The matter of attribution: Curry vs Schmidt

For twenty years and more, sceptical or dissident accounts of global warming, and more recently of ‘climate change’, have been ignored by the orthodoxy. Because governments had accepted the orthodoxy, at least in public, there was no need for debate about the science. That was settled. Now for the really important and exciting action: carbon taxes, subsidies for alternative energy, and the rest! The highpoint was probably 2009, just before the Copenhagen climate summit.

But things have changed more than a little. The lack of significant warming for a decade and more while greenhouse gas emissions have risen steadily, the real cost of the orthodoxy in practice when governments are short of money, the rise of international troubles that have nothing to do with ‘climate change’ — these factors have greatly reduced the confidence of the orthodox that they are are the Truth, and that they will prevail. The Abbott Government’s ending of the Australian carbon tax has been widely noted overseas, and heads of state of Germany, India and China are not going to the next climate summit on 23 September.

One consequence has been the generation of even shriller accounts of the doom that awaits us, as I pointed out in my last post. As interest in the whole issue wanes the orthodox feel the need to scream even more loudly, which may put even more people off. In economics this is known as a backward-sloping supply function, and you can see  it today in our mineral exports: as iron ore prices decline our big miners are shipping even more of it abroad, which will further reduce the price.

Another consequence, to which I first referred a couple of posts ago, is that for the first time there exists a real debate about one quite important issue — the extent to which human activity has been responsible for any of the warming over the last century. The debate has taken place on two websites, Climate etc., whose hostess is Professor Judith Curry of Georgia Tech, and RealClimate, whose presiding genius is Gavin Schmidt, the new head of the Goddard Institute of Space Studies, which produces one of the important historical temperature datasets, GISTEMP. The two important links are here and here, but each has lots of comments and many references elsewhere.

Professor Curry has from the beginning of her website, four years ago, maintained that the level of real uncertainty in the whole global warming debate is much greater than the IPCC and  the orthodox accept, and believes that the most recent IPCC Assessment Report, AR5, is disingenuous in both recognising (in the most oblique way) that there has been a long ‘hiatus’ in the warming and being even more confident that the orthodoxy is right. She highlights this little assertion on the question of attribution:

The AR5 provides an answer:

‘The best estimate of the human induced contribution to warming is similar to the observed warming over this period.’

She interprets it to imply that the IPCC’s best estimate is that 100% of the warming since 1950 is attributable to humans, and they then down weight this to ‘more than half’ to account for various uncertainties. And then assign an ‘extremely likely’ confidence level to all this.

She then wonders whether this is equivalent to ‘making things up’. The essay has drawn so far more than 800 comments, several of them from supporters of the orthodoxy, though not from Gavin Schmidt, who writes at RealClimate. He has taken Professor Curry to task in 2012 and 2013, essentially on the same issue: how much warming can be attributed to human activity? He supports the IPCC reports, and works hard, and ingeniously, to show that they must be right, and Professor Curry simply wrong: The bottom line is that multiple studies indicate with very strong confidence that human activity is the dominant component in the warming of the last 50 to 60 years, and that our best estimates are that pretty much all of the rise is anthropogenic.

Though I admire the way he argues, I don’t think he has won. He is committed to the AR5 statement that the world has warmed by so much since 1950, which (at least in my view) was the way in which the IPCC set to one side the real problem of the ‘hiatus’. And Dr Schmidt then ignores the other observational finding, that this warming didn’t really begin until 1975 — the earlier period, 1950 to 1975 — was one of a very small cooling. He states that the even earlier warming period, in the first half of the 20th century, was not caused by human activity (the models tell him so), but then dismisses the implication: that if the planet could warm without human activity, why could not the recent period of warming have been caused by whatever caused the earlier? Or was it a mixture of natural and human causes, and if so, by how much of each?

It is the impossibility of finding observational answers to these questions that caused Professor Curry to shake her head and state that the IPCC was simply over-confident in its statements about attribution.

I’ve done my best to summarise, but the exchange is really worth reading and reflecting on. Read Curry first, is my suggestion, then Schmidt.


Join the discussion 34 Comments

  • dlb says:

    Thanks Don, I will reluctantly head over to RC to see what Schmidt has to say. It was his website that turned me from a curious observer into a climate sceptic. After about 20 minutes there I realised that no genuine scientist would hold the certainties that he and his colleagues did on the results of their computer models. Are they are so wedded to their models they can’t see the real world? or is it that they are political activists? My guess is that is probably a bit of both.

