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A Climate Sensitivity Tail

Posted on 6 February 2013 by dana1981

A blog post by climate scientist James Annan and his comments on Andrew Revkin's Dot Earth blog have drawn a lot of attention, despite not being particularly controversial.  For example,

"...a high climate sensitivity [is] increasingly untenable. A value (slightly) under 2 is certainly looking a whole lot more plausible than anything above 4.5."

Annan is mostly critical of the Intergovernmental Panel on Climate Change (IPCC) for maintaining the "long tail" of high possible climate sensitivity values, for example as shown in Figure 1.

sensitivity-big.gif
 
Figure 1: Probability distribution of climate sensitivity to a doubling of atmospheric CO2, from Roe and Baker (2007)

Uncontroversial Comments

Annan's own work has focused on constraining the range of climate sensitivity values.  For example, Annan and Hargreaves (2009) investigated the question using a Bayesian statistical approach, and concluded that

"the long fat tail that is characteristic of all recent estimates of climate sensitivity simply disappears, with an upper 95% probability limit ... easily shown to lie close to 4°C, and certainly well below 6°C."

Annan appears to feel that the IPCC has been too slow to let go of the sensitivity 'long tail' and incorporate a more tightly-constrained probability distribution into their reports.  However, from a purely policy standpoint, it is important to consider all possible scenarios, and a very high climate sensitivity cannot yet be ruled out, as Chris Colose explains (via personal communication):

"From an IPCC/policy perspective, however, I'm not convinced the longer tails should be completely disregarded, even if they have very low probability of being consistent with the present-day evidence.   Ray Pierrehumbert pointed to the Pliocene-case where a much different climate prevailed at a time with very similar climatic boundary conditions as present.  The issue runs deeper than this, since such a regime shift could indicate the low-but-finite-probability of some sort of bifurcation point in the system ... I have seen this behavior in GCM's, not necessarily dor a doubling of CO2, but for higher concentrations, and as others have noted, the world doesn't end once we double CO2."

Nevertheless, in general, Annan's comments are consistent with the body of mainstream climate science research, and most of his colleagues believe that climate sensitivity is most likely close to 3°C surface warming in response to doubled CO2; unlikely to be more than 4.5°C or less than 2°C.  One good example of this was Hansen et al. (2008), which concluded that equilibrium climate sensitivity is

"3 ± 1°C for the 4 W/m2 forcing of doubled CO2."

And the probable range of equilibrium climate sensitivity cited by Annan is fully consistent with the body of scientific literature (Figure 2).

Various estimates of climate sensitivity

Figure 2: Distributions and ranges for climate sensitivity from different lines of evidence. The circle indicates the most likely value. The thin colored bars indicate very likely value (more than 90% probability). The thicker colored bars indicate likely values (more than 66% probability). Dashed lines indicate no robust constraint on an upper bound. The IPCC likely range (2 to 4.5°C) is indicated by the vertical light blue bar.  Adapted from Knutti and Hegerl (2008).

Really all Annan is disputing is the 'long tail' of possible climate sensitivity values above 4.5°C, which Annan believes are more improbable than the IPCC report has stated.  Nevertheless, the 'long tail' represents very low probability scenarios even in the IPCC report.

Memo to Contrarians: "High"  Isn't What You Think it is

It's something of a mystery why the climate contrarian blogosphere lit up in response to Annan's fairly mainstream, uncontroversial comments, other than the fact that he was rather critical of the IPCC.  Annan said equilibrium climate sensitivity is unlikely to be higher than 4.5°C - there are few if any mainstream climate scientists who would disagree with this.  He also said that sensitivity is unlikely to be much less than 2°C.  This rules out the beliefs of many prominent climate scientist contrarians, like Roy Spencer (who believes equilibrium sensitivity is around 1.3°C) and Richard Lindzen (who believes it's less than 1°C).

In short, the comments Annan made which were celebrated by climate contrarians are incompatible with the beliefs of the most prominent climate contrarian scientists, but entirely compatible with mainstream climate scientists like James Hansen.  Is this really an argument you want to get behind, contrarians?

