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The Sun Has Cooled, So Why Are The Deep Oceans Warming?

Posted on 28 October 2013 by Rob Painting

Key Points:

  • In keeping with scientific expectations, the ongoing emission of greenhouse gases from human industrial activity is causing the Earth to build up heat - a process known as the increased (enhanced) greenhouse effect.
  • Though this is not widely-known, the increased greenhouse effect traps more of the sun's energy in the ocean, causing the upper ocean to grow warmer over time.
  • Despite a much smaller increase in warming of the global atmosphere since the year 2000, the ocean, and the deep ocean in particular, has warmed rapidly.
  • The following series of posts will explain the fundamentals of the wind-driven ocean circulation, including a natural multidecadal oscillation in the circulation that, when it intensifies, is responsible for removing heat from the surface ocean and mixing it into deeper layers below.
  • The combination of the greenhouse gas-forced warming of the surface ocean, and the intensified (but temporary) vertical mixing of heat into the deeper layers, is consistent with the deep ocean warming which has been measured.

Figure 1 - Spin-up of the South Pacific subtropical ocean gyre centered near New Zealand between 1995-2004. As the wind-driven ocean circulation intensifies surface water is pushed toward the centre of the gyre where it piles up and raises the local sea surface height (anomalies shown are in cm). With the gyre spinning faster, and nowhere else to go, the water is pumped down into the ocean interior by a process known as Ekman pumping, and thereby warms the deep ocean. Animation created from images in Roemmich (2007).

The Motion of the Ocean  

The anomalous warming of the deep ocean over the last few decades has received a great deal of attention on climate-related blogs and in the mainstream media, but there has been little discussion of the mechanisms driving this deep ocean warming. Contrarians often seem to think that the warming of the deep ocean can only be accomplished by magic, or Star Trek-style teleportation devices, however that probably stems from basing their expectations on some gut instinct of how they feel the ocean should behave. The oceans don't behave according to gut instincts, they behave according to the laws of physics and, sadly, much of the physics is not very intuitive at all. 

It is hoped that some of the fundamentals of oceanography conveyed in this series of posts will enable readers to appreciate, for instance, why the surface currents (shown in Figure 1) converge around New Zealand and are directed downwards into the ocean - taking surface heat with them. But more importantly, it is hoped that readers realize that, despite an incomplete picture of all the details, there is a great deal known about the ocean circulation, and that it plays a large role in recent global temperature trends.

First up, the Relevant Background Context: Greenhouse Gases are Warming the Oceans

Figure 2 - Land, atmosphere, and ice heating (red), 0-700 meter OHC increase (light blue), 700-2,000 meter OHC increase (dark blue). From Nuccitelli (2012).

Over the last 3 decades the sun has seen a very slight decrease in the amount of solar radiation it puts out. In spite of this, there has been a tremendous build-up of heat in the ocean (see Figure 2), especially the deep ocean (Levitus [2012], Nuccitelli [2012], Balmaseda [2013]). Given that the heating of the oceans is almost entirely due to sunlight (in the form of shortwave radiation) entering the surface layers, this raises the question of how this ocean warming is possible. Why are the oceans warming so much when the main source of heat input into the upper ocean has diminished slightly?

As discussed in this SkS post, and the Real Climate post by Professor Peter Minnett it is based upon, the oceans are warming due to an increase in the greenhouse effect. The oceans are heated from sunlight entering the surface, and because they are typically warmer than the overlying air, the net flow of heat is from the warmer surface ocean to the cooler atmosphere above. Turbulence is suppressed at the ocean-atmosphere boundary, so heat has to travel through a conductive layer within the cool-skin - the thin surface layer of ocean in contact with the atmosphere. The rate of heat flow is, therefore, determined by  the thermal gradient through the cool-skin layer (Saunders [1967], Grassl [1976].

As additional greenhouse gases accumulate in the atmosphere, they trap more heat (longwave radiation) and reflect more of it back toward the ocean surface. Strong absorption of longwave radiation occurs in the cool-skin, but is compensated by the powerful emission (loss) of longwave radiation to the atmosphere at the ocean surface (e.g. Konda [2004]) This blocks heat from reaching the ocean below the cool-skin, but it does warm the upper portion of the cool-skin layer and the thermal gradient through the layer is therefore reduced. In doing so, this lowered thermal gradient slows the flow of heat out of the ocean and causes the oceans to grow ever warmer over time. So, in a similar manner to that where greenhouse gases trap heat in the atmosphere by reducing heat lost to space, they carry out a similar function in warming the surface oceans by reducing heat lost to the atmosphere.

