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Global warming slowdown retrospectively “predicted”

The views expressed are those of the author and are not necessarily those of Scientific American.

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The ocean acts as a large heat sink (Image: Wikipedia Commons)

When I was in graduate school I once came across a computer program that’s used to predict the activities of as yet unsynthesized drug molecules. The program is “trained” on a set of existing drug molecules with known activities (the “training set”) and is then used to predict those of an unknown set (the “test set”). In order to make learning the ropes of the program more interesting, my graduate advisor set up a friendly contest between me and a friend in the lab. We were each given a week to train the program on an existing set and find out how well we could do on the unknowns.

After a week we turned in our results. I actually did better than my friend on the existing set, but my friend did better on the test set. From a practical perspective his model had predictive value, a key property of any successful model. On the other hand my model was one that still needed some work. Being able to “predict” already existing data is not prediction, it’s explanation. Explanation is important, but a model such as mine that merely explained what was already known is an incomplete model since the value and purpose of a truly robust model is prediction. In addition, a model that merely explains can be made to fit the data by tweaking its parameters with the known experimental numbers.

These are the thoughts that went through my mind as I read a recent paper from Nature Climate Change in which climate change modelers “predicted” the last ten years of global temperature stagnation. The lack of global warming since about 2000 does not disprove everything we know about climate change; the discovery of global warming is based on much more than just computer models (Weart, 2008). But models are still an integral tool for predicting future changes, and the fact that the current stagnation was not accurately encompassed by the models did pose an inconvenient truth for climate change scientists. In the latest paper scientists from Spain and France seem to have located the reason for the failure; it seems that the models were underestimating the contribution of the oceans in acting as a sink for the heat. Heat absorption by the ocean is a long established mechanism for the cessation or slowing down of atmospheric warming but it seems that the models were not accounting for this natural variability well enough. What happens is that when human induced global warming and ocean absorption reinforce each other you get a net warming signal. However when they oppose each other then the ocean sink puts a brake on the warming, which is what we see during recent years. From what I can tell, once they bumped up the value of the parameters dealing with ocean absorption of heat they could use one particular model to reproduce the observed stagnation of temperatures.

That’s fair enough. This kind of retrospective calculation is a standard part of model building. But let’s not call it a “prediction”, it’s actually a “postdiction”. The present study indicates that models used for predicting temperature changes need some more work, especially when dealing with tightly coupled complex systems such as ocean sinks. In addition you cannot simply make these models work by tweaking the parameters; the problem with this approach is that it risks condemning the models to a narrow window of applicability beyond which they will lack the flexibility to take sudden changes into account. A robust model is one with a minimal number of parameters which does not need to be constantly tweaked to explain what has already happened and which is as general as possible. Current climate models are not useless, but in my opinion the fact that they could not prospectively predict the temperature stagnation implies that they lack robustness. They should really be seen as “work in progress”.

I can also see how such a study will negatively affect the public image of global warming. People are usually not happy with prediction after the fact, and there is little doubt that skeptics and deniers will play up the futility of climate change models based on this study to varying degrees. But this is really a problem with any models that are designed to make predictions about complex systems. The right thing to do is to honestly own up to the failures of your models and suggest modifications, and it’s only through such constant feedback that the models can be improved. The next assessment of the IPCC should clearly state this discrepancy. Georgia Tech professor Judith Curry puts the issue in context:

“The flawed assumption behind the orthodoxy was that natural variability is merely ‘noise’ superimposed on the long term trend.  The natural variability has been shown over the past two decades to have a magnitude that dominates the greenhouse warming signal.  It is becoming increasingly apparent that our attribution of warming since 1980 and future projections of climate change needs to consider natural internal variability as a factor of fundamental importance.  I sincerely hope that the (IPCC) AR5 provides an assessment of what we know and what we don’t know and areas of disagreement, rather than trying to manufacture a consensus.”

Unfortunately this standard process of introspection and improvement is subverted when a topic like climate change becomes highly politicized. Proponents are often wary of publicizing limitations as part of a healthy process of scientific give and take for fear of retribution by denialists. The politicization of science harms both proponents and honest skeptics and we are all worse off for it.

Ashutosh Jogalekar About the Author: Ashutosh (Ash) Jogalekar is a chemist interested in the history and philosophy of science. He considers science to be a seamless and all-encompassing part of the human experience. Follow on Twitter @curiouswavefn.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. rkipling 1:00 am 05/15/2013

    Interesting. While you make some good points about models, you still seem to be saying something like, “Well yes, the models can’t reliably global warming, but gosh darn it I still believe.”

    Unquestionably this topic is highly politicized. I don’t see why it has to be politicized here, however? Also without question, the usual crowd will show up to insult each other’s intelligence, motives, etc. and this comment area will be filled with political posturing dressed up as science.

    It’s unfortunate that all the effort that is typically applied to the “Is TOO!” “Is NOT” comments can’t be applied to rational discussion of solutions to the potential problem. Whether one believes it will be catastrophic or not, atmospheric CO2 increasing at the current rate should give a rational person pause.