Another issue highlighted by Joe Romm – climate sensitivity is not the same thing as future projected warming, unless we limit ourselves to a doubling of atmospheric CO2.  At the moment, our emissions are tracking along some of the worst case scenarios (Figure 3), and if this continues, we will blow well past a doubling of atmospheric CO2.

IEA vs. SRES 2011

Figure 3: IEA fossil fuel CO2 emissions estimates vs. IPCC SRES emissions scenarios.

This highlights a mistake that climate contrarians make frequently, for example claiming that we will only see 1°C warming over the next century.  The amount of future warming depends on two factors - climate sensitivity, and human CO2 emissions.  Even if climate sensitivity is on the lower end, if we don't get our emissions under control, we will still see a dangerous amount of global warming (more details on this to come in a future blog post).

Do We Have "A Bit More Time"?

Annan has also made the case that the most likely equilibrium climate sensitivity value may be closer to 2.5°C than 3°C.  This case appears to be based on recent research taking two different approaches: looking at recent climate changes, and changes during the Last Glacial Maximum (LGM) about 20,000 years ago.  As with the press release and media attention surrounding the Norwegian climate sensitivity project we recently examined, this has resulted in some suggestions that perhaps climate sensitivity is toward the lower end of possible values, which might buy us a bit more time to reduce human greenhouse gas emissions.

If true this would be good news, because our current efforts to reduce global human CO2 emissions have been woefully inadequte.  They continue to climb with no international climate agreement in sight.

However, caution is advisable here.  As we discussed regarding the Norwegian paper, studies estimating climate sensitivity based on recent data may be biased low due to a failure to account for increased heat transfer to the 700–2000 meter ocean layer (Figure 3).

OHC

Figure 3: Comparison of Global Heat Content 0-700 meters layer vs. 0-2000 meters layer, from the National Oceanographic Data Center.

Kevin Trenberth similarly notes (via personal communication),

"Global surface temperature is but one manifestation of warming and not a very good one as it is subject to a lot of natural variability.  The increasing evidence for much more heat going deeper into the ocean has major implications and that pattern can easily be reversed.  I think that any assessment of climate sensitivity based on the short term temperatures record is fraught with major difficulties and the implied assumptions do not stand up.  Simple box models that keep mixing into deep ocean fixed are wrong!

Another interpretation is that given a certain energy imbalance at the top of atmosphere, if the heat is not manifested as surface temperature rise then it goes elsewhere.  Another place it goes is into the more vigorous hydrological cycle which has a whole new set of implications."

There are also significant uncertainties associated with some radiative forcings (aerosols in particular), and the possibility that climate feedbacks are not linear (e.g. discussed in Long and Collins 2013).

As for the LGM, equilibrium climate sensitivity estimates depend strongly on the temperature data used.  Research by Schmittner et al. (2011) and Annan and Hargreaves (2012) found most likely equilibrium sensitivity values close to 2.5°C based on LGM changes, whereas as noted above, Hansen et al. (2008) estimated 3 ± 1°C sensitivity based on the LGM.  The Schmittner and Annan studies used ocean temperature data from the Multiproxy Approach for the Reconstruction of the Glacial Ocean (MARGO) project, about which Richard Alley noted:

"MARGO made a solid effort, which indicates very small temperature changes. But, there are other ways to do it, and indeed, [Schmittner et al.] coauthor Alan Mix has published independent papers indicating that the temperature changes were larger in some regions than indicated by MARGO.   David Lea and others have also obtained larger temperature shifts….

In short, the MARGO data for the ocean show very small temperature change from the ice age to today, and thus lead to the low climate sensitivity, but they disagree with some independent estimates showing larger temperature change.  They also lead to disagreement with the pollen-based land temperature data.  Furthermore, they lead to an answer that disagrees with many other lines of evidence for climate sensitivity."

A smaller temperature change estimate will result in a lower climate sensitivity estimate, so if MARGO data are biased low, that could result in a too-low climate sensitivity estimate.

Nevertheless, 2.5°C equilibrium sensitivity is certainly a possibility, well within the IPCC range.  And it would essentially give us an extra 10 to 15 years of greenhouse gas emissions before we become committed to 2°C warming above pre-industrial levels, for example – in roughly 2038 as opposed to 2027 in Representative Concentrations Pathway (RCP) 4.5, which represents a scenario in which we slowly reduce our greenhouse gas emissions.  This also depends on how our aerosol and soot emissions change in the future.

