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Global warming and ocean heat content

Filed under: — gavin @ 3 October 2011

The connection between global warming and the changes in ocean heat content has long been a subject of discussion in climate science. This was explicitly discussed in Hansen et al, 1997 where they predicted that over the last few decades of the 20th Century, there should have been a significant increase in ocean heat content (OHC). Note that at the time, there had not been any observational estimate of that change (the first was in 2000 (Levitus et al, 2000)), giving yet another example of a successful climate model prediction. At RC, we have tracked the issue multiple times e.g. 2005, 2008 and 2010. Over the last few months, though, there have been a number of new papers on this connection that provide some interesting perspective on the issue which will certainly continue as the CMIP5 models start to get analysed.

The most recent paper was a new study from NCAR out last week that looked into what happens to OHC in models when there are occasional 10 year periods with no trends in global surface temperatures (Meehl et al, 2011).

It is well-known (or at least it should be) that simulations for late 20th C and early 21st Century do not produce monotonically increasing temperatures at the annual or decadal time-scale. For the models used in AR4, the decadal trends expected under estimates of present day forcings are roughly N(0.2,0.14) (i.e. a Gaussian distribution centered on 0.2 ºC/decade with a standard deviation of ~0.14ºC/decade. This implies that one would expect an 8% probability of getting surface temperature trends less than zero in any one decade.

The Meehl et al study looked at the changes in ocean heat content during these occasional decades and compared that to the changes seen in other decades with positive surface trends. What they found was that decades with cooling surface temperatures consistently had higher-than-average increases in ocean heat content. This makes perfect sense if there is internal decadal variability in the fluxes that connect the deeper ocean to the surface ocean (which of course there is). An anomalous downward heat flux reduces the ocean surface temperature (and hence global surface temperature), which generates an anomalous heat flux into the ocean from the atmosphere (because the flux into the ocean is related to the difference between atmospheric and ocean temperature). And this of course increases total OHC.

A related study from the UK Met. Office looked at the relationship between the ocean heat content changes in the top 700m and the total ocean heat content change in models (Palmer et al, 2011). They found that (unsurprisingly) there is more variability in the top 700m than in the whole ocean. This is important to quantify because we have better estimates of the upper ocean OHC change than we do of changes in the whole ocean. Observational studies indicate that the below-700m increases are not negligible – but they are poorly characterised (von Schuckmann et al, 2009). The Palmer study indicates that the uncertainty on the decadal total OHC change is about 0.15 W/m2 if one only knows the OHC change for the top 700m.

So what can we infer about the real world from these tests? First, we can conclude that we are looking at the right quantities. Total OHC changes are a good measure of the overall radiative imbalance. Second, there is likely to be a systematic issue if we only look at the 0-700m change – this is a noisy estimate of the total OHC change. Third, if the forcings are close to what we expect, we should anticipate that the deeper ocean (below 700m) is taking up some of the slack. There are of course shorter term sources of variability that also impact these measures (OHC changes associated with ENSO, solar irradiance variability over the solar cycle) which complicate the situation.

Two further points have come in comment threads recently that are related to this. The first is whether the changes in deep ocean heat content have any direct impact other than damping the surface response to the ongoing radiative imbalance. The deep ocean is really massive and even for the large changes in OHC we are discussing the impact on the deep temperature is small (I would guess less than 0.1 deg C or so). This is unlikely to have much of a direct impact on the deep biosphere. Neither is this heat going to come back out from the deep ocean any time soon (the notion that this heat is the warming that is ‘in the pipeline’ is erroneous). Rather, these measures are important for what they tell us about the TOA radiative imbalance and it is that which is important for future warming.

The second point is related to a posting by Roger Pielke Sr last week, who claimed that the Meehl et al paper ‘torpedoed’ the use of the surface temperature anomaly as a useful metric of global warming. This is odd in a number of respects. First, the surface temperature records are the longest climate records we have from direct measurements and have been independently replicated by multiple independent groups. I’m not aware of anyone who has ever thought that surface temperatures tell us everything there is to know about climate change, but nonetheless in practical terms global warming has for years been defined as the rise in this metric. It is certainly useful to look at the total heat content anomaly (as best as it can be estimated), but the difficulties in assembling such a metric and extending it back in time more than a few decades preclude it from supplanting the surface temperatures in this respect.

Overall, I think these studies show how we can use climate models to their best advantage. By looking at relationships between key quantities – those that can be observed in the real world and those that are important for predictions – we can use the models to interpret what we are measuring in the real world. For these cases the inferences are not particularly surprising, but it is important that they be quantified. Note that the assumption here is akin to acknowledging that since the real world is more complicated than the (imperfect) models, inferences in the real world should at least be shown to work in the models before you confidently apply them to reality.

However, it is the case that none of these studies prove that these effects are happening in the real world – they are merely suggestive of what we might strongly expect.


References

  1. S. Levitus, "Warming of the World Ocean", Science, vol. 287, pp. 2225-2229, 2000. http://dx.doi.org/10.1126/science.287.5461.2225
  2. G.A. Meehl, J.M. Arblaster, J.T. Fasullo, A. Hu, and K.E. Trenberth, "Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods", Nature Climate change, vol. 1, pp. 360-364, 2011. http://dx.doi.org/10.1038/nclimate1229
  3. M.D. Palmer, D.J. McNeall, and N.J. Dunstone, "Importance of the deep ocean for estimating decadal changes in Earth's radiation balance", Geophysical Research Letters, vol. 38, pp. n/a-n/a, 2011. http://dx.doi.org/10.1029/2011GL047835
  4. K. von Schuckmann, F. Gaillard, and P. Le Traon, "Global hydrographic variability patterns during 2003–2008", J. Geophys. Res., vol. 114, 2009. http://dx.doi.org/10.1029/2008JC005237

283 Responses to “Global warming and ocean heat content”

  1. 1
    Magnus W says:

    A common pseudo skeptic punchline is that the heat is coming from the deep ocean… got any quick easy accessible data to show that this is not the case?

