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Christy Crock #3: Internal Variability

Posted on 14 April 2011 by dana1981, Albatross

Christy Crocks (200 x 70 pixels)In the recent U.S. House of Representatives Committee on Science Space and Technology climate hearing, Dr. John Christy was the main witness presenting the opinions of the global warming "skeptics."   As we previously noted, the quality of Dr. Christy's testimony was extremely disappointing, as he frequently repeated and affirmed climate myths.  Among them, Dr. Christy touted the myth that internal variability could be the cause of the current global warming:

"When you look at the possibility of natural unforced variability, you see that can cause excursions that we've seen recently"

As we will see here, this statement is simply false.  Natural variability cannot account for the large and rapid warming we've observed over the past century, and particularly the past 40 years.

Swanson and Tsonis

One of the most widely-circulated papers on the impact of natural variability on global temperatures is Swanson et al. (2009) which John has previously discussed

Although Swanson 2009 was widely discussed throughout the blogosphere and mainstream media, the widespread beliefs that the study attributed global warming to natural variability and/or predicted global cooling were based on misunderstandings of the paper, as Dr. Swanson noted:

"What do our results have to do with Global Warming, i.e., the century-scale response to greenhouse gas emissions? VERY LITTLE, contrary to claims that others have made on our behalf. Nature (with hopefully some constructive input from humans) will decide the global warming question based upon climate sensitivity, net radiative forcing, and oceanic storage of heat, not on the type of multi-decadal time scale variability we are discussing here. However, this apparent impulsive behavior explicitly highlights the fact that humanity is poking a complex, nonlinear system with GHG forcing – and that there are no guarantees to how the climate may respond."

In their paper, Swanson et al. use climate models to hash out the role internal variability has played in average global temperature changes over the past century (Figure 1). 

Swanson Tsonis variability

Figure 1: Estimation of the observed signature of internal variability in the observed 20th century global mean temperature in climate model simulations

As you can see, over periods of a few decades, modeled internal variability does not cause surface temperatures to change by more than 0.3°C, and over longer periods, such as the entire 20th Century, its transient warming and cooling influences tend to average out, and internal variability does not cause long-term temperature trends.

Additional Studies

A number of other scientific studies have also examined the impact of internal variability on global temperatures, and arrived at a very similar conclusion to Swanson et al.  For example, here are the findings of DelSole et al. (2011)(emphasis added):

"The amplitude and time scale of the IMP [internal multidecadal pattern] are such that its contribution to the trend dominates that of the forced component on time scales shorter than 16 yr, implying that the lack of warming trend during the past 10 yr is not statistically significant...While the IMP can contribute significantly to trends for periods of 30 yr or shorter, it cannot account for the 0.8°C warming that has been observed in the twentieth-century spatially averaged SST."

This conclusion directly contradicts Christy's statement that natural variability can account for all of the recent warming.  This is not a new finding, as it is consistent for example with Stouffer et al. (1994):

"throughout the simulated time series no temperature change as large as 0.5°C per century is sustained for more than a few decades. Assuming that the model is realistic, these results suggest that the observed trend is not a natural feature of the interaction between the atmosphere and oceans."

and with Wigley and Raper (1990):

"Simulations with a simple climate model are used to determine the main controls on internally generated low-frequency variability, and show that natural trends of up to 0.3°C may occur over intervals of up to 100 years. Although the magnitude of such trends is unexpectedly large, it is insufficient to explain the observed global warming during the twentieth century."

These studies are also consistent with Bertrand and van Ypersele (2002), Rybski et al. (2006), and Zorita et al. (2008), among others.  There is a strong consensus that natural variability cannot account for the observed global warming trend, which raises the question: on what is Christy basing his unsubstantiated claims to the contrary?  Although he does not provide any supporting evidence in his congressional testimony, Christy is likely relying on the work of his University of Alabama at Huntsville (UAH) colleague and fellow "skeptic," Roy Spencer.

Spencer's Hypothesis

Dr. Roy Spencer has proposed a hypothesis whereby some unknown internal mechanism causes cloud cover to change, which in turn changes the reflectivity (albedo) of the planet, thus causing warming or cooling.  Spencer also attributes most of the global warming over the past century to this "internal radiative forcing."  There are some significant flaws in this hypothesis.  For one thing, it fails to explain many of the observed "fingerprints" of human-caused global warming, such as the cooling upper atmosphere (stratosphere and above) and the higher rate of warming at night than during the day.