Figure 3 - observations carried out aboard the New Zealand research ship Tangaroa shows upper ocean warming when the cool-skin layer is warmed by stronger downward longwave radiation (heat) brought about by increased cloud cover (i.e. more heat is reflected back toward the ocean surface by clouds than comparable clear-sky conditions). The vertical axis is the temperature difference between the skin layer and the ocean bulk 5 cm's below as a function of longwave radiation (horizontal axis). Image from Real Climate.

The observations depicted in Figure 3 show that as the longwave radiative forcing increases (becomes less negative - as plotted), the surface oceans become warmer. Despite this cloud-forcing effect being a great deal stronger than greenhouse gas forcing, the measurements offer up observational evidence of the tendency of the reduced thermal gradient in the cool-skin layer to warm the surface ocean. 

Ocean Temperature & Carbon Dioxide: A Committed Long-Term Relationship 

It's now possible to understand why the ice core records over the last almost-million years show such a strong correlation between CO2 (the main greenhouse gas) and global temperature - not only do greenhouse gases trap heat in the atmosphere, they trap heat in the ocean, warming up the entire Earth system over time.

Figure 4 - the relationship between atmospheric CO2 and global temperature (and therefore ocean heat content too) over the last 400,000 years, as indicated by the Vostok ice core in Antartica.

This mechanism also helps to explain why the Earth has gradually cooled over the last 50 million years (Lear [2000], Zhang [2011], Anderson [2011]), despite the sun growing warmer over this period of time. As the atmospheric concentration of carbon dioxide has gradually declined, so too has the temperature of the atmosphere and oceans. 

The beauty of the concept of the increased greenhouse effect is that it has great explanatory power and enables us to solve some pieces of the great climate puzzle. CO2 is not the only factor of course, but for many periods of the past we can see why the oceans warmed or cooled in deep time, why they warmed and cooled over the last million or so years, and why they are warming now. But it doesn't tell us everything about the changes we have observed over recent decades. That requires knowledge of the wind-driven ocean circulation, a natural phenomenon that moves back-and-forth between phases of intense or sluggish circulation over decadal timescales, and therefore alters the short-term rate of ocean heat uptake.

This natural oscillation in the global oceans is largely responsible for the recent deep ocean warming because, at times when the circulation is strong, it removes heat from the surface layers and pushes it down into the ocean interior. But before examining the wind-driven ocean circulation, it makes sense for us to first look at some of the factors which influence its general behaviour - appreciating why it works the way it does, rather than just how it works. A greatly neglected consideration in blog-related discussions is the Earth's rotation, but this is probably the largest dynamical influence over the global oceans. This will be the subject of the next installment. 

Next: Deep Ocean Warming and the Coriolis Effect

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Comments

Comments 1 to 16:

  1. This mechanism also helps to explain why the Earth has gradually cooled over the last 50 million years (Lear [2000], Zhang [2011], Anderson [2011]), despite the sun growing warmer over this period of time. As the atmospheric concentration of carbon dioxide has gradually declined, so too has the temperature of the atmosphere and oceans.

    Cue someone claiming that "Skeptical Science shows the Earth is on a long-term (50+ million year!) cooling trend - we'd better burn more fossil fuels before we all freeze permanently!!!"

    On a serious note, this is a good start to what will surely be a fascinating primer on the behaviour of the ocean (at larger scales).

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  2. But Composer, the Earth has been cooling, and it's because of increasing solar radiation, or so an engineer chap, with whom I was discussing climate recentely, claims.  To be fair, I don't think he actually put the two together in a cause-effect relationship.  He also claimed that the theory of the greenhouse effect was 20 years old, water vapor feedback is strongly negative, and the greenhouse effect is a giant fraud.  Again, no reconciliation between these claims, but then in the postmodern world having a consistent physics is "old school" and worthy of a chortle from one's betters. 

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  3. DSL

    I would be interested to know what branch of engineering this chap was from. Civil, Electrical, Chemical - maybe they can be forgiven.

    Mechanical and they should know better - thermodyamics is meat and potatoes to a mechanical engineer.

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  4. Glenn- Chemical Engineers do more thermo than mechanical engineers, who can get stuck with gear trains, transmissions and the like.  Chemical engineers convert lab chemistry to functioning chemical plants- and must use heat transfer, mass transfer, kinetics Daily.  Gotta run.

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  5. Here's his attempt at intimidation:

    "Nice try. BTW, I am a Chemical Engineer who graduated from the top-ranked undergraduate engineering school in the country. I was awarded U.S. Patent #5,348,662 for the development of a wastewater recycling process.
    And I obviously know more than you do, because I know how to navigate hyperlinks on the Internet."

