}}_{2}}/\partial {\rm{DIC}})\varDelta {\rm{DIC}}\). b, ESM2M zonal-mean mixed layer change, relative to the pre-industrial control, in ∆DIC (green), fixed-circulation added temperature ∆Tad (red), and ∆CFC 12 (purple). The blue line shows the sensitivity term \(\partial {p}_{{{\rm{CO}}}_{2}}/\partial {\rm{DIC}}\). c, ESM2M zonally integrated Cant (green), CFC 12 (purple) and Had (red) storage in the upper 2,000 m relative to the pre-industrial control. Solid single lines show year 70 from the fixed-circulation 1%CO2 and double lines show year 2060 of the ESM2M RCP8.5 simulation. d, ESM2M \(\partial {p}_{{{\rm{CO}}}_{2}}/\partial {\rm{DIC}}\) with a latitudinally varying temperature (solid) and a latitudinally constant temperature (dotted). e, Observed zonally integrated Cant (green) and CFC 11 (purple) storage in the upper 2,000 m in year 2017, from ref. 24. The quantities in b–d have been scaled by the global mean value for each tracer, and the blue line in b has been offset upwards by a scaled value of 1 for clarity.

Extended Data Fig. 7 Latitudinal \({p}_{{{\rm{CO}}}_{2}}\) changes.

a, b, Zonal-mean contribution of DIC (green), DIC + ALK (blue) and DIC + ALK + T (total; red), towards the change in ocean surface \({p}_{{{\rm{CO}}}_{2}}\) at year 70 of the 1%CO2 simulation with IPSL-CM5A-LR, relative to the pre-industrial control (a), and at year 2011 best-estimate from observations, relative to year 1765, using GLODAPv265,66,67 for mean-state fields and ∆DIC (Cant) from ref. 24. c, d, Zonal-mean contribution of ∆DIC (green) and \(\partial {p}_{{{\rm{CO}}}_{2}}/\partial {\rm{DIC}}\) (blue) towards ocean surface \({p}_{{{\rm{CO}}}_{2}}\) changes at year 70 of the 1%CO2 simulation with IPSL-CM5A-LR (c), and year 2011 from observations (using GLODAPv2 for mean states fields37 and ∆DIC from ref. 24; d). The thin lines in b and d show the 66% confidence interval. \({p}_{{{\rm{CO}}}_{2}}\) values from observations are calculated using CO2SYS68. The correlation coefficients between ∆DIC and \(\partial {p}_{{{\rm{CO}}}_{2}}/\partial {\rm{DIC}}\) are −0.88 and −0.82 for IPSL-CM5A-LR and observations, respectively.

Extended Data Fig. 8 Fixed-climate Cant.

a–c, Year 61–80 mean upper 2,000 m Cant in the fixed-climate (a), fixed-circulation (b) and free-circulation (c) 1%CO2 ESM2M simulations. In the fixed-climate simulation, atmospheric CO2 is treated as a non-radiative gas29. The correlation coefficients of the fixed-circulation and fixed-climate Cant, with free-circulation Cant as shown, are 0.98 and 0.98, respectively. d, The difference between the free-circulation and fixed-climate experiments. e, The difference between the fixed-circulation and fixed-climate experiment. f, The difference between the free-circulation and fixed-circulation experiments. g–l, As in a–f, but for zonal-mean Cant.

Extended Data Fig. 9 MITgcm ocean circulation.

a, b, The mean-state (a) and overturning (b) barotropic streamfunctions in the MITgcm control simulation, where 1 Sv = 106 m3 s−1. c, d, The perturbed (c) and overturning (d) barotropic streamfunctions in the MITgcm climate change simulation after 70 yr of warming (taken as the mean of years 61–80).

Extended Data Fig. 10 MITgcm simulated anthropogenic tracer changes.

a–d, Upper 2,000 m column-integrated anomalies in the MITgcm climate change experiments, for free-circulation carbon (a), free-circulation heat (b), fixed-circulation carbon (c), and fixed-circulation heat (d). e, f, The re-distributions of carbon (e) and heat (f) are taken as the difference between the free-circulation and fixed-circulation anomalies. Anomalies are taken as the difference between years 61–80 of the perturbed run and a tandem control. g, h,  The fixed-circulation heat estimated using only the free-circulation experiment quantities (g), and the corresponding estimated redistributed heat (h).

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Bronselaer, B., Zanna, L. Heat and carbon coupling reveals ocean warming due to circulation changes. Nature 584, 227–233 (2020). https://doi.org/10.1038/s41586-020-2573-5

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