Realism is Important

Ultimately while 2.5°C equilibrium sensitivity would certainly be better news than 3°C, it's not something we can bank on.  In either scenario we need to take serious action to reduce human greenhouse gas emissions in order to avoid dangerous and potentially catastrophic climate change.

James Annan's comments and research are also incompatible with an equilibrium climate sensitivity much less than that – certainly nowhere near as low as the most prominent climate contrarians would have us believe.

The danger with acting as though we have plenty of time to reduce our emissions is that if this turns out not to be the case, we may find ourselves beyond the point where potentially catastrophic climate change is avoidable.  At the moment, the body of scientific research points to 3°C as the most likely equilibrium climate sensitivity value.  It's possible that it's 2.5°C, or even 2°C, but there's also evidence that it may be closer to 4°C, and it's certainly not much below 2°C, contrary to contrarian beliefs.  In any realistic scenario, we need to take serious and immediate action to reduce human greenhouse gas emissions.

Hopefully sensitivity is on the lower end of possible values, but in any case, we are running out of time to solve the problem.  We will have a blog post examining the various possible climate change scenarios (from best case to worst case) in the near future.

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Comments

Comments 1 to 36:

  1. As the Oregonian thread here demonstrated, people can easily read what they want instead of reading what is actually written.

    So instead of what Annan & Hargreaves are actually saying, which (if I am reading their paper correctly) is that we can more confidently (though still not completely) rule out possible climate sensitivity greater than 4°C, pseudoskeptics & contrarians appear to be reading it as supporting their notions of climate sensitivity (which vary, but as far as I can see a sensitivity of 1.5°C appears to be the ceiling).

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  2. Composer @1 - yes, it appears that contrarians interpreted Annan's comments as "climate sensitivity is not very high, therefore it's low".  Hence the 'memo to contrarians' section in the OP.  I saw several contrarians saying that Annan's comments were confirming what 'skeptics' have believed all along.  Unless they think Lindzen and Spencer are extremists, that is simply untrue.

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  3. My sense was the (fake) skeptics homed in on only this part: "A value (slightly) under 2 is certainly looking a whole lot more plausible..."

    And that's all the wanted, all they needed.  Everything else didn't fit their notions of CS and so they excluded it.  

    I've had exchanges with a bunch of them this week on various websites and they were completely unaware that Annan had also made statements regarding the central estimate still being 2.5C to 3C.  It totally flew by them without notice.

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  4. "Annan has also made the case that the most likely equilibrium climate sensitivity value may be closer to 2.5°C than 3°C."


    It's worth noting that the current estimate from NASA's GISS Model E, to be used in the upcoming IPCC model ensemble, is 2.5C.  The version used for the previous IPCC report was about 2.7C.  "About 3C" includes the possibility of "a bit less than 3C" ... contrarians seem to have missed just how mainstream Annan's view is that it might be closer to 2.5C than 3C.


    Not to mention just how uncomforting this mainstream (though slightly on the low side) view is to anyone who cares about the future.


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  5. As illustrated in this OP and noted in comments above, "skeptics" seem to be expert at mental cut-and-paste to build their understanding. Maybe they all grew up eating out at smorgasborgs where their parents allowed them to choose the ice cream for the main course and for desert. If only the world really did work that way....

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  6. One doesn't need to misinterpret James Annan's comment, he's writing some unsupported nonsense all on his own. For example this comment over at the Dot Earth blog:

    "combined with the stubborn refusal of the planet to warm as had been predicted over the last decade, all makes a high climate sensitivity increasingly untenable"

    Over the last 50 years around 93% of global warming has gone into heating the ocean. Compare the "noughties" decade to the 1990's and you find that the ocean heating rate increased. In other words the observations contradict what Annan claims.

    Then there's all his strange rants about the IPCC. He seems peeved that he's being ignored by those involved in preparing the latest IPCC assessment. Given that he is making claims contrary to the evidence, I'm not surprised his protestations may be falling on deaf ears.

    By the way, what is the basis for claiming that the negative aerosol forcing estimate has been reduced? 