  • David says:



    Measurement rears its ugly head again.

    The saga about measurement of temperature, in the United States, at least, is growing in reach and noise, and there are now dozens of websites with thousands of posts about it….

    All this worries me, as a data-cruncher from way back, because anyone who works with data needs to be confident that the data are valid and reliable…..

    [Finishing with a flourish]
    “It is absurd that a global policy is being decided by our governments on the basis that they think we know to a considerable degree of accuracy the global temperature of land and ocean over the last 150 years.” Amen.

    CURRY (Respected scientist etc.)

    The absence of a statistically significant difference indicates that these networks
    of stations can reliably discern temperature trends even when individual stations have nominally poor quality rankings.

  • David says:

    Team Schmidt

    For all those skeptics whop don’t believe it is possible for humans to modify a green house gas to affect the environment both negatively and positively,…..

    • dlb says:

      So David do you implicitly believe everything printed in the Guardian?

      I could using the same data and come up with the following statement:

      Since the ban in Fluorocarbons in 1987 over 25 years ago, spring ozone levels in Antarctica have continued to fall. It is only in this past year that a slight uptick was noted. The continuing fall for 26 years since the ban has baffled scientists who would have expected a recovery by now. The slight improvement even if continued in subsequent years may be evidence of natural cycles rather than “industrial ozone depleting chemicals”.

      Like it? perhaps I should send it to Graeme Lloyd over at the OZ, I’m sure there would be plenty of interested readers.

      A few things to know about ozone, outside the polar regions ozone levels show no correlation with CFCs but instead vary with solar activity and volcanic eruptions. The so called ozone hole is not a hole it is a depletion which happens in the Antarctic spring. Since the mid 60s spring levels have dropped from 300 ppm in the mid 1960s to 100 ppm at present. Summer levels have dropped from 350 ppm to 250 ppm.

      • David says:

        “…has baffled scientists…” Nice try dlb :).

        Except that it has not baffled scientists. See for example this prediction from Hofman published in 1996!. In the past I have seen a turn around in the size of the ozone hole predicted for 2016.

        “Research and subsequent prediction of the anticipated recovery period by Hofmann (1996) estimates that the total concentration of ozone depleting substances ‘will peak in the stratosphere between 1997 and 1999 and the ozone hole would begin to heal slowly, reaching pre-ozone-hole levels by about the year 2050’. Hoffman also predicts that it is possible that the first signs of healing could be detected early in the twenty-first century, and more specifically that detection of the turnaround should be possible by the year 2008.)

        (p. 8)

        A few things to know about ozone, outside the polar regions ozone levels show no correlation with CFCs

        Yes well, that is where the “hole” is. 🙂

        • dlb says:

          “It can be seen from the state of current research and knowledge that there is considerably more work yet to be done in order to accurately make two important predictions: If and when the Antarctic ozone hole is likely to undergo a full recovery; and what the long term implications of such a recovery might be for the Antarctic system, and also possible effects on the wider global climatic system”

          In other words they just don’t know, I’m afraid all these predictions whether it be on ozone or Schmidt’s GCMs just leave me cold.

          One small correction to my last comment I mentioned Ozone in ppm, I meant to say Dobson Units.

        • dlb says:

          “Baffled Scientists” that was a bit of poetic license David. As a scientist myself I was unsurprised that there has not been much change in Antarctic ozone levels. I suspect ozone levels are much like global warming more influenced by natural cycles.

          In the paper you link to, Hoffman (1996) expected ozone levels in Antarctica to reach a minimum in 2000. It is now 2014, so when is this turnaround supposed to happen? (looks like a pause to me)

          Re low ozone levels retarding melting of the Antarctic ice sheet, I don’t know enough to comment. However my instinct would be to regard this as fanciful speculation. See my comment below on an important caveat mentioned in the paper you linked to. In other words they don’t really know.

          • David says:

            “It is now 2014, so when is this turnaround supposed to happen? (looks like a pause to me)”
            Now. A pause is the beginning of a turnaround.

      • David says:

        1. CFC’s go up

        2. Ozone hole gets bigger

        3. CFCs go down after treaty

        4. Ozone hole gets smaller

        Smell the coffee

        • dlb says:

          Well its taking an awful long time to get smaller, yet another pause!

