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Lessons from Past Climate Predictions: Wallace Broecker

Posted on 18 July 2011 by dana1981

Wallace Broecker was among the first climate scientists to use simple climate models to predict future global temperature changes.  His 1975 paper Climatic Change: Are We on the Brink of a Pronounced Global Warming? is widely credited with coining the term "global warming".

In that paper, Broecker modeled the effects of the expected future increase of CO2 due to humans burning fossil fuels, combined with a natural climate cycle which he estimated based on Greenland ice core records, and tweaked to match the observed temperature record at the time (Figure 1). 


Figure 1:  Broecker's global temperature prediction

This was a very simple model, excluding the effects of the sun, volcanoes, other greenhouse gases, aerosols, and so forth, which Broecker acknowledged:

"In this report only the interaction of the CO2 effect and natural climatic change is considered.  As other anthropogenic effects are shown to be significant and as means to quantitatively predict their future influence on global temperatures are developed, they can be included in models such as this."

As it turns out, Broecker has been fortunate, because the cooling effects of human aerosol emissions have roughly cancelled out the warming effects of human non-CO2 greenhouse gas emissions since 1975, and solar activity has been flat over that period.  So the net effect of the factors which he did not take into account has been close to zero.  However, Broecker was also smart; the dominant effect on temperature since 1975 has been from CO2, as he expected.  It's better to be lucky than good, but it's best to be both.

Broecker anticipated the actual increase in CO2 very closely, predicting 373 ppm in 2000 and 403 ppm in 2010 (actual values were 369 and 390 ppm, respectively).  Broecker also used an equilibrium climate sensitivity of 3°C for doubled CO2; however, his model's transient climate sensitivity worked out to be 2.4°C for doubled CO2.  Current climate models put equilibrium sensitivty at 1.5 times transient sensitivty, so Broecker effectively underestimated the thermal lag of the climate system, and the equilibrium sensitivity in his calculations was approximately equivalent to 3.6°C for doubled CO2 - a bit higher than today's best estimates of 2°C transient sensitivity, 3°C equilibrium sensitivity.

We digitized Broecker's prediction from Figure 1, and compared it to the observed global temperature change since 1975 (Figure 2).  We adjusted it slightly to reflect the current atmosperic CO2 concentration (390 ppm) as opposed to his predicted 403 ppm, because we're interested in the accuracy of Broecker's temperature predictions, not his CO2 predictions.

Broecker Prediction vs Observations

Figure 2: Broecker's temperature prediction, adjusted to reflect measured CO2 changes, vs. GISTemp observed global surface temperature changes.

As you can see, Broecker's prediction has matched the net global temperature change quite closely over the past 35 years.  His 'natural cycle' estimate held his prediction below the actual global temperature increase for most of the period, but  as illustrated in Figure 1, he predicted its effects would approach zero after 2000.  Not coincidentally, this is when his prediction most closely matches the observed global temperature.  Broecker overestimated the amount of global warming by 2010 slightly, by a bit less than 0.2°C.  This is probably mainly due to his slight overestimate of climate sensitivity, and potentially due to the increased cooling effects over the past decade.

It's quite remarkable that a prediction made in 1975 using such a simple model of the climate system could so accurately match the observed global temperature change.  It's a testament to the dominant effect of CO2, and the fact that we have had a solid understanding of the fundamental workings of the Earth's climate for many decades.

Nevertheless, those who are "skeptical" that humans are driving global warming, including the few climate scientists in this category, often emphasize and exaggerate what we don't know about how the Earth's climate functions.  In his testimony before US Congress earlier this year, "skeptic" climate scientist John Christy compared the state of climate science research in the 1970s to that today, saying "our ignorance about the climate system is just enormous".

While it's true that there remain some features of the climate system which we still don't fully understand, Broecker's success illustrates that climate scientists have long had a good grasp on the main drivers of the global climate.

