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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 101 to 125 out of 138:

  1. krisbaum @ 95... Upon a quick read of the paper you cited I can't find any reference even to the atmospheric lifetime or spacial distribution of black carbon. I don't think you've read the paper.
  2. krisbaum... I believe if you're talking carbon monoxide aerosols that would be generally accepted as anthropogenic, pretty much without saying. Unless you can think of other large sources of carbon monoxide.
  3. To clarify, as can be seen in this image of various emissions with short dwell times in the atmosphere, they have, as Krisbaum puts it "a very heterogeneous spatial distribution": However, just because concentrations are strongest near there source, and fall of rapidly with distance does not mean emissions do not result in significant concentrations hundreds of miles away, or that relatively remote locations will not have sufficient concentration to track changes in emissions. Indeed, if you look at the record for aerosol emissions (center top), you will see that individual cities are not clearly delineated by the concentrations, as would be the case if their typical transit distance was limited to 10 kilometers. In that way, aerosols are different from NO2 (left top), where individual industrial centers are easily delineated. What is more, significant concentrations of aerosols are still found a couple of hundred miles of the coast, again showing the absurdity of the 10 km claim made by Krisbaum. So, once again, while Krisbaum purports to have a problem with the IPCC citing Popper's "The Logic of Scientific Discovery" on falsification, and Kuhn's "The Structure of Scientific Revolutions" on paradigm shifts (just two of the very dubious claims of inadequate literature from Krisbaum's favourite source), he has no problem with citing non-academic websites and simply making "facts" up in support of his argument. Why then do we continue to tolerate his sloganeering? His own words have thoroughly discredited him; as has his refusal to look at the particular use made of particular citations for which he condemns the IPCC.
  4. Rob @101 - page 3 of the report Rob @102 - Bushfires for example generate huge amounts of carbon monoxide.
    Response: [DB] You have yet to fulfill the challenges you have assigned yourself in comments 81 and 94 earlier in this thread. You will be held accountable to fulfill those commitments.
  5. krisbaum @ 104... Ummm... No. Just read page three and there is nothing there that talks about the atmospheric lifetime or spacial distribution of black carbon. They discuss accumulation over time. There is certainly nothing there to support your 10 km statement.
  6. Tom@103 I was talking bout anthropogenic aerosols not emissions. Why post a diagram of CO, CH4 etc when they are NOT aerosols?
  7. krisbaum @ 104... And so you think that the large spacial distribution of CO presented in DB's gif is... bush fires? Maybe I've not been watching the news closely enough. Are there massive bush fires occurring in Beijing and Tianjin?
  8. @Rob 105 'Anthropogenic atmospheric aerosols are believed to be a significant climate forcing agent, probably second only to greenhouse gases in their effect on global temperature in the past century [Houghton et al., 2001]. However, the history of atmospheric aerosols is not nearly as well known as that of most gases because of the short atmospheric lifetime of aerosols and thus their very heterogeneous spatial distribution. Moreover, the climate effect of aerosols is complex, as some aerosols cause cooling while others are believed to cause warming.' page 3.
  9. Rob @107 - CO is not an aerosol.
  10. krisbaum @ 109... We are clearly not looking at the same paper. You said the title of the paper was, "Large historical changes of fossil-fuel black carbon aerosols."
  11. krisbaum @106, I explicitly referred you to the top center image for the discussion of aerosols. You will notice that it is labelled "AOT", standing for Aerosol Optical Thickness. There are certain minimum standards of conduct necessary for any rational discussion - honesty, consistent standards for your self and others, a willingness to cite relevant information etc. You have shown yourself consistently unwilling to abide by any such standard. Consequently my discussion with you is over. Until you undertake to show why it was reprehensible of the IPCC to cite Agassiz as the originator of the theory of ice ages; or to cite Newton's letter to Hooke in 1675 as the source of the famous quotation about standing on the shoulders of giants (two more of the supposedly dubious citations according to your favourite source), or admit that the quality of citations cannot be determined simply by looking at the immediate source and that your favourite source used a massively flawed methodology which is completely unable to justify the conclusions you, and they draw from it, there is nothing more to say.
  12. It is duly noted that krisbaum blatantly is ignoring the tasks he has given himself in 81 and 94. Just as he ignored the text description for the animated GIF found on the host page:
    "The animation below shows the fire activity and smoke plumes in Russia and China in June/July 2012 and the consecutive transport of smoke across the Pacific. The MACC GFAS assimilation of Fire Radiative Power observations are superimposed over the combined organic matter and black carbon fields in the MACC global aerosol assimilation, which combines the smoke emissions calculated in GFAS with inventories of anthropogenic emissions and aerosol optical depth observations from space. Both types of observations are performed by the satellite-based MODIS instruments and NASA has provided the observation data."
    Also note that aerosols travel enormous distances across continents and oceans. And note that they are not evenly-mixed or distributed, unlike CO2: [Source]
  13. Very interesting, there are actually two different versions of this paper. The one that krisbaum cites includes the sentence about short atmospheric lifetime, etc. The version I found was published in GRL specifically has that line deleted. It looks to me like kris is citing from (maybe) a pre-press version of the paper (?). I find it interesting that that line was removed.
  14. This has to be the most pointless discussion I've encountered in several years, and that includes the engagement with Tim Curtin at OM, the SLoT thread, and everything Dan H has started at RC. What will be the ultimate bottom line if krisbaum is correct in feeling misled by Pachauri? Would it mean anything where WG1, WG2, or WG3 are concerned? And although it's probably against comment policy, I'll wager krisbaum has no problem with (-Snip-), and those are much more damaging than anything Pachauri has to say about bloody grey literature (as one can see from the variety of mainstream media outlets that are seemingly plugged directly into WUWT).
    Response: [DB] Comments policy violation snipped.
  15. DSL does have a point. Krisbaum, the figure of 10km is not only arbitrary, it's also ludicrous and baseless. None of the papers you have referred to can allow to support this number. It is obvious you made it up on the moment to serve your argument. I see nothing in your various arguments that deseves any consideration and I'm done wasting my time on them.
  16. '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.' It is fairly obvious from recent discussions that my quotation above still applies, everbody is correct in disputing '10km' - as an arbitrary number like this, argue you may but the point still stands that (-Snip-).

