xkcd Goes Off the Rails

I like xkcd. It's a fun, nerdy webcomic that almost everyone has seen. It is usually interesting and is often insightful. Today, however, it is just nuts. I refer you to a comic which looks more like a Greenpeace scare poster than the normal xkcd fare:


There is so much wrong with this comic, I couldn't hope to give an exhaustive list. It's so wrong people on both sides of the global warming debate should all be able to agree it's wrong. That's an accomplishment.

I've been reading xkcd for years. I never thought to sign up for its forum. This comic changed that. I was so shocked by this comic I immediately registered an account for the xkcd forum and left a comment. I think it speaks for itself:

I'm not looking for a debate on global warming, but there is so much wrong with this comic. I'll just focus on the big point. Projections of future warming depend on two things: 1) The level of future greenhouse gases; 2) The planet's sensitivity to greenhouse gases.

The former is described by Representative Concentration Pathways (RCPs), estimates of how greenhouse gas levels might evolve in the future. These are given in the form of RCPx, where x is the change in radiative forcing from pre-industrial times to 2100. The IPCC uses four RCPs: RCP2.6, RCP4.5, RCP6 and RCP8.5. Of these, only RCP2.6 requires prompt, aggressive action.

Once you've chosen an RCP, there is still the question of the planet's sensitivity. I won't delve into that save to point out the IPCC gives the likely range for the sensitivity as 1.5-4.5°C. You can't simply ignore that level of uncertainty.

Regardless, I'll just accept the IPCC's values. That's convenient because the IPCC gives estimated temperature changes for each RCP as of 2100. The IPCC has this to say about the worst case scenario, RCP8.5:

Warming above 4°C by 2081–2100 is unlikely in all RCPs (high confidence) except RCP8.5. Under the latter, the 4°C global temperature level is exceeded in more than half of ensemble members, and is assessed to be about as likely as not (medium confidence).

And that's in reference to temperatures of 1850-1900. Even the IPCC agrees there's basically no chance we'll see 4°C of warming relative to current temperatures (by 2100), even in the worst case scenario. We certainly won't see 5°C of warming. Moreover, plenty of climate scientists think RCP8.5 is unrealistic, saying RCP6 is more like what we'd see with no climate related policies. If they're right, there's no chance we'll see even 4°C of warming by 2100. None at all.

Even if you feel RCP8.5 is an accurate estimation of what would happen with no climate related policies, we don't need prompt, aggressive action to avoid it. Worst case scenario, without prompt, aggressive action, we can still follow RCP4.5 or RCP6. According to the IPCC, neither of those will result in the sort of warming this comic indicates.

In other words, this comic cites the IPCC, yet it exaggerates even the worst case scenario offered by the IPCC while portraying itself as discussing the best case scenario.

Note: Here's a link to the IPCC report chapter I quoted. I couldn't include it in my comment their because it made my comment get flagged as spam.


  1. Thanks for catching the missing link. I'm not sure how I forgot to include it, but I'm sure it had something to do with it being three in the morning. I was actually in bed, trying to sleep, at the time I saw the comic.

    As for what he'll say, I'm not sure if we should expect any response. People rarely seem interested in correcting exaggerations. Maybe I'm just cynical though.

  2. Brandon

    Dr Brown at Duke also took Randall to task, if somewhat more forcefully, here

    But will he author of XKCD publish a retraction when it is pointed out, repeatedly (as is now happening on the blog devoted to today’s comic) that the actual warming expected by the IPCC is currently around 2 C and falling as the “hiatus” continues and the central estimate of climate sensitivity continues to diminish?
    What will it take to call somebody on something like this, stated in a public forum? One single, maximally extreme scenario, one in which human CO_2 actually plays the MINOR role, has been modelled to produce this much warming as a median result, and and not even the IPCC assigns it any “confidence” — not that their assertions of confidence have the slightest weight from any actual theory of statistical analysis, as the statistical average of a large stack of non-independent models that are unchecked against the phenomena they are trying to predict (and that largely fail such a check if it is performed) is a completely meaningless quantity.