  2. 2
    brian m flynn says:

    I understand Dr. Pielke Sr. has for some time now claimed the Argo system has yet to detect an increase in upper ocean heat content or an ocean heat transport to the deep. Do you concur with his claim and, if not, can you explain how and where this transport to the deep is accomplished? Thank you for your time.

    [Response: Argo measures temperatures, not heat flux. You can calculate a net heat flux into the top 700m of the ocean given the changes in temperature in this region, but Argo cannot measure the heat flux through that region. The latest data from Willis and others indicates that ocean heat content (top 700m) is increasing, although a lower rate than in the last decade, and the (less comprehensive) studies related to below-700m oceans indicate an increase as well. Most heat transport into the deep ocean will occur in the down-welling branches of the overturning circulation, centered in the North Atlantic and the Southern Oceans. Diffusive fluxes in the rest of the ocean will be much smaller. - gavin]

  3. 3
    Alex Harvey says:

    Dear Gavin,

    Thank you for the update. I note with interest that you have read Roger Pielke Sr’s recent post on this topic but have not discussed his first “major issue”.

    He asks:

    1) “If heat is being sequested in the deeper ocean, it must transfer through the upper ocean. In the real world, this has not been seen that I am aware of. In the models, this heat clearly must be transferred (upwards and downwards) through this layer. The Argo network is spatially dense enough that this should have been seen.”

    Do you agree with this?

    [Response: Obviously heat going below 700m must have passed through the upper ocean. However, the notion that Argo could see this is odd. Argo measures temperature, not flux. The net flux into a layer is calculated by looking at the change in temperature. It cannot tell you how much came in at the top and left at the bottom, only how much remained. - gavin]

    I would add that I have never seen any comment on any blog that addresses this point. I have always wondered – perhaps naively – how Kevin Trenberth and others can believe that heat gets to the deep ocean without being seen passing through the upper 700 metres.

    [Response: Of course they don't believe that. You are setting up a strawman. - gavin]

    Pielke also raises another interesting point at the end of his post.

    “…if heat really is deposited deep into the ocean (i.e. Joules of heat) it will be dispersed through the ocean at these depths and unlikely to be transferred back to the surface on short time periods, but only leak back upwards if at all. The deep ocean would be a long-term damper of global warming, that has not been adequately discussed in the climate science community.”

    [Response: This is discussed all the time going back to Hansen's paper referenced above. I have never heard any scientist claim that the heat 'leaking back' is an issue. - gavin]

    I would be interested in your thoughts on this point as well.

    Finally, aside from these questions about Pielke’s post, I am also interested in the recent (unpublished I think) Hansen, Sato, Kharecha & von Schuckmann paper entitled, “Earth’s Energy Imbalance and Implications”. In that paper, especially in sections 6 & 7, it appears – to me anyway – that James Hansen and his colleagues have given up on the search for the so-called “missing heat” in the deep ocean and have instead concluded it must have been radiated away as a result of the negative anthropogenic aerosol forcing. I take this as suggesting that Hansen has parted company with Kevin Trenberth and others and has conceded that the IPCC models are flawed – flawed in their “climate response functions”.

    Do you know if the model used by Meehl suffers the same problem with the “climate response function” that Hansen discusses? Do you have any other comments on the Hansen et al. paper?

    [Response: I don't see any contradiction. Meehl et al are looking at a generic behaviour which will exist in all models, while Hansen is thinking about the specific forcings and response for the last decade. Different issues. - gavin]

    Thank you in advance and best regards,
    Alex Harvey

  4. 4
    Ray Ladbury says:

    Magnus, I am not a climate scientist, now do I play one on TV. However, in order for there to be heat coming from the deep ocean, there must be mixing of deep ocean with shallower water. Such upwelling of deep water only happens in a few select locations. If it were occurring, those locations ought to be warming faster. They aren’t

    A more plausible denialist meme is that the warming has occurred because there is less mixing of warm surface water with the briny deep. Here again, though, we’d expect to see the changes first in the oceans. We don’t. We’d also expect to see changes in chemical composition of solutes that would accompany the changes in energy flow.

    Saying “It’s the oceans” is just another way of saying it’s not CO2. It’s a little more sophisticated than saying it’s the sun, because it’s a little harder to rule out.

  5. 5
    wili says:

    Do we have any idea how long the heat is likely to stay sequestered in the ocean? Does it stay in a particular layer that then comes back up, or is it likely dissipated into the rest of the ocean by then?

    [Response: The circulation time for the deep ocean is on the order of hundreds to thousands of years. Change there is very slow - which makes the changes seen so far quite surprising. At any new (warmer) equilibrium, there will be a significant increase of OHC over what there was before. The damping of the rate of surface warming or the warming in the pipeline isn't anything to do deep ocean heat coming back out. I have no idea where this idea originated, but it is not accurate. - gavin]

  6. 6

    #1 Magnus W

    I would say that they have no proof that it is deep ocean upwelling of heat and that the science ‘suggests’ that the increased radiative forcing is heating the oceans.

    Generally, for them to have substance in their argument they need to provide scientific evidence that holds up to scrutiny. Otherwise, they can make claims all day long, that does not make the claims right, it just means they like talking about unproven ideas. Simply put, they have to show mechanism not hyperbole.

  7. 7
    Gorm R. Larsen says:

    In brief – is it thus correct to emphasize that ‘stratified ocean heat content variations’ appear to be a major cause for the natural large scale atmospheric temperature variability ?

  8. 8
    grypo says:

    Gavin,

    The damping of the rate of surface warming or the warming in the pipeline isn’t anything to do deep ocean heat coming back out. I have no idea where this idea originated, but it is not accurate

    I believe confusion about that could have come from Kevin Trenberth’s statement in the press release for the Meehl study.

    Kevin Trenberth, a study author and NCAR scientist, said: “… this study suggests the missing energy has indeed been buried in the ocean, the heat has not disappeared and so it cannot be ignored. It must have consequences.”

    It confused me. I was under the impression he was saying that it had warming consequences on short timescales. He was not as as explicit as you in discussing that the implications were on the TOA radiative imbalance. Or do you think he meant something else?