    [DB] It is very obvious from recent discussions that your quotation above is still invalid. The intercontinental reach & dispersion of aerosols has been amply demonstrated.

    Either finish your homework from comments 81 and 94 above or cede those points by declaring your position invalid.

    Off-topic diversion snipped.

  17. krisbaum, I'm not going to immediately discard your claim. I do, however, want to see some evidence. What you claim is not common knowledge. Here's a study that casts doubt on what you say just from the abstract. The study measures the 72 hour trajectory of a cloud of SO2 that extends 1600 meters from the surface. If the mass of SO2 travels for 72 hours and only goes 10km, that means it is traveling at the extremely fast rate of about 139 meters per hour. Possible, but not probable. This study worries about an oil refinery being built 30km away from the Taj Mahal and what the emissions will do to the structure. Why did they even perform the study? It's common knowledge that human-sourced aerosols only travel 10km. These were just two of the first few on google scholar. I can do more of your work for you if you require it.
  18. DSL; My original assertion about aerosols was to do with a lack of records pertaining to their levels - hence the high amount of high uncertainty in their values. [Anthropogenic] Aerosols are concentrated at their source and depending on their type (eg. Sulphates or Black Carbon etc) - they travel varied distances from their source but a geo-spacial representation of their concentrations in the atmosphere is required to estimate their impact on climate change.

    [DB] Actually, your original assertion about aerosols was made here and was this:

    "Has anybody actually investigated further, for example the findings on Aerosols and why the IPCC believe what the range of aersol forcing is thats stated in their 4AR?"

    You did not make the assertion about aerosols that you claim until 4 ½ hours later, here.

  19. (-Snip-)

    [DB] This has gone on long enough. As has been noted several times already, finish the work you have claimed for yourself in comments 81 and 94 above. You will be allowed no further podium to waste the time of others here until that work is completed.

