Radiocarbon observations suggest that the deep Atlantic Ocean takes up to several centuries to fully respond to changes at the sea surface. Thus, the ocean’s memory is longer than the modern instrumental period of oceanography, and the determination of modern warming of the subsurface Atlantic requires information from paleoceanographic data sets. In particular, paleoceanographic proxy data compiled by the Ocean2k project indicate that there was a global cooling from the Medieval Warm Period to the Little Ice Age over the years 900−1800, followed by modern warming that began around 1850. An ocean simulation that is forced by a combined instrumental-​proxy reconstruction of surface temperatures over the last 2,000 years shows that the deep Atlantic continues to cool even after the surface starts warming. As a consequence of the multicentury surface climate history, the ocean simulation suggests that the deep Atlantic doesn’t take up as much heat during the modern warming era as the case where the ocean was in equilibrium at 1750. Both historical hydrographic observations and proxy records of the subsurface Atlantic are needed to determine whether the effects of the Little Ice Age did indeed persist well after the surface climate had already shifted to warmer conditions.

Gebbie, G. 2019. Atlantic warming since the Little Ice Age. Oceanography 32(1):220–230, https://doi.org/10.5670/oceanog.2019.151.

Abram, N.J., H.V. McGregor, J.E. Tierney, M.N. Evans, N.P. McKay, D.S. Kaufman, K. Thirumalai, B. Martrat, H. Goosse, S.J. Phipps, and others. 2016. Early onset of industrial-era warming across the oceans and continents. Nature 536 (7617):411–418, https://doi.org/10.1038/nature19082.

Banks, H.T., and J.M. Gregory. 2006. Mechanisms of ocean heat uptake in a coupled climate model and the implications for tracer based predictions of ocean heat uptake. Geophysical Research Letters 33(7), L07608, https://doi.org/​10.1029/2005GL025352.

Bradley, R.S., and P.D. Jones. 1993. ‘Little Ice Age’ summer temperature variations: Their nature and relevance to recent global warming trends. The Holocene 3(4):367–376, https://doi.org/​10.1177/095968369300300409.

Caesar, L., S. Rahmstorf, A. Robinson, G. Feulner, and V. Saba. 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature 556(7700):191–196, https://doi.org/10.1038/s41586-018-0006-5.

Cappelen, J., ed. 2014. Greenland-DMI Historical Climate Data Collection 1784–2013—With Danish Abstracts. Technical Report 14-04, Ministry of Climate and Energy, Copenhagen, Denmark, 90 pp.

Deacon, G.E.R. 1937. The Hydrology of the Southern Ocean. Discovery Reports Series 15, Cambridge University Press, 124 pp.

DeVries, T., and F. Primeau. 2011. Dynamically and observationally constrained estimates of water-mass distributions and ages in the global ocean. Journal of Physical Oceanography 41(12):2,381–2,401, https://doi.org/​10.1175/JPO-D-10-05011.1.

Dickson, R.R., H.H. Lamb, S.-A. Malmberg, and J.M. Colebrook. 1975. Climatic reversal in northern North Atlantic. Nature 256:479–482, https://doi.org/​10.1038/256479a0.

Dickson, R.R., J. Meincke, S.A. Malmberg, and A.J. Lee. 1988. The “great salinity anomaly” in the northern North Atlantic 1968–1982. Progress in Oceanography 20:103–151, https://doi.org/​10.1016/​0079-6611(88)90049-3.

Durack, P.J., P.J. Gleckler, F.W. Landerer, and K.E. Taylor. 2014. Quantifying underestimates of long-term upper-ocean warming. Nature Climate Change 4(11):999–1,005, https://doi.org/10.1038/nclimate2389.

Fukumori, I. 2002. A partitioned Kalman filter and smoother. Monthly Weather Review 130(5):1,370–1,383, https://doi.org/​10.1175/​1520-0493​(2002)130​<1370:APKFAS>2.0.CO;2.

Gebbie, G., and P. Huybers. 2010. Total matrix intercomparison: A method for resolving the geometry of water-mass pathways. Journal of Physical Oceanography 40(8):1,710–1,728, https://doi.org/​10.1175/2010JPO4272.1.

Gebbie, G., and P. Huybers. 2011. How is the ocean filled? Geophysical Research Letters 38(6), https://doi.org/​10.1029/2011GL046769.

