Where Did the IPCC Get its Numbers From?

I was recently reminded of an issue I discovered several months ago. It seems worth revisiting. The Summary for Policymakers of the latest IPCC report (Working Group II) says:

Global economic impacts from climate change are difficult to estimate. Economic impact estimates completed over the past 20 years vary in their coverage of subsets of economic sectors and depend on a large number of assumptions, many of which are disputable, and many estimates do not account for catastrophic changes, tipping points, and many other factors. With these recognized limitations, the incomplete estimates of global annual economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0% of income (±1 standard deviation around the mean) (medium evidence, medium agreement).

The reference given for this text is Chapter 10, Section 9. When we look at that chapter, we find the same text in its Executive Summary. Like in the SPM, it cites 10.9. However, nothing in the text of 10.9 gives those numbers. They seem to have been pulled out of thin air.

Another troubling issue is the text in Chapter 10’s Executive Summary was changed to match that in the SPM. In the “Final Draft” of Chapter 10, the text originally said:

Globally aggregated economic impacts of global warming are a small fraction of income up until 3°C [10.9.2, medium evidence, high agreement]. A global mean average temperature rise of 2.5C may lead to global aggregated economic losses between 0.2 and 2.0% of income (medium evidence, medium agreement) and losses increase with greater warming. Little is known about aggregate economics impacts above 3°C. Impact estimates are incomplete and depend on a large number of assumptions, many of which are disputable.

General differences aside, the thing which stands out is the previous version of the text said a rise of 2.5C may cause 0.2 – 2.0% economic losses. The new version says “economic losses for additional temperature increases of ~2°C are between 0.2 and 2.0%.” They’ve shifted the amount of warming necessary for these damages up by half a degree, and they’ve done so without any explanation.

A clue might be present in a previous version of the SPM. It said:

Global mean temperature increase of 2.5C above preindustrial levels may lead to global aggregate economic losses between 0.2 and 2.0%

Notice it says a rise above preindustrial levels. We could perhaps assume the 2.5C figure is in relation to preindustrial levels while the 2C figure is in relation to modern levels even though there has been a greater amount of warming than .5C. This assumption requires assuming the “Final Draft” of Chapter 10’s text was wrong to refer to a “rise of 2.5C” as ~20% of that rise had already happened.

Even with that assumption though, nothing much is explained. Even if we know exactly what value the IPCC intends, we have no explanation for where the values came from. It seems they were pulled out of thin air. However, if they were somehow based upon Section 10.9, then they would be affected by the changes I highlighted in my recent post. It’s difficult to see how they could get the exact same results for these two sets of data:


So where did the IPCC get its numbers from? Did it just make them up? Did somebody, somewhere, perform some secret calculations we’re not allowed to see?

It’s impossible to read Section 10.9 and think the numbers came from it. Does that mean the IPCC lied about where it got those numbers from?



  1. Brandon: The difference is the reference point. The chapter has warming relative to pre-industrial, the SPM relative to recent.

  2. Richard Tol, there are so many problems with that claim, I don’t know where to begin. To start, I’ll note the difference between pre-industrial temperature and modern temperatures is not .5C. It is greater. If the IPCC wanted to shift baselines like you claim, it would have had to shift them by closer to .75C.

    For another, when asked what the baseline temperature used for your graph was:

    From what baseline is this increase measured? Is it 1971-2000? Pre-industrial?

    You said:

    I don’t think there is an answer to that question.

    Which you presumably said because you know each of the values you plotted in your figure have a different baseline period. The temperatures in that figure are all relative to different times. You, yourself know these estimates don’t share a single baseline. And obviously, you can’t shift a baseline which doesn’t exist.

    I was about to say more, but you know what? You clearly have no interest in actual discussions. I say this because you aren’t even aware of changes made to your the chapter you were responsible for. If you had been aware, you’d know this statement is false:

    The chapter has warming relative to pre-industrial

    As the text in the chapter is identical to the text in the SPM – a point I explicitly highlighted in this post. If you can’t be bothered to know what is secretly changed in your chapter, or be bothered to read what is written in the posts you respond to, there’s no reason to think you could be bothered to give an honest response.

  3. I should point out a couple things. According to the IPCC WGI SPM (page five), there has been ~.85C of warming. That means if Richard Tol’s claim is true, the first .85C of his figure have already happened. We’d have to shift the baseline for anything related to his paper by .85C. This shows the lie of claiming the IPCC SPM shifted the baseline for the figure it gave by .5C to align it to modern times. It would have had to change 2.5C to ~1.65C if it had wanted to do that.

    More importantly, the Tol 2002 paper, the only one which shows any sort of notable benefit, is an estimate for 1C of warming. If that is relative to pre-industrial times as Tol Claims, then relative to today, it is for a temperature change of ~.15C. Every estimate after that point shows either no meaningful benefit, or harms.

    I’ve actually dealt with this point in the past. As I noted then, this is a strange issue. In Tol 2009, Tol plotted many of these same values with the caption:

    Figure 1 shows 14 estimates of the global economic impact of climate change, expressed as the welfare-equivalent income gain or lose, as a function of the increase in global mean temperature relative to today.

    When he was forced to publish a correction and update of his work, the caption changed to:

    Figure 2 shows 21 estimates of the global economic impact of climate change, expressed as the welfare-equivalent income gain or loss, as a function of the increase in the annual global mean surface air temperature relative to preindustrial times.

