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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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How the IPCC is more likely to underestimate the climate response

What the science says...

Select a level... Basic Intermediate

Numerous papers have documented how IPCC predictions are more likely to underestimate the climate response.

Climate Myth...

IPCC is alarmist

"Unquestionably, the U.N. Intergovernmental Panel on Climate Change (IPCC) was formed to build the scientific case for humanity being the primary cause of global warming. Such a goal is fundamentally unscientific, as it is hostile to alternative hypotheses for the causes of climate change." (Roy Spencer)

At a glance

The Intergovernmental Panel on Climate Change (IPCC) is a United Nations body founded in 1988. Its purpose is to inform governments about the status of our scientific knowledge with regard to our changing climate. In order to accomplish this role, it gathers and summarises evidence, producing an Assessment Report (AR) every few years. Each AR is an up-to-date account of the impacts and risks of a changing climate. However, because it takes 6-7 years to bring an AR to publication, by the time one is produced, the science is already moving ahead - as is the climate. The laws of physics wait for nobody.

It is important to clear up a couple of serious misunderstandings about the IPCC that are often encountered in online discussions. Firstly, the IPCC does not conduct original scientific research. That includes modelling. But how often do we see commentators ranting about 'IPCC models'?

In fact, climate models are managed by multiple modelling groups around the world. Together, these groups form the Coupled Model Intercomparison Project (CMIP). In AR6, published in 2022-23, the latest generation CMIP6 output was featured. The modellers, however, did the modelling, not the IPCC.

The above example illustrates the depth of confusion that is out there. The confusion was sown by the same merchants of doubt who created and distributed all the other denialist talking-points that we deal with here at Skeptical Science.

A second frequently-cast aspersion is that the IPCC is alarmist, exaggerating the threat of climate change to cause needless worry or panic. Let us repeat: it merely collates what the science is saying. And what the science is saying is very worrying.

We have understood the heat-trapping properties of certain gases such as water vapour, methane and carbon dioxide for more than 100 years. Yet we have raised the concentration of atmospheric carbon dioxide from a pre-industrial level of ~280 parts per million (ppm) to 420 ppm (in 2023). That is a 50% increase.

A CO2 level of 420 ppm last occurred on Earth during the middle of the Pliocene division of geological time, some 3.5 million years ago. Back then, the Polar ice-sheets were much smaller and vegetation distribution, detailed by the fossil record, differed dramatically from that of today. As an example, mixed woodlands were able to grow in Arctic Siberia, where today there is just stunted tundra. Sea levels were metres higher than today's. In AR6, the IPCC summarises, in its typically non-dramatic language:

"While present-day warming is unusual in the context of the recent geologic past in several different ways, past warm climate states (i.e. the Pliocene) present a stark reminder that the long-term adjustment to present-day atmospheric carbon dioxide concentrations has only just begun. That adjustment will continue over the coming centuries to millennia."

If you're not worried about the threat of climate change, then you haven't been paying attention.

Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!

Further details

Roy Spencer, an advisor to evangelical lobby-group the Cornwall Alliance, is our myth-provider in this instance. He is insinuating that the IPCC has an agenda that distorts the reports they produce. Specifically, that the IPCC exaggerates what the science says in favour of anthropogenic global warming. It's a frequently encountered argument from climate science deniers who know that there is a sector of the populace receptive to conspiracy-theories that they can play. Yet those same deniers offer no credible evidence to support it.

Some critics go even further down this road, implying that the IPCC actively suppresses science that doesn’t support the theory that climate change is being caused by human activities. In response to this, one has to ask, "what science". If a bundle of poor, demonstrably error-ridden papers in dubious journals is the answer (it is), then that's why such material doesn't pass muster. And there are a fair few such journals out there, some created purely to misinform.

So: to the IPCC. It was founded in 1988 in order to collate a broad range of scientific research into the climate and our effects on it and to summarise the science for policymakers. It's a UN body, bringing together the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP). The science they summarise has already been published. That means it is straightforward for a scientifically-literate reader to follow the references. They can compare the primary science with the IPCC reports and check them for consistency.

Another criticism of the IPCC is in the opposite sense - that they are too conservative. To a lay-person, this may seem reasonable on the grounds that a proportion of the people who finalise IPCC reports are government representatives, not scientists. These represent 195 member-states and as we know, governments prefer the status quo wherever possible. In the early decades of the IPCC there was also resentment about the disproportionate representation of climate scientists from OECD countries. This was discussed in a very readable paper following the release of the Fourth Assessment Report (AR4) (Hulme & Mahony 2010).