     

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  7. I think part of the contrarian reaction is based in the psychology of Going Down the Up Escalator.  Just as any sort-term drop in temperatures becomes a claim that global warming has "stopped", any study that mentions a decrease in climate sensitivity is grabbed as if it is evidence that everything about climate sensitivity is decreasing. As part of the uncertainty monster, it confirms the contrarians' belief that mainstream science is wrong, and that all the errors are accumulating in one direction. To the contrarian, it is just a matter of time before other uncertainties will yield even lower estimates of sensitivity, until eventually it will fall to the point that it is nothing significant.

    That Annan's statements do nothing to support the low sensitivity favoured by the Spencers and Lindzens is irrelevant: i just another "nail in the coffin" for a  confirmation-biased view that the sensitivity just has to be lower than what the proper science is saying.

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  8. Rob P @6 - yes, the quote from Trenberth in the OP is in response to Annan saying that warming has slowed over the past decade.  That's just not true if you include ocean heat content data to 2000 meters.

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  9. The key issue that stands out for me is Annan's crticism of using a uniform prior in the Bayesian models.

    This has been criticised by statistician Steve Jewson in comments at RealClimate, who claims that the IPCC should not use sensitivity studies that use uniform priors. This happens to include quite a lot of them.

    The use of a uniform prior has the effect of shifting the modal or median values of sensitivity higher, and also fattens the tail of the pdf.

    Nic Lewis made this point about Forster and Gregory, which the IPCC subsequently applied a uniform prior assumption that S had equal probabilities of lying between 0 and 18.5 deg C, thus skewing the original data.


    There's a lot of discussion here that is outside my comfort zone, but for me this is the key issue, not that CS is necessarily lower.

     

     

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  10. (sarc) The correct answer to all political questions about TCR global sun-induced ozone recovery sensitivity (50% undefined or rather obscurely neglecting the modelling of bark beetles and their modelled 'black carbon' on ice sheets.)  here. :-). I'm rather surprised noone has proposed π since it would make calculations easier on this flat earth. (/Otherwise nice editor still missing the sarc tag)

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  11. It's not a big deal, but I think that mainstream climate blogs have failed to convey the idea put forward by James Annan. In his own words, that:

    "the additional decade of temperature data from 2000 onwards [...] can only work to reduce estimates of sensitivity"

    I think that climate blogs have rather argued that recent data are just natural variability, and thus don't have any effect on long-term trends nor sensitivity.


    However, regarding the lower temp change since LGM (4K instead of 6K), an important fact is that the changes observed in deglaciation are anyway the same. So a warming of 4K caused impacts that we previously thought requiered a 6K-warming. In other words: impacts should be considered higher than before for any given sensitivity. Ken Hedlin puts it in a nice way over at Annan's blog:

    "Given that in your Dec. 21 2013 post, "How cold was the last glacial maximum", your conclusion was 4C colder, and that in your comment here ,you estimate a sensitivity of 2.5 - 3C, then with a doubling of CO2, we can expect a temperature increase of about 2/3 of the warming since the LGM."

     

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  12. Rob Painting #6,

    You are right. There was a lot of sense in what Annan wrote, but also a lot of drivel as well. He seems to have been stung somewhere along the way. I thought gavin Schmidt comment to Andy Revkin summed up matters:

    Indeed, the consensus statements in the IPCC reports have remained within the 1.5 – 4.5 range first set by Charney in 1979. James’ work has helped improve the quantifications of the paleo constraints (particular for the LGM), but these have been supported by work from Lorius et al (1991), Kohler et al (2010), etc. and therefore are not particularly radical.

    By not reflecting that, you are implying that the wishful thinking of people like Ridley and Lindzen for a climate sensitivity of around 1 deg C is tenable. It is not, and James’ statement was simply alluding to that. For reference, James stated that his favored number was around 2.5 deg C, Jim Hansen in a recent letter to the WSJ quote 2.5-3.5 (based on the recent Palaeosens paper), and for what it’s worth the CMIP5 GISS models have sensitivities of 2.4 to 2.7 deg C. None of this is out of the mainstream.

    Revkin complains the scientists have not "adequately conveyed the reality." Isn't that your job too, Andy?

