Why greenhouse gases heat the ocean

Guest commentary by Peter Minnett (RSMAS)

Observations of ocean temperatures have revealed that the ocean heat content has been increasing significantly over recent decades (Willis et al, 2004; Levitus et al, 2005; Lyman et al, 2006). This is something that has been predicted by climate models (and confirmed notably by Hansen et al, 2005), and has therefore been described as a ‘smoking gun’ for human-caused greenhouse gases.

However, some have insisted that there is a paradox here – how can a forcing driven by longwave absorption and emission impact the ocean below since the infrared radiation does not penetrate more than a few micrometers into the ocean? Resolution of this conundrum is to be found in the recognition that the skin layer temperature gradient not only exists as a result of the ocean-atmosphere temperature difference, but also helps to control the ocean-atmosphere heat flux. (The ‘skin layer‘ is the very thin – up to 1 mm – layer at the top of ocean that is in direct contact with the atmosphere). Reducing the size of the temperature gradient through the skin layer reduces the flux. Thus, if the absorption of the infrared emission from atmospheric greenhouse gases reduces the gradient through the skin layer, the flow of heat from the ocean beneath will be reduced, leaving more of the heat introduced into the bulk of the upper oceanic layer by the absorption of sunlight to remain there to increase water temperature. Experimental evidence for this mechanism can be seen in at-sea measurements of the ocean skin and bulk temperatures.

During a recent cruise of the New Zealand research vessel Tangaroa, skin sea-surface temperatures were measured to high accuracy by the Marine-Atmospheric Emitted Radiance Interferometer (M-AERI), and contemporaneous measurements of the bulk temperature were measured at a depth of ~5cm close to the M-AERI foot print by a precision thermistor mounted in a surface-following float. The M-AERI is a Fourier Transform Infrared spectroradiometer that has very accurate, NIST-traceable, calibration. The skin temperature can be measured with absolute uncertainties of much less than 0.1ºK The thermometer in the surface following float is accurate to better than 0.01ºK. Both are calibrated using the same equipment at the University of Miami.

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