Changing temperature throughout the oceans is a key indicator of climate change. Since the 1960s about 90% of the excess heat added to the Earth’s climate system has been stored in the oceans1,2. The ocean’s dominant role over the atmosphere, land, or cryosphere comes from its high heat capacity and ability to remove heat from the sea surface by currents and mixing. The longest interval over which instrumental records of subsurface global-scale temperature can be compared is the 135 years between the voyage of HMS Challenger3 (1872–1876) and the modern data set of the Argo Programme4 (2004–2010). Argo’s unprecedented global coverage permits its comparison with any earlier measurements. This, the first global-scale comparison of Challenger and modern data, shows spatial mean warming at the surface of 0.59 °C±0.12, consistent with previous estimates5 of globally averaged sea surface temperature increase. Below the surface the mean warming decreases to 0.39 °C±0.18 at 366 m (200 fathoms) and 0.12 °C±0.07 at 914 m (500 fathoms). The 0.33 °C±0.14 average temperature difference from 0 to 700 m is twice the value observed globally in that depth range over the past 50 years6, implying a centennial timescale for the present rate of global warming. Warming in the Atlantic Ocean is stronger than in the Pacific. Systematic errors in the Challenger data mean that these temperature changes are a lower bound on the actual values. This study underlines the scientific significance of the Challenger expedition and the modern Argo Programme and indicates that globally the oceans have been warming at least since the late-nineteenth or early-twentieth century.
We acknowledge the beginning of global oceanography by HMS Challenger and the 140th anniversary in 2012 of its departure from Sheerness, Kent, UK, on 7 December 1872. The Argo data used here were collected and are made freely available by the International Argo Programme and by the national programmes that contribute to it. D.R. and J.G. and their part in the Argo Programme were supported by US Argo through NOAA grant NA17RJ1231 (SIO–JIMO).