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The IPCC Fourth Assessment SPM

Filed under: — group @ 2 February 2007 - (Français) (Português) (Türkçe) (Español)

We’ve had a policy of (mostly) not commenting on the various drafts, misquotes and mistaken readings of the Fourth Assessment report (“AR4” to those in the acronym loop) of the IPCC. Now that the summary for policy makers (or “SPM”) has actually been published though, we can discuss the substance of the report without having to worry that the details will change. This post will only be our first cut at talking about the whole report. We plan on going chapter by chapter, hopefully explaining the key issues and the remaining key uncertainties over the next few months. This report will be referenced repeatedly over the next few years, and so we can take the time to do a reasonable job explaining what’s in it and why.

First of all, given the science that has been done since the Third Assessment Report (“TAR”) of 2001 – much of which has been discussed here – no one should be surprised that AR4 comes to a stronger conclusion. In particular, the report concludes that human influences on climate are ‘very likely’ (> 90% chance) already detectable in observational record; increased from ‘likely’ (> 66% chance) in the TAR. Key results here include the simulations for the 20th Century by the latest state-of-the-art climate models which demonstrate that recent trends cannot be explained without including human-related increases in greenhouse gases, and consistent evidence for ocean heating, sea ice melting, glacier melting and ecosystem shifts. This makes the projections of larger continued changes ‘in the pipeline’ (particularly under “business as usual” scenarios) essentially indisputable.

Given all of the hoopla since the TAR, many of us were curious to see what the new report would have to say about paleoclimate reconstructions of the past 1000 years. Contrarians will no doubt be disappointed here. The conclusions have been significantly strengthened relative to what was in the TAR, something that of course should have been expected given the numerous additional studies that have since been done that all point in the same direction. The conclusion that large-scale recent warmth likely exceeds the range seen in past centuries has been extended from the past 1000 years in the TAR, to the past 1300 years in the current report, and the confidence in this conclusion has been upped from “likely” in the TAR to “very likely” in the current report for the past half millennium. This is just one of the many independent lines of evidence now pointing towards a clear anthropogenic influence on climate, but given all of the others, the paleoclimate reconstructions are now even less the central pillar of evidence for the human influence on climate than they have been incorrectly portrayed to be.

The uncertainties in the science mainly involve the precise nature of the changes to be expected, particularly with respect to sea level rise, El Niño changes and regional hydrological change – drought frequency and snow pack melt, mid-latitude storms, and of course, hurricanes. It can be fun parsing the discussions on these topics (and we expect there will be substantial press comment on them), but that shouldn’t distract from the main and far more solid conclusions above.

The process of finalising the SPM (which is well described here and here) is something that can seem a little odd. Government representatives from all participating nations take the draft summary (as written by the lead authors of the individual chapters) and discuss whether the text truly reflects the underlying science in the main report. The key here is to note that what the lead authors originally came up with is not necessarily the clearest or least ambiguous language, and so the governments (for whom the report is being written) are perfectly entitled to insist that the language be modified so that the conclusions are correctly understood by them and the scientists. It is also key to note that the scientists have to be happy that the final language that is agreed conforms with the underlying science in the technical chapters. The advantage of this process is that everyone involved is absolutely clear what is meant by each sentence. Recall after the National Academies report on surface temperature reconstructions there was much discussion about the definition of ‘plausible’. That kind of thing shouldn’t happen with AR4.

The SPM process also serves a very useful political purpose. Specifically, it allows the governments involved to feel as though they ‘own’ part of the report. This makes it very difficult to later turn around and dismiss it on the basis that it was all written by someone else. This gives the governments a vested interest in making this report as good as it can be (given the uncertainties). There are in fact plenty of safeguards (not least the scientists present) to ensure that the report is not slanted in any one preferred direction. However, the downside is that it can mistakenly appear as if the whole summary is simply up for negotiation. That would be a false conclusion – the negotiations, such as they are, are in fact heavily constrained by the underlying science.