  3. No prob. I posted malformed links on a semi-regular basis. Normally it's easy to fix because the correct link is simply mixed in with other stuff. Yours was missing altogether though.

    Anyway, it's good to see other people spotting what that comic did. I'm still dumbfounded someone who makes so many intelligent comics could make such a stupid mistake.

  4. I'm still dumbfounded intelligent climate scientists make so many stupid mistakes publishing comical papers...

    the human minds capacity for self deceit is beyond belief

    Arthur C Clarke

  5. Since Randall describes "his neighborhood" as having been under half a mile of ice, he must in the Northern United States (e.g. Boston where the annual average overnight low is 43.9°F and the average daytime high is 59.2°F. http://www.rssweather.com/climate/Massachusetts/Boston/ )

    Instead of a large, ominous question mark in the third box, he could simply have drawn a pretty picture that depicted the climate in any one of the numerous cities in the country where the average annual temperature is 7-9°F warmer than in Boston. For instance, Norfolk, Virginia which is 7.5 to 8.6°F warmer: http://www.rssweather.com/climate/Virginia/Norfolk/ Not especially different.

    For the sake of accuracy, he could have drawn the high-water line a foot or two higher to allow for 86 years of continued sea level rise, but I doubt that would have been sufficiently alarming or apocalyptic.

  6. Yeah, I finally gave up trying to discuss the subject over there after the glo> pointed out

    bal warming thread spiraled into pages and pages of quibbling and random interjections of poetry from someone else.

    I did discover an interesting fact while researching posts there.

    It is never really discussed, but the oft-presented solar vs terrestrial spectrums suggest that the upward radiation at some wavelengths exceeds that of the downward solar radiation.

    In particular it is implied that upward IR radiation is equal or greater than that from the Sun. The chart with the solar and terrestrial scaled to both be visible is particularly egregious about this: as seen here.

    Generally they give the impression that the emissions from the ground at those wavelengths exceed the solar emissions, but in all those pictures the solar emissions are scaled down several times.


    The little snipped sections are for emissions at that wavelength above the atmosphere, and at the surface.

    Continuing the curves and doing calculations shows that even at the ground the downwelling IR from the Sun is greater than the upwelling from the surface, and thus greater than any possible downwelling IR from the air could ever be.

  7. I thought it was quite wrong also, initially, and said so on Twitter and at another blog. However, based on some discussions then, and since, I've looked much more closely at the CMIP5/AR5 forcing data for all major forcings, and the newer AR4 TCR (transient sensitivity) numbers, and I don't think it's as extreme as I first thought it was. I'm going to post on it, with R code, maybe next week.

  8. Jim Bouldin, I wouldn't be bothered by the comic accepting the IPCC ranges. It's reasonable to accept the IPCC estimates, whether or not they're right. The problem is the comic didn't accept the IPCC estimates. It exaggerates them.

    As you say, RCP8.5 can justify the numerical values given in the comic. The problem is the IPCC says RCP8.5 is said to be what we'd get if we took no action to combat global warming. We don't need to take "prompt, aggressive" action to avoided it. We can take basically any action, any time in the next half century.

    In other words, the comic compares the actions necessary to reach RCP2.6 with the outcomes of RCP8.5. It only looks at the most extreme points, pretending the range between 2.6 and 8.5 is non-existent.

  9. With my analyses, I'm actually getting point estimates, for all four RCPs, that are below the midpoints of the AR5 ranges. I'm not sure why though. I'm using the exact CMIP5 forcing numbers used. I'm still unclear on the exact differences between the straight CMIP5 model runs and the IPCC numbers though, which are not the same. Could be related to that.