Are the IPCC too conservative? In AR4, the global sea level rise prediction amounted to 18-59 centimetres over the 1990-2090 period, plus an unspecified amount that could come from the Greenland and Antarctica ice-sheets. That prompted robust criticism from within the glaciology and oceanography communities. A central theme to the critique was that sea level rise was clearly accelerating and that the acceleration was not taken into account (e.g. Rahmstorf 2010).

That criticism has continued into recent years. There is discussion of how decision makers would benefit from the reframing of IPCC terminology. After all, it is important to avoid unintentionally masking worst-case scenarios (Siegert et al. 2020). Prominent climate scientist James Hansen has called this issue ‘scientific reticence’.

However, others (e.g. Solomon et al. 2008) have argued that AR4 stated that no consensus could be reached on the magnitude of the potential fast ice-sheet melt processes that some suspect could lead to 1–2 m of sea-level rise this century. At the time of AR4, these feasible but relatively data-poor processes were not included in the quantitative estimates. This takes us into the territory of uncertainty.

What is not perhaps appreciated by the general public is how science deals with uncertainty. Uncertainty in science is what drives it along, since any uncertain area deserves thorough investigation. This is the case even where a phenomenon is well-understood - such as the core fact that CO2 without doubt warms the planet. It's the details, the minutiae, where the uncertainty problem rears its head.

Here's an example of uncertainty and how it's handled. We can answer different questions with different levels of certainty. For example, how do we reply if asked, "how much is glacier X going to retreat by 2100?" We look at the data and see if the current rate of retreat is documented. If so we have a baseline. But we are still uncertain how emissions will pan out in the future. Therefore we plot a forward extrapolation of the current rate, plus a range of possible outcomes if emissions accelerate at one end, stay the same or plummet at the other. These were originally expressed as Representative Concentration Pathways (RCPs). Four such pathways were used for the IPCC Fifth Assessment Report (AR5), published in 2014. The pathways describe different climate change scenarios, depending on the amount of greenhouse gases emitted in the future. They are named after a possible range of radiative forcing values in the year 2100: RCP 2.6 = 2.6 Watts/square metre, with RCP 4.5, 6, and 8.5 having a similar structure, with RCP the worst case scenario of a continued fossil fuels binge.

Since AR5, this structure has been revised into Shared Socioeconomic Pathways (SSPs; fig. 1).

Emission trajectories for different SSPs.

Fig. 1: emissions trajectories on the different Shared Socioeconomic Pathways (SSPs), from IPCC AR6 WGI SPM box SP1.

Reports released by the IPCC over the years have used a very specific terminology to express the certainty level of specific outcomes, tabled in fig. 2, again from AR6.

IPCC language to express levels of uncertainty.

Fig. 2: currently-used IPCC language to express levels of uncertainty. Advice on how to describe risk for IPCC authors can be found here (PDF).

Other questions are a lot harder to answer because there are so many independent variables involved. But what about possible future events that carry a vague but non-negligible probability of occurring? A good example is the rapid collapse in the coming decades of the West Antarctic Ice Sheet. In IPCC terminology, such a high-impact event would be labelled as “unlikely” or “very unlikely” in the cited time-frame. The question therefore has to be, "do these terms used by the IPCC convey the right message to policymakers?" Scientists, for whom such terminology is everyday, are different to policymakers. There's the risk that the latter will react to such words by thinking, "oh that's okay then, not going to happen on my watch".

Language clearly matters here because we're dealing with different people who have differing reference frames. Climate scientists tend to work with decades to centuries whereas palaeoclimatologists deal with tens of thousands to millions of years. But politicians typically think in terms of years to decades at the most. The next election cycle is what matters to a lot of them, with some honourable exceptions.

Furthermore, there are serious risks associated with language because of the way the media interprets statements. In particular, a recent study into media treatment of part of AR6 found that denialist responses to IPCC output are largely confined to TV channels and other media with a right-wing worldview (Painter et al. 2023 - open access). The trouble is that the right-wing media is a formidable machine with a lot of reach. There is certainly a case for plain speaking here in order to counter their messaging.

Clearly there is always room for improvement in any organisation and the IPCC is no exception to that rule. But claims that the IPCC is alarmist are not supported by evidence. If anything, the published criticisms from the peer-reviewed literature suggest the opposite. The IPCC may - in certain areas - be erring on the side of caution.