    The link he's referring to (and referring me to) is Steve Goddard's claim of recovery based on one year's growth in 1m+ ice.  No comment when I pointed out the flaw in SG's implied argument--and the implied error in judgment by this guy for even reading such garbage.

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    Moderator Response:

    [JH] You, Dave123, & Glenn Tamblyn are skating on the thin ice of being off topic. This thread is not a chat room. 

  6. Composer99 #1 There's a less fanciful aspect that I've not seen discussed. If the coal was used judiciously it could help mitigate many thousands of years of the approaching glaciation period. I would think (I've no time now to work it out) that it's far less than required to prevent any "ice age" at all, but it sure would help. There is some ideal rate (such as 0.05 ppmv/year for 80,000 years if that largest coal reserves estimate is correct) that might have a noticeable mitigating effect through that period. Instead our under-evolved species will likely burn the whole lot in a spasm lasting the next 600 years and cause a big killing temperature spike following by a decline into a killer ice age.

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  7. Back on topic…

    Having read the RealClimate post (and avoiding all the off topic comments posted there…) a simplistic perspective seems to be as follows:

    Assuming an idealised model, where there is equilibrium, and using average temperatures in ocean and atmosphere (lower troposphere) so that the top few meters of the ocean are warmer than the atmosphere (because the top few meters of the oceans absorb SW radiation), LW radiation from the ocean to the atmosphere is controlled by an ocean skin layer whose gradient is negative (i.e the bottom of the skin layer is warmer than the top). This gradient controls the amount of LW radiation emitted by the oceans into the atmosphere.

    So let me stick my neck out a bit…

    We now consider the effect of an increase in GHG which in turn warms the atmosphere.

    This reduces the gradient of the skin layer, because the increased LW radiation from GHG will get absorbed by the skin layer.

    The reduced temperature gradient reduces the heat flux from the oceans into the atmosphere.

    Since less heat goes from the oceans into the atmosphere, the increase in atmospheric temperature due to increasing GHG declines.

    And since less heat is emitted by the oceans, the oceans start to warm, with ocean dynamics carrying that heat down to greater depths.

    Eventually the increase in ocean temperature is going to re-establish a larger skin layer gradient, causing the oceans to increase the heat flux into the atmosphere, which in turn will cause the atmosphere to start warming up again, with ‘eventually’ being the key word!

    So sticking my neck out even more…

    At a very simplistic level, one could look at atmosphere vs ocean temperatures, and guess that, as a first approximation, we will have no more than 20 years of ‘stable’ atmospheric temperatures. Why 20 years? Temperature records indicate a ‘stable’ atmospheric temperature between 1950 and 1970. Although research has suggested that this, and the current hiatus in temperature changes may be due to aerosols (Wilcox et. Al. Environmental Research Letters, June 2013), it seems obvious that if the oceans are emitting less heat into the atmosphere because of a reduced skin temperature gradient, then that could be a contributing factor to the current hiatus. If that is true, we can expect to see atmospheric temperatures to start increasing within the next few years (since the hiatus started about 15 years ago, give or take...). Hm, could be an interesting topic for some real research…

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  8. ClimateChangeExtremist #7 That doesn't sound right because the LWR has energy and it must go somewhere. Though I just found out about the details of this effect last week, I suggest it is that heat from ocean to atmosphere that would otherwise come from very slightly deeper (from SWR warming) is being replaced by LWR energy to atmosphere (perhaps mostly evapotranspiration, need to check that) so heat to atmosphere is not being reduced as you appear (note 1) to suggest by rather the LWR energy is blocking/slowing part of it from leaving oceans and replacing it with its own energy. That causes warming of both atmosphere and oceans. I don't have knowledge to comment on a 20-year rationale --- go for it. 

    Note 1: "less heat is emitted by the oceans" yes if you divide "oceans" into this skin layer and the part beneath. Extra heat emitted by the skin, less by the oceans, I suggest. Brings a definition issue though because every place oceans is mentioned it must not include this skin to remain consistent. I've not seen indications that it's done that way. 

     

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  9. ClimateChangeExtremist - A reduced viscous skin layer thermal gradient leads to less atmospheric warming over the oceans, but that gradient is determined by both the incoming LWR and by the amount of circulation in the ocean. A series of La Nina's, for example, bring more cool water to the surface and decrease the gradient. 

    The 1940's-1975 'hiatus' (more than 20 years, note!) appears to be clearly driven by the forcings; a resumption of more normal volcanic activity (1910-1950 being quite low in that respect), variations in anthropogenic forcings, and the natural variations like the ENSO.