    [Response: You'd have to ask him. 'Consequences' is pretty vague. However, for context, a flux of 0.5 W/m2 into the deep ocean (700 - 3700m (average depth)), is around 0.5*365*24*3600*10/4000/1000/3000 = 0.013ºC/decade on average. This is a much bigger heat flux then we expect, but we don't expect changes to be evenly spread across the ocean. In some places temperature rises will be higher (N. Atl., S. Ocean etc.), but while significant in terms of heat flux, the change is not likely to be important in terms of deep ocean temperature. - gavin]

  9. 9

    Two questions:

    First, will this tendency for heat to flow into the deep ocean appreciably slow the rate of warming? That is, if some value of climate sensitivity says we have “set the thermostat” to X for a given level of CO2, will this possibly mean that we don’t actually see the effects of at least some that warming for a much longer time, and if not, how much longer? Can this be well quantified?

    This could be a godsend or a debacle. If the time it takes to actually realize climate change is long enough it could give us more time to transition away from fossil fuels and let CO2 start to drop out of the atmosphere (before truly dangerous temperatures are reached).

    Alternately, it could just provide fodder for skeptics to claim that climate change isn’t happening or isn’t a real problem, leading the world to adopt a business as usual times five strategy and ultimately make things far, far worse (much like touching a hot stove, because by the time your nerve endings tell you “hot,” you’ve already burned your skin).

    Second question: Gavin has twice already in these comments talked about the transport of heat into the deep ocean not being the “warming in the pipeline.” This phrase gets thrown around a lot without clarification. Can anyone explain exactly what the phrase does mean, and what mechanisms delay warming?

  10. 10
    Andrew says:

    Can anyone familiar with the climate literature say if the estimation resulting in the N(0.2, 0.14) distribution accounted for the possibility of long memory/long range dependence (Cf. Samorodnitsky, Taqqu, etc.)? The trick here is that if you have LM/LRD then you can’t use vanilla statistics, and it seems to me that in climate, the whole point is that you are trying to observe a dependence on a time scale long compared to many of the durations of the data series. I’m not suggesting there is anything wrong with the quoted result; just asking about the supporting analysis.

    [Response: N(0.2,0.14) is just an empirical fit to the IPCC model output - not a statement about the underlying pdf. All of the GCMs exhibit LRD of various sorts - particularly in ocean temperatures and particularly in the North Atlantic, yet still manage to have a well-defined climate sensitivity and predictability. - gavin]

  11. 11
    Tom Fiddaman says:

    Re #5 – Gavin – Isn’t it an overstatement to say that the damping of the rate of surface warming has nothing to do with the “heat coming back out”? I agree that the implied notion of the ocean as a literal pipeline, where you put heat in and it comes out after some discrete delay, is misleading. However, a better explanation – e.g., the ocean hasn’t warmed enough to come into equilibrium with new, higher surface temperatures, so that there’s a net downward flux from the surface, still involves an influence of ocean heat on surface temperature. Right?

    [Response: There is no 'heat coming back out' - that's what a net OHC gain means. As you say in your second point, there will continue to be a net flux into the ocean until the SST has risen high enough so that it reaches a new balance with the incoming radiation. The problem might be in an over-literal interpretation of the 'pipeline' comment though. - gavin]

  12. 12
    Andrew says:

    @Magnus: People have studied the heat exchange between the bottom of the ocean and the surface for many decades under the name of “Abyssal Circulation”. It was intensely studied as part of the explanation of Quaternary Glaciations. The observations extend back a long time (using “Swallow floats” – neutral buoyancy devices). In fact, one of the first papers to try and explain the observations is Arons, A. B. and Stommel, H. “On the abyssal circulation of the world ocean- I. Stationary planetary flow patterns on a sphere.” Deep-Sea Research, 6, 140-154, (1961). Now if you have a peek in that paper, the explanation involves considering the way that any explanation of upwelling must conserve vorticity. There are going to be limits to how easy to understand that sort of thing is going to be. Arons and Stommel’s model is now regarded as too simple (not a surprise given this is it’s 50th birthday is it?) So I think “accessible” observations will be a bit much to expect given that what you must observe includes significant nonlinear fluid dynamic effects.

  13. 13
    Hank Roberts says:

    This gives a feel for the challenge of getting data:

    http://www.pmel.noaa.gov/pubs/outstand/john3037/john3037.shtml
    Recent Bottom Water Warming in the Pacific Ocean
    Gregory C. Johnson et al.
    J. Climate., 20 (21), 5365–5375, 2007

    —-excerpt—

    … data for estimates of deep (>2000 m) ocean heat storage changes will still be very sparse.

    As might initially be expected for the case where heat is simply mixed down from the surface of a stratified fluid like the ocean, heat content changes do appear to be surface-intensified (Willis et al. 2004; Levitus et al. 2005). For example, simple linear fits to World Ocean heat content variations for 0–300-m and 0–700-m analyses of Levitus et al. (2005) between 1955 and 1998 have, respectively, slopes that are 35% and 59% of the slope for the 0–3000-m analysis (not shown), even though those layers span only 10% and 23% of the depth of the 0–3000-m layer, respectively.

    However, the ocean is not ventilated solely by mixing from a shallow surface mixed layer into the thermocline. At high latitudes in locations such as the Labrador Sea (Lazier et al. 2002) and the Greenland Sea (Karstensen et al. 2005), very dense waters occasionally form where cooling in the open ocean is sufficiently strong to overcome the weak local stratification and create a surface mixed layer that extends deep into the water column, thus locally exposing the abyss to surface forcing. In addition, very dense waters are formed on ocean shelves around Antarctica, which then cascade down into the abyss (Orsi et al. 1999). Combinations of these North Atlantic Deep Waters and Antarctic Bottom Waters ventilate the cold deep abyss, mixing with waters above them (Mantyla and Reid 1983). As a result, while middepth waters in the Pacific and Indian Oceans are some of the “oldest” waters in the world in terms of the time since they have last been exposed to the surface (or ventilated), the bottom waters are significantly newer (England 1995).

