An oceanographer ponders the difficulty of accurately estimating abyssal-ocean warming.

Estimating how much oceans are warming and where within them heat is stored is a fascinating challenge for me and my fellow oceanographers. So far, most studies comparing observations and models of changing ocean temperatures have focused on the upper 1 kilometre of water. But what about the abyssal depths, from about 3,000 metres to the bottom? Are changes in those waters really so slow as to be essentially irrelevant to atmospheric warming?

The most comprehensive surface-to-bottom measurements of ocean temperature were collected by research ships over many months during the World Ocean Circulation Experiment in the 1990s. By comparing these observations with more recent ones from the World Climate Research Programme's CLIVAR Project, Greg Johnson and his colleagues have shown that the Pacific Ocean's abyssal waters have warmed during the past two decades (G. C. Johnson et al. J. Clim. 20, 5365–5375; 2007).

Although the temperature increase is small — up to about 0.01 °C — compared with the much larger changes in the upper 1,000 metres of the ocean, it has occurred over a thickness of several kilometres, implying a huge quantity of heat storage. The deep warming is strongest in the south-west Pacific, where newly ventilated abyssal waters enter from the south.

The Pacific warming, and abyssal warming elsewhere, means that we should start considering abyssal waters when estimating sea-level rise and the climate's sensitivity to increasing greenhouse-gas concentrations. There is plenty to find out: how does the heat reach abyssal waters? Is the warming human-induced? Designing and implementing an adequate abyssal-water-observing system is a high priority.

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