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Climate Hustle

How Increasing Carbon Dioxide Heats The Ocean

Posted on 18 October 2011 by Rob Painting

Much like a heated kettle of water takes some time before it comes to the boil, it seems intuitive that the world's oceans will also take some time to fully respond to global warming. Unlike a kettle, however, it's not obvious how the oceans warm.

Adding further greenhouse gases to the atmosphere warms the ocean cool skin layer, which in turn reduces the amount of heat flowing out of the ocean. Reducing the heat lost to the atmosphere allows the oceans to steadily warm over time - as has been observed over the last half century.

Warming on sunshine

Sunlight penetrating the surface of the oceans is responsible for warming of the surface layers. Once heated, the ocean surface becomes warmer than the atmosphere above, and because of this heat flows from the warm ocean to the cool atmosphere above. This process is represented in the graphic below:

Figure 1 - simplified steps of ocean heating 

The 'cool skin' layer

The rate of flow of heat out of the ocean is determined by the temperature gradient in the 'cool skin layer', which resides within the thin viscous surface layer of ocean that is in contact with the atmosphere. It's so named because it is the interface where ocean heat is lost to the atmosphere, and therefore becomes cooler than the water immediately below. Despite being only 0.1 to 1mm thick on average, this skin layer is the major player in the long-term warming of the oceans. 

Curious behavior in the cool skin layer

The cool skin behaves quite differently to the water below, because it is the boundary where the ocean and air meet, and therefore turbulence (the transfer of energy/heat via large-scale motion) falls away as it approaches this boundary. No longer free to jiggle around and transfer heat via this large scale motion, water molecules in the layer are forced together and heat is only able to travel through the skin layer by way of conduction. With conduction the steepness of the temperature gradient is critical to the rate of heat transfer.

Greenhouse gas-induced warming of the ocean

Greenhouse gases, such as carbon dioxide, trap heat in the atmosphere and direct part of this back toward the surface. This heat cannot penetrate into the ocean itself, but it does warm the cool skin layer, and the level of this warming ultimately controls the temperature gradient in the layer. 

Increased warming of the cool skin layer (via increased greenhouse gases) lowers its temperature gradient (that is the temperature difference between the top and bottom of the layer), and this reduces the rate at which heat flows out of the ocean to the atmosphere. One way to think about this is to compare the gradient (steepness) of a flowing river - water flows faster the steeper the river becomes, but slows as the steepness decreases.

The same concept applies to the cool skin layer - warm the top of the layer and the gradient across it decreases, therefore reducing heat flowing out of the ocean.

The ever-present effect of the cool skin layer

An important point not be be glossed over here, is that changing the temperature gradient in the cool skin layer by way of greenhouse gas warming is a worldwide phenomenon. Once the gradient has changed, all heat leaving the ocean thereafter has to negotiate its way through the layer. With the gradient lowered, the ocean is able to steal away a little bit more from heat headed for the atmosphere. It is in this ever-present mechanism that oceans are able to undergo long-term warming (or cooling).

Experimental evidence for greenhouse gas heating of the oceans

Obviously it's not possible to manipulate the concentration of CO2 in the air in order to carry out real world experiments, but natural changes in cloud cover provide an opportunity to test the principle. Under cloudy conditions, the cloud cover radiates more heat back down toward the ocean surface than happens under clear sky conditions. So the mechanism should cause a decline in skin temperature gradients with increased cloud cover (more downward heat radiation), and there should also be a decline in the difference between cool skin layer and ocean bulk temperatures - as less heat escapes the ocean under increased atmospheric warming. 

This was observed in an experiment carried out in 2004, aboard the New Zealand research ship Tangaroa. Using intruments to simultaneously measure the 'cool skin', the ocean below, and the amount of heat (longwave radiation) reaching the ocean surface, researchers were able to confirm how greenhouse gases heat the ocean. It should be pointed out here, that the amount of change in downward heat radiation from changes in cloud cover in the experiment, are far greater than the gradual change in warming provided by human greenhouse gas emissions, but the relationship was nevertheless established.  

Figure 2 -The change in the skin temperature to bulk temperature difference as a function of the net longwave (heat) radiation. The net forcing is negative as the atmosphere is cooler than the ocean skin layer, but approaches zero under cloudy conditions. See Real Climate post "Why Greenhouse Gases Heat The Ocean" by Professor Peter Minnett.