Gebbie, G., and P. Huybers. 2012. The mean age of ocean waters inferred from radiocarbon observations: Sensitivity to surface sources and accounting for mixing histories. Journal of Physical Oceanography 42(2):291–305, https://doi.org/​10.1175/​JPO-D-11-043.1.

Gebbie, G., and P. Huybers. 2019. The Little Ice Age and 20th-century deep Pacific cooling. Science 363(6422):70–74, https://doi.org/10.1126/science.aar8413.

Giese, B.S., and S. Ray. 2011. El Niño variability in simple ocean data assimilation (SODA), 1871–2008. Journal of Geophysical Research 116(C2), https://doi.org/​10.1029/​2010JC006695.

Giese, B.S., H.F. Seidel, G.P. Compo, and P.D. Sardeshmukh. 2016. An ensemble of ocean reanalyses for 1815–2013 with sparse observational input. Journal of Geophysical Research 121(9):6,891–6,910, https://doi.org/​10.1002/​2016JC012079.

Gleckler, P.J., P.J. Durack, R.J. Stouffer, G.C. Johnson, and C.E. Forest. 2016. Industrial-era global ocean heat uptake doubles in recent decades. Nature Climate Change 6:394–398, https://doi.org/​10.1038/nclimate2915.

Gouretski, V., and K. Koltermann. 2004. WOCE Global Hydrographic Climatology. Technical Report 35, Berichte des Bundesamtes für Seeschifffahrt und Hydrographie.

Haine, T.W.N., and T.M. Hall. 2002. A generalized transport theory: Water-mass composition and age. Journal of Physical Oceanography 32(6):1,932–1,946, https://doi.org/​10.1175/​1520-0485​(2002)032​<1932:​AGTTWM>​2.0.CO;2.

Helland-Hansen, B., and F. Nansen. 1909. The Norwegian Sea: Its Physical Oceanography Based upon the Norwegian Researches 1900–1904. Det Mallingske bogtrykkeri, 390 pp.

Huang, B., V.F. Banzon, E. Freeman, J. Lawrimore, W. Liu, T.C. Peterson, T.M. Smith, P.W. Thorne, S.D. Woodruff, and H.-M. Zhang. 2015. Extended reconstructed sea surface temperature version 4 (ERSST. v4): Part I. Upgrades and intercomparisons. Journal of Climate 28(3):911–930, https://doi.org/​10.1175/JCLI-D-14-00006.1.

IPCC (Intergovernmental Panel on Climate Change). 2005. IPCC Special Report on Carbon Dioxide Capture and Storage. Prepared by Working Group III of the Intergovernmental Panel on Climate Change. B. Metz, O. Davidson, H.C. de Coninck, M. Loos, and L.A. Meyer, eds, Cambridge University Press, Cambridge, UK, and New York, NY, USA, 442 pp.

Jackson, L.C., K.A. Peterson, C.D. Roberts, and R.A. Wood. 2016. Recent slowing of Atlantic overturning circulation as a recovery from earlier strengthening. Nature Geoscience 9(7):518–522, https://doi.org/10.1038/ngeo2715.

Kawase, M. 1987. Establishment of deep ocean circulation driven by deep-water production. Journal of Physical Oceanography 17:2,294–2,317, https://doi.org/​10.1175/​1520-0485(1987)017​<2294:​EODOCD>​2.0.CO;2.

Key, R.M., A. Kozyr, C.L. Sabine, K. Lee, R. Wanninkhof, J.L. Bullister, R.A. Feely, F.J. Millero, C. Mordy, and T.H. Peng. 2004. A global ocean carbon climatology: Results from Global Data Analysis Project (GLODAP). Global Biogeochemical Cycles 18(4), https://doi.org/10.1029/2004GB002247.

Khatiwala, S., M. Visbeck, and P. Schlosser. 2001. Age tracers in an ocean GCM. Deep Sea Research Part I 48(6):1,423–1,441, https://doi.org/10.1016/S0967-0637(00)00094-7.

Kleppin, H., M. Jochum, B. Otto-Bliesner, C.A. Shields, and S. Yeager. 2015. Stochastic atmospheric forcing as a cause of Greenland climate transitions. Journal of Climate 28(19):7,741–7,763, https://doi.org/​10.1175/​JCLI-D-14-00728.1.