Last updated on 5 November 2023 by John Mason. View Archives

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Denial101x video

Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial


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Comments 26 to 50 out of 138:

  1. The sulfate measurements over the last few centuries krisbaum @25 claims do not exist: (Source)
    "Figure 1: (a) Sulfate concentrations in several Greenland ice cores and an Alpine ice core. Also shown are the total SO2 emissions from sources in the US and Europe. The inset shows how peaks due to major volcanic eruptions have been removed by a robust running median method followed by singular spectrum analysis."
  2. Krisbaum, There is ample opportunity to challenge the "experts" appointed to write the IPCC reports if you can document that they are not qualified. Can you provide an example of a single one who was not qualified? Please do not waste my time claiming anyone who has a different opinion from you is unqualified, suggest only people who are really not experts in the field. The entire IPCC report is put online for comments before the final review. You, Exxon-Mobile, Anthony Watts and anyone else interested can comment and ask for changes in the wording or conclusions. How could you get more unbiased review than that? The SPM is reviewed word by word by representatives of all the countries. It is released in advance so that anyone interested can read it and develop their arguments (the draft of the scientific report is already on line for review when the SPM is written). Are you suggesting that the Bush Administration did not look out for oil interests and keep in mind the deniers arguments in this word by word review? In fact, the governments of oil producing countries, with help from the USA, diluted the conclusions of the scientists in the SPM. Your suggestion that the IPCC is alarmist is backwards, it reduces the conclusions to make AGW seem less of a problem. Are you suggesting a committee to review the work of the IPCC (which is essentially a big committee)? Who will appoint the members of this committee? If not the governments (who set up the Intergovernmental PCC) than who? Maybe you can select the members of this duplicate committee. Perhaps I can appoint a committee to oversee the one you appoint. Your suggestion is absurd. Please consider your arguments before you present them here and make sure that they make sense. Do you realize that the IPCC has only a handful of employees? The work is all done by volunteer scientists. The IPCC only coordinates the process.
  3. Tom Curtis addressed the claims in post 25 above, I thought I'd add that, historically, the serious study of atmospheric aerosols dates back to the time when atmospheric sciences started being a field of their own and separated from geology, inthe late 1800s. Seminal work on the subject was published by Aitken in 1888, 1891, 1894, 1895. As of today, aerosols are the focus of intense work, and several scientific journals are exclusively dedicated to the subject.
  4. Tom Curtis; Greenland aerosol measurements tell you nothing, it is fairly common knowledge that aerosols do not travel far from their source typically 10km or so. You need localised measurements to get any kind of global pattern. Michael Sweet; 'The entire IPCC report is put online for comments before the final review. ' Yes it is, but the final version is the responsibility of authors, and they declare what changes will be incorporated into the final version and which wont. Sure here's an example of appointing non-experts; Richard Klein .. An IPCC Lead Author at the age of 25 - back in 1994 after completing a geology degree 2 years prior. at 28 he was promoted to the most senior role - Coordinating Lead Author..
  5. 'The SPM is reviewed word by word by representatives of all the countries. It is released in advance so that anyone interested can read it and develop their arguments (the draft of the scientific report is already on line for review when the SPM is written). Are you suggesting that the Bush Administration did not look out for oil interests and keep in mind the deniers arguments in this word by word review? ' Micheal Sweet; The point is; they hold a plenary with politicians and together create the SPM for release into the world before the main report has been released!!! How can the summary be validated against the main report or scrutinised correctly by external scientists? (-Snip-) (-Snip-).
    Response: [DB] All-caps usage (forbidden) converted to bold; moderation complaints and sloganeering snipped.
  6. Michael; And to reply to your final part. My suggestion is that, considering the report produced by the IPCC is being used to decide one of the most critical fates of humanity - spending trillions on alternative energy in effect - there could be an external policing of their processes. I'm sure you can work out for yourself how this could work in the future - the rest of the world seem to be capable of such ideas - take law enforcement and the state for example. Do you see judges doubling as policemen and appearing on their own jury for a crime they've been accused of?
  7. krisbaum at 09:49 AM on 4 August, 2012 Man... you imply the IPCC is doubling as police force and judge?! For a crime they commited?! What kind of power do you think the IPCC has, besides gathering scientific papers and putting them together in a report?
  8. sorry guys i'd love to communicate more, but the moderator keeps 'snipping' my views

    [DB] Comments constructed to comply with the Comments Policy never need moderation. Comments in violation of it, such as yours, invite moderation. As do complaints about moderation.

