The Greenhouse Effect in a Water World

The natural world is complicated. Therefore, so is the science that tries to understand it.

Complexity, uncertainty, ambiguity are all a part of the story that describes processes that are as extraordinary as they are mundane. While these are the very characteristics of scientific study that motivate professional and amateur alike, they are also the characteristics that give delayers, doubters and liars in the climate debates ample material for confusing and misrepresenting reality.

One such complexity is the interaction of infra-red or long wave radiation (IR or LW), the ocean surface and the atmosphere. This is what gives rise to one of the more esoteric arguments in the climate change denier’s arsenal, the argument that the Greenhouse Effect does not operate over 70% of the earth’s surface, the portion that is covered in ocean rather than land and therefore ocean warming can not be a result of an enhanced GHE from rising CO2. Since ocean heat content is by far the lion’s share of climate system warming…um..Not the IPCC.

The grain of truth in this line of reasoning is that ocean warming is primarily from solar short wave (SW) radiation, the visible light, that freely propogates through the atmosphere and penetrates 10’s of metres into the ocean’s upper layers. This sunlight directly warms the ocean waters just as it directly warms the earth’s surface. Recall now that the greenhouse effect is what happens next: all warm bodies, land and water, emit longwave radiation and these IR photons are trapped and re-radiated by greenhouse gases (H20, CO2, CH4 among others). Roughly half of the energy the surface of the earth tries to send back out to space returns this way. So, the GHE increases down-welling LW but not SW energy. Well, it just so happens that this returning LW energy, unlike its SW cousin, does not penetrate water beyond a few micrometres and therfore it can not directly warm anything but an insignificant portion of the upper ocean layers. No increase in SW radiation, no ocean warming.

To be honest, I am not sure what is so compelling about this argument. Firstly, the climate we inhabit exists in the bottom of the troposphere over land, so while not insignificant, warming of the ocean is not our primary concern. Secondly, energy is not destroyed, so absorbed directly or not, the fact is it is having its escape from the earth’s surface, be it land or water, impeded and this must show up as warming somewhere. Logically, some of it must be finding its way back into ocean water via some means or another. Skin layer water molecules that evaporate rather than allow the IR energy to pass through will immediately or eventually give that heat up again as they condense back to liquid form. This results in warmer air which can transfer its heat via conduction. Admittedly, this is probably a minor factor as the heat capacity of air is so much less than water, but that heat does not just disappear. And finally, whatever is going on, the fact is that the heat content of the upper ocean is rising, as expected in an enhanced greenhouse environment. A truly inquiring mind would search for the mechanism that explains the reality, not declare that reality is impossible so… “drill baby, drill!”

Well, it turns out that reality is possible and is happening for a reason, no matter how complicated or surprising. The reason is that that very thin skin layer of water on top of the ocean surface is a strong controller of the heat flux between atmosphere and ocean. Specifically, the size of the temperature gradient between the surface layer and the bulk temperature of the mixed water below it determines how well heat can propogate through it. Now, there are two possible directions heat might propogate. In polar regions or winter in temperate climates, the air is much colder than the water and so heat will flow out of the ocean, whereas in the tropics or summer temperate climates the air will be warming the ocean, drawing energy out. If increased down welling LW is causing a warming of the skin layer, then in the first situation, where the ocean is warmer than the air, the gradient is increasedreduced and in the second, where the ocean is colder, the gradient is reducedincreased. The larger the temperature gradient, the faster heat will move across it.

This image and caption, taken from a RealClimate article on this subject, shows the relationship we are looking for:

Figure 2: The change in the skin temperature to bulk temperature difference as a function of the net longwave radiation.

Please check out that RC article and the first couple of screens of comments for the rather gory details.

Because of the principal of larger temperature gradients causing faster heat transfer, by warming the skin layer, down welling IR will decrease the heat transfer from warm ocean to cold air and increase the heat transfer from warm air to cool water, both of which effects contribute to warmer upper ocean waters. Ocean waters warmed by the direct sunlight find that, in an enhanced greenhouse environment, more of this heat is retained rather than passed into the atmosphere.

It is a bit of a brain twister, I’ll admit, but it is all consistent with well known physical properties of energy transfer and well established observational evidence of warming upper oceans.