Finally, a few people have asked why the SPM is being released now while the main report is not due to be published for a couple of months. There are a number of reasons – firstly, the Paris meeting has been such a public affair that holding back the SPM until the main report is ready is probably pointless. For the main report itself, it had not yet been proof-read, and there has not yet been enough time to include observational data up until the end of 2006. One final point is that improvements in the clarity of the language from the SPM should be propagated back to the individual chapters in order to remove any superficial ambiguity. The science content will not change.

Had it been up to us, we’d have tried to get everything together so that they could be released at the same time, but maybe that would have been impossible. We note that Arctic Climate Impact Assessment in 2004 also had a similar procedure – which lead to some confusion initially since statements in the summary were not referenced.

How good have previous IPCC reports been at projecting the future? Actually, over the last 16 years (since the first report in 1990), they’ve been remarkably good for CO2 changes, temperature changes but actually underpredicted sea level changes.

When it comes to specific discussions, the two that are going to be mostly in the news are the projections of sea level rise and hurricanes. These issues contain a number of “known unknowns” – things that we know we don’t know. For sea level rise the unknown is how large an effect dynamic shifts in the ice sheets will be. These dynamic changes have already been observed, but are outside the range of what the ice sheet models can deal with (see this previous discussion). That means that their contribution to sea level rise is rather uncertain, but with the uncertainty all on the side of making things worse (see this recent paper for an assessment (Rahmstorf , Science 2007)). The language in the SPM acknowledges that stating

“Dynamical processes related to ice flow not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.”

Note that some media have been comparing apples with pears here: they claimed IPCC has reduced its upper sea level limit from 88 to 59 cm, but the former number from the TAR did include this ice dynamics uncertainty, while the latter from the AR4 does not, precisely because this issue is now considered more uncertain and possibly more serious than before.

On the hurricane/tropical strorm issue, the language is quite nuanced, as one might expect from a consensus document. The link between SST and tropical storm intensity is clearly acknowledged, but so is the gap between model projections and analyses of cyclone observations. “The apparent increase in the proportion of very intense storms since 1970 in some regions is much larger than simulated by current models for that period.”

We will address some of these issues and how well we think they did in specific posts over the next few weeks. There’s a lot of stuff here, and even we need time to digest it!

364 Responses to “The IPCC Fourth Assessment SPM”

  1. 351
    greg meyerson says:

    hi: did anyone here read the recent article (2/13) in the ny times by john tierney on the richard branson offer of 25 million to invent a carbon eating technology?

    in the course of that report, the author made assertions about the ipcc report–suggesting among other things that the issues around sea level rise are hyped and that we will see only slow changes, nothing rapid, no tipping points.

    I think that in his discussion of sea level rise he may have made the mistake referred to above–he asserts that predictions of sea level rise has gone down (instead of making the point above about apples and oranges comparison where one measurement leaves out dynamical effects)

    anyway, are there any serious criticisms the experts here would make of this article, either on grounds of interpreting the ipcc report or on plausibility of seeking out miracle (carbon eating nano) technologies to deal with global warming as a substitute for reducing ghgs?



  2. 352

    [[Thank u very much for your kind response. But i just want to tell u that i’m not afraid of all these earth changes. i was just eager to know what’s going on… Moreover, my main hobby is collecting information about earth, beyond space, the unexplained phenomenon, black holes, Bermuda Triangle, underworld civilizations & several other spots that has wierd science & mysteries behind it. I hav been doing this for about 5yrs(since i was 14), but i have’nt got any clear evidences.]]

    All interesting stuff, but not really relevant to this blog, which is mostly about global warming. If you want to e-mail me about any of this I’ll try and tell you what I know, which may not always be much. :)

  3. 353
    Priya says:

    Yes, i know Barton. These are really not relevant to this blog. But i tried through email. It did’nt reach u…

  4. 354

    [[Yes, i know Barton. These are really not relevant to this blog. But i tried through email. It did’nt reach u… ]]

    It reached me, I just didn’t have anything to say in response.