In order for internal variability to account for the global warming over the past century (especially over the past 40 years), it requires that the large greenhouse gas radiative forcing can't have much effect on global temperatures.  For this to be true, climate sensitivity must be low.  But as discussed in Swanson et al. (2009), if climate is more sensitive to internal variability than currently thought, this would also mean climate is more sensitive to external forcings, including CO2.  This is a Catch-22 for Spencer's hypothesis; it effectively requires that climate sensitivity is simultaneously both low and high.

Dr. Andrew Dessler published a study (Dessler 2010) which casts further doubt on Spencer's hypothesis, as detailed in an email exchange between the two scientists.  In short, Dessler argues that cloud cover change is a feedback to a radiative forcing, for example increasing greenhouse gases, while Spencer argues that clouds are changing due to some other, unknown cause, and acting as a forcing themselves.  Unlike Spencer, Dessler explains the mechanism and supporting evidence behind his cloud feedback research:

"My cloud feedback calculation is supported by a firm causal link: ENSO causes surface temperature variations which causes cloud changes. This is supported by the iron triangle of observations, theory, and climate models."

El Niño Southern Oscillation (ENSO)

Although he is very coy about the physical mechanisms behind his hypothesis, Spencer does seem to believe that his hypothesized internal radiative forcing will cause "ENSO-type behavior," such as warming surface air temperatures.  However, Trenberth et al. (2002) examined the role ENSO has played in the global warming over the past half-century, and their conclusions do not bode well for Spencer's hypothesis:

"For 1950–1998, ENSO linearly accounts for 0.06°C of global surface temperature increase."

This 0.06°C accounts for approximately 12% of the warming trend over the timeframe in question.  Foster et al. (2010) also examined the effects of ENSO on global temperature and arrived at the same conclusion.

"It has been well known for many years that ENSO is associated with significant variability in global mean temperatures on interannual timescales. However, this relationship (which, contrary to the claim of MFC09, is simulated by global climate models, e.g. Santer et al. [2001]) cannot explain temperature trends on decadal and longer time scales."

Foster et al. examine a number of previous studies which assessed and removed the effects of ENSO on the global surface temperature (emphasis added):

"In all of these previous analyses, ENSO has been found to describe between 15 and 30% of the interseasonal and longer-term variability in surface and/or lower tropospheric temperature, but little of the global mean warming trend of the past half century."

Pacific Decadal Oscillation (PDO)

ENSO is part of the PDO, which Spencer has also tried to blame for the current global warming.  In a post on his blog following up on Spencer and Braswell (2008), Spencer claims:

"The evidence presented here suggests that most of that warming might well have been caused by cloud changes associated with a natural mode of climate variability: the Pacific Decadal Oscillation."

However, as detailed here by Dr. Barry Bickmore in a three part series, and by Dr. Ray Pierrehumbert at RealClimate, Spencer's attribution of the recent global warming to PDO is no more than an example of how to cook a graph.  As Dr. Bickmore put it,

"Spencer's curve-fitting enterprise could (and did!) give him essentially any answer he wanted, as long as he didn't mind using parameters that don't make any physical sense."

Further, as we have previously discussed, like ENSO, PDO physically cannot cause a long-term global warming trend.  It is an oscillation which simply moves heat from oceans to air and vice-versa, so even if there were a period of predominantly positive PDO over the long-term, the oceans would cool as a consequence of the transfer of heat to the overlying air.  That is not the case: the oceans are warming as well.

It's not Internal Variability

In conclusion, there is simply no supporting evidence or physics behind Christy's unsubstantiated claim that the global warming over the past century could simply be attributed to internal variability.  Studies on the subject consistently show that internal variability does not account for more than ~0.3°C warming of global surface air temperatures over periods of several decades.  Internal variability also tends to average out over longer periods of time, as has been the case over the past century, and cannot account for more than a small fraction of the observed warming over that period.  Spencer's hypothesis cannot account for numerous observed changes in the global climate (which are consistent with an increased greenhouse effect), does not have a known physical mechanism, and there are simply better explanations for interactions between global temperature and cloud cover.

Dr. Christy was simply wrong to tell our policymakers that natural variability can account for all of the observed global warming over the past century, and particularly the past several decades.   One can't help but suspect that Christy was simply telling the Republican policymakers what they wanted to hear.  What is more, he knows the state of the science on this issue, and cannot plead ignorance.  As such, his false statement was disingenuous and the very antithesis of good science.