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  13. shoyemore @12, I would say it is entirely the role of the science journalists to convey the science to the public.  It took me half an hour on googlescholar to determine that recent publications on climate sensitivity bracket a range between 2.5 and 3.5 C per doubling of CO2, with some outliers; and that portrayal of recent results as being predominantly in favour of low outcomes represents cherry picking.  If Revkin cannot spend that half hour before writting on the topic, what is he being paid for?

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  14. The following from Rohling et al 2012 also strongly suggests CS is around 3+


    Over 30 studies, looking at paleo climate from the last 10,000 years back to 420 million years ago. CS between 3.1 to 3.7

     

    An S value of 1 corresponds to a CS of 3.7. Not much tail there. And not much to  suggest low CS either.

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  15. Jesús Rosino,

    I have a lot of respect for James Annan's statistical acumen and often read his blog, but the comment "the additional decade of temperature data from 2000 onwards [...] can only work to reduce estimates of sensitivity" puzzled me also.

    If the current instrumental record is a very poor constraint on climate sensitivity — which Figure 2 suggests is the case — then it's entirely possible for the additional decade of temperature data to have no effect whatsoever on estimates of sensitivity. Indeed, given the magnitude of the internal variability, how could it? There are plenty of ways of looking at the surface air temperature record that all show no statistically significant change in trend from earlier decades, so any study that concludes sensitivity is different just with the addition of the past decade must be automatically suspect, and that's not even taking into account the heat going into the oceans.

    As I said, it's puzzling.

    As for "...a high climate sensitivity [is] increasingly untenable. A value (slightly) under 2 is certainly looking a whole lot more plausible than anything above 4.5", it would be hard to craft a statement that would be guaranteed to excite the denialati even more. :-) It's practically begging to be taken out of context and misinterpreted. If I was a well known so-called "team member", I might be tempted to make such statements now and then just so I could point at all the idiots who misunderstood it afterwards.

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  16. I also find Annan's post-2000 comment surprising.

    My 2-box+enso model (which has a number of limitations which I won't go in to here, but can at least address the question) using GISS forcings gives a TCR of 1.67 with the data to 2010/12, and 1.60 with data to 2000/12. That's a long way from being proof but does suggest that Annan's claim should not be accepted without significant supporting evidence.

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  17. JasonB,

    There are some discrpenacy among probability distribution functions on Figure 2 based on the instrumental record, but all of them tend to point to the lower end of the sensitivity spectrum. I'm prone to trust sensitivity based on instrumental record rather than paleo, as there are significantly less uncertainties regarding both temp and forcing changes.

    It's true that there's no change in the warming trend, but, as Annan says in the comments, high-end sensitivities should show a gradual acceleration. He adds that "quite a sustained steadying, with the limited ocean warming and changes to forcing estimates all points in the same direction".

    I don't say that Annan is right, my point is that I don't think he's saying just the same as mainstream climate blogs, and that he is indeed suggesting a (slight) change in the way climate sensitivity is portrayed in scientific reviews. In fact, I think he is suggesting that the IPCC authors have a more critical approach when reporting about papers, but this is a different war.

    It's true that his statement is prone to controversy, but the denialist point that he is suggesting a sensitivity lower than 2 is easily debunked, and I think that this controversy may have the positive effect of attracting more climate scientists to the discussion put forward by Annan. :) Constraining sensitivity is an interesting issue.

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  18. This item was recently covered on WattsUp With That?

    http://wattsupwiththat.com/2013/02/01/encouraging-admission-of-lower-climate-sensitivity-by-a-hockey-team-scientist/#comment-1214311

     

    I submitted a comment pointing out that there was not much difference between what Annan was saying and the IPCC position. My post appeared as the following:

    Philip Shehan says:

    [oh, shut up with your whining - mod]

     

     

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  19. Tom asks,

     If Revkin cannot spend that half hour before writting on the topic, what is he being paid for?

    Revkin says:

    I'm saying my read is that analyses finding a sensitivity lower than 3oC are more likely to end up right, not based solely on the quality of the work, but the history of ideas. See my 1985 article on nuclear winterfor an example of how things can play out. (Things trend toward "nuclear autumn," although there are stillsome researchers seeing big climatic impactsfrom even a "small" nuclear war.)

     

    Apparently comparisons to an irrelevant selection of biased history is more important.