    Even though the data points remained in the same spots. That means Tol has claimed two entirely different baselines for the same data set with no explanation for the difference.

    (I personally don’t think either claim is correct. I think the baselines for each estimate are different. I’d go beyond that to say some of the differences make it impossible to set all of them to a single baseline.)

  4. Brandon: According to the IPCC, the difference between pre-industrial and recent is 0.61K. Don’t ask me why they introduced a new and strange reference point.

  5. Richard Tol, you’re not even trying to address anything I say. I suppose I shouldn’t be surprised given that’s your standard behavior. Happily, you reminded me of another silly issue with these numbers. I cited a value given by the IPCC for the amount of warming that’s been observed. I’ll quote it now because this is important:

    The globally averaged combined land and ocean surface temperature data as calculated by a linear trend, show a warming of 0.85 [0.65 to 1.06] °C, over the period 1880 to 2012, when multiple independently produced datasets exist. The total increase between the average of the 1850–1900 period and the 2003–2012 period is 0.78 [0.72 to 0.85] °C, based on the single longest dataset available

    The amount of warming we’ve seen is .85C (or .78C depending on methodology). The only way to get your .61C is to use the word “recent” knowing most people will never catch what you’re doing. You see, I happen to know the IPCC does a lot of comparisons with the “recent” reference period of:

    Projections in this Summary for Policymakers are for the end of the 21st century (2081–2100) given relative to 1986–2005, unless otherwise stated. To place such projections in historical context, it is necessary to consider observed changes between different periods. Based on the longest global surface temperature dataset available, the observed change between the average of the period 1850–1900 and of the AR5 reference period is 0.61 [0.55 to 0.67]°C. However, warming has occurred beyond the average of the AR5 reference period. Hence this is not an estimate of historical warming to present (see Chapter 2)

    Your “recent” is actually the period of 1986-2005. I’m not sure how the year of my birth could be considered “recent.” But even if we suppose it were, the period 1850-1900 is not “pre-industrial.” The Industrial Revolution started ~1750 and is commonly considered to span the period of 1750-1850. You can’t take a time 50 years after the Industrial Revolution ended as “pre-industrial.”

    The IPCC seems to recognize this as the SPM I linked to says:

    Carbon dioxide concentrations have increased by 40% since pre-industrial time…
    The atmospheric concentrations of the greenhouse gases carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) have all increased since 1750 due to human activity. In 2011 the concentrations of these greenhouse gases were 391 ppm, 1803 ppb, and 324 ppb, and exceeded the pre-industrial levels by about 40%, 150%, and 20%, respectively.

    If the IPCC had defined “pre-industrial” as 1850-1900 like you claim, there’s no reason it would refer to “pre-industrial” here as 1750. I imagine I could find other examples which show the point more explicitly.

    What this shows is not only does each value in your figure have a different temperature baseline, numbers are being arbitrarily adjusted to fit baselines other than those being described.

    Here’s a simple and direct question. Surprise us all by answering it. What is used as the temperature baseline in the figure you added to the IPCC report?

  6. Steven Mosher, a rule I’ve informed people of before is if you want to contribute a link to a discussion, you need to discuss the contents of the link. Any comment consisting merely of a link is considered spam here.

  7. Actually I understood what Mosher was saying without clicking on the link, so I think he has discussed the contents sufficiently.

  8. Something I meant to go through for the latest report, but I think there is generally a built in contradiction any time they talk about damages from global warming. It goes like this:
    1) To get high amount of damages, you need a high amount of warming.
    2) To get high amount of damages, you need poor countries that can’t handle the impacts.
    3) To get a high amount of warming, you need a high amount of CO2.
    4) To get a high amount of CO2, you need lots of economic growth.

    With all this growth, the countries are better able to adapt and the damages are not as great.

  9. MikeN, you may “think he has discussed the contents sufficiently,” but he has not, as a site rule. As a rule here, a link offered without any explanation of what it contains is not content, and a comment with no content is spam.

    It’s a very simple rule. If people want to participate in a discussion, they have to discuss something. Putting up a sign that says, “Look here,” is not discussing anything.

  10. On the issue of net damages from global warming, I find the more you drill into the arguments for such damages, the less reasonable the arguments appear. One thing which has come from spending some time with the literature on calculating economic damages from global warming is I’ve realized the economic aspects of these models are often terrible. It’s interesting to look at what some of these papers from 15+ years ago give for projections of the world in regard to economics, population, etc.

    For instance, one of the estimates in the tables/figures I discuss (Nordhaus and Yang 1996) comes from their projections for 2100. They project the population of the United States will be 294. The current population of the United States is something like 320 million. It’s almost certain to break 400 million by 2050. The idea it’d only be 294 million in 2100 is just silly.

    It’s funny to think even if the damages from global warming were perfectly calculated in these models, the results would likely still be way off because the economic projections in the models are wrong.

  11. The link to the principle of charity is self evident.
    Mike gets it. He gets it because he followed the principle. Brandon
    You rarely follow the principles which is why you don’t get it
    And which explains why you establish the uncharitable forum that you do.

  12. Steven Mosher, you can say whatever you want about me as a person. You’ve said plenty of things about me in the past which are wrong, and I’m sure you’ll continue saying things about me which are wrong for many years to come. It doesn’t matter.

    What matters is the rule I stated is a simple and fair one. It is used in many forums, and it is not burdensome. There is nothing unreasonable about me expecting people to abide by it.

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