Observations of changes in the properties of ocean waters have been restricted to surface1 or intermediate-depth waters2,3, because the detection of change in bottom water is extremely difficult owing to the small magnitude of the expected signals. Nevertheless, temporal changes in the properties of such deep waters across an ocean basin are of particular interest, as they can be used to constrain the transport of water at the bottom of the ocean and to detect changes in the global thermohaline circulation. Here we present a comparison of a trans-Pacific survey completed in 1985 (refs 4, 5) and its repetition in 1999 (ref. 6). We find that the deepest waters of the North Pacific Ocean have warmed significantly across the entire width of the ocean basin. Our observations imply that changes in water properties are now detectable in water masses that have long been insulated from heat exchange with the atmosphere.
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We thank the officers and crew of the three research vessels (RV Kaiyo-maru of JFA, RV Mirai of JAMSTEC, and CCGS John P. Tully) and all technical assistants. The Canadian contribution was funded by the Strategic Science Fund of the Department of Fisheries and Oceans, and the Japanese contributions were funded by the Promotional Foundation for Science and Technology of the Science and Technology Agency of Japan (now known as the Ministry of Education, Culture, Sports, Science and Technology).
The authors declare that they have no competing financial interests.
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Fukasawa, M., Freeland, H., Perkin, R. et al. Bottom water warming in the North Pacific Ocean. Nature 427, 825–827 (2004). https://doi.org/10.1038/nature02337
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