While the "skeptics" enjoy criticizing climate scientists, they rarely put their money where their mouths are in terms of making predictions of their own.  We previously examined one of the exceptions - Don Easterbrook, who has been predicting imminent global cooling since approximately 2000.  He stood by that prediction in December 2008, presenting his projections of future global temperature change at the American Geophysical Union annual conference.  Easterbrook's model is even simpler than Broecker's, only taking into account his estimates of past natural climate cycles, assuming they will continue in the future, and effectively presuming that CO2 has no effect on global temperatures (throwing out the centuries-old physics of Tyndall and Arrhenius).  Figure 3 compares Broecker's prediction and two of Easterbrook's to the observed global temperature.

Broecker vs. Easterbrook

Broecker's 1975 prediction was within 0.2°C of the observed global temperature in 2010, while Easterbrook's, last made in December 2008, were off by 0.3 to 0.5°C.  This illustrates the importance of basing future predictions on solid physical footing, and also shows that climate scientists understand the inner workings of the global climate much better than the "skeptics" would have us believe.  In fact, in the 1970s, climate scientists understood how the Earth's climate works better than many "skeptic" scientists do in 2011!

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Comments 1 to 18:

  1. Wow - Broecker's 1975, after correcting to actual CO2 looks even more accurate than Hansen 1988. Thank you for the official right to laugh in the face of the next denier who claims "models are inaccurate" - 35 years and counting! (they won't be informed enough to realize it was part luck, part skill)
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  2. To put some perspective on Broecker's 1975 work, remember that the Cray-1 back then had a maximum sustained performance rating of about 140 MFLOPS. Fast-forward to the present time, and we have the Apple iPad-2 producing LINPACK benchmark results in the neighborhood of 170 MFLOPS. So the best computing facilities available to Broecker when he published his 1975 paper were equivalent to something like a slightly crippled iPad-2. Something else to keep in mind when the "skeptics" start nit-picking Broecker's work.
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  3. ... if we go back to “old” papers, you should remember about these uncertainties. They are still current ...
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  4. Arkadiusz, Thank you for your demonstration of the hypocrisy of "skeptic" arguments. The proper place to put a projection is starting at the time it was made (1975) and going into the future, as the graphs above show clearly. Dr. Broecker's accurate projections of future temperatures demonstrates that his model has skill. If you have a problem with his skill demonstration please provide data that supports your objections. Random criticism of someone who was correct does not add to the conversation.
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  5. Arkadiusz Semczyszak Please can you provide a link to that graphic in its original context, so that we can see that the uncertainties you mention are actually relevant to Broecker's projection.
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  6. Excellent post. I'm glad to see Broecker get a little attention for his outstanding work -- he's been seriously overlooked in discussions, even among the climate geeks, for far too long.
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  7. Thanks Lou. Agreed, I had no idea Broecker had even made this prediction, let alone its impressive accuracy, until Ari pointed me towards the paper.
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  8. "you should remember about these uncertainties..." hehe... it´s great to see people clinging to straws to avoid seeing the obvious: the science that predicted AGW is around for quite a while, has resisted the test of time and has improved in the meantime. On the other hand, clumsy predictions of cooling based on nothing more than graph eyeballing are deemed as worthy mavericks. It's really a "debate" that does not excite me anymore. Fortunately, renewables seem to be getting close to a tipping point economically (like this) and I hope to see big changes within this decade.
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  9. I think that, in the development of science, there comes a time where it is simply not legitimate any longer to neglect theories that are firmly based on standard physical understanding, and have stood the test of time quite well. While they may, ultimately, still be wrong in many (if not all) ways, we are simply not allowed to deny the possibility that they are right, and still stay within the realm of science. It's fully legitimate to believe something different than consensus, and to find and present the evidence supporting that alternative view. It is _not_ legitimate, however, to overlook all the evidence supporting the opposing views. One may propose alternative interpretations, but those interpretations may have a very hard time getting accepted. When we have a simple (in terms of physical understanding) model, we won't reject that for a more complicated one until that one is shown to be significantly better. It is _not_ a competition on even terms here. To throw established parameters out of a model, you have to substitute something that is demonstrably better, subject to model complexity. Here: Given the understanding and the evidence re greenhouse gases, it is a very big task to establish valid explanations that they don't matter in practice, after all. Akasofu's cyclical explanation model may be a case in point here. It would have to perform brilliantly on new data to be accepted, which it can hardly be said to do now. In the context of confirmation, there is no need to fully specify a hypothesis (here: the totality of forcings from gases), and it's therefore not quite fair to judge the merits of an only partially specified hypothesis on comparison with observations alone. Rather, we can look at the original formulation, and make some estimates about how a more complete specification, based on the present state of knowledge, would modify the original predictions. It seems that Broecker's work then looks even better.