    Argumentative and sloganeering snipped.

  20. [finish the work you have claimed for yourself in comments 81 and 94 above.] I just did and you deleted them.
    Response: [DB] Comments deleted were due to inflammatory/ad hominem comments and sloganeering. You continually focus on newspaper accounts of what Pachauri is reputed to have said. You continue to fail to show (from 81) where the IPCC has stated that it only bases its work on peer-reviewed literature. It is clear you posture.
  21. A final comment on the dispersion of sulfates. Fiedler et al, 2009 discuss the observation of a SO2 pollution plume just of the west coast of Ireland. That pollution plume came from China. In fact, it was not the only plume they found at that location. They found another, lower plume from North America as well, as shown in this diagram: You will notice the Chinese plume has approximately half the concentration North American Plume. Its greater altitude and thickness, however, suggests it would have greater than half the cooling effect of the North American plume. This suggests that it would make an appreciable impact on the atmospheric energy balance at that time and location. Below is a modeled distribution of Chinese SO2 based on known weather patterns at the time of observation. Again, notice that the concentration of SO2 in the plume is only around 40% of that over China, again signifying an appreciable impact on the energy balance: Despite the high concentration of the plume half way around the world, occurrences of plumes from China at that location would be sporadic, whereas the plume over China itself would be constant throughout the year. As a result, averaged over the year, concentrations over China would be large compared to those over the North Atlantic. But that in no way justifies Krisbaum's rather silly claims, and it certainly does not justify his supposition that SO2 concentrations in Greenland ice cores are a poor proxy of SO2 emissions over Europe and North America. As we have seen, there is no question that SO2 plumes can be carried more than half way around the world - let alone half way across the world's narrowest ocean to Greenland.
  22. Hi all,

    This may be a dumb question, but I'm wondering if someone could clear up my confusion about the rate of change in CO2 concentrations we're seeing today vs. the rate at which they've varied in the past.

    In this video, Michael Mann states that the rates today are changing a million times faster than what we've seen in the past, but this article above states that "atmospheric CO2 is increasing ten times faster than any rate detected in ice core data over the last 22,000 years."

    Thanks for your help,


  23. Woops, forgot the link:

  24. Tough to say, Andrewii.  While it's true that CO2 hasn't increased as rapidly as present in at least 300 million years, it would take some selectivity to make the math come out right on "1,000,000 times faster."  For example, we could find a period where CO2 advanced 120ppm over the course of 150,000,000 years.  Such a period might be found if one used the right analysis (e.g COPSE).  Then one could say, "We've put up 120ppm in 150 years, one million times as fast as in period x."  Or one could go the other route and find a much shorter period with a much more neutral trend. Without knowing the comparison period, it's hard to say what Mann is referring to.

    It's hard to be alarmist with the rate of CO2.  As Honisch et al. point out, it's possible there's no precedent for the current rate of increase.

  25. Andrewwii, when studying climate topics and associated facts, try not to view or read individual statements out of the larger context (note also that the video is pasted together from a longer one). It is easy to cherry-pick a single statement taken out of context and get confused by it, or even take it to mean somebody is dishonest or similar.

    In this particular case, we are talking about a rise from a preindustrial level of 280 ppm to today's 400 ppm in roughly 200 years (average 1.2 ppm per year, accelerating; currently 2 ppm/yr). In that video, Michael Mann refers to the last time the CO2 level was that high (several million years ago) and how much time it took then to change average levels by roughly as much (e.g. by 100 ppm), "10s of millions of years" (long-term average data here). Taking an average slope (ppm CO2 change over time) from that graph over the last 100 million of years gives roughly 10 ppm/million years ((1300-300) / 100), so about a 5th to a 10th of a million times as fast as today. If you take a shorter period, say only the last 30 million years, the slope is less, nearly matching what Mann referred to.

    So you see, it is easy to accuse him of saying something wrong. Human nature. It is more difficult though to stand back, look at the bigger picture, ask a clarifying question if possible, and avoid taking things out of context. So thanks for asking.

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