Greenhouse Gases: On duty 24/7 

The effect of greenhouse gases on ocean heat isn't confined to daylight hours however, they toil away around the clock. The warming of the oceans by sunlight, makes the daytime surface waters more bouyant than the cooler waters below and this leads to stratification - a situation where the warmer water floats atop cooler waters underneath, and is less inclined to mix. At night much of the heat accumulated during the day is lost back to the atmosphere (the overling air still being cooler than the ocean), and this cooling leads to the stratified surface layers sinking and mixing with lower layers. This allows the remaining heat to be transported down deeper into the ocean, causing an increase in ocean heat content over the long-term. The typical diurnal (day/night) cycle is seen in the figure below:

Figure 3 - Schematic showing the upper ocean temperature profiles during the (A) nighttime or well mixed daytime and (B) daytime during conditions conducive to the formation of a diurnal warm layer. Image from Gentemann & Minnett (2008)

Warming in the pipeline

Given the atmospheric lifetime of carbon dioxide is many hundreds to thousands of years, we can now understand that long-lived greenhouses will also continue to exert a warming influence on the worlds oceans for a very long time. Indeed, climate models suggest that ocean warming will continue for at least a thousand years even if CO2 emissions were to completely stop. See below:

Fig 5 - Time series of the (modeled) climate response to a cessation of CO2 emissions. a) global mean thermosteric sea level anomaly (b) and zonal mean ocean temperature at 792.5mtrs, 66 S (the Southern Ocean). Green line = cessation of CO2 at 2010 & red line = cessation at 2100. From Gillett (2011).

Ocean warming not just skin deep   

Because of their effect on lowering the temperature gradient of the cool skin layer, increased levels of greenhouse gases lead to more heat being stored in the oceans over the long-term. This ocean warming mechanism has been observed experimentally, and is also supported by numerical modeling.  

So although greenhouse gases, such as carbon dioxide, don't directly warm the oceans by channeling heat down into the oceans, they still do indeed heat the oceans, and are likely to do so for a very long time. 

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Comments 1 to 50 out of 68:

  1. What about mixing from wave activity? Is it enough to matter? Does it vary significantly from year to year on a global scale?

    A series on this topic similar to your ocean salinization series, would be a great thing.
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  2. Harald - for sure mixing can vary significantly on short timescales - ocean temperatures would not be held constant in an "idealized" stable climate, they would still fluctuate, but without any long-term trend.

    The post only deals with one aspect of the ocean warming process, but perhaps the most important one, given the general confusion exhibited by some commenters on how the oceans warm.

    As for a series, I'll be covering some other aspects of the warming oceans, but have no intention of writing a series. Perhaps another SkS author might decide to do so, but I won't be holding my breath.
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  3. Logically wave energy should be small since it is third hand solar energy (Solar -> Wind -> Wave). Then you have the issue of how far it penetrates and causes any heating.
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  4. Rob, thanks for this post. It's a bit of a missing link for lay people, I think. When you know that the average temperature of the ocean is warmer than that of the atmosphere, and that heat flows from a warmer body to a cooler one, the idea that a warming atmosphere can warm the oceans is not straightforwardly intuitive. You've clarified it well.

    Still have questions, though. When you say,

    "Indeed, climate models suggest that ocean warming will continue for at least a thousand years even if CO2 emissions were to completely stop."

    It makes me wonder why equilibrium is considered to be reached after 30 - 40 years of oceanic lag, rather than 1000. Why is the long tail not included in equilibrium climate sensitivity?
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  5. Excellent post, Rob. Off to learn more about it.
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  6. Harald Korneliussen at 22:10 PM on 18 October, 2011

    There's an interesting series about this subject on ScienceOfDoom, beginning here:

    Does Back-Radiation “Heat” the Ocean? – Part One

    At some point into the series he explores some simple models to help understand the role of the upper turbulent layer you mentioned, that yes, also plays its role.
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  7. Barry @4

    Climate Sensitivity is defined in terms of three different time scales.

    - Transient CS is days/weeks/ maybe a year. This is mainly radiative effects - GH gases, Water Feedback, Aerosol changes due to pollution or volcanoes.

    - Short Term CS is decades out to around 50 years - this is the one you are referring to. This takes acount of changes in sea ice, movements in weather patterns, upper ocean heat changes etc. This is the one commonly referred to as Climate Senstivity in the literature when they aren't being more specific. It is also called the Charney Sensitivity after the scientists who first coined the term in the Charney Report.

    - Long Term CS is centuries to even millenia long. This includes effects like whole of ocean warming, Land ice changes, major changes in vegetation patterns, desertification, changes in ocean circulation pattern, methane feedbacks from permafrost and clathrates etc
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  8. A couple of questions I came up with while skimming this.
    (1) How was the cooling effect of evaporation controlled for in the Tangaroa study? I think I read that light (not just heat) increases evaporation considerably.
    (2) There's something counter-intuitive here -- ignoring the mechanistic explanation, it seems like you're saying that stratification increases the mixing down of heat. I'm wondering how much this relies on diurnal heating. If just not diurnally heating and cooling, but just receiving an intermediate amount of solar energy constantly, wouldn't stratification slow the warming of the ocean? Cooling allows the warm layer to sink? Doesn't it sink below a warmer layer which then loses its heat to the atmosphere?

    I suppose I could read the links provided before asking ignorant questions!
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  9. One concept that is useful in understanding these sorts of exchanges is the "surface energy balance". For the case of the ocean, think of the ocean-atmosphere interface as a plane surface separating the ocean and atmosphere. This plane has no mass, so it can't store energy. With no capability of storing energy, the fluxes of energy towards and away from the surface must sum to zero.