        • Peter Kemmis says:

          Hi David

          You might care to have a look at this reference from a couple of NASA’s scientists (you’ll have to excuse my accessing the report through Anthony Watts’ site, but I don’t think I can find it on John Cook’s.). I had thought that the reduction in CFCs had led successfully to a reduction in the size of the ozone hole, but evidently this has not been the case. So now I’m not at all sure that the CFCs were the culprit.

  • dlb says:

    “As interest in the whole issue wanes the orthodox feel the need to scream even more loudly, which may put even more people off”

    Ha! evidence of negative feedback in the climate debate.

  • BoyfromTottenham says:

    Let’s see – an IPCC climate report cites evidence that only 3% of atmospheric CO2 is anthropogenic in nature, and that increasing CO2 raises the global temperature. So how the heck can human C)2 emissions possibly be responsible for ALL the global warming ostensibly caused by CO2 in any given year? Did the other 97% of naturally-caused CO2 amazingly not cause any warming for some unexplained reason, or did the other 97% of CO2 amazingly fall just enough to exactly offset the warming effect human-caused 3%? Personally, I’m really amazed that science is so perverse!

    • Gus says:

      IPCC *assumes*, of course, this assumption is completely groundless, that the planet somehow deals with its natural CO2 emissions, and that the additional 3% emitted by humans is beyond the planet’s ability to absorb and so it accumulates.

      • David says:


        Seems like a pretty good assumption to me. Atmospheric CO2 has increased from 280 ppm to 400 ppm since industrial revolution.
        “Do the Math”, as they like to say in the US. 🙂

        • Gus says:

          Yes, but most of this increased CO2 derives from natural sources and is not produced by humans. Why doesn’t the globe absorb it back? This is because it has warmed due to high solar activity compared to what it was 200 years ago, so the ocean outgassed in response, as did tropical soils. As it gets cooler, this excess CO2 will go back to where it came from and pretty soon too. It’ll be gone within a couple of decades–depends on how fast and how much it cools.

          The balance between the ocean and atmospheric CO2 is dynamic and highly dependent on temperature. It varies all the time, even over the diurnal cycle. This is governed by Henry’s law.

          For more on this see three papers by Segalstad:

          • David says:

            Seriously Gus!

            The first article was published in 1992 and cited once. The second in 1995 and also cited only once. The third reference
            was published in 1998 and has been cited 28 times, which is acceptable. Unfortunately, it does not support your argument!

          • Gus says:

            It doesn’t matter how often the articles were cited. What matter is if they are correct, and they are. These are conference articles, which you can download in PDF nevertheless. Conference articles are seldom cited generally. From all three, you will learn more about Henry’s law.
            Specifically, the articles talk about CO2 atmospheric residence time. I can point you to other articles if you are interested. But to evaluate the residence time, you need to apply Henry’s law, because by far the largest CO2 exchange surface is that of the ocean, which covers 72% of the Earth’s surface. Henry’s law describes how CO2 partitions between atmosphere and water in function of temperature. It is from the application of Henry’s law that we infer that human contribution to the atmospheric CO2 is 3% only.

          • David says:

            Gus, they don’t support your argument either. Your
            claim that the reason increased atmospheric CO2 (caused by unspecified natural processes) has not returned to natural levels is because of high solar activity, is not outlined in any of three references you supplied.

          • Gus says:

            The references are about Henry’s law, and show how it is used. Henry’s law partitions CO2 between the atmosphere and the ocean and is strongly temperature dependent. The rest is obvious inference. The atmospheric CO2 has come from the ocean mostly. It’s been coming up since the end of the Little Ice Age. The CO2 level at the time, i.e., during the Little Ice Age, was way below the norm, because temperatures were way below the norm. As it got warmer, the ocean degassed and the atmospheric CO2 concentration increased. It would have happened with or without humanity. Human contribution to its present level is tiny.

          • David says:

            Yes but you are banging on about “high solar activity” .

            “This is because it has warmed due to high solar activity compared to what it was 200 years ago, so the ocean outgassed in response, as did tropical soils”
            You have provided no supporting evidence for this claim.

