Science of Doom has raised this question, which does crop up more frequently than it ought. Doug Hoyt, who should know better, has given it impetus.
It goes like this. Water is opaque to thermal IR, and not a good conductor. IR penetrates only a few microns. Therefore heat from incident IR from above must ...
be all converted to latent heat
or, be reflected, or ...
Land is also opaque to IR, but the idea that IR can't warm it doesn't seem to have the same attraction
Some thoughts below the jump.
Can the ocean emit thermal IR?
This is an equally good question. But it certainly does. In fact, that's how satellites measure SST (skin temperature).
A large part of the insolation that penetrates the sea, to a depth of several meters, is later radiated in this way. It's a big part of the thermal balance. Somehow, that heat is conveyed to the surface, and is emitted by the surface layer.
That pathway is a mix of turbulent advection, conduction in the top few mm, and radiation in the top microns. All those mechanisms are bi-directional. If heat can be emitted, it can be absorbed.
On average, the surface loses heat, by evaporation and radiation (and some conduction to air). Incoming IR does not generally need to be absorbed. It simply offsets some of the emission.
There is some diurnal effect, so that there depth intervals where the heat flow during the day is downward.
Thermal gradientsI've said that heat can be transported to and across the interface. But of course there is some resistance, and this produces a temperature gradient. The temperature differences are noticeable, but not huge. Dr Piotr Flatau has posted some results on Wikipedia:
Note the nearly logarithmic depth scale. The top 10 microns, in which there is nett daytime emission, shows a drop of a bit less than 1°C. There is another drop of about half a degree in the conductive layer. Then the gradient changes, reflecting presumably the upflux of heat from the more rapidly absorbed solar wavelengths. The gradient then diminishes as the source fades, and the turbulent eddies grow in size, increasing turbulent advection.
At night, the gradient becomes monotonic, with the advected flux uniformly upward. But again the temperature difference is less than a degree:
ConclusionIn a very technical sense, the sea is heated by sunlight rather than downwelling IR, as is the land. That's just trivial arithmetic - the sun is the heat source. And the upward IR flux generally exceeds the downward. But downwelling IR does add joules to the sea just as effectively as SW - there is no blocking thermal resistance.