    Abyssal cooling of about 0.02°C has been reported in the southwest Pacific Ocean in 1990/91 relative to 1968/69 (Johnson and Orsi 1997). It should be borne in mind that the deep stations they analyzed were widely spaced in the horizontal, not all these deep stations were occupied all the way to the bottom, the 1968/69 stations had about 500-m vertical spacing between samples in the abyss, and 0.01°C is about the best instrumental accuracy expected for the reversing thermometers (Emery and Thomson 1998) that were used in 1968/69. In contrast, more recent analyses of modern closely sampled high-quality repeat hydrographic section data taken over the last decade or so reveal an abyssal warming of 0.005°–0.01°C at decadal intervals in the very coldest, nearly vertically homogenous abyssal waters of the main deep basins of the Pacific Ocean that are ventilated from the south (Fukasawa et al. 2004; Kawano et al. 2006b).

    Here deep ocean temperature differences are presented from analyses of modern high-accuracy closely spaced hydrographic section data taken in the Pacific Ocean from the Antarctic Circumpolar Current to the Alaskan Stream and occupied at least twice during the past few decades (Fig. 1).

    —end excerpt—-

    Note in particular — deepest =/= oldest:

    “middepth waters in the Pacific and Indian Oceans are some of the “oldest” waters in the world in terms of the time since they have last been exposed to the surface (or ventilated), the bottom waters are significantly newer (England 1995).”

  14. 14
    Tom Fiddaman says:

    Re 11 – OK, glad I understand you. Perhaps one should clarify further that there’s no net heat coming back out. I find that these little terminology items are always a struggle when communicating with the public, and even slight problems with analogies, like the pipeline, cause confusion.

    On a related note, the IBTimes article that Pielke links ( http://www.ibtimes.com/articles/216084/20110919/global-warming-deep-ocean-research-science.htm – his link is wrong, I think ) is also misleading. It says, “The last decade saw an incessant growth in greenhouse gas emissions which ideally should have increased Earth’s temperature.” This implies that one can pattern-match the emissions trend to the temperature trend, which is incorrect due to the intervening integrations of GHG and heat accumulation. This kind of basic misconception gives skeptics undeserved traction at times.

    [Response: Well, 'ideally' temperature would be insensitive to increasing GHGs - but unfortunately that is not the planet we live on. ;-) -gavin]

  15. 15
    vukcevic says:

    [……particularly in ocean temperatures and particularly in the North Atlantic, yet still manage to have a well-defined climate sensitivity and predictability. - gavin]
    (my highlighting above)
    North Atlantic is the most investigated and data analysed of the oceans, with some odd things ‘concealed’ in the data, shown here as an extract from a short article I am currently writing about aspects of the N. Atlantic’s data.
    http://www.vukcevic.talktalk.net/Data.htm

  16. 16
    Andrew says:

    @Gavin: “All of the GCMs exhibit LRD of various sorts – particularly in ocean temperatures and particularly in the North Atlantic, yet still manage to have a well-defined climate sensitivity and predictability.”

    Well, in that case, there could be an issue. Having a well-defined climate sensitivity and predictability is good, but it doesn’t mean you LRD is taken care of at all.

    The thing about LRD is that it will likely manifest itself as a parameter drift as the realizations get longer, so until you have, e.g., realizations which are twice as long, etc., you won’t see the effect. However, the point of climate sensitivity is that you want to say things about longer times in the future than the length of your series, which is long enough for the LRD to come home to roost. And, since it’s pretty much the LRD that is the issue here – attribution of the main effect to a persistent forcing (e.g. atmospheric carbon dioxide) – then it’s probably worth bothering with LM/LRD capable methods of estimation.

    It’s not likely to mess up the qualitative conclusion – since we know from rock weathering, etc., that even over very long times we have some bounds on carbon dioxide sensitivity. It’s even possible that the correction for LM/LRD is quite modest, although it’s not easy to square that possibility with the main effect being due to a source of LM/LRD. I would actually expect someone has already considered this point.

    [Response: I disagree. LRD is a statistical measure that people are fitting to a physical process. The fact of the matter is that, in particular, N. Atlantic temperatures have long memories and variability that projects onto the statistics used to characterise LRD (ie. large Hurst coefficients). Yet climate models with this statistical property do not show a long term drift in statistics. The mismatch is due to the over simplification of statistical models so that they are not representative of the physical models (and probably not the real world either). All of this is empirically demonstrable - climate models have a well-defined sensitivity and also have high Hurst coefficients in certain regions/parameters. - gavin]

  17. 17
    John Hartz says:

    Gavin: Kudos on another excellent article.

    By coincidence, Rob Painting covers some of the same ground in “The Deep Ocean Warms When Global Surface Temperatures Stall” posted on Skeptical Science, Oct 2, 2011

    http://www.skepticalscience.com/The-Deep-Ocean-Warms-When-Global-Surface-Temperatures-Stall–.html

  18. 18
    Andrew says:

    “The fact of the matter is that, in particular, N. Atlantic temperatures have long memories and variability that projects onto the statistics used to characterise LRD (ie. large Hurst coefficients). Yet climate models with this statistical property do not show a long term drift in statistics.”

    Well then the answer to my original question is apparently that people HAVE considered the statistical issues that come with LRD, (you seem to be referring to known results using models with large Hurst coefficients). One would expect “vanilla” estimations to be exposed to parameter drift due to LRD; but models which accommodate it should not have this specious drift, which suggests, (and you seem to be confirming) that models that people have used which accommodate LRD work out OK.

    So I’m not sure we disagree.

    [Response: See this previous post on Hypothesis testing and long range memory for some discussion of the issue. - gavin]

  19. 19
    Martin says:

    @Gavin,

    “However, it is the case that none of these studies prove that these effects are happening in the real world – they are merely suggestive of what we might strongly expect.”

    Could you elaborate why this should be so?

    [Response: Because we don't have the data that we would ideally want. They would tell us whether what happened in the models is really happening. Meanwhile, we just have inferences that we know work in the models. So they are suggestive, but not conclusive. - gavin]

  20. 20
    Utahn says:

    I think I understand the point being made about lost heat, but will try to express it here to make sure I do.