    This is a moderated forum in which the science of climate research and climate change is discussed. The no-holds-barred, anything-goes discussions commonplace on fake-skeptic blogs have no place here. FYI.

    Moderation complaints struck out.

  9. Alexandre; They produce the report used by governments to convince the world about global warming & the science behind what we know at present. It has to be one of the most important documents on the planet at present. It is used to decide the fate of trillions of dollars of investment through carbon taxes or emissions trading schemes.
  10. Oh and this New Scientist article talks a little bit about the use of grey references (reference to un-peer reviewed sources and other grey literature).
  11. krisbaum at 10:08 AM on 4 August, 2012 Actually, 1)it's not really being used in practice 2) and governments struggle to ignore it, instead of convinceing themselves, let alone anyone else but that's beside the point. They gather information from openly available sources. Certainly not the offender, or judge, or police officer of the world. "Ignored scientific comittee" would be closer to the mark, I guess. If you disagree, could you please make my day and point to any meaningful measure to cut down emissions that has actually been taken? With or without the IPCC's report help?
  12. Alexandre, It is here in Australia - the IPCC is quoted as the definitive place to go to for climate change science/recommendations. It was pivotal in the stern report brought out in the UK, and without it there wouldnt be a Carbon Tax in Australia. I think the police analogy came across wrong. What I mean is, they produce a report but there is no external auditor or group of people that have no vested interests, scrutinising the report and how its generated. So Alexandre - ther's your meaningful example - the Carbon Tax in Australia.
  13. Michael Sweet; ' In fact, the governments of oil producing countries, with help from the USA, diluted the conclusions of the scientists in the SPM. Your suggestion that the IPCC is alarmist is backwards, it reduces the conclusions to make AGW seem less of a problem' Well, this alarms me just as much as the opposite. No government should have their hands in diluting or concentrating such a report. Its my point exactly. At least we agree that politics is getting in the way ;).
  14. krisbaum: "My suggestion is that, considering the report produced by the IPCC is being used to decide one of the most critical fates of humanity - spending trillions on alternative energy in effect - there could be an external policing of their processes." This represents a fundamental error in analysis on your part, krisbaum. The IPCC ARs gather the existing science and integrate it into a readable summary. If the IPCC did not exist, the science would still be there saying the same thing, and the probable outcomes would be just the same (or arguably worse). You imply the IPCC is a bad idea and subject to corruption. I say not having the IPCC is a worse idea. I can imagine a version of the IPCC that is worse than nothing at all, but the current form is far, far from that. I agree that the process can improve, but I fail to see how a few errors and loose practices (recognized and in correction) are justification for condemning the whole process. You also seem to assume that all of the regular SkS posters bow before the IPCC and are oblivious to its problems. No. In fact, they are so aware, from having to explain it daily, that they know where practice is sound and where it needs work. It is a large project with many aspects. It does inform the world, and it does a much better job than individuals trying to piece together the science and then explain it and the implications to the world.
  15. DSL; There is no error in my analysis - the problem is much wider than 'a few erros and loose practices'. You have countless IPCC staff who are inexperienced scientists and not experts. They determine what to include and what to not include from the science - and that doesnt just mean peer reviewed, they also include other grey literature. Something like 1/3 of the references in the last report are grey literature - WWF reports, Greenpeace, news, un-peer reviewed.. etc.. If you believe this kind of working practice is acceptable then fair enough, I on the other hand think its not. The world needs people who believe and are passionate. I have no problem with activists, climate activists or environmentalists - on the contrary - if it wasnt for some of these people, but - you have to consider people's vested interests and take precautions to make sure those interests dont get in the way. It would be like the IPCC obtained funding and had 1/3 of its staff filled with Exxon employees. What would you think then?

    [DB] "You have countless IPCC staff who are inexperienced scientists and not experts."

    Participants here are not allowed to simply make things up as they please. You will need to furnish source citations for extravagant claims such as this. Else moderators will intervene.