    The main problem I see with the comic is their imprecise wording. If you want to evaluate T changes due to humans, use pre-industrial as the baseline, not "the 20th century norm", since the latter 20th C is affected by GHGs.

    I don't think I agree on your statement that we can take any action at any time point. If it were all methane driven then you could say something like that (maybe), but not with CO2. When you put it there, it's there to stay.

  10. Jim Bouldin, it'll be interesting to see if you can figure it out. The IPCC has done some strange things with model results in the past.

    As for my statement, I didn't say "we can take any action at any time point." I said "any time in the next half century." That gives ~40 years for the actions to take effect. That's plenty of time to shift off the RCP8.5 pathway.

    I do agree their wording is imprecise though. I just don't think it is anywhere near as big a problem as the fact we do not need to take prompt, aggressive action to avoid radiative forcings increasing to 8.5 by 2100. Prompt, aggressive actions are needed to keep it down to 2.6. Less immediate and/or less drastic actions can keep it to a value anywhere from 2.6-8.5.

  11. My brain is *really* mis-firing right now. I *am* in fact getting fairly large discrepancies between the cartoon's midpoint estimate (4.5 C) and my RCP85 point estimate (3.5 for GHG forcings only, 3.7 for all anthro forcings), as well as between the AR5 and mine. And of course, all other RCPs give something < 3.5 degrees.

    I'll post a notice when I put my post up.

  12. Cool. That's definitely easier to work with.

    By the way, I left a comment over there a little while ago with an idea of something that might be causing the discrepancy you got, but I think it got flagged as spam.

  13. Good thing you said that--I never check the spam folder. And sure enough...I really don't know what's going on, but at least I now know to check.

  14. Just stumbling by

    (1) As the comic notes, we have already warmed some since the 'modern average' (think more stable, average global temp we've seen for human civilization) and so the distance to "+1 AH" is only in the neighborhood of 3.5°C (or 3.7), which is probably enough to justify comic under RCP8.5

    (2) Arguing about 8.5 vs. 6 is fine, but likely only shifts the point of hitting "+1 AH" by a small number of decades.

    If this is your 'glaring' issue that reveals this to be 'nuts', should be clear you're overreaching by a lot.

  15. waxliberty, I'm not sure you read this post in its entirety. If you had, you would have seen:

    Moreover, plenty of climate scientists think RCP8.5 is unrealistic, saying RCP6 is more like what we'd see with no climate related policies. If they're right, there's no chance we'll see even 4°C of warming by 2100. None at all.

    Even if you feel RCP8.5 is an accurate estimation of what would happen with no climate related policies, we don't need prompt, aggressive action to avoid it. Worst case scenario, without prompt, aggressive action, we can still follow RCP4.5 or RCP6. According to the IPCC, neither of those will result in the sort of warming this comic indicates.

    The post clearly highlights the only scenario in which the warming this comic claims will happen unless "prompt, aggressive limits on CO2 emissions" are put into place is RCP8.5, an extreme scenario which many scientists don't think is even possible.

    The only scenario in which this much warming could happen is one in which we ramp up emissions far more than there is any reason to think will happen. Despite that, the comic portrays the results of that extreme scenario as being inevitable if we don't take strong action to combat global warming. That point "should be clear" from what this post says. I am not sure why you missed it. Similarly, I don't know why you would say:

    (2) Arguing about 8.5 vs. 6 is fine, but likely only shifts the point of hitting "+1 AH" by a small number of decades.

    When this isn't remotely true. The RCP6.0 scenario involves far less warming than RCP8.5, and it could be followed even if we didn't take any action to combat global warming for the next 20 years. Under that scenario, we'd likely only see ~2 degrees of warming by 2100. Indeed, under that scenario we wouldn't be expected to see the amount of warming this comic refers to until maybe 2200. What you call "a small number of decades" is in reality a century or more.

    By the way, right now even with how little action is being taken to combat global warming, we're still coming in under the RCP6.0 scenario.

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