    If some of the expected heat gain is in the deep ocean (is not “lost”), so that the total planetary heat gain is as predicted, it suggests that our estimates of TOA imbalance are accurate, and so global warming would proceed to eventually warm the planet as expected.

    The reason the heat, despite being buried so quickly in the deep ocean, cannot “come back to haunt us”, is that the deep ocean is so vast that the heat does not stay in the same one packet or current it came down in (if it did it might pop back up just as fast), but is basically conducted away into the great abyss.

    If so it means interruptions or slow downs of warming may occur, because of tranfer of heat from the shallower layers, allowing the shallower layers to “accept” more heat from the atmosphere. This might slow down the pace at which global warming as measured in the atmosphere occurs (even though the eventual temperature rise should be the same), and we shouldnt’ expect a “make-up” more rapid warming later, because that heat will not reemerge to warm the shallower layers and affect the atmosphere.
    So possibly, climate models may overestimate the rapidity, if not the final magnitude, of atmospheric warming, due to this effect?

    Is that anywhere near right?

  21. 21
    CM says:

    Bob #9,

    IIUC, ‘warming in the pipeline’ generally refers to ‘committed warming’ minus ‘warming to date’. That is, it’s how much more the surface must warm up for the planet to reach a new radiative equilibrium, given the forcing from what we’ve already emitted. (Corrections welcome, I’m just testing my understanding.)

    Incidentally, Pielke Sr seems to be prone to thinking it refers to an assumption about warming coming back out of the oceans. I chanced across a blogpost of his where he slammed somebody’s paper for making that assumption. (From context in the paper, though, I’m pretty sure they used it properly, and not as Pielke understood a clumsy formulation of theirs.)

    As for whether we can hope the occasional decade of heat going deep into the ocean will slow the pace of warming: Rob Painting over at SkepticalScience sees those hiatus periods as alternating with periods of greater-than-average heating — suggesting, I guess, that we might get just as much heating, just in fits and starts. I dunno, I can’t see a physical reason why surface warming would have to “catch up” after a period of heat going into the deep, per se, but I guess that would depend on the mechanism behind that heat transport.

    Which brings me to the question, what mechanism, acting where, would account for these modeled deeper-sea-warming periods? I only gather from the Meehl et al abstract (no full-text access) that they seem to be associated with La Niña conditions.

    Oh, and do these model results together with the last decade’s OHC measurements mean we’re closer to accounting for the relatively slow surface warming that decade?

  22. 22
    Icarus says:

    Thanks for this post Gavin, it has clarified a few things for me. I think I understand the point that the oceans are still coming into equilibrium with surface temperatures, and that therefore there will be no heat ‘coming out of the oceans’ as such, but if the rate at which the oceans are taking up surface heat were to decline at some point in the future (perhaps because of circulation changes or simply a reduction in the imbalance), then we would see a steeper rise in surface temperatures even with other factors unchanged. The same TOA radiative imbalance would have a greater impact on surface warming in that case. Is that correct? The ‘consequences’ would be greater surface warming at a later date, not because of heat coming out of the oceans but because of less heat going *into* the oceans at that time. As Trenberth says, that heat is not going to go away, and it would make it harder to stop and reverse the rise in surface temperature if and when we start making serious efforts to do so.

  23. 23
    brian m flynn says:

    I have understood that altimetry data have been used a proxy for OHC in the upper ocean. If heat transport to and thermal expansion of the deep has occurred, is reliance on such data (for the upper ocean) been misplaced? Has thermal expansion of the deep also been considered in determining the sea level budget? Thanks again for your time.

  24. 24
    Jim Prall says:

    I’m catching up on some podcasts and this morning’s subway trip brought up a podcast from UCTV’s excellent oceanography series, entitled “Modeling Ocean Circulation in the Age of Supercomputers” by Paola Cessi, professor of oceanography at Scripps. Readers here might find the video a good intro to ocean circulation and the state of our understanding and modelling of it:

    http://www.uctv.tv/search-details.aspx?showID=20912

    Her talk mentions, but does not focus on quantifying, vertical heat flux; it does offer useful background to this q. by giving a good overview of horizontal heat transport by the overturning circulation, particularly in the Atlantic basin. Horizontal net heat flux is on the order of a petawatt, though atmospheric transport is even larger at around 2-3 PW. See her slide starting at 8:45, which cites Trenberth and Caron 2001.

  25. 25
    isotopious says:

    “A common pseudo skeptic punchline is that the heat is coming from the deep ocean… got any quick easy accessible data to show that this is not the case?”

    Interesting punchline. Not because it is right, or very likely wrong, but because it does not strengthen the AGW case one iota.

    Gavin, any ‘experts’ using this example of why skeptics are wrong, are following their political interests?

  26. 26
    MapleLeaf says:

    Pielke Sr. was recently on tour in Canada and he is still claiming the following in an interview with the Waterloo Record:

    “In fact, Pielke argues that various data sets, including upper-ocean heating, don’t show any global warming for the past seven years or so.”

    Now maybe the reporter got it wrong, but that is doubtful because as recently as June 2011 Pielke claimed on his blog that the top 700m of the oceans have accumulated ZERO Joules of energy since 2003.

    SkepticalScience has looked at OHC data for several datasets and they found the following:

    “In another blog post, Dr. Pielke said that the upper 700 meters of ocean have accumulated no heat since 2003. However, we examined the data from several studies on the subject (provided by NOAA), and found that between 2003 and 2009, the upper 700 meters accumulated between 1.1 x 1021 Joules (Levitus – though this reference may be slightly out of date), and 5.6 x 1022 Joules (Palmer), with Willis et al. falling in between at 5.1 x 1021 Joules.

    And of course there’s nothing special about the upper 700 meters; von Shuckmann and Le Traon (2011) estimate an increase of 5.9 x 1022 Joules between 2005 and 2010 for 10 to 1,500 meters, and heat is accumulating in the deep oceans as well (i.e. see here and here and here).