  16. *if it wasnt for these people we'd have polluted water, rivers and probably be dead by now.
  17. Krisbaum - governments govern. That's what they do. They do it will if their policy is well-informed. To suggest that government should not be involved is absurd. They requested the report. Yes, the editors make their decisions but every decision is transparent. Which one do you agree with. You want audit by others with no vested interests - what like the scientists of the world? That was the idea in drawing up the IPCC. Who do you propose should do this audit? the uninformed? And whose choses the auditors? And would you then demand audit of the auditors if you still didnt like the answer. Note that "grey literature" was only used by WG2 in the absence of any other. However, you are challenging WG1 but so far I dont see a basis for that.
  18. "You have countless IPCC staff who are inexperienced scientists and not experts. They determine what to include and what to not include from the science - and that doesnt just mean peer reviewed". The IPCC doesnt have countless staff - it has next to none. The review decision is by lead authors who are not IPCC staff and are chosen as the experts in their field.
  19. krisbaum at 10:23 AM on 4 August, 2012 Actually, you have made my day. Thanks. I think these news broke here at SkS too but I somehow missed it. And ok, you realize the police/judge/offender juxtaposition was a bit too much. Thank you again for being this open minded. So your main criticism is that the process does not garantee the quality of the information. You even imply that it has vested interests, and is therefore biased (correct me if I read too much from your comment). I will set aside for a moment the fact that it is painfully and repeatedly reviewed by thousands of scientists from all over the world before publication - including countries that make great efforts afterwards to ignore it in pratice. After 22 years, we have had time to check some of the main projections, and they are basically right. Even too conservative sometimes (sea level rise). Where would you say the report missed the point and got carried away by its "bias"? (it's bedtime here in Brazil, so see you tomorrow)
  20. 'To suggest that government should not be involved is absurd. ' Thats correct, the government should not be involved in generating the report. Like I mentioned before, do you have the police judging their own court cases? Do you have the government enforcing their own laws? We have separate entities for a reason. 'what like the scientists of the world? That was the idea in drawing up the IPCC. Who do you propose should do this audit? the uninformed?' Its pretty easy to find people that have no vested interests in the IPCC's conclusions. You dont need to audit what the auditors write - you just need a cross-check from another external party. Over a 3rd of their sources are grey like i mentioned, yet Pachauri declares all sources 'peer reviewed literature'. So he says one thing and does another. Let it be clear the shortfallings of the report, or enforce your standards. Grey literature is used throughout the whole IPCC report. The fact you admit there was grey literature is enlightening. Dont you therefore see it a problem Pachauri tells everybody that its peer reviewed when its not?

    [DB] "Grey literature is used throughout the whole IPCC report."

    Please provide a link to a reputable source that documents the usage of such literature in WG1, or, failing that, provide a link to that portion of WG1 in question and to the grey literature on which it was based. Note that fake-skeptic blogs are not reputable.

    Failure to provide such a link will result in further moderation of your comments here.

  21. DB - i cant furnish your request unless I know what your interpretation of 'reputable' is ????
    Response: [DB] Either something like a commentary paper in a peer-reviewed journal or showing directly which section of WG1 was based on which specific piece of grey literature. It was your statement; it is incumbent upon you to be able to furnish documentation supporting it...or to withdraw the assertion.
  22. How about showing us a portion in WG1 (the scientific basis) that is based on 'grey literature.' Failure to do so will lead to the inescapable conclusion you simply made that up.
  23. Don't expect SkS readers to fossick around in that mess. Cite one portion from WG1 that is based on 'grey literature.' If you're familiar with the details it won't take long. We're waiting.............
  24. Rob, I think it would fairer to suggest the krisbaum is reading frothing-at-mouth denialist sites and prefers to believe what he is told there. krisbaum, you are implying lead authors have "vested interests" in results. What is the basis of this belief? By the way, I would be perfectly happy for Exxon staff to be reviewing and writing what they would so long as they are Exxon's scientists. I am oil man. By your interpretation of the world, I should be a rabid anti-AGW activist because AGW threatens my current position. In fact, I mostly find widespread acceptance of the IPCC conclusions among my colleagues and client's scientists.
  25. 'Rob, I think it would fairer to suggest the krisbaum is reading frothing-at-mouth denialist sites and prefers to believe what he is told there.' I think its a little unfair you can write such things about me - I havent been nasty to anybody here.

    [DB] "I havent been nasty to anybody here"

    Actually, your very first comment in this forum contained sufficient vitriol towards a regular member of this forum that the entire comment necessitated immediate deletion. Nevertheless, can all participants exercise more discretion in their comment formulations, please?

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