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  20. Dana:  ...quote from Trenberth in the OP is in response to Annan saying that warming has slowed over the past decade.  That's just not true if you include ocean heat content data to 2000 meters.

    Annan inadvertently taking his cue from NOAA, who do insist on making surface temperature their main message when communicating w/the public. "12th warmest year" etc., without proper clarification or prioritizing, or at least emphatically reminding us  that one year's surface temperature is irrelevant in the larger scheme of things. 

    Is it any wonder the public at large is confused, when NOAA on the one hand tells us that energy is relentlessly accumulating in the Earth system while the other hand NOAA loudly and repeatedly implies it isn't? 

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  21. doug @20 - fair criticism, but as surface dwellers, people do tend to focus on surface temps, and there are different groups in NOAA looking at surface temps and ocean heat content.  And surface temps are updated monthly, while OHC data are updated quarterly. 

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  22. Hi, I have a related question to the climate sensibility.

    I am searching a reference on the thermal inertia of the Earth. If possible, I would like to have "pure version" not the convolution with the CO2 growth curve.

    I guess I could use the heat transfer rate to the ocean as a proxy and the mass of the ocean itself as a heat capacity, but I would like to know if something more formal exist in the scientific literature.

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  23. If  the long expected tipping points await us in the future then the temperature vs CO2 graph is unlikely to be a smooth continuous function.  The most obvious tipping point is when the Arctic ocean is ice free in, say, the beginning of August.  With the huge increase in Arctic ocean sea temperature, the Arctic becomes a zone of rising air, sucking climate zones northward in the fall.  Temperature readings from met stations all over the northern hemisphere lurch upward and this appears as a discontinuous or at least a sharp increase in temperature in the global record. The ice free condition then comes earlier and earlier each subsequent year until a new regime is established.  Temperature then continues to increase gradually according to climate sensitivity and our output of Carbon dioxide (which may be considerably reduced due to the resulting collapse in our ecology and hence our economy)  Gaia wins.

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  24. Yeah, Dana, but here's NOAA's headline statement to the public, as found by the first Google hit on "NOAA state of the climate":

    " 2012 global temperatures 10th highest on record "

    Thereby entirely ignoring the overwhelming majority of the energy added to the Earth system during 2012. 

    What does yelling out "10th warmest year" say to John Q. Citizen? Superficially (which, given our busy lives is the consideration most of us can afford) it says that global warming is an intermittent phenomenon, that in some years we find the world cooler than it was the year before. This is of course not only wrong but plays perfectly into the hands of "global warming has stopped." 

    It's not as though this communcation effort started yesterday, that we don't already know the surface temperature expresses only a tiny fraction of accumulated energy in the Earth system. In any case, including "oceans" in "surface" isn't such a huge conceptual leap; the oceans are an integral part of the functioning of the atmosphere and compared to the rest of our sphere are a surface feature. 

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  25. I agree, a lot of people (including climate scientists and climate science organizations) use 'global temperature' when they should say 'global surface temperature'.  It's a very important distinction, and careless to ignore it.

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  26. I should stop because this is going off-topic but if the total energy accumulation of the planetary surface (aka "global warming" which of course includes the oceans) is not continuous then... what? What would be the full implications? 

    So either conveying the impression to the listening public that energy accumulation is not a steady, continuous, uninterrupted process is astoundingly wrong or... what? 

    When an organization armed with the single loudest (even if ineffective and self-defeating) press arm performing climate communication makes this error they're wasting -everybody's- time, most especially that of relative pipsqueaks such as SkS. We can talk ourselves blue in the face here for a year but a single thoughtless press release from NOAA will nullify that work.  Everybody listens to NOAA, everybody repeats what they say. A search on press results from NOAA's most recent bullet-to-the-foot is as depressing as it is pretty damned annoying.  

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  27. I think James' point about the last decade is not that global warming has stopped (implying low or zero climate sensitivity) but that it has not accelerated to the extent that it would have if climate sensitivity were very high (above, say, 4).

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  28. And I should add to the last post that by global warming I mean increases in the global surface temperature, which is certainly not the only climate metric, or necessary the best one, but is the one for which we have the best data.