    While the trends listed above are not statistically significant due to the short timeframe, they are nevertheless most likely positive, and we certainly can’t be confident that they are zero, particularly given the order of magnitude discrepancy between the various estimates. Given this data we wonder, will Dr. Pielke agree that his previous assessment of zero Joules accumulated during this period was incorrect, and that the timeframe (since 2003) and depth (700 meters) is insufficient for a suitable assessment of the climatological trend?”

    Why is Pielke Sr. continuing to make this fallacious claim about OHC when the observations show otherwise?

  27. 27
    Jim Prall says:

    Further to my last comment, estimates of horizontal heat transport do not in themselves give us much info to constrain the vertical transport by the overturning circulation. Instead, I presume the key is to quantify the mass fluxes of density-driven deep water formation by sinking at the few key locales in the North Atlantic, Antartic shelf, etc., plus key upwelling sites, then to use in-situ measurements to start to constrain the local vertical heat fluxes by advection (heat carried in parcels of sinking/rising water) as distinct from diffusion/convection across stratified layers elsewhere around the world ocean. Short-term changes in the rate of upwelling and of deep water formation (sinking) could lead to large variations in vertical advective fluxes, I’d imagine.
    Some obersvational work on this is reported in Lavender, Davis & Owens 2002 J. Phys. Oceanography
    http://journals.ametsoc.org/doi/pdf/10.1175/1520-0485(2002)032%3C0511%3AOOOODC%3E2.0.CO%3B2

  28. 28
    Greg Wellman says:

    Correct me if I’m wrong (please!) but heat transport in the ocean is dominated by bulk movement of water rather than say, radiative transport.

    Assuming that is so, then there’s a nearly linear relationship between how quickly heat can be sequestered in the deep ocean and how quickly dissolved CO2 can be (because it too is transported by bulk movement more effectively than by diffusion… right?).

    So the rate of ocean acidification could be used to constrain the physically possible rate of ocean heat vertical transport, and vice versa, right? After all, if the mixing was fast, we could worry a lot less about both warming and acidification, and conversely if the mixing slowed down, we’d be in serious trouble. Is there any literature on how these two fluxes are related or constrain each other?

    [Response: You make an excellent point, and this coupling of the carbon and heat uptake is certainly something people have been thinking about. I'll see if I can find a paper that discusses it... - gavin]

  29. 29
    Bill Ruddiman says:

    I’ll give the pipeline-confusion issue a shot here.

    In order to write a (paleo)climate textbook in 2001/2007, I had to grapple with this issue, and the highly respected source of the “pipeline” term (Jim Hansen) was not as clear about this as he could have been. So here is what I eventually came to.

    The small amount of heat that sinks into the very deep ocean (1000′s of meters) will not be seen by the atmosphere for a very long time (many hundreds up to 1000 years). So: very deep sinking is irrelevant to this discussion/thread.

    But very large (and much less publicized) volumes of water sink to depths not far below the surface layer (not far below 100-200 meters) at subpolar and northern subtropical latitudes (example: so-called 18C water just north of Bermuda in winter) Several of these large masses of sinking water are called ‘mode waters’. They reach subsurface depths of several hundred meters and carry the temperature signature of the overlying (mainly winter) atmosphere in the region where they sink.

    So consider two cases:

    A few decades ago, and for centuries before that, with cooler temperatures in these mode-water sinking regions, those shallow subsurface waters were cooler than now. As a result, the normal vertical overturn (that occurs by deep winter mixing by cold air masses and by slow diffusion) brought some of this chilly shallow subsurface water to the surface during cold winter months and kept the surface ocean cooler.

    Now consider the present>future. The same regions in which those relatively shallow waters sink have warmed and will warm more (some more than the planetary average). Because those ‘mode waters’ are warmer when they sink, they warm the shallow subsurface layer, and those same processes of winter mixing and diffusion bring to the surface subsurface waters that are warmer than they were in previous decades.

    The net (current and future) result is that the atmosphere eventually ‘sees’ more of the warming that had once been partly hidden in the shallow subsurface waters — “in the pipeline”. So the delayed benefit of this ocean pipeline heat comes back and adds an extra increment to future warming.

    This does not mean that the ocean is suddenly going to start belching back “extra heat” for no reason. Rather, normal ocean circulation is going to start exposing to the atmosphere part of the extra heat we have for decades been burying in the ocean, and will ‘feel’ this increment of “extra” heat coming in.

    But it’s our own heat — the heat we previously put there.

    Bill

  30. 30
    Lawrence McLean says:

    A feasible mechanism by which heat in the upper ocean may be transported to the bottom of the ocean could be by the sinking of more dense water made that way by evaporation on the warm surface. In order for it to happen would most likely need strong circular winds to create the drain pipe effect that would be required to enable the mechanism that I suggest. To visualize what I an trying to describe, think of water going down a drain (which I call the drainpipe effect), it is like an upside down tornado.

  31. 31

    #29 I think I may see what Dr Rudiman is suggesting with sea water right next to the Arctic ice pack.
    The contradiction of ice NOT cooling sea water with a low sun seems to make a case of it.

    http://www.osdpd.noaa.gov/data/sst/anomaly/2011/anomnight.10.3.2011.gif

  32. 32
    Edward Greisch says:

    “Heat coming back out of the ocean deeps”

    Simple explanation why this is absurd: The temperature at the bottom of the ocean is 4 degrees C, 39 degrees f, if my memory is correct. c-c-cold for a swim.

    It will be quite a while before heat comes back out.

  33. 33
    Buzz Belleville says:

    I just want to say thanks again for this site and the efforts you all make to even respond to oafs like me who ask questions thru the comments. There are a lot of really smart non-scientists who can get most of the technical posts here, and that’s what we need. Skeptical science gives us the ammunition to rebut the stupid stuff. And Climate Progess gives us the current event spin on climate change, as well as the blurbs about new studies. But when there’s a new study in which I am interested, coming to this site is invaluable. Many of us are “fighting the fight” on other fronts, and this site gives us the needed ammunition.