    You can express this in mathematical terms. There are variations depending on what you decide to call + and - for fluxes, but one convenient form is:

    net radiation = sensible heat + latent heat + ocean heating

    where net radiation is the sum of incoming radiation (solar and IR) minus outgoing (reflected solar and upward-emitted IR), sensible heat is thermal energy transported into the atmosphere, latent heat is the energy of evaporation transported into the atmosphere (i.e., the flux of water vapour, expressed in energy terms), and ocean heating is the flux of heat into the ocean.

    With the water surface allowing penetration of solar radiation, things are really a bit more complex than this, but it still serves as a useful model. I'm also ignoring chemical reactions, for simplicity. (Photosynthesis would be the largest of these, and is still quite small.)

    The sensible heat flux is dependent on the temperature gradient between the surface and the lower atmosphere (and wind and a few other details). The latent heat flux is dependent on the humidity gradient between the surface and the lower atmosphere, but due to the condition of vapour saturation at the water surface, the latent heat is also dependent on surface temperature. Lastly, the flux of heat into the ocean is also dependent on the temperature gradient away from the surface (and the rate of mechanical mixing). Of the radiation terms, only the outgoing IR is strongly dependent on surface temperature (unless you run into freezing or thawing).

    The usefulness of this model is that it helps illustrate how surface temperature is linked to the energy fluxes. Stop mixing the air, and you reduce the fluxes into the atmosphere. Response? Surface temperature rises, increasing the fluxes again - but more will go into the ocean. Restrictions on evaporation (easier to image on a land surface) will cause a rise in atmospheric and sub-surface heating. Mix the ocean more (wind), and more heat is carried into the ocean leaving less flux into the air (both heat and vapour).

    Both the ocean and the atmosphere will exhibit stratification effects. The atmosphere, when heated from below, will become unstable and mix more easily; when cooled from below to the point of a strong temperature inversion, heat (and vapour) transfer is limited due to the reduced mixing.

    Wind affects waves, which affect solar absorption, too, so there is lots to keep track of in developing a comprehensive mathematical model. But you can still end up expressing it in a form where each component is a function of surface temperature plus some other terms, and if you can define the other terms you can solve for surface temperature.
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  10. @9: Great addendum, Bob.

    The exposition on this sub-topic really highlights, for me, the challenges when deciding how much complexity (and uncertainty) to sacrifice in order to describe scientific understanding to lay people. There will always be genuine questioners who want more detail, as well as queries from people less curious and more disposed to find or expose fault, incompetence or iniquity.

    So there will always be a need for new posts here. Having lurked here for several years, I enjoy SkS as a conversation as well as resource for rebuttals. Topics are returned to and refined. It has been a pleasure watching arguments and explanations unfold and evolve, and I really appreciate the time taken to engage with my own queries.
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  11. @7: Glenn Tamblyn

    Thanks for the breakdown. I'm familiar with TCS and ECS, and was curious about the choice of running with the Charney sensitivity as opposed to effective (immediate) response or long-term sensitivity. I googled about after reading your reply and rediscovered this post from realclimate.
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  12. This is an interesting post Rob. I wasn't aware of the particular impact of skin temperature changes on the heat transfer balance.

    Bob Loblaw @9
    The interesting challenge is actually the movement of heat from the skin to just below that without this heat returning to the atmosphere. Once it is down in the sub-surface waters there are a range of mixing processes that will move it down further, down to 50-100 metres. This upper layer of the ocean is referred to as the Well-Mixed Layer. Its depth varies around the world and the seasons but is typically less than 100m. Here various mixing processes dominate; wind, waves, tides, even the movement of organisms - megatonnes of krill, zoo-plankton and phyto-plankton moving vertically over the course of a day can have a surprising impact.

    Once additional heat has reached this depth, over-turning currents within the oceans can carry it further down. The major overturning current flows, mixing the surface with the abyssal depths happen in several defined regions in the oceans. But lesser flows and gyres exist in some locations that can move heat further down without reaching the bottom.

    Also, this mechanism explains well the underlying thermodynamics of 'in-the-pipeline' warming. This is sometimes portrayed as if at some future time all this heat will start 'coming back out' from the oceans. But this doesn't make thermodynamic sense.

    Prior to AGW, the temperatures of the air and ocean would have been in some sort of thermal balance. Not necesarily the same temperatures, but at temperatures where the heat fluxes between ocean <-> air where in balance.

    So as AGW starts adding heat to the atmosphere & oceans the much lower thermal mass of the atmosphere would mean that the temperature of the atmosphere starts rising faster than the oceans - which we have observed. Thus the temperature differential between oceans and air starts to change, creating a 'brake' on the rate at which atmospheric temps can rise.

    If atmospheric temps rise faster than ocean temps, this will tend to increase the energy flow from air to oceans, limiting the rise that is possible in the air until the temp change in the oceans 'catches up'. Not a heat flow from oceans to air 'later' warming the air. Rather a heat flow from air to oceans limiting the rise in air temps 'now' until this flow subsides when the temp imbalance is rectified as the oceans warm.

