Are Scientists Overestimating — or Underestimating — Climate Change, Part I

A study by Stephen Schwartz of Brookhaven National Lab to be published in the Journal of Geophysical Research (JGR) has the denyers and doubters delighted.

“Overturning the ‘Consensus’ in One Fell Swoop” gloats Planet Gore, which says the study “concludes that the Earth’s climate is only about one-third as sensitive to carbon dioxide as the IPCC assumes” and so we “should expect about a 0.6oC additional increase in temperature between now and 2070″ [0.1oC per decade] if CO2 concentrations hit 550 parts per million, double preindustrial levels.

Is this possible? Aren’t we already warming up 0.2oC per decade — a rate that is expected to rise? Has future global warming been wildly overestimated by the Intergovernmental Panel on Climate Change (IPCC) consensus?

Or, as I argue in my book, has future global warming been underestimated by the IPCC. This is perhaps the central issue in the climate change debate, so this will be a long post. To cut to the chase, it is not possible for one study to overturn the consensus, and in any case this inadequately researched, overly simplistic, and mistake-riddled study certainly doesn’t.

Climate sensitivity expert James Annan points out key mistakes that rip the guts out of Schwartz’s analysis. That is strike one. Now I’ll offer my 2 cents worth.

I have always considered it ironic that the Denyers — who don’t believe the consensus, which is based on hundreds of studies that they obviously reject out of hand — are so enamored of the very few studies that suggest the consensus overestimates climate change (while ignoring the great many studies that suggest an underestimate). Even more ironic, let me quote from the end of the Schwartz paper, which is painfully aware how dubious its main conclusion is:

Finally, as the present analysis rests on a simple single-compartment energy balance model, the question must inevitably arise whether the rather obdurate climate system might be amenable to determination of its key properties through empirical analysis based on such a simple model. In response to that question it might have to be said that it remains to be seen. In this context it is hoped that the present study might stimulate further work along these lines with more complex models…. Ultimately of course the climate models are essential to provide much more refined projections of climate change than would be available from the global mean quantities that result from an analysis of the present sort.

Yes, the Denyers routinely attack the IPCC consensus for using elaborate computer models that they claim are still far too simplistic to model the real climate — claiming those models omit key variables and negative feedbacks that would reduce future climate change. But now they would have us embrace a self-acknowledged “simple model” — one far more simplistic than the climate models the Denyers repeatedly denounce as too simplistic. That’s chutzpah.

There is both a simple reason and a more complicated reason why I firmly believe that IPCC scientists are underestimating future climate change (and hence that Schwartz is very wrong). First, the simple reason — Scientists have underestimated current climate change:

So what are the chances that the IPCC has overestimated the climate sensitivity by a factor of three as Schwartz’s overly simple model would have us believe — that the rate of warming in the next several decades will be under half that of the rate of the past 16 years? Zilch. Does Schwartz mention any of these data points? Not one. Shame on the JGR editors for letting this go by. Strike two.

Now on to the more complicated reason I am convinced scientists are underestimating future climate change.

First let’s define theequilibrium climate sensitivity” as the “equilibrium change in global mean surface temperature following a doubling of the atmospheric (equivalent) CO2 concentration. Schwartz’s simple model deduces this is 1.1 ± 0.5 degrees° Kelvin (or °C). Schwartz notes that the IPCC’s 2007 Fourth Assessment Report concluded it was “2 to 4.5 K with a best estimate of about 3 K and … very unlikely to be less than 1.5

Planet Gore touts the fact that “Schwartz’s results … are based on the empirical relationship between trends in surface temperature and ocean heat content,” implying that Schwartz’s simple model is based on empirical data but other estimates of the climate sensitivity aren’t. In fact, as noted, Schwartz ignores the abundant real world data of the past two decades that the climate is more sensitive than the models.

And the IPCC’s numbers are derivable from empirical data from the past century as well as paleoclimate data as well as complex climate models. Back in 2004, Science magazine published an excellent article, Three Degrees of Consensus, which explains that “almost all the evidence points to 3°C” as the most likely climate sensitivity, as climate researcher Alan Robock explained. The article notes, for instance, that we have very good empirical data from the temperature dip and rebound following recent volcanoes:

From the magnitude and duration of the Pinatubo cooling, climate researcher Thomas Wigley of NCAR and his colleagues have recently estimated Earth’s sensitivity to a CO2 doubling as 3.0ºC. A similar calculation for the eruption of Agung in 1963 yielded a sensitivity of 2.8ºC. And estimates from the five largest eruptions of the 20th century would rule out a climate sensitivity of less than 1.5ºC.