          • Gus says:

            Do I have to explain everything like to a child? … hmmm … are you? Anyhow, here’s how it works:

            Ever since the end of the little Ice Age, the worst of which occurred during the times of Maunder and Dalton Minima, solar activity has picked up and continued on the increase throughout all of the 20th century, having reached the peak in the last two decades of it, whereupon it subsided. At its peak, the sun was at its most active in 9000 years.

            What happens when the sun is active? There are two major effects. The first one is that the solar magnetosphere bloats, thus shielding the whole of the inner solar system from cosmic rays. As cosmic rays seed clouds upon entering the atmosphere (this is the same thing that happens in cloud chambers), there is less cloud formation during high solar activity periods. This is called Svensmark Effect, because Svensmark was the guy who drew physicists’ attention to this mechanism. It’s been since confirmed by experiments, some carried out by Svensmark himself, others by physicists at CERN, and the results published.

            The second effect, also very important, is that during high activity periods, the sun actually gets a little darker, fancy that, this is because of sun spots. But the total energy emitted by the sun increases. How can this be? This is because the energy shifts to ultraviolet. What effect does this have on the Earth’s atmosphere and the ocean? It so happens, you see, that ultraviolet absorption by water is between eight and ten orders of magnitude stronger than absorption at visual wavelengths. So, this shift of solar energy to ultraviolet packs a real wallop of heat at the atmosphere, which is full of water vapor, and the ocean.

            That this is so and that this is profound we know from countless observations that correlate solar activity periods with global climate and ocean responses. For example, ENSO is driven by the solar cycle. The flow of Parana river in South America and storminess over Brazil, are driven by the solar cycle. Algae blooms in the Mediterranean are driven by the solar cycle. The response is universal and very strong.

            Now, what if you had a long period of subdued activity in the 17th, 18th and the early 19th centuries, followed by gradually increasing activity, to reach the highest activity in 9000 years in the last two decades of the 20th century? Of course, this would manifest, and has manifested, in globally increasing temperatures. They have been on the increase since roughly 1850 or so, and most of it, some 80%, has occurred prior to 1980. And, of course, it is not just the atmosphere that has warmed, the ocean has warmed as well, not from the atmosphere by from the direct absorption of solar UV and from diminished cloud cover. These two effects compound, you see.

            Now, we invoke Henry’s law, which tells us how CO2 concentrations partition between water and atmosphere. The law is strongly temperature dependent: the warmer the water, the less CO2 water can hold, therefore as the ocean becomes warmer, it has to release excess of CO2, excess compared to the equilibrium at a given temperature, into the atmosphere. This is referred to as “ocean degassing.”

            In summary: The increase in solar activity since 1850, with its peak in the last two decades of the 20th century, warmed the ocean and the atmosphere, in effect forcing the ocean to degas a part of its CO2 reservoir. This is a huge amount of CO2. According to current estimates, it accounts, together with soil degassing, for 97% of all observed CO2 emissions, leaving 3% only to human activities. Nearly all of the currently observed atmospheric CO2 concentration derives from the ocean and soils.

            It is a godsend, because CO2 is a life giving gas: it is the foundation of the whole food chain of the earth bio-system. The more CO2 then, the more life on the planet’s surface. And we are seeing this too: in greening of the desert, in improved yields. If humans can contribute to this beneficial development by adding further 3%, so much the better.

          • Mike says:

            Very much appreciated Gus and food for thought. Over the years I have delved into these explanations on both sides since I don’t have a scientific background it has been with some difficulty. I do have considerable expertise in the production of computer software and long ago realised that even if there was an exact provable theory on how climate works the current crop of climate models would still be hopeless. They deny the rules of software engineering principally that they are untestable.

            I rely on the fact that there hasn’t been a measurable increase in world temperature for at least the last 13 years. It seems to me that ties in with what you’re saying. So I’m curious about what you think regards David Evans and his theory that there will be a significant drop round about the beginning of 2017.

            I think though the activist lobby is not concerned with all this and is using it to distract us from the main game. Which is to do anything that will cause a major reduction in human population.

    • David says:

      I’m not sure you are going to get this, but lets try.

      Suppose Tottenham and Arsenal are playing a game of football. Each has team has 11 players. Substitutes are coming on and off, but always
      11 players from each team on park. The game is in balance. The score at half time is 1 all.

      Then in second half Arsenal decides to run on a reserve and start playing with 12 players. That’s “just” a 9% increase in player numbers. But the game
      is no longer in balance. Arsenal wins 4-1.