    Looking at Rob's 4 step process, step 3 - 'surface layers become warmer than atmosphere'. If the atmosphere warms, then the surface layers are warmer compared to the oceans, but not as much as before. Therefore step 4 - 'ocean looses heat to cooler atmosphere above'. They don't loose as much and retain more. Since the oceans aren't losing as much to the air, heating of the air is restricted while heating of the oceans is enhanced. But there is a lot of ocean.

    Its not heat coming out of the oceans that is 'in the pipe-line'. It is heating of the oceans eventually taking the brake off heating of the atmosphere that 'is in the pipeline'
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  13. Steve L @ 8 - Question 1: solar radiation, temperature, humidity and wind speed, all affect the rate of evaporation from the ocean surface. The giveaway is figure 2 which shows a clear relationship with longwave radiation and the cool skin layer temperature gradient. This would not likely be the case if evaporation were controlling the temperature gradient.

    Also, if you're saying that solar radiation (via evaporation) cools the ocean (as your post seem to suggest), then how would the oceans ever undergo long-term warming or cooling, as we know they have in the past? Perhaps I've misinterpreted your question.

    2. No. It's just a description of the diurnal cycle. The important point to remember, is that lowering of the temperature gradient in the skin layer reduces heat lost by the ocean to the atmosphere, and thereby enables the ocean to steadily accumulate heat over time.
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  14. Barry @ 4 - Equilibrium Climate Sensitivity, you have me there. Seems a bit of a misnomer. The Earth System Sensitivity approach (as discussed in the Real Climate link) seems better, although harder to estimate.
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  15. Glenn Tamblyn@12

    Interesting point you make here:

    *Also, this mechanism explains well the underlying thermodynamics of 'in-the-pipeline' warming. This is sometimes portrayed as if at some future time all this heat will start 'coming back out' from the oceans. But this doesn't make thermodynamic sense.*

    All the heat in the land, atmosphere and oceans must be represented in some form - existing water, air and land temperatures, ice melt, air/water vapour content.

    It seems to me that melting ice takes heat out of the surrounding water and heat should flow to these sinks until all the ice is melted.

    Would anyone like to explain the effect of sea ice in moderating ocean temperatures? Would sea ice anywhere on the planet prevent the oceans warming until it all melted?
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  16. Rob @ 14 - I'd hazard the choice to go with the Charney sensitivity makes sense for a policy document like the IPCC reports. The estimate is better constrained and the feedback processes are 'fast' in that their impact can be felt in decades. If the concern is the rate more than the magnitude of change, then slower feedbacks perhaps don't need so much attention in the IPCC.
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  17. This post, for me, appears to ignore one primary property of seawater and that is its salinity.
    Both salinity and temperature determine buoyancy. Resulting density imbalances lead to vertical thermohaline adjustment.
    I have always assumed that diurnal warming leads to evaporation with water mass loss in the surface skin. We would expect both temperature and salinity to increase during the daytime radiation.
    At night, surface cooling through radiative heat loss should allow the increased salinity to create a density gradient down which the surface waters travel to reach an equilibrium depth.
    In the tropics evaporation is reported to be of the order of 2m per year. But we still get evaporation as evidenced by coastal fogs in more poleward latitudes.
    Has there been any observational work on Tangeroa of these temperature and salinity gradients?
    I am very interested in near-surface dynamics in the ocean and its role in distributing heat and salt.
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  18. As far as I am aware temperature dominates in lower latitudes and salinity in higher in relation to buoyancy. I don't think diurnal effects have much impact on surface waters due to the inertia of water.
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  19. Since the mechanism by which greenhouse gases heat the ocean is through slowing the escape of heat from deeper depths by heating the "cool skin" layer , wouldn't that mean that the amount heat accumulating in the oceans is larger than the amount of heat being trapped by greenhouse gases in the atmosphere?
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  20. Barry@ 14 - I understand the rationale, but it comes down to a values-based judgement I guess. Why only consider negative consequences that will unfold in the (relatively) short-term? Are those yet to be born in the next hundred years any more important than those born in following centuries? I wouldn't have thought so, but sea level, and it's attendant problems, will only get higher over the coming centuries.

    Micawber @17 - knock yourself out. Click on some of the links I've provided and chase up the cited peer-reviewed literature. I doubt much of it is of interest to a general audience.

    Karamanski @ 19 - Absolutely! Over 90% of global warming is going into the oceans.

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  21. Glenn Tamblyn @ 12:

    Yes, the dynamics of heat transfer into the ocean are complex - much more so than land. On land, you just have simple conduction for the most part. As the surface heats, energy is conducted downwards (or if the surface cools, heat is conducted back upwards when the gradient reverses). Conduction is a slow process, so 20-30cm down you won't even see the daily cycle - it is damped out. At 10m depth, you won't even see the annual cycle. (Exact depths vary, depending on the conductivity and heat capacity of the soil.)

    Contrast with the oceans - the active mixed layer (forced by winds, mixing due to salinity or temperature gradients, whatever) is much thicker, as Glenn discusses. Water also has a much higher heat capacity than soil, so it takes a lot more energy to change the temperature of the ocean that is tied to surface temperature (compared to land). In the surface energy balance model I discussed in #9, an ocean will see a lot more of the net radiation (think of it as "available energy") end up in the ocean than would go into land in a similar situation (and thus also less sensible and latent heat is transferred to the atmosphere).