Doh! Annan himself co-authored a terrific article in 2006, “Using multiple observationally-based constraints to estimate climate sensitivity,” which combines many different pieces of data and research to conclude that the climate sensitivity has a 68% chance of being 2.5-3.5ºC and a 95% chance of being 2-4ºC, as he explains in this blog post.

And climatologist Barry Pittock cites several articles that suggest a climate sensitivity range of “around 2º–6°C” in his important EOS article, “Are Scientists Underestimating Climate Change” (All Pittock’s citations can be found here). These include Annan’s paper, as well as:

  • Forster, P. M. D., and J. M. Gregory (2006), The climate sensitivity and its components diagnosed from Earth Radiation Budget Data, J. Clim., 19, 39–52.
  • Hegerl, G. C., T. J. Crowley, W. T. Hyde, and D. J. Frame (2006), Climate sensitivity constrained by temperature reconstructions over the past seven centuries, Nature, 440, 1029–1032.
  • Murphy, J. M., D. M. H. Sexton, D. N. Barnett, G. S. Jones, M. J. Webb, M. Collins, and D. A. Stainforth (2004), Quantification of modelling uncertainties in a large ensemble of climate change simulations, Nature, 430, 768–772.
  • Piani, C., D. J. Frame, D. A. Stainforth, and M. R. Allen (2005), Constraints on climate change from a multi-thousand member ensemble of simulations, Geophys. Res. Lett., 32, L23825, doi:10.1029/2005GL024452.
  • Stainforth, D. A., et al. (2005), Uncertainty in predictions of the climate response to rising levels of greenhouse gases, Nature, 433, 403–406.

I list them all here because Schwartz doesn’t cite a single one of them in his paper. That’s right — he has written an article purporting to overturn perhaps the central number in the climate change debate, and he doesn’t cite any of these recent articles on the subject. I agree with Annan:

It’s surprising that Schwartz didn’t check his results with anyone working in the field, and disappointing that the editor in charge at JGR apparently couldn’t find any competent referees to look at it.

Strike Three. This paper is called out swinging. The Denyers can keep citing it, but it is too severely flawed to be considered a serious contribution to the scientific literature. The paper should be withdrawn.

Annan explains how Schwartz made a simple but important mistake — the paper “grossly underestimates [by a factor of 3] the time scale of response of climate models to a long-term forcing change” — that led his results to be off by a factor of 3.

So Schwartz’s paper is easily dismissed.

There’s a very important twist to this discussion, however, which gets to the heart of how the IPCC underestimates future climate change — and why “equilibrium climate sensitivity” is very poorly named. That will be the subject of Part II of “Are Scientists Overestimating — or Underestimating — Climate Change?”


12 Responses to Are Scientists Overestimating — or Underestimating — Climate Change, Part I

  1. Cliff says:

    Unfortunately, all the Denyers need is an excuse to throw more statements of doubt into the public media pool. To most people, the prospect of catastrophic climate change is a bad dream they’d just as soon wake up from, and the Denyers offer them that escape route — “Ah. I can stop worrying now.”

    In “The Bourne Ultimatum” Scott Glenn plays the CIA Director, a corrupt but coldly practical man. Bad as he is, he makes one statement that is spot on for climate change scenarios:

    Hope for the best, but plan for the worst.

    If we don’t plan that way, hindsight from 50 years hence won’t provide us any comfort.

  2. Kevin Kvisle says:

    If only they applied the same standard to the fake war on terror as they do for global warming, we would see some of that security spending being used to reduce emissions. This is not necessarily my own thought, I just want it to become a well known retort to the corporate humps denials.

  3. Daryl T says:


    I have one question for you. If the scientists are underestimating Anthropogenic Global Warming, as you believe. What are you actually prepared to do about it?
    It seems to me if we are facing the level of calamity that is being dicussed, the old CFL and Hybrid really are not going to cut it.
    We need a complete shut down of all CO2 emissions by 2025 so we can start a cycle of reduction not sustainability, as CO2 has 200 year atmospheric lifespan, leaving the earth in an elevated temperature, such as the sustainable levels of 550ppm suggested by the IPCC, will cause all the same disasters just in the next generation (maybe this one).
    Ice will not stop melting at the new temperature, storms will not subside, droughts will not disappear. So would you a writer and advocate of AGW put pen to paper in your Blog and advocate the complete elimination of CO2 emissions from all fossil fuels by 2025, with all the implications that will come with said stance? If not, then are you saying the effects of AGW do not warrant this action and it is not the single biggest threat to the planet we have ever faced.