  5. 355
    Danny says:

    Global Warming is bad! =]

  6. 356
    mark says:

    i,m not a scientist and not really up on all the jargon that they use….after several years of listening to theorys about how the planet is being destroyed,my thoughts have dedused a theory of my own,one that i have not heard anyone comment on before,maybe i am not well read enough or it is something that may have been brushed under the to speak….not dismissing that polution and all the other things that have been commented on help,but to me it seems the rapidness over the last 30 years of global warming coencides with mans overwhelming desire to venture into space….now as i said,i am no scientist but,if you continually keep breaking through the atmosphere with rockets and satalites,not only are you burning all that fuel so close to the ozone layer….the thing they say which is causing the problem….maybe momentarily you are creating a hole,all those harmful radiations must be able to get through?how many and how many times have we done this?this has got to weaken our ozone layer surely?how many planes are flying in the skies now?burning all that fuel so close to the ozone,think we need to look higher to solve this problem we have…..dont you?

  7. 357

    [[if you continually keep breaking through the atmosphere with rockets and satalites,not only are you burning all that fuel so close to the ozone layer….the thing they say which is causing the problem….maybe momentarily you are creating a hole,all those harmful radiations must be able to get through?how many and how many times have we done this?this has got to weaken our ozone layer surely?how many planes are flying in the skies now?burning all that fuel so close to the ozone,think we need to look higher to solve this problem we have…..dont you? ]]

    It’s an interesting idea, but I don’t think the holes created by spacecraft last long enough to have an effect on the planet’s climate. That, and consider how physically small they are compared to the size of a planet. The biggest spacecraft out there in terms of width might be the US space shuttle, which is what, 50 meters across? Compare that to an Earth with an area of 510 trillion square meters. Similar comments for airplanes, I think, although of course there are a lot more of them.

  8. 358
    yartrebo says:

    Re #356:

    At the present time, there aren’t enough spacecraft launched per year to have any major effect, but I do believe that airplanes have an effect on ozone levels via their stratospheric emissions.

    If space travel becomes much more frequent, perhaps because of space tourism, then it does have the potential to cause serious harm as it is orders of magnitude more harmful per passenger or per kg of payload than commercial aircraft flights.

  9. 359
    rick hanheide says:

    I’m trying to read the SPM and got all the way to page 5 before getting stuck. Table SPM-0 is a list of sources of sea level rise (thermal expansion, glaciers and ice caps, Greenland, and Antartica, with numerical values for each). The next line is “sum of the individual climate contributions to sea level rise”. I was expecting that to be the “sum” of the four listed components, but it’s certainly not.

    I’m sure you have noticed and explained this already – please just point me to where..thanks

  10. 360
    rick hanheide says:

    Hmmm. Maybe no one is reading this topic anymore. I’ll try my question on a current discussion.

  11. 361
    WJG says:

    What sort of people work for the IPCC? [edit]

    Page 5 of the summary, the first table, none of it makes any sense. First, its in meters, then they sum up the results in millimeters and exxagerate the numbers by a factor of 10. Then, the estimated contributions to sea level rise, simple do not add up. Just do the math.

    Are the authors and everyone, everyone that reviews the summary making up numbers, or are they unable to do simple math? In addition to models being completely wrong. Conversions do not make sense, addition is wrong, coupled with a wild model that is no where close to the observed rate.

    0.18 meters is 1.8 millimeters?

    0.16+.077+0.21+0.21 = 0.28?? (coincidenatlly close to the observed rate?)

  12. 362
    WJG says:

    my mistake – the authors made a table on a scale of 100 years, while discussing the result son a scale of one year.

    However, it still stands that the math does not add up.

    Do the reviewers own a calculators? As it stands – the summary for sea level can only be discarded because no one in the IPCC can do math.

    [Response: See comment #11. This typo is already fixed in the downloadable version. -gavin]

  13. 363
    Henk Lankamp says:

    @Rick and WJG
    Please download a newer version. At first there was a mixture of units in the table, this is corrected now, all values in mm/yr: SPM2.

  14. 364
    Dan says:

    re: 361 and 362.
    It is not all that difficult to do a quite simple search here at the top of the page and find the SPM reviewers are listed in post 48 at

    So you can now contact through Google Scholar each scientist listed and specifically ask them if they are “making up numbers” or “are (you) unable to do simple math?”.