    The inertia in warming or cooling the ocean is what leads to reduced daily and annual temperature ranges compared to land - especially the further you get away from the coast. (Look up "continental climate".)

    As to comments about evaporation - technically, the flux of water vapour away from the surface into the atmosphere is controlled by the gradient of water vapour and the transfer coefficient in air (also dominated by turbulent mixing), not radiation input. The catch is, that to sustain a vapour flux, you have to convert liquid to vapour, and that takes energy. But you can get that energy from any of the other parts of the surface energy balance - radiation input, sensible heat transferred down from the atmosphere (warm air passing over cold water), or from the water (i.e., cool the water). Or any combination of the three. If you don't have radiative input, sucking heat out of the air will rapidly cool the air (nighttime inversions). When the air isn't warm, then you have to cool the water, which then drops the saturation vapour pressure at the surface, which reduces the vapour gradient between the surface and the air, and reduces the flux of vapour - a sort of feedback process. (Take a deep breath - what a long sentence!!) It's a mini-version of the earth radiative balance - where increasing CO2 upsets the balance with space and the system changes temperature to compensate.
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  22. Bob Laidlaw @21 Thanks for the very helpful comments especially on evaporation. I am interested in heat and salinity partly because of its influence on CO2 absorption. It is sensitive to alkalinity and temperature. It therefore has a bearing on ocean acidification processes I believe.
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  23. victull @15

    Sea ice is made up of the part that freezes and remelts in a year, and the longer lasting semi-permanent ice. By definition the annual ice cycle can't contribute to the heat balance on longer timesacles. In contrast the decline of older ice is taking up heat that would end up in some other part of the system, probably the ocean. However the slow circulation time of the oceans would mean that a change in this thermal distribution is more likely to be limited to the oceans in the vicinity of the ice rather than the entire planet.

    The other factor, probably larger than the energy involved in melting the ice, is the effect that the ice melt has on Albedo - how much sunlight gets reflected by the Earth rather than absorbed. Ice reflects 80-90% of sunlight depending on how dirty it is. In contrast water absorbs about 90% of sunlight. So a major factor is the change in total heat absorption by the environment due to net ice melt exposing open water. This is probably more significant than the actual amount of heat needed to melt the ice.
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  24. From what I read from Saunders, Peter M., 1967: The Temperature at the Ocean-Air Interface. J. Atmos. Sci., 24, 269–273, the long wave radiation loss is a significant factor only in a dead calm, with clear skies - otherwise wind and insolation are the dominant factors... Ever been to sea?
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  25. 24, guinganbresil,

    I do not see anything in the provided paper that supports your statement that "long wave radiation loss is a significant factor only in a dead calm, with clear skies."

    I think you are misreading the paper.

    Can you quote the passages which you believe lead to that interpretation?

    Also, please note that your supplied paper discusses the ocean-air interface in the top centimeters of the ocean only. Even if your interpretation were correct, it would only relate to how energy is exchanged between the ocean surface and the atmosphere. It would in no way change any statements or conclusions drawn in the post above:

    • The rate of flow of heat out of the ocean is determined by the temperature gradient in the 'cool skin layer'
    • Despite being only 0.1 to 1mm thick, on average, this skin layer is the major player in the long-term warming of the oceans.
    • The same concept applies to the cool skin layer - warm the top of the layer and the gradient across it decreases, therefore reducing heat flowing out of the ocean.
    • Under cloudy conditions, the cloud cover radiates more heat back down toward the ocean surface than happens under clear sky conditions.
    • The effect of greenhouse gases on ocean heat isn't confined to daylight hours however, they toil away around the clock.
    • Given the atmospheric lifetime of carbon dioxide is many hundreds to thousands of years, we can now understand that long-lived greenhouses will also continue to exert a warming influence on the worlds oceans for a very long time.
    • Because of their effect on lowering the temperature gradient of the cool skin layer, increased levels of greenhouse gases lead to more heat being stored in the oceans over the long-term.
    • This ocean warming mechanism has been observed experimentally, and is also supported by numerical modeling.
    • So although greenhouse gases, such as carbon dioxide, don't directly warm the oceans by channeling heat down into the oceans, they still do indeed heat the oceans, and are likely to do so for a very long time.
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  26. Guiganbresil - clearly wind and solar radiation are important factors over short timescales, that much is discussed in the literature cited. But think about it, how would they alone cause long-term warming?

    How did the oceans get much warmer than they are now during the Paleocene-Eocene Thermal Maximum (PETM) 55 millions years ago, when radiation from the sun was weaker than it is now? Readily explained by the higher levels of greenhouse gases (in the PETM) altering the gradient in the cool skin layer.
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  27. Sphaerica -

    "Our discussion is based on a recognition that except for very light winds (perhaps less than 2 m/s) the Richardson number for the upper meter of the ocean is very small. As in the atmosphere immediately above the surface, heat transfer is forced or passive rather than free."

    The discussion that follows for the next two pages discusses the interaction of wind with the surface... forced heat transfer.