Lazier, J.R. 1980. Oceanographic conditions at Ocean Weather Ship Bravo, 1964–1974. Atmosphere-Ocean 18:227–238, https://doi.org/​10.1080/​07055900.​1980.​9649089.

Levitus, S., J.I. Antonov, T.P. Boyer, O.K. Baranova, H.E. Garcia, R.A. Locarnini, A.V. Mishonov, J. Reagan, D. Seidov, E.S. Yarosh, and others. 2012. World ocean heat content and thermosteric sea level change (0–2000 m), 1955–2010. Geophysical Research Letters 39(10), https://doi.org/​10.1029/​2012GL051106.

Lund, D.C., J. Lynch-Stieglitz, and W.B. Curry. 2006. Gulf Stream density structure and transport during the past millennium. Nature 444(7119):601–604, https://doi.org/10.1038/nature05277.

Lyman, J.M., and G.C. Johnson. 2014. Estimating global ocean heat content changes in the upper 1800 m since 1950 and the influence of climatology choice. Journal of Climate 27(5):1,945–1,957, https://doi.org/10.1175/JCLI-D-12-00752.1.

Marshall, J., J.R. Scott, K.C. Armour, J.-M. Campin, M. Kelley, and A. Romanou. 2015. The ocean’s role in the transient response of climate to abrupt greenhouse gas forcing. Climate Dynamics 44(7–8):2,287–2,299, https://doi.org/​10.1007/s00382-014-2308-0.

McCartney, M., and L. Talley. 1982. The subpolar mode water of the North Atlantic Ocean. Journal of Physical Oceanography 12(11):1,169–1,188, https://doi.org/​10.1175/​1520-0485​(1982)012​<1169:TSMWOT>​2.0.CO;2.

McDougall, T.J. 2003. Potential enthalpy: A conservative oceanic variable for evaluating heat content and heat fluxes. Journal of Physical Oceanography 33(5):945–963, https://doi.org/​10.1175/​1520-​0485(2003)033​<0945:PEACOV>​2.0.CO;2.

McGregor, H.V., M.N. Evans, H. Goosse, G. Leduc, B. Martrat, J.A. Addison, P.G. Mortyn, D.W. Oppo, M.-S. Seidenkrantz, M.-A. Sicre, and others. 2015. Robust global ocean cooling trend for the pre-​industrial Common Era. Nature Geoscience 8(9):671–677, https://doi.org/10.1038/ngeo2510.

Mjell, T.L., U.S. Ninnemann, H.F. Kleiven, and I.R. Hall. 2016. Multidecadal changes in Iceland Scotland Overflow Water vigor over the last 600 years and its relationship to climate. Geophysical Research Letters 43(5):2,111–2,117, https://doi.org/​10.1002/2016GL068227.

Morley, A., M. Schulz, Y. Rosenthal, S. Mulitza, A. Paul, and C. Rühlemann. 2011. Solar modulation of North Atlantic Central Water formation at multi-decadal timescales during the late Holocene. Earth and Planetary Science Letters 308(1–2):161–171, https://doi.org/​10.1016/j.epsl.2011.05.043.

Morley, A., Y. Rosenthal, and P. deMenocal. 2014. Ocean-atmosphere climate shift during the mid-to-late Holocene transition. Earth and Planetary Science Letters 388:18–26, https://doi.org/10.1016/​j.epsl.2013.11.039.

Murray, J. 1895. A Summary of the Scientific Results Obtained at the Sounding, Dredging and Trawling Stations of HMS Challenger, vol. 1. HM Stationery Office, London.

Oppo, D., Y. Rosenthal, and B. Linsley. 2009. 2,000-year-long temperature and hydrology reconstructions from the Indo-Pacific warm pool. Nature 460(7259):1,113–1,116, https://doi.org/10.1038/nature08233.

Orsi, A.H., T. Whitworth, and W.D. Nowlin. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep Sea Research Part I 42(5):641–673, https://doi.org/​10.1016/​0967-0637(95)00021-W.

Paasche, Ø., and J. Bakke. 2010. Defining the Little Ice Age. Climate of the Past Discussions 6(5):2,159–2,175, https://doi.org/​10.5194/​cpd-6-2159-2010.