  4. john says:

    Another oak stake in the heart of the denyer’s is that this experiment has been run several times in the geologic past, and each time it reveals a real empirical sensitivity on the extreme upper range of what we’re projecting.

    One of the most thoroughly studied of these, the Paleocene-Eocene Thermal Maximum, was inititated by an unusually active period of volcanic activity. Using isotope analysis we can trace the amount of carbon introduced and its rate.

    The PETM was catastrophic — sea level rise was more than 20 meters; extincition rates were off the charts. And yet we are introducing carbon into the atmosphere at more than 30 times the rate that volcanic activity did back then.

    Hansen cites other examples of warming in the geologic record and notes the consequences of that warming was far worse than IPCC projections.

    At some point when the experiment keeps yeilding the same results, isn’t it time to chuck the models and deal with reality? Or at least adjust them so they reflect reality?

    And one last note: I can see some denyer pointing to these geologic incidents and saying, “See, it’s natural; it’s all happened before, so we don’t need to worry.”

    Well, here’s the deal: the real question is what happens when human-induced warming ocurrs on top of one of these cycles? We don’t have the vocabularly to describe this — it’s a nightmare.

  5. Joe says:

    I don’t think we need complete elimination of CO2 emissions from fossil fuels by 2025, but I do think we need to stay at 450 ppm or less, so we need an 80% reduction by 2050 — and that could include carbon capture and storage, if that proves practical and feasible.

  6. Ron says:

    Over or under estimating isn’t the point. When the climate doesn’t behave the way a model predicts it means the model is flawed. It doesn’t tell you why, it only means that the scientists who designed the model didn’t fully understand climate.

    We are a long way from fully understanding how climate works. Real scientists will tell you the same thing. Even most of the devout AGW believers admit to a lot of uncertainty.

    I’m not saying the failure of an arctic sea ice model or hurricane season model is proof that AGW is a farce. In fact, you can’t use an inaccurate model as proof of anything.

    Come on, guys. Let’s do some scientific thinking here.

  7. robert says:

    Ron, I don’t think the point here is whether or not the models are getting all of the details right – of course they’re not! From a policy perspective, the issue is risk management.

    I wouldn’t bet my savings that the models have it right; but more importantly, I wouldn’t bet my savings that they’re wrong. But by refusing to take meaningful action, that’s exactly what we’re doing: betting the farm that the models are wrong – when there’s ample reason to believe they’re getting it essentially right. I hope they’re wrong (on the high side). But from a risk management perspective, I’m going to behave as though they’re right. And the beauty of it is, it’s a no-regret approach: we come out on the other end with cleaner, renewable energy. What’s the down side?

  8. Jacob says:

    “But by refusing to take meaningful action”
    What meaningful action ?
    Declaring we won’t exceed 450 ppm CO2 ? Declaring is fine, but what do you do ?
    Research ? It’s being done. We don’t have solutions yet. We cannot know when there will be.
    Carbon sequestration ? No yet clear it will work, and how much it will cost. Won’t be around until 2025…

    The Government can prohibit the opening of new power plants, or refineries, or steel mills, that’s something that can be done. As a consequence – people will suffer blackouts, and will install home generators, producing 3 times the CO2 saved. And will use Chinese steel… no carbon reduction here….

    I know Joe has written a book on it. Writing books is the easy part.

  9. Hank Roberts says:

    Joe, as a favor to readers, could you please put

    Part I Part II Part III

    or “First” and “Last” and “Next” and “Previous” or something — as links at top and bottom of the multiple page articles, to make it easy to get the whole article and read back and forth? Rereading helps comprehension.

    With this one, the one link to “next” at the bottom of parts I and II is hard to find and only one-way navigation, unless I miss something. This would be a kindness for all multiple-page articles.

  10. DT says:

    In case you still come across any denialoholics out there citing the Schwartz 1.1 ± 0.5 ° K estimate it is worth pointing out to them that he has now revised this to 1.9 ± 1.0 ° K, which (while still low) now overlaps the IPCC estimates.

    The updated paper on his website is called:
    Reply to comments by G. Foster et al., R. Knutti et al., and N. Scafetta on “Heat capacity, time constant, and sensitivity of Earth’s climate system”

  11. John1031 says:

    Very nice site!