    Thanks
    Buzz Belleville
    Professor of Sustainable Energy Law
    Appalachian School of Law

  34. 34
    Rob Painting says:

    CM @ 21 – the climate modeling by Meehl (2011) suggest that the current hiatus may be due to natural variability. In the absence of any long-term warming this natural cycle would be like a sine-wave oscillating about a long-term average of zero – up and downs around a zero trend line. With a warming trend, the line follows an incline so that dips now become hiatus periods and the peaks become steeper and higher. Translated, that may mean “heat coming back to haunt us” as the climate climbs to the next peak of the cycle.

    The hot-coloured ocean regions in Fig 4 of my post at SkS are where heat is converging and being driven down into deeper layers. This downwelling component is quite intense in the model, and seems to be taking place at mid-latitudes. La Nina-like is reminiscent of the negative (cool) phase of the PDO.

  35. 35
    Icarus says:

    Heating ‘in the pipeline’ doesn’t refer to any heat already absorbed by the planet, but instead to heat that *will* be absorbed before the TOA radiative imbalance reaches equilibrium.

  36. 36
    ldavidcooke says:

    RE: 28,29

    Hey Greg and Bill,

    It may help to keep in mind that for warm water to sink near the higher latitudes, it would have to be denser then the cooler water. In short, it is unlikely we should see “mode water”, it has to reach the bottom of the Arctic basin for the THC to be unaffected.

    As to the top 100 meters, looking at the Pacific ITCZ zone the heat content has not necessarily exceeded a +/-1 std. deviation. It appears to have no visable trend, given the measuring tool accuracy, since roughly 1996, except for the ’97 peak and ’98 trough at several TRITON sample sites. Though the N. Atlantic ITCZ region does demonstrate currently up to a 1 deg. C SST increase over historic records and higher variability in the isotherm patterns, over time since about 2005, the trend is not significantly higher. Both which suggests, most of the energy being put in near this region must be coming out prior to a complete cycle of the N. Atlantic gyre.

    I believe the key will be the salinity. Follow the surface salinity and by products (wv) and you should see the heat flow. This may help explain why the ocean surface heat may both reduce CO2 uptake and mainly only gain from the absorbed radiant energy captured at the surface in the lower latitudes.

    Cheers!
    Dave Cooke

  37. 37
    CM says:

    Rob Painting #34,

    I’m sorry, I failed to read your SkS post properly through before blabbering about it. I note that the surface temperature map in your figure 4 has warming at sites of deep water formation (Norwegian sea, Kamchatka peninsula) and intermediate water formation (mid-Atlantic), maybe that’s part of the explanation?
    Anyway, I see from the SkS comments that you’re planning a follow-up post on the mechanism and look forward to it.

  38. 38
    Utahn says:

    “heat coming back to haunt us”

    Rob, I think I’m understanding what you’re saying. Tell me if I have it right:

    At times, more heat than previously expected is buried in the deep ocean. This heat will not return literally to the surface, but periods of greater “shallow” ocean heating will be expected, because sometimes, much less of the heat is transported deeply (due to the natural variability in whatever mechanism is driving heat deeper than expected some of the time).

    Relatedly, since we have some expectation that this deep ocean heat transfer has always been occurring, and since our climate models have a decent handle on the sensitivity of the climate, past and present, one might expect the ebbs and flows of deep heating to even out to the expected trend based on knowledge of climate sensitivity. How’s that for a run-on sentence?

    Does this (esp the first part) represent correctly what you mean?

  39. 39
    D. Robinson says:

    Re “A common pseudo skeptic punchline is that the heat is coming from the deep ocean… got any quick easy accessible data to show that this is not the case?”

    That statement is dead wrong and I think this comment thread is setting up a strawman argument that skeptics are not making.

    Skeptics are reacting to the “heat is in the pipeline” statement of Trenberth which skeptics widely assume [rightly or wrongly] to mean ‘the heat is hiding in the deep ocean’. Pielke Sr. also is responding to this meme by saying the heat can’t be hiding in the deep ocean if we haven’t seen it transit through Argo. (Gavin’s comments on heat flux would seem to be in response the skeptic argument that ‘the heat is not hiding in the deep oceans’.)

    Now if the ‘heat is in the pipeline’ theory is not meant to say hiding in the deep ocean, then it is RC (and Trenberth?) that need to state exactly what “heat is in the pipeline” is supposed to mean.

    But please don’t blame the skeptics for stating the heat is hiding in, or coming from the deep ocean. They are not making that case, they are saying this is not the case.

  40. 40
    Greg Wellman says:

    D. Robinson, I believe Icarus above @35 defined ‘in the pipeline’ best:

    “Heating ‘in the pipeline’ doesn’t refer to any heat already absorbed by the planet, but instead to heat that *will* be absorbed before the TOA radiative imbalance reaches equilibrium.”

    To clarify, the relevance of the deep ocean to this definition is that if heat wasn’t being sequestered in the deep, we’d already be much closer to equilibrium at the surface. It would be hotter now with less ‘in the pipeline’. Heat transport to the deep ocean delays the surface response to the TOA imbalance, resulting in more ‘in the pipeline’ at this time.

    On the separate topic of what arguments skeptics make … well as the taxonomy of those arguments at Skeptical Science shows, many are contradictory and you can’t attribute all arguments to all skeptics. One of the sillier arguments is that undersea volcanoes are responsible for ocean warming, not CO2. (I won’t bother explaining all the ways that’s silly.) But that’s probably what Magnus was referring to.

  41. 41
    Meow says:

    @39: I’m pretty sure that the proper term is “warming in the pipeline”, and that it means that we haven’t yet seen all the surface warming from the CO2 that we’ve already emitted. One of the reasons we haven’t seen all that warming is that the oceans absorb some of the radiative excess, moving it outside the areas (surface air and surface sea) measured by the usual temperature indexes like HadCRUT.

    As the various ocean reservoirs absorb all the heat they can, the remaining heat from the radiative imbalance will heat the surface, thus emerging from the “pipeline”. That surface heating will continue until the earth as a whole reaches radiative equilibrium.

    That’s a simplified explanation, because there are multiple ocean heat sinks that operate on different time scales.

  42. 42
    isotopious says:

    As for ocean records, I all ways prefer a little context.
    Take for example:
    Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic d18O records, Paleoceanography,20, PA1003, doi:10.1029/2004PA001071.

