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.

Clearly it is not possible to alter the concentration of greenhouse gases in a controlled experiment at sea to study the response of the skin-layer. Instead we use the natural variations in clouds to modulate the incident infrared radiation at the sea surface. When clouds are present, they emit more infrared energy towards the surface than does the clear sky. The incident infrared radiation was measures by a pyrgeometer mounted on the ship, and the emission from the sea surface was calculated from the Stefan-Boltzmann equation using the skin temperature measurements of the M-AERI. The difference between the two is the net infrared forcing of the skin layer. If we can establish a relationship between the temperature difference across the skin layer and the net infrared forcing, then we will have demonstrated the mechanisms for greenhouse gas heating the upper ocean. That is seen in the flow chart on the right.

The figure below shows just the signal we are seeking. There is a clear dependence of the skin temperature difference on the net infrared forcing. The net forcing is negative as the effective temperature of the clear and cloudy sky is less than the ocean skin temperature, and it approaches values closer to zero when the sky is cloudy. This corresponds to increased greenhouse gas emission reaching the sea surface.

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