    On the bottom of page 271, Saunders discusses free convective loss case: "In a dead calm, when the Richardson number is very large..."

    At the top of page 272, he states: "If we suppose that the long-wave radiation to clear skies is the major agency for heat exchange between the ocean and the atmosphere... then the temperature difference derived from Eq.(4) is about 0.5C"

    So, free heat transfer vs. forced heat transfer. In one case, wind and humidity - the other dead calm and clear skies.
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  28. Sphaerica -

    "The rate of flow of heat out of the ocean is determined by the temperature gradient in the 'cool skin layer'"

    [ - semantics/word-gaming snipped -]

    "Under cloudy conditions, the cloud cover radiates more heat back down toward the ocean surface than happens under clear sky conditions."

    Yes, so the heat loss from the surface goes down, and the temperature difference of the 'cool layer' goes down per Eq. (5)

    I notice that data from Minnett et al 2001, Figure 12 shows that under low wind conditions (during the day), the cool layer vanishes and becomes warm due to solar heating.

    So I should amend my previous statement - the long wave emission supports the cool skin layer at a dead calm, with clear skies, at night.
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  29. Rob -

    I noticed that the graph you show from RealClimate (Figure 2 above) is from R/V Tangaroa in 2004, but the paper you link to in the text above the figure was published in 2001.

    Unfortunately Minnett does not source his graph (was it published and peer reviewed?) - I am curious if the researcher controlled for wind speed or humidity - given that both are factors in the forced heat transfer case.

    Looking more closely at the orders of magnitude, I see that the effect of clouds shown in figure 2 above is ~100 W/m2 with a slope of ~0.002 K/(W/m2), so a 4 W/m2 change due to a doubling in CO2 would provide a 0.008K change in the skin layer temperature difference. Sun, wind, humidity and clouds jerk the value from -0.6 to +1 K - that is two orders of magnitude larger...
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  30. guinganbresil -..... but the paper you link to in the text above the figure was published in 2001

    The paper linked to is a description of MAERI, the measuring instrument, and it details how measurements are carried out.

    "Unfortunately Minnett does not source his graph"

    I believe the data were obtained during a survey with a different purpose in mind. I don't think this has ended up in the peer-reviewed literature, but Professor Minnett informed me he has a postdoc taking up work on it.

    "Looking more closely at the orders of magnitude..........."

    I can see you're not really following this, sorry but its a very complex subject and not easy to put into layman terms (at least not for me) but I hope this helps:

    - The ocean cool skin is a surface layer where molecular processes dominate, i.e conduction is the form of heat transfer. Therefore a temperature gradient needs to exist (confirmed by Khundzhua [1977]) .

    - Greenhouse gases, such as CO2, warm the cool skin and lower its gradient. The lowered gradient slows the heat flow of heat out of the ocean into the atmosphere. This mechanism allows the oceans to build up heat.

    - This mechanism is pervasive because the increased greenhouse effect is a global phenomenon. In other words, all parts of the ocean surface are subjected to its influence.

    - Changes in heat associated with changing winds, humidity, evaporation etc, can indeed be much larger on short timescales, but are local effects, causing the ocean to either store more or less heat over these shorter intervals. However, the greenhouse gas effect on the skin layer gradient is still toiling away in the background, it's ever-present 24 hours a day (except for those rare occasions when it breaks down momentarily), and is global in scale. So any heat headed for the atmosphere has to run the gauntlet of this greenhouse gas-warmed ocean cool skin layer, and in the process a little bit of heat is stolen away and stored in the ocean.

    - Because of the very long-lived nature of carbon dioxide, it can affect the cool skin layer temperature gradient for hundreds to thousands of years. Of course in Earth's deep past, higher levels of carbon dioxide corresponded with higher ocean temperatures. So the whole conceptual framework shows coherency - elevate carbon dioxide, and the oceans warm - just like they did in the past, and just like they are now.

    - Now what you, and a couple of earlier commenters, seem to be inferring doesn't make any sense whatsoever. If wind speed, humidity etc (there's a bunch of other factors) were the long-term controls on the cool skin layer, wouldn't the ocean temperature just fluctuate within a set boundary determined only by those factors? (windspeed etc).

    - In your hypothesis (?) how could you account for the 20,000 year ocean warming that took place in the Paleocene-Eocene termal Maximum (PETM)? Windspeed? How are the oceans warming now?
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  31. 30, Rob,

    I have to admit, as a result of guinganbresil's comments, I went back to the post and started to try to work things out, and realized there's some ambiguity that led me to a state of mild confusion.

    I think my problem lies primarily in that there are two gradients involved (ocean/air and deep-ocean/cool-skin) and it's not always clear which is being discussed. I think at most points that when you talk about the gradient being decreased, you are referring to the latter (ocean/cool-skin), not the former (ocean/air), but it's not always clear in the text.

    For example, when you talk about GHGs warming the cool-skin layer, this will increase the ocean/atmosphere gradient (the ocean being warmer) which implies the ocean will cool faster, but it decreases the deep-ocean/cool-skin gradient, which I think is the real issue, because it blocks heat loss from the bulk of the ocean through the cool-skin layer. But that's not always clear.