Palmer, M., C. Roberts, M. Balmaseda, Y.-S. Chang, G. Chepurin, N. Ferry, Y. Fujii, S. Good, S. Guinehut, K. Haines, and others. 2017. Ocean heat content variability and change in an ensemble of ocean reanalyses. Climate Dynamics 49:909–930, https://doi.org/10.1007/s00382-015-2801-0.

Rahmstorf, S., J.E. Box, G. Feulner, M.E. Mann, A. Robinson, S. Rutherford, and E.J. Schaffernicht. 2015. Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nature Climate Change 5(5):475–480, https://doi.org/​10.1038/nclimate2554.

Rayner, N., D.E. Parker, E. Horton, C. Folland, L. Alexander, D. Rowell, E. Kent, and A. Kaplan. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108(D14), https://doi.org/​10.1029/2002JD002670.

Roemmich, D., and C. Wunsch. 1984. Apparent changes in the climatic state of the deep North Atlantic Ocean. Nature 307:447–450, https://doi.org/​10.1038/307447a0.

Roemmich, D., J. Church, J. Gilson, D. Monselesan, P. Sutton, and S. Wijffels. 2015. Unabated planetary warming and its ocean structure since 2006. Nature Climate Change 5(3):240–245, https://doi.org/​10.1038/​nclimate2513.

Rosenthal, Y., B.K. Linsley, and D.W. Oppo. 2013. Pacific Ocean heat content during the past 10,000 years. Science 342(6158):617–621, https://doi.org/​10.1126/science.1240837.

Rosenthal, Y., J. Kalansky, A. Morley, and B. Linsley. 2017. A paleo-perspective on ocean heat content: Lessons from the Holocene and Common Era. Quaternary Science Reviews 155:1–12, https://doi.org/​10.1016/j.quascirev.2016.10.017.

Sievers, H.A., and W.D. Nowlin Jr., 1984. The stratification and water masses at Drake Passage. Journal of Geophysical Research 89:10,489–10,514, https://doi.org/​10.1029/JC089iC06p10489.

Stommel, H. 1979. Determination of water mass properties of water pumped down from the Ekman layer to the geostrophic flow below. Proceedings of the National Academy of Sciences of the United States of America 76:3,051–3,055, https://doi.org/10.1073/pnas.76.7.3051.

Thornalley, D.J., D.W. Oppo, P. Ortega, J.I. Robson, C.M. Brierley, R. Davis, I.R. Hall, P. Moffa-Sanchez, N.L. Rose, P.T. Spooner, and others. 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature 556(7700):227–230, https://doi.org/10.1038/s41586-018-0007-4.

Tomczak, M., and J.S. Godfrey. 1994. Regional Oceanography: An Introduction. Pergamon Press, 422 pp.

Våge, K., R. Pickart, V. Thierry, G. Reverdin, C. Lee, B. Petrie, T. Agnew, A. Wong, and M. Ribergaard. 2008. Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008. Nature Geoscience 2(1):67–72, https://doi.org/​10.1038/ngeo382.

Walin, G. 1982. On the relation between sea-​surface heat flow and the thermal circulation in the ocean. Tellus 34:187–195, https://doi.org/​10.1111/j.2153-3490.1982.tb01806.x.

Williams, R.G., J. Marshall, and G. Nurser. 1995. Does Stommel’s mixed layer “demon” work? Journal of Physical Oceanography 25:3,089–3,102, https://doi.org/​10.1175/​1520-0485​(1995)025​<3089:DSMLW>​2.0.CO;2.

Wunsch, C. 2002. Oceanic age and transient tracers: Analytical and numerical solutions. Journal of Geophysical Research 107(C6), https://doi.org/​10.1029/2001JC000797.

Wunsch, C., and P. Heimbach. 2007. Practical global oceanic state estimation. Physica D: Nonlinear Phenomena 230(1–2):192–208, https://doi.org/​10.1016/​j.physd.2006.09.040.

Yashayaev, I., and A. Clarke. 2008. Evolution of North Atlantic water masses inferred from Labrador Sea salinity series. Oceanography 21(1):30–45, https://doi.org/​10.5670/oceanog.2008.65.

Zanna, L., S. Khatiwala, J.M. Gregory, J. Ison, and P. Heimbach. 2019. Global reconstruction of historical ocean heat storage and transport. Proceedings of the National Academy of Sciences of the United States of America 116(4):1,126–1,131, https://doi.org/​10.1073/pnas.1808838115.

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