NOTE: this post has also been adapted into the Intermediate rebuttal to "it's internal variability"

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Comments 101 to 113 out of 113:

  1. "Giles@99 If you point is that models used to have drifts because of their limitations, but now they have improved so that they don't, then that is an entirely vacuous point. You have provided no evidence whatsoever that the drift are problematic, just assertion." YOU provide the evidence that they're problematic, since you say they have "improved", and that "It was an indicator of a shortcoming of the models". Darwinian evolution is not a rude word in my mouth - of course scientific theories evolve like species, by try and error. The point here is that the issue to be discussed is precisely the question if large timescale evolutions are reproduced by the models, or not - and precisely the quality of the models has been evaluated following this criterion, an unstable climate being considered as "unphysical" - without precise reason. Again, it's a generic feature of numerical models to be unable to quantify precisely the amount of unforced variability for non linear chaotic systems it's not restricted to climate models - it's generic And so the argument that "variability does not exist because we don't see it in our computations" is practically devoid of any significance. And comparison with planets is irrelevant since we know precisely the constraints they must obey - which is not the case for unforced variability.
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  2. Gilles@101 There you go again, I am not the one suggesting that the drift is problematic. The need for flux corrections is an indication that there models had inadequacies in exactly the same way that epicycles were and indication that heliocentric and Copernican models have inadequacies. The comparison with planetary motion was a comment on the way that models are developed, I didn't imply in any way that the problems were physically related. It is only your supposition that "large timescale evolutions" actually exist, you have provided precisely zero evidence to support that supposition. The unforced stabilty of the climate seems well established by the paleoclimate record. Can you give an example of a major shift in climate that cannot be attributed to a change in forcing? If not, your argument seems to be as follows: Models with known shortcomings exhibit unforced drift. Therefore the real climate may have unforced drift. Therefore the model projections may be unreliable. That isn't a very convincing argument. Maybe the problem is with your second language issues. In which case, as I suggested you need to spend more time both reading other peoples posts (which you repeatedly appear to misunderstand) and more time writing your posts to explain yourself better (so that others don't misinterpret yours).
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  3. I make no supposition : I just say that the use of computer simulation is not a good criterion to exclude long term variations, since both computation methods *and* initial conditions have been carefully selected with this criterion. Now you use a totally different argument by saying :"The unforced stabilty of the climate seems well established by the paleoclimate record. " because this argument doesn't rely at all on computer simulations but on observations ! so you change the argument and say now " but paleoclimate observations exclude a natural 0.15°C/yr during 50 years". Which is again wrong : paleoclimate data don't exclude this kind of variations. " Can you give an example of a major shift in climate that cannot be attributed to a change in forcing?" "being attributed" being not an objective variable, but kind of a sociologic feature of current science, I can't answer this question. I stick on facts. But a 0.5 °C variation in 50 years wouldn't certainly be called a "major shift in climate" for paleoclimatic data (after all, who in the world is living in a different "climate" from his parents ?) - and yes of course there are plenty of such variations in the past - that cannot clearly be attributed to a change in forcing.
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  4. 0.015°C/yr of course.
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  5. Gilles@103 I don't recall anyone saying that computer models were the only line of evidence suggesting that long term unforced variations are unlikely. So the comment about changing stories is a strawman and you know it. This discussion stems from your question about initialisation of models on post 67. I have already explained to you that initial conditions do not affect the conclusion of model ensembles and why. If you want to show that they do, then you need to provide evidence. So far you have provided none whatsoever. You have not provided any evidence of long term unforced variability. Pointing out there is variability in the data does not mean that it is unforced. You can't have it both ways, if you dismiss the attribution of changes to forcings as being a "sociological feature of modern science", then it is equally a "sociological feature of modern science" for you to attribute it to unforced variability. So what evidence do you have that model projections are sensitive to the initial conditions? What evidence do you have that long timescale unforced variability is a significant part of the climate system? No word games, just give straight answers to those three questions.
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  6. "I have already explained to you that initial conditions do not affect the conclusion of model ensembles and why." which is wrong, or rather, depend on the relaxation time scale of the model. "So what evidence do you have that model projections are sensitive to the initial conditions? Again, search for "initialize" in, and you will find plenty of occurences such as : "should initialize from a point early enough in the pre-industrial control run to ensure that the end of all the perturbed runs branching from the end of this 20C3M run end before the end of the control. This will enable us to subtract any residual drift in the control from all runs that will be compared to it." if initialization is immaterial, why give a prescription ? "What evidence do you have that long timescale unforced variability is a significant part of the climate system?" I don't have to "provide" evidence of long timescale variability - the burden of the proof is for the one who claims there isn't. Do you claim that, or not ? you can provide either strong theoretical , or strong observational evidence. I would accept both. I just don't see where they are.
