"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."The RealClimate post then shows the experimental evidence [a single paper] for the remaining claim that greenhouse gases reduce the size of the temperature gradient to reduce heat flow from the oceans to the atmosphere, showing this graph:
|Figure 2: The change in the skin temperature to bulk temperature difference as a function of the net longwave radiation.|
"There is an associated reduction in the difference between the 5 cm and the skin temperatures. The slope of the relationship is 0.002ºK (W/m2)-1. Of course the range of net infrared forcing caused by changing cloud conditions (~100W/m2) is much greater than that caused by increasing levels of greenhouse gases (e.g. doubling pre-industrial CO2 levels will increase the net forcing by ~4W/m2), but the objective of this exercise was to demonstrate a relationship."According to the IPCC, a doubling of CO2 levels allegedly increases forcing by 3.7 Wm-2 at the top of the atmosphere and by only about 1 Wm-2 at the surface. The paper cited by RealClimate is measuring the effect of longwave forcing at the surface, therefore we assume 1 Wm-2 from doubled CO2 at the surface. Using the slope of the relationship, 0.002ºK (W/m2)-1, we find that doubling of CO2 concentrations could only reduce the temperature gradient 0.002*1 = 0.002ºC.
Furthermore, a reduced temperature gradient of 0.002ºC could at the very, very most result in an increase in bulk ocean temperature of 0.002ºC. In reality, this will never happen since the heat capacity of the ocean is more than 1000 times greater than the atmosphere, and therefore the ability for a doubling of CO2 to warm the oceans is essentially zero.
Related: New paper finds world's oceans have warmed only 0.09°C over past 55 years