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  29. Yvan #22:

    Hansen et al (2011) have a nice comparison with different heat uptake sinks in addition to the ocean: atmosphere, land, ice-on land, floating ice (see Fig 8 and the discussion in section 10.1). They calculate that the ground uptake today is about 0.03 W/m^2 averaged over the Earth's surface. Look there and you'll also find references to other works, which perhaps can be of some use for you.

    Hansen, J., Sato, M., Kharecha, P., and von Schuckmann, K.: Earth's energy imbalance and implications, Atmos. Chem. Phys., 11, 13421-13449, doi:10.5194/acp-11-13421-2011, 2011 Full paper here.

     

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  30. Grypo #19

    I agree that it is a strange comparison. The nuclear winter was hardly as well researched in the 1980s as the climate sensitivity issue is today.

    BTW: The late Stephen H Schneider has a really interesting story to tell about this in his book "Science as a contact sport" (National Geographic Society, 2009) He did some modelling on the climatic effects of a thermonuclear war, and found that it would probably not lead to as much cooling as Carl Sagan thought. Sagan chose to ignore Schneider’s results, instead promoting the "nuclear winter" hypothesis, causing a schism between these two great scientists and science communicators of our time. Schneider's book is recommended reading for anyone who is interested in the history of climate science. It is really well written and provides an exiting first hand account from someone who was in the centre of both the science and the debate over climate change for more than four decades.

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  31. dana @ 25:

    There is even a slightly finer distinction that is almost always skipped: when it comes to the land record, we're talking about global surface air temperature. The air temperature close to the surface is not the same as the temperature of the actual ground surface. Ocean temperatures are measured differently, so it is a real surface temperature representation. This is one of the reasons why the analysis methods continue to distinguish between ocean and land data sets.

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  32. The problem for those with vested interests in fossil fuel production and use is that arguments supportive of low climate sensitivity, usually put forward by contrarian scientists, often on their behalf, are unsustainable and dangerous.  There are three reasons for this:

    1.  The most widely accepted value for climate sensitivity is 3°C, a value proposed by the IPCC.  Yet, as the author notes, there is evidence that climate scientists in general and the IPCC in particular tend to be conservative in their estimates.  One suspects that this could be true of climate sensitivity.

    2.  Increase of CO2 concentration and their effects this century are an underestimation because they can not – and do not – accurately reflect the effects of feedbacks initiated by anthropogenic emissions.  It is quite possible that CO2 concentration will rise more rapidly and be higher than anticipated because of carbon emissions from thawing Arctic permafrost.

    3.  Likewise, the effects on average global temperature and climate of rapidly diminishing albedo evidenced by loss of Arctic sea ice and retreating glaciers, is not accurately known.  However, it is more likely than not that it will contribute to accelerated global warming and Arctic amplification, increasing carbon emissions further and producing undesirable climate change.

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  33. Bob @31 - true, right after I submitted that post I thought to myself "surface air temperature, technically".  I generally stick to "surface temperature" just to keep it simple, and distinguish from overall global temperautre.

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  34. HadfieldThe problem with trying to draw any inference about CS from decade scale surface temperature changes is that what we are often seeing is short term variability. Or mwe might be seeing fluctuations due to changing patterns of where the heat is going in the oceans with a flow-on impact on surface temperatures. Any measure of longer term CS must include the longer term processes. However we can't assume that because a long term process may have a certain long term impact that we can then assume that some linear proportion of that impact will appear quickly.Long termimpacts may well occur in very non-linear ways over longer time scales. For this reason I tend to prioritize types of studies in terms of how much weight I give them:1. Long term Paleo studies.2. Models3. Short term CS estimates.
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  35. Jesús Rosino #17,

    There are some discrpenacy among probability distribution functions on Figure 2 based on the instrumental record, but all of them tend to point to the lower end of the sensitivity spectrum.

    They all have peaks at the lower end of the PDF, but the problem is that they all have long, fat tails — in other words, they are a very poor constraint on climate sensitivity, as I mentioned. The Last Glacial Maximum, on the other hand, is a much better constraint.

    The fact that the estimates based on the instrumental period tend to peak low has probably more to do with the fact that the climate has not been in equilibrium during that entire instrumental period and so therefore converting the sensitivity computed into an equilibrium climate sensitivity (ECS), which is what is being discussed, requires some guesswork (and, dare I say it — modelling).