    In addition to this, there are obviously different states in different time frames (e.g. during the day, temperatures are X, Y, and Z, and the system is dominated by solar radiation, while that changes at other times and under other conditions).

    Is it possible to clarify all of this? Perhaps a diagram showing the relative temperatures and net flows in different situations?

    [In my mind the system has 5 possible components, the deep ocean, the cool-skin layer, the atmosphere, and then one of two sources of radiation, either the sun or clouds/atmosphere/GHGs.]

    I think that guingabresil's problems come from focusing on the ocean/atmosphere interface rather than the more important (in this situation) deep-ocean/cool-skin interface... or so I think. Again, I haven't completely wrapped my head around it enough to say this with authority (it's Friday, and it's been a very long week!).
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  32. Sphaerica - fair comment. I'll go over it in the next day or two and tweak a few things to make it clearer. I did produce a graphic but wasn't happy with it - I'll reassess that too, but I'm in the middle of writing a few other posts which rate higher on my agenda.
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  33. 32, Rob,

    Maybe jg can help, if you could just give him a napkin version of what you envision?
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  34. Rob, Sphaerica -

    I think I understand now... Lets start with a picture:

    There are several gradients:

    1 - Air to Surface = very small
    2 - Surface to bottom of 'skin layer' = ~0.5 mm
    3 - Bottom of 'skin layer' to the peak of the diurnal warm layer (when warmed by sunlight)
    4 - The thermocline
    5 - Deep ocean gradient below the thermocline

    I imagine that there is a depth where the diurnal swing in temperature is not observable due to a combination of mixing and the fact that you are too deep to get any direct sunlight warming. I think this depth is still in the well mixed layer in the top...

    The heat transfer between the air and the ocean is related to the air-surface gradient. A warming of the skin layer just increases this gradient, increasing the heat transfer - it is essentially a wash.

    The heat transfer to the deep oceans (by deep I mean 100's of meters not 100's of microns...) is determined by the gradient of the thermocline.

    Now, you can clearly see the gradients shown in the top figure would show a constant buildup of energy in the ocean... As I understand it, the deep, cold water (in the tropics) turns out to be the warm upwelling water in polar regions - rejecting the energy it picked up in the form of latent heat, so you don't see much of a temperature change...

    Remember the currents:

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  35. 34, guinganbresil,

    See Trends in Observation and Research of Deep Ocean Circulation and Heat Transport for more details on currents. They act as an additional mechanism for transporting heat downward into the depths.
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  36. guinganbresil - ""A warming of the skin layer just increases this gradient, increasing the heat transfer - it is essentially a wash."

    No, warming decreases the gradient. Warming from the surface (longwave radiation) means the temperature difference between the bottom of the layer and the (now warmed) top is smaller - therefore the gradient decreases and slows the heat transfer process (ocean to atmosphere).

    The point to remember is that this greenhouse gas-warmed cool skin layer is global in scale - all heat transferred from the ocean has to go through this layer.

    Anyway I'll get around to giving this post a re-write, and include some diagrams.
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  37. Rob - I was imprecise in my language.

    The warming of the skin layer increases the gradient between the cooler air and the top of the warmer skin layer - increasing the heat transfer from the water to the air.
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  38. This skin layer would be cool as a result of either heating the atmosphere above, or from evaporation. Either way it would be thermally unstable, sink, mix and cool the water below. One cannot think of it as an oil slick that stays on top. It is really the mixed layer. It is mixed mechanically by waves and shear at current boundaries. The mixed layer varies considerably in depth, extending to 2000m (Levitus).
    It can certainly become warmer when it is unable to radiate as much energy to a warmer air mass above.
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  39. truckmonkey - it looks to me like some basic misunderstandings here. Have a look at Does back-radiation heat the ocean series at SoD.
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  40. Scaddenp-OK I read it, including the comments. Don't see any basic misunderstandings. What I do see is that something so apparently simple as heat transfer between the ocean and the atmosphere pushes us beyond our understanding of the physics.
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  41. Truckmonkey, your comments about sea being "warmed by the atmosphere" seemed to imply you were thinking of conductive or convective heating, not radiative heating. If this is not so, then no issue. Also, the issue of skin layer The cool skin was raised in the last of the series.
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  42. Responding to trunkmonkey from another, inappropriate, thread: Your contention that UV from the atmosphere cannot transfer energy to the ocean is incorrect, as scaddenp noted months ago in the comment immediately above this one. For yet another place to learn how that works, see this RealClimate post.
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  43. trunkmonkey, I apologize for my comment to you being off base; I neither read nor typed carefully. I saw your response briefly before it was deleted for being off topic. Please repeat your comment here on this thread.
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    Moderator Response: Sorry, I might have incorrectly deleted trunkmonkey's response for being off topic, because I was skimming and thought it was posted on the other thread rather than this one. If so, please comment again here, trunkmonkey.
  44. Trunkmonkey,

    The ocean does heat proportionately to the warming atmosphere. You have forgotten to take into account the fact that the heat capacity of the ocean is 1000 times greater than the atmosphere. You would expect 1000 times as much energy to go into the ocean as into the atmosphere. The graph in the other thread shows energy absorbed. As expected, the ocean has absorbed much more energy.

