135 years of global ocean warming between the Challenger expedition and the Argo Programme

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 oceans(1,2). 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 Challenger(3) (1872-1876) and the modern data set of the Argo Programme(4) (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 degrees C +/- 0.12, consistent with previous estimates(5) of globally averaged sea surface temperature increase. Below the surface the mean warming decreases to 0.39 degrees C +/- 0.18 at 366m (200 fathoms) and 0.12 degrees C +/- 0.07 at 914 m (500 fathoms). The 0.33 degrees 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 years(6), 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.