    At a resolution of 1000 years, we see that from 0 to 10 kya we have 11 proxy data points, with lower values corresponding to higher temperature:
    3.23
    3.23
    3.18
    3.29
    3.30
    3.26
    3.33
    3.37
    3.42
    3.38
    3.52

    As you can see, it has been warming steadily for thousands of years. It is very likely that changes we observe today are a residual of much larger changes that have occurred in the past. Over that time, the average sea level rate of rise has been 3.6 mm per year.

    So is global warming a fraud? Clearly not!

    [Response: At this resolution, the timeseries of this stack says nothing about Holocene trends. Most of the cores don't even have a well defined Holocene section, and very few have good dates therein. Lisiecki's work is great, but it is great at the long time scales, not short. For a better view, look at Wanner et al, review paper in QSR (i think). - gavin]

  43. 43
    Ray Ladbury says:

    D. Robinson,
    I think that a 2-box model is somewhat instructive when trying to understand the “heat-in-the-pipeline” issue. It is only the top of the upper box that actually radiates heat away, and the amount is determined by the temperature there. Thus, heat can accumulate in the lower box (ocean) and it doesn’t raise the temperature of the upper surface of the upper box.

  44. 44
    isotopious says:

    Yep, Lisiecki’s work is great….for the deep ocean, which has very long time scales.

    The question should be more like ‘why shouldn’t that robust ocean warming continue for several more thousand years?’

    Very likely it will in my opinion. Nothing says it wont.

    [Response: Brilliant logic. Actually, it hasn't been warming since the beginning of the Holocene (and in the Northern Hemisphere, it's definitely been cooling (as a function of the orbital change to perihelion), so if you want to extrapolate, that's the way you would go. I, on the other hand, prefer to understand why something changes, and then, based on how those things will change in the future, make a projection. Linear extrapolation based on your opinion is likely to be a touch less reliable. - gavin]

  45. 45
    Chris G says:

    Greg @ 28,
    I had a similar thought at some point. Anyway, I’d also be interested in anything Gavin finds.

    I’m wondering if there are gravity data sensitive enough to filter out changes in down-flow regions. Sinking water would not sink if if were not denser, regardless of if that is because it is colder or more saline; in either case, there might be enough signal to detect periods of relatively higher and lower density in these regions. Possibly technically challenging to couple that a fluid dynamics model as well, but if it could be done, it might add a useful dimension.

    Rob Painting has the same model that I have (FWTW), little waves on big waves and noise over all. It isn’t as though there is a “catching up” so much as reduction in a damping effect. (Though, I can’t think of an easy way to tell the difference just using math.) If the damping effect oscillates, you get something that looks like normal waves overlapping a ‘tidal’ wave. There are numerous local minima in the temperature record; it could be speculated that the slope of the larger wave has increased enough that it appears that the local minimum that we are in is merely flat rather than a minor valley. I have come to think Trenberth is looking for an oscillation, and Hansen is looking at noise.

    D Robinson @ 39,
    I don’t think it is accurate to paint with so broad a brush; there is a lot of diversity within the skeptic camp. But, stepping back a bit, in general, if a reasonably intelligent person says something that doesn’t makes sense to you, it is possible they are wrong, or that what they said was not interpreted correctly. It the absence of knowing which, it is better to give the benefit of doubt, or, at least, not assume they are wrong.

    I think Gavin’s response is more properly isolated to identifying the flaw in the logic about not much warming in the upper ocean implying that not much heat is being transferred to the depths.

    Heat in the deep ocean. I suppose it is possible that even if we get the current warming under control, there might be something like a distorted echo of warming that shows up in hundreds to thousands of years, not because the deep ocean upwelling is releasing heat, but because it isn’t damping quite as much as it used to.

  46. 46
    tamino says:

    I was under the impression that the LR04 stack (Lisiecki & Raymo 2005) was d18O from benthic foraminifera, and as such is was NOT a proxy for temperature (ocean or otherwise), rather a proxy for global ice volume. I was also under the impression that global ice volume continued to decline even after temperatures peaked very early in the holocene, due to the (albeit brief) persistence of the Laurentide (and other) ice sheets.

    I have studied the LR04 stack data, and I know that the changes since 10ky ago are *tiny* compared to the typical swings during glaciation/deglaciation. Characterizing the last 10ky of that data set as indicating “warming” is definitely misleading. I suspect it’s deliberately so.

  47. 47
    jyyh says:

    (treating ocean as a stack of blocks (maximally stratified))
    sunlight reaches to the depth of about 100m on locations, if this surface layer is warming then it takes at minimum 4000m (average depth of the ocean)/100m(warming layer) = 40 times that much energy to warm the deepest 100m as much assuming no downwelling nor heat escaping to the atmosphere. assuming half the radiation absorbed by the surface layer of the ocean is radiated back to atmosphere it would take some 80 years of constant heating of the upmost layer of the ocean to get a similar response in the deepest layer than the surface gets in one year. currents, downwelling and upwelling just mix thing up a bit. similarly with cooling. i’m not expecting GW to go away quickly. and by global i mean the troposphere-ocean-soil-biosphere system.

  48. 48
    David B. Benson says:

    The ratio of 18O to 16O is used to tell the temperature of the surrounding water of the time solidified, indirectly. The ratio varies slightly depending on the temperature of the surrounding water, as well as other factors such as the water’s salinity, and the volume of water locked up in ice sheets. from
    http://en.wikipedia.org/wiki/%CE%9418O
    so one should also check other sources. [My understanding is that d18O is indeed a mixture of temperature and water volume.]

  49. 49
    David B. Benson says:

    I forgot to mention that there is a good presentation of the d18O proxy in Ray Pierrehumbert’s “Principles of Planetary Climate”
    http://geosci.uchicago.edu/~rtp1/PrinciplesPlanetaryClimate/index.html

  50. 50
    Chris Colose says:

    Tamino, benthic d18O records measure both global ice volume and deep ocean temperature, and often a second, independent proxy like Mg/Ca is required to disentangle the effects.


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