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  10. Dikran Marsupial @ 5 Arkadiusz' graph comes from a German language site ultimately sourcing a NASA website circa 2000 - hence, no doubt the reference to "old" uncertainties. Wasn't hard to trace - took all of three minutes. It took me much longer to try to work out how best to post the link, which ended up a dismal failure so apologies for the resulting clutter. The site incidentally is basically pro-AGW.
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    Moderator Response: [Dikran Marsupial] Link activated
  11. Follow-up to myself: Huge uncertainties are very relevant to the actual estimation problems, but focussing on them, which is a well-known defensive tactics nowadays, completely misses the important point: Having established a phenomenon, the important issue is no longer one of precise estimation (in which case I, among others, would not have been qualified to comment), but of science-based risk management. And in that context, the significance question turns the other way round: It's not about establishing the theory beyond doubt, but about refuting it. For example, when in vitro studies strongly indicate carcinogenicity of a substance, regulations are indicated, only to be lifted if strong in vivo/population studies show notihing of the expected effect. This is quite analogous to the climate issue: Ever since the first theoretical and practical radiation studies on CO2 ("in vitro studies"), there has been good reason to expect a CO2 climate effect, and regulations on the release of CO2 have been indicated. This concern could only be set aside by _repeated_ failures to detect the expected effects "in vivo" (e.g. observed effects on actual radiation spectra) or "population" - observations on climate changes. But "population" studies, generally, only establish associations, not causality. If CO2 was about carcinogenicity, the mass of evidence would leave no regulatory body in any state of doubt about it: This must be kept in check. Serious risk management is not about established facts (then it's too late), but about suspicions and probabilities. It should also be kept in mind that uncertainties, generally, is a two-sided business. So stressing the uncertainties in a risk-management situation could in fact imply stronger measures to be taken. "So, you say, the standard deviation of the estimated pdf for sensititity (expected=3) is 1.6? Gosh, then we must regulate even harder: It's a >5% chance it will be >6, and we can't live with a CO2 doubling or more in that case."
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  12. chris1204 You are missing the point. It is for Arkadiusz to provide the proper context for the graph he used; as it happens the article it comes from doesn't really support his contention as Broecker's projection was based on physics, and was pretty accurate given the known uncertainties in natural variability. Essentially I was hinting (perhaps too subtly) that Arkadiusz was posting in a style indistinguishable from trolling. If he wants to make a scientific point, it is better to do so explicitly so that a scientific response to the point can be made.
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  13. Excellent and very important post!
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  14. I agree Bart, Glad to see you noticed it. Posts like this need to be trumpeted far and wide.
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  15. Can anybody explain what remains as a natural forcing if Sun and Volcanoes are excluded, and how this remnant is able to form a cycle (as used in the model)? It must be an important one to drive temperatures in a range of nearly 0.8K within 100 years, but I've never heard of it and I found no explanations of it in any of the IPCC publications. But it is of course necessary to know about to call the model discussed here a proper one.
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  16. bart and Albatross - thanks. JoeRG - what's left is mainly internal variability, which does contain some cycles. Bear in mind that Broecker's "Camp Century Cycles" are very rough estimtes used to try and fit the model to the data. As such, they actually do indirectly include solar and volcanic forcings in the early-mid 20th Century.
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  17. dana1981 Then you made a mistake in the post, because you wrote: This was a very simple model, excluding the effects of the sun, volcanoes,... what obviously is not true. Even if they are indirect signals, they are indeed part of the model. As it is written, it is a bit confusing. Sorry.
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  18. Please check if you could update the correct URL for the "missing link" to the paper, in the first sentence:

    ... to predict future global temperature changes. His 1975 paper Climatic Change: Are We on the Brink of a Pronounced Global Warming? is widely credited with coining the term "global warming". ...

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    Moderator Response:

    [PS] Thanks for that heads up. In meantime try this link.

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