    GISS model forcings - SeparateGISS model forcings - Net

    [Source]

    20 years doesn't seem (IMO) to be a limiting/driving time span. Temperatures will ramp up, regressing to the mean trend, when natural variations such as ENSO swing back. At that time we should be better able to determine whether the underlying forcings such as aerosols have changed.

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  10. KR #9 The concept that LWR causes "less atmospheric warming over the oceans" does not seem correct because it breaks the law of conservation of energy. If there is no LWR at all then SWR-produced ocean heat warms the atmosphere. If LWR is added then that is adding energy in the very thin layer. It must go somewhere. It cannot cause less heating either up or down. I think the concept is that it blocks and replaces some ocean heat going up. 

     

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  11. grindupBaker - Increased downward LWR decreases ocean cooling (oceans heat faster), while higher circulation of cool water to the surface ocean layers decreases atmospheric warming (and again, the oceans heat faster). In both situations the skin layer gradient is reduced and a higher percentage of incoming solar energy ends up being retained by the oceans. 

    The fact that atmospheric warming and increased downward LWR increases energy retention in the oceans is in fact the central aspect of how GHG's warm the ocean

    Since the oceans act as a thermal "flywheel", accelerating the warming of the oceans just speeds equilibrium with forcing imbalances, meaning that overall warming of the climate occurs faster. 

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  12. KR #11 My issue is not addressed. To clarify: I am in agreement with "decreased ocean cooling",  increased ocean warming, all the ocean stuff. I have specific disagreement with "A reduced viscous skin layer thermal gradient leads to less atmospheric warming over the oceans" because, by straightforward logic, this would depend on the cause of the reduced thermal gradient. If the cause was more circulation of cool water to the surface then " less atmospheric warming over the oceans" I agree. But if the cause was more LWR hitting the skin layer then there's no reason for the atmospheric warming over the oceans to reduce even though ocean cooling reduces --- because there's more energy overall due to more LWR.

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  13. grindupBaker - There's no contradiction here. A warm atmosphere decreases the skin layer gradient, retaining more energy in the oceans and warming them faster, but in the balance reducing the amount of energy going from the oceans to the atmosphere. The rate of incoming sunlight->ocean->*->atmosphere energy flow is directly dependent on that gradient, at the '*' (as, of course, one of a number of factors in total energy)

    Warming of the atmosphere, a change in temperature, depends on an energy imbalance, an energy accumulation in the atmosphere. If energy rising from the ocean decreases (because it's warming the oceans instead) incoming energy to the atmosphere decreases, as does the atmospheric warming rate. 

    It's really a matter of where at any time the top of atmosphere (TOA) energy imbalance is accumulating - more in the oceans (water warming fast, air warming slowly if at all), or more in the atnosphere (air warming fast, water warming more slowly)

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  14. KR #13 I'm quite sure that I'm correct (or at the very least annoying) on this one because it's fundamental, requiring no special knowledge. There's no contradiction here. The SKS post I found on the topic says nothing about reduced ocean cooling causing reduced atmospheric warming just above the oceans and there's no reason why it would given that the reduced ocean cooling is not caused by ocean mixing in this case. The +LWR warms both. The skin layer temperature is increased, slowing the rate of heat leaving the oceans, but that same skin layer temperature increase must increase warming to the atmosphere.

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  15. grindupBaker - I'm afraid I'm going to have to disagree with you, in particular with "...that same skin layer temperature increase must increase warming to the atmosphere". The 'skin layer' is less than a millimeter thick, almost no thermal mass to speak of. The ongoing warm/cooling of the oceans and atmosphere is determined by the rate of energy transfer, not the instantaneous temperature of any component. And the thermal gradient, the temperature difference top/bottom of the ocean surface has a direct bearing on that rate - if the gradient is decreased for any reason, whether higher exchanges with deep water or with a warm air mass (for that matter, cloud changes), the rate of energy flow from the ocean to the atmosphere decreases. 

    I think this exchange is getting bent around terminology and implied meanings - but I believe what I've said is correct. 

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  16. The one factor missing in all this talk about the Skin Layer is that it is a System which contains a myriad of Life. It is known. for example, that lifeless ( distilled ) water and air transpires at a vastly decreased rate ( about 100x) 

    It seems very likely that the processes of long-wave radiation, surface mixing, etc will be fundamentally altered by the mix of organisms at the boundary, just as transpiration is.

    These surface Lifeforms could, in turn, be affected by factors such as pH, the presence of pollutants such as Oil, and Plastic objects, and the status of Lifeforms slightly lower down (e.g. krill ).

    Much to think about.

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