    You now agree that a mechanism for ocean warming exists but claim the magnitude is incorrect. Previously you claimed that "The problem is there is no mechanism for greenhouse gasses to warm the oceans". You are not arguing in good faith.
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  45. I know that this thread is old, but some comments are still appropriate.

    As I understand this article, the decrease in temp gradient in the cool skin layer is what allows increases in atmospheric CO2 concentrations to further warm the oceans.

    This can only be possible if conductive warming of the cool skin layer from the ghg warmed air above can prevent more heat loss than an increase in evapoaration heat loss due to a ghg warmed atmosphere.

    Greenhouse gases, such as carbon dioxide, trap heat in the atmosphere and direct part of this back toward the surface. This heat cannot penetrate into the ocean itself, but it does warm the cool skin layer, and the level of this warming ultimately controls the temperature gradient in the layer.

    From other threads, it is known that the increase in evaporation heat losses is 4%.  This is substantial.  Since conductive heat transfer from gas to liquid is quite small, it is obvious that the increase in evaporative losses shall dominate.


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    Moderator Response: (Rob P) Note the experiment carried out by Professor Minnett which effectively debunks your claim - it is a central plank of the post.

    Also read Fairall (1996)- their observations & modelling demonstrated the the net effect of the cool-skin is to warm the ocean in the tropics. This is where the bulk of sunlight enters the ocean.
  46. Kevin, have you been over to SoD on this subject?

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  47. Kevin,

    You wrote:

    Since conductive heat transfer from gas to liquid is quite small, it is obvious that the increase in evaporative losses shall dominate.

    Are you really trying to say that the dominant effect of a warmer atmosphere is to increase evaporation so much it cools the ocean? Or did I misread your post at #45?

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  48. Kevin @45: 

    1)  If there is no increase in skin surface temperature, there is no increase in evaporation (by your argument), and hence no evaporative cooling.  Therefore while an increase in evaporation may limit the increase in temperature (by your argument), it cannot prevent there being an increase.

    2) In a confined volume, an increase in evaporation will result in an increased vapour pressure of H2O in the atmosphere above the water surface.  The increased vapour pressure results in an increased frequency of water molecules in the amosphere striking the surface, and being absorbed, carrying there energy of motion into the water as heat.  After warming stops, an equilibrium will be reached in which the frequency of water molecules entering the atmosphere from the liquid will equal the frequencey of molecules entering the liquid from the atmosphere resulting in an equilibrium of transfer of water molecules and (if atmosphere and liquid are the same temperature) of energy transfers.

    If the atmosphere is warmer than the liquid, on average the energy transferred to the liquid by water molecules being absorbed will excede the energy transfer to the atmosphere by evaporation.  Warming the atmosphere without warming the liquid will result in an increased energy transfer to the liquid by this means.

    The Earth's atmosphere is slightly more complex.  It is closed for practical purposes, but some of the water vapour in the atmosphere precipitates out.  The increase in evaporative cooling with increased surface temperature is therefore limited by the increase in precipitation, not by the increase in sea surface temperature.  As Kevin has shown nothing about how much precipitation will increase, his argument does not even get of the ground.

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  49. Kevin, even with greater evaporation, when one considers all the energy fluxes into and out of the ocean cool skin layer, as long as the change in net energy flux causes the cool skin to warm, the temperature gradient between the cool skin layer and the bulk ocean below it will decrease.

    Conduction from atmosphere to ocean is not the only (and I suspect not even the primary) manner by which energy transfers from the atmosphere into the ocean cool skin layer.

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  50. On average the oceans are always warmer that the atmosphere and net transfer is skewed 14% from the oceans to the atmosphere. Theoretically the thermal mass of the atmosphere, if it were warming, would reduce the margin and warm the oceans. (snipped)

    The oceans on average have continued to warm, quite a bit. This is really wierd. But what is even wierder is that all of the net ocean warming can be accounted for by the North Atlantic, the Indian, and the Arctic oceans. All the other oceans are flat or cooling.

    So if you can explain to me how the atmosphere could cause these three oceans to warm and allow the rest to languish or cool, I would be very interested to hear it.

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    Moderator Response: (Rob P) - Sloganeering snipped.

    Do you have a reputable scientific link to back up your claim? I'm aware the the Pacific Ocean has not warmed in the last few years, but this is nothing new. It's happened many times before, each time followed by abrupt warming.

    The oceans are dynamic and shuffle a lot of heat around - even while the oceans are gaining heat though the greenhouse gas forcing of the cool-skin layer. There is no expectation that year-to-year warming is continuous because changes in ocean circulation, such as ENSO (La Nina/El Nino), can temporarily act against the long-term warming trend.

    Therefore, over short time frames, it can appear very noisy, but thus far, over longer (decadal) timescales, the warming pattern is smooth and incompatible with natural variation. See Sedlácek & Knutti (2012).

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