    I'm prone to trust sensitivity based on instrumental record rather than paleo, as there are significantly less uncertainties regarding both temp and forcing changes.

    It's important to consider both signal and noise.

    There is no doubt that the temperature accuracy and many of the forcing accuracies during the instrumental period are much better than temperature and forcing reconstructions of the Last Glacial Maximum. In other words, the noise is lower.

    However, the change in temperature and forcing between the LGM and now is so much greater than the change during the instrumental period — in other words, the signal — that the signal:noise ratio is in fact much better using paleo reconstructions than it is using the instrumental period. That's why the range of estimates for climate sensitivity derived from the LGM are so much tighter than the range of estimates from the instrumental period, the last millennium, and volcanic eruptions, easily shown in that figure. The final result, combining the different lines of evidence, owes much more to the LGM results than it does to the instrumental period.

    Also don't forget that during the instrumental period,

    1. We still don't have a very good constraint on the influence of aerosols, and
    2. The climate has not been in equilibrium, as I already stated.

    For these reasons I trust the sensitivity based on the paleo data far more than I trust the sensitivity based on the instrumental period, and that trust is vindicated by the level of uncertainty associated with the sensitivity derived from each.

    It's true that there's no change in the warming trend, but, as Annan says in the comments, high-end sensitivities should show a gradual acceleration.

    I don't think that's true — not yet, anyway. There are a wide range of sensitivities in the models used in the IPCC reports yet the individual model forecasts haven't separated themselves into different temperature ranges based on their sensitivities yet, they're still all mixed together. So why should we expect the acceleration in the trend to have separated itself from the noise by this date?

    He adds that "quite a sustained steadying, with the limited ocean warming and changes to forcing estimates all points in the same direction".

    I don't say that Annan is right, my point is that I don't think he's saying just the same as mainstream climate blogs, and that he is indeed suggesting a (slight) change in the way climate sensitivity is portrayed in scientific reviews.

    I guess my point is that to the extent that he's saying something different to the mainstream climate blogs, he's wrong.

    I'm not talking about his claims that the IPCC and others have failed to damp down that long, fat tail as much as they should have — as far as I knew, that was "settled" years ago by his work, among others, and Figure 2 shows very clearly that combining the different lines of evidence very effectively shows very high sensitivities to be unlikely, despite the instrumental period on its own not being able to show that, so I wasn't aware that this was the problem that he claims it is — but rather his claims that the last ten years or so tell us anything useful. Think about it — if the various estimates of climate sensitivity based on the instrumental period still had such fat tails just five years ago, then why would an extra five years suddenly turn that around and allow calculations of sensitivity based on the instrumental period to now rule out high sensitivities?

    It's true that his statement is prone to controversy, but the denialist point that he is suggesting a sensitivity lower than 2 is easily debunked, and I think that this controversy may have the positive effect of attracting more climate scientists to the discussion put forward by Annan. :) Constraining sensitivity is an interesting issue.

    It's trivially easy to debunk, but that doesn't mean that the usual suspects will actually do so and won't instead blindly repeat it as if it supports them. Yet another example of how "fake" their scepticism really is.

    Constraining sensitivity is an interesting issue, but I suspect you'll find that there's little more progress that can be made on that front until the signal during the instrumental period is so great that the signal:noise ratio becomes higher than with the LGM.

    In other words, the only way to really get a much more precise value on it is by waiting until Bad Things Happen.

    In the meantime, even the lower limits of the likely range suggest urgent action. "Lukewarmists" like to pretend that we only need to worry if the sensitivity is really high, and that if it's somewhere in the middle of the range or lower (Steven Mosher defines "lukewarmists" as those who think there is a >50% probability of it being less than 3°, apparently not realising that that includes pretty much everyone) then we can sit back and relax.

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  36. JasonB,

    Thanks for your insighful explanations, I especially liked the one about signal/noise in the LGM. I have to reflect on them for a while, but your arguments seem convincing to me. It would have been nice anyway if Annan had been around here to add something :).

    Of course, I agree that this discussion is interesting for the sake of accuracy, but doesn't have political implications. The main uncertainty in long-term proyections is rather the emission scenario we follow.

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