    On further reflection, few if any commenters here actually have a skill set which could usefully be applied to searching for solutions. I put myself in that category as well. I’m working with renewable energy, but my efforts will not amount to half a drop in the bucket toward CO2 stabilization. So, never mind what I just wrote. Have fun yelling at each other.

    Link to this
  2. 2. Steven Dobbs 8:25 am 05/15/2013

    Should be possible, with more earth monitoring to adjust the forecasts.

    previously there was insufficient measuring of ocean heat content and salinity distribution.

    It would be helpful if forecasters could then predict when the oceans will spill back their stored heat.

    Link to this
  3. 3. Shoshin 9:51 am 05/15/2013

    “You didn’t cool that” – Barack Obama

    Link to this
  4. 4. Jack Wolf 10:38 am 05/15/2013

    We are not only “worse off” by a small minority of deniers, they put us in harms way through inaction on emissions, the ultimate culprit.

    Link to this
  5. 5. Foremast Jack 10:47 am 05/15/2013

    So a model with hundreds of interdependent feedback loops, many of which cannot be directly measured, produces a forecast that fails to match instrument data and we are surprised? I do CFD modeling and see this all the time. My rule of thumb is the more complex a model becomes, the less reliable their forecasts and the less useful they become.

    While I won’t say that climate models are useless, their use in crafting policy is useless. If the decision was up to me, I wouldn’t make a decision effecting trillions of dollars of global economic activity based on the results from a computer model.

    Link to this
  6. 6. rkipling 11:24 am 05/15/2013

    I really don’t believe deniers as you call them are that much of a problem. Some of them are the same people who believe the earth is 6,000 years old. They believe dinosaur bones were placed here by the devil to confound the righteous. The righteous nitwits who blindly voted for the Obama are no better.

    New technology is needed; not Solyndras to nowhere. How exactly are the nitwits of either variety holding us back? Politicians of any persuasion are no help.

    Wouldn’t our time be better spent discussing solutions rather than cursing the nitwits? If we are going to spend taxpayer money, let’s spend it on apolitical basic research. A step change in technology, which is affordable, seems to be needed.

    Although I disagree with some of his political positions, we need scientists like Dr. Jogalekar, to bring us new technologies by advocacy, research, implementation, etc. So Doc, times a wastin’. Please redouble your efforts to get the best minds on this problem.

    If as some say, we actually NEED more CO2 in the atm. at some point, we seem to have figured out very effective ways to make that happen.

    Link to this
  7. 7. Leroy 12:07 pm 05/15/2013

    “On further reflection, few if any commenters here actually have a skill set which could usefully be applied to searching for solutions. I put myself in that category as well. I’m working with renewable energy, but my efforts will not amount to half a drop in the bucket toward CO2 stabilization.”

    Don’t count us out so easily. I think you’re making the same mistake we (humans) made to cause the problem in the first place… you aren’t appreciating the magnitude of collective action. Maybe you’re not going to make a big break on solar tech (but maybe you are)… I’m certainly not the man for that job… but I try to buy more things locally, grow more of my own food, turn things off when they aren’t being used… all the little things add up.

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  8. 8. M Tucker 12:26 pm 05/15/2013

    “The lack of global warming since about 2000 does not disprove everything we know about climate change”

    This statement is inaccurate. The past decade was the warmest on record for atmospheric temperatures and when the oceans are included it is obvious that we still have warming. If you have a look at this graph you can see how data can be manipulated to show cooling or a plateau.

    Models are not evidence. Models will never replicate the real world. We have evidence of what CO2 has done to warm the Earth in the past 3 million years. That is recent enough so that the geography of the Earth has not changed. We have evidence of current warming and the past evidence shows we will get even more. Those who wish to slow or obstruct a transition away from burning fossil fuels are a danger to future generations. I know of plenty of conservatives who are not scientifically challenged and who also think that continued delay is criminal. The politics is only evident with the Republicans in congress and those who elect them. Plenty of Republican mayors and state representatives are in favor of addressing the issue.

    Link to this
  9. 9. Chryses 5:19 pm 05/15/2013

    “The politics is only evident with the Republicans in congress and those who elect them.”


    Link to this
  10. 10. string_beery 5:48 pm 05/15/2013

    “The lack of global warming since about 2000…”
    The graph in the link above that 8. M Tucker provides shows that a clear warming trend appears only over periods of at least 20 years. Over shorter periods, the trend is not clear against “short-term noise”.

    “the models were underestimating the contribution of the oceans in acting as a sink for the heat.”
    it would be interesting to know more about how much actual data is available re ocean temperatures.

    Link to this
  11. 11. Dr. Strangelove 10:22 pm 05/20/2013

    I don’t think climate models can do “postdiction.” Read this article.

    I don’t believe models can ever do predictions, at least not to the level of accuracy claimed by IPCC. But that doesn’t disprove AGW.

    Link to this
  12. 12. shinn 5:39 pm 06/16/2013

    One of the best articles I’ve read recently!
    I found more info here:
    Causes Of Global Warming

    Link to this

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