The twentieth-century trend in global-mean surface temperature was not monotonic: temperatures rose from the start of the century to the 1940s, fell slightly during the middle part of the century, and rose rapidly from the mid-1970s onwards1. The warming–cooling–warming pattern of twentieth-century temperatures is typically interpreted as the superposition of long-term warming due to increasing greenhouse gases and either cooling due to a mid-twentieth century increase of sulphate aerosols in the troposphere2,3,4, or changes in the climate of the world’s oceans that evolve over decades (oscillatory multidecadal variability)2,5. Loadings of sulphate aerosol in the troposphere are thought to have had a particularly important role in the differences in temperature trends between the Northern and Southern hemispheres during the decades following the Second World War2,3,4. Here we show that the hemispheric differences in temperature trends in the middle of the twentieth century stem largely from a rapid drop in Northern Hemisphere sea surface temperatures of about 0.3 °C between about 1968 and 1972. The timescale of the drop is shorter than that associated with either tropospheric aerosol loadings or previous characterizations of oscillatory multidecadal variability. The drop is evident in all available historical sea surface temperature data sets, is not traceable to changes in the attendant metadata, and is not linked to any known biases in surface temperature measurements. The drop is not concentrated in any discrete region of the Northern Hemisphere oceans, but its amplitude is largest over the northern North Atlantic.
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D.W.J.T. and J.M.W. are supported by the National Science Foundation Climate Dynamics Program under budget numbers ATM-0132190 and ATM-0613082 (D.W.J.T.) and ATM-0812802 (J.M.W.). J.J.K. is supported by the Joint Department of Energy and Climate Change and Defra Integrated Climate Programme: DECC/Defra (GA01101).
The authors declare no competing financial interests.
This file contains Supplementary Information comprising a) The adjustment Methodology, b) Examination of the SST metadata and c) Generation of the regression map in Figure 3a. Also included are an additional reference and Supplementary Figures 1 - 2 with legends. (PDF 405 kb)
This file contains all the data used in this study. (A .tar file concatenates multiple files and places them into a single file. To extract the data type ‘tar–xvzf DATA.tar.gz’ at the ‘Unix’ or ‘Linux’ command prompt. This command will extract all of the files contained within DATA.tar.gz and place them in the current working directory. Descriptions of the data files and data sources are located in the extracted README.txt file.). (ZIP 51592 kb)
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Thompson, D., Wallace, J., Kennedy, J. et al. An abrupt drop in Northern Hemisphere sea surface temperature around 1970. Nature 467, 444–447 (2010). https://doi.org/10.1038/nature09394
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