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  7. Gilles@106 I explained earlier that the initial conditions don't affect the model ensembles provided you let them "burn-in". This "burn in" phase is presumably what you mean by "relaxation time scale". This is of the order of a few weeks. That is why numerical weather prediction is only accurate to a prediction horizon of a few days; that the models rapidly forget their initial conditions. Now if you have evidence that is not the case, lets hear it. The initialisation prescription given is basically just saying the models need to be properly burned in, while considering the need for the comparison. Nothing more. I am not cliaming there is no long timescale unforced variability, just that there is little evidence to suggest that such a thing exists and therefore it is not rational grounds for significant doubt on the model projections. If you want to assert that long timescale unforced variability is a reason to doubt the model projections then the burden is on you to give credible evidence that it even exists.
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  8. "This is of the order of a few weeks." If this were a generic feature of GCM models, then there wouldn't be any drift : everything should be "burnt-in" in a few weeks. What you don't understand is that when running a numerical model, you can only measure the characteristic variability timescales of the model itself, not of the reality it is supposed to describe. The limit cycle that can arise in a non linear ,chaotic model are generically *not* well predicted, again- and this is extremely difficult, and so to say impossible, to make a reliable diagnosis from this kind of computation. You're using an argument for "evidence" with a tool that is simply not appropriate for this use. Of course, there are plenty of signs of variabilities at hundreds or thousand years levels -and this is totally compatible with the order of magnitude of thermohaline circulation and heat transport timescales by the oceans.
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  9. Gilles #106: "I don't have to "provide" evidence of long timescale variability " Of course not. You can just throw out the teaser of 'long timescale variability could be ... ' and not bother with evidence, proof or theory. That's the advantage of saying 'no' to everything you don't like. One could ask what relevance 'long timescale variability' has to the current forced climate change?
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  10. Gilles@108 "If this were a generic feature of GCM models, then there wouldn't be any drift : everything should be "burnt-in" in a few weeks." The obvious answer to this point, is that as the references you yourself provided clealy state, modern GCMs don't exhibit these drifts and no longer need flux adjustments to correct them*. *Although the drifts are not solely due to initialisation anyway, so the argument was invalid to beign with Sorry Gilles, I am bored with your trolling, it is obvious you have no substantive point to make, are not even reading your own sources, just quote mining, and are merley trying to disrupt sensible discussion. Sadly you have achieved your goal.
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  11. "One could ask what relevance 'long timescale variability' has to the current forced climate change?" It "just" changes the influence of forcings accordingly . "The obvious answer to this point, is that as the references you yourself provided clealy state, modern GCMs don't exhibit these drifts and no longer need flux adjustments to correct them." I already answered that precisely here . No need to say it again - you just prove with your models what you have carefully worked out to get.
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  12. and an example of publication (but I'm sure there are others and you know it very well) " Several recent studies suggest that the observed SST variability may be misrepresented in the coupled models used in preparing the IPCC's Fourth Assessment Report, with substantial errors on interannual and decadal scales (e.g., Shukla et al. 2006, DelSole, 2006; Newman 2007; Newman et al. 2008). There is a hint of an underestimation of simulated decadal SST variability even in the published IPCC Report (Hegerl et al. 2007, FAQ9.2 Figure 1). Given these and other misrepresentations of natural oceanic variability on decadal scales (e.g., Zhang and McPhaden 2006), a role for natural causes of at least some of the recent oceanic warming should not be ruled out. Regardless of whether or not the rapid recent oceanic warming has occurred largely from anthropogenic or natural influences, our study highlights its importance in accounting for the recent observed continental warming. Perhaps the most important conclusion to be drawn from our analysis is that the recent acceleration of global warming may not be occurring in quite the manner one might have imagined. The indirect and substantial role of the oceans in causing the recent continental warming emphasizes the need to generate reliable projections of ocean temperature changes over the next century, in order to generate more reliable projections of not just the global mean temperature and precipitation changes (Barsugli et al. 2006), but also regional climate changes." No need to say that if even the decadal variability may be "misrepresented", this can only be worse at the century timescale.
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  13. GIlles#111: "It "just" changes the influence of forcings" Explain please. In specific terms, with specific forcings, in the context of this thread: over periods of a few decades, modeled internal variability does not cause surface temperatures to change by more than 0.3°C, and over longer periods, such as the entire 20th Century, its transient warming and cooling influences tend to average out "I already answered that precisely ... models converge towards an absence of drift " This should go to the 'models are unreliable' thread, where you will find your 'answer' makes little sense. But your position has descended into the circular: If models didn't converge, you'd be screaming they are unreliable; if they do converge, you claim that's what they are designed to do. Life must be good when you get to argue both sides.
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