      So boy-from-Tottenham what would you say to a boy-from-Arsenal who argued their win had nothing to do with extra player because most of the time the ball was being played by the original 11 players?

  • whyisitso says:

    It’s a shame that idiot “David” has hi-jacked the comments facility here.

  • Mike says:

    There are a number of world authorities on global temperature it is often stated that the temperature has not increased for so many years. As an exercise let us look at each global temperature record and see how far back since the last significant rise.

    Gistemp 2001 13 years
    HADCRUT3 1998 16 years
    HADCRUT4 1998 16 years
    RSS 1995 19 years
    UAH 1998 16 years

    It is argued that AGW is because the human virus insists on emitting GHG so therefore we must assume that for at least the last 13 years this hasn’t been happening. But CO2 is nearly 80% of GHG and we do have a measure of that going back more than 13 years. Currently it is 400 ppm and 13 years ago it was 370 ppm.

    That means that it changed by 8% but the world temperature hasn’t changed at all so if it is us despicable little viruses that are causing it please explain how. There seems to be no justification to blame it on CO2.

  • DaveW says:

    Instead of going for a walk on a beautiful, sunny Sunday morning, I spent the time trying to figure out if Schmidt or Curray made more sense on attribution of global temperature change to human activities: 110% vs ~50%, respectively. I won’t complain that the time was wasted, because as Don points out at least there seems to be a debate now, and this is the crux of the debate on AGW. As a disinterested observer (i.e. I don’t think either estimate has much to stand on), I think I would have to give the win to Curry. She writes clearly, made her case for why she preferred 50% in a way that even I could understand (although not agree with), and best summed up the whole brouhaha with her comment about living on planet earth vs planet model.

    I found Schmidt’s prose tends to be obtuse and his arguments seemingly reflecting a ‘bubble’ type mentality where he is used to speaking to like minds in a kind of shorthand. He tends to make authoritative statements and rarely explains them – sometimes he actually seems to be very coy about his meaning. For example, he uses two figures in his rebuttal (Don links to this above):

    1. The Probability Density Function with the peak at 110%. Schmidt apparently doesn’t realize that most people would find this a bit strange and only hints at what it may mean in the text and later in reply to commentators trying to guess. Of course, he probably cannot explain how the IPCC could be 95% certain that the climate would have cooled without Anthropogenic forcing. Also, this would imply a benefit from AGW which is not allowed. His use of this figure came across to me as disingenuous.

    2. Later Schmidt gives the Fig 10.5 on which the PDF graph is based without a legend and makes no attempt to explain how the bars were calculated or why we should believe them (except for, as William Briggs is wont to say, an itsy, bitsy p value). A dense paragraph of arguments from authority and handwaving follows that left me feeling especially distrustful of his conclusions. Wasn’t there a Super El Nino in 1998? What were oceanic cycles doing? Has there really been no net natural warming since 1950?

    I found many of his other comments equally obscure (as did several commentators), e.g.: “It is worth pointing out that there can be no assumption that natural contributions must be positive – indeed for any random time period of any length, one would expect natural contributions to be cooling half the time.” Why would natural contributions be cooling half the time (and from what?, a peak or the mean?). I and others thought he meant some equilibrium temperature model, but when questioned about it he said ‘it means exactly what I said’.

    Here are a couple where parenthetical insertions seem to contradict the leading phrase: “The sensitivity argument is irrelevant (given that it isn’t zero of course).” So sensitivity can be relevant? “they [GCMs] are scaled to get the best fit to the observations (along with the other terms)” – so they aren’t just scaled to the data, but also to ‘the other terms’. This is important because of a comment where Matt Skags points out that normally attribution studies deal with observations, not parameterizations.

    I see no reason to change my conclusion that no one actually knows what is going on with the climate, what the transient climate response to rising CO2 may be, or if it is something we really need to be worried about. Based on current data, the sensitivity of the climate to anthropogenic CO2 may be small, possibly zero, and the GCMs seem to be GIGO. If we’d gotten some real alternatives to fossil fuels out of this huge waste of time and money, then perhaps it would have been worth it, but we haven’t.

    • Mike says:

      “I see no reason to change my conclusion that no one actually knows what is going on with the climate, what the transient climate response to rising CO2 may be, or if it is something we really need to be worried about” spot on Dave the idea that they know is a pretense unfortunately Gaia says no I will do as I like.

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