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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

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Comments 126 to 138 out of 138:

  1. andrewii

    This article discusses the findings by Lee Kump and colleagues wrt the rate of inctrease of CO2 during the Paleocene/Eocene Thermal Maximum, around 55 million years ago.

    PETM CO2

    This period is considered a reasonable analogue for today. The world was originally warmer than today by perhaps 4-6 DegC, then it experienced a doubling or more of CO2 very quickly (in geological time scales). Temps climbed another 4-6  DegC, a small Extinction event occurred and an Anoxic event occcurred in the ocean.

    Kump et al found that CO2 levels today are climbing 10 times faster than during the PETM.

  2. Thanks DSL, gws and Glenn Tamblyn!

  3. I'd like to suggest a subject for a future post:

    IPCC figures are virtually all limited to 2100. And all IPCC scenarios assume some emmission curbing at some point (ranging from slow to fast). That's unrealistally optimistic at this point, since there are no signs of leaving carbon reserves unexplored and buried undergound. They're even exploring new possibilities in hydrocarbons (like methane clathrate), and looking for new oil reserves (like in the Arctic).

    If we burn every reserve we know, this paper below projects a 16 ºC warming eventually, making "much of the planet uninhabitable by humans".

    "Climate sensitivity, sea level and atmospheric carbon dioxide"

    Hansen et al. 2013

  4. The reason the IPCC projections are necessarily conservative is that all participating countries have to agree on the information included!

  5. Hope this thread is appropriate.

    Over the years I've used the SKS escaltor in my blog posts from time to time.  Recently I recieved a reply "it is a great example of cherry picking noisy high/low points along a period when first the PDO and then the AMO moved into their positive phases. If you remove the noise then the escalator magically disappears. What's left is two step rises. The first from 1976-1980 was the PDO going positive. The 2nd from 1993-1995 was due to the AMO going positive.


    [PS] Try here. Cherry picking by the way is making an argument by selecting only the part of the dataset that supports your argument. Real statistician have removed noise rigourously. See here. Sks escalator is that only deniers believe in step changes. The science shows that when internal variability is removed, then temperatures steadily rise with the increase in CO2.

  6. I've downloaded the 2018 IPCC report 

    Changes in Climate Extremes and their Impacts on the Natural Physical Environment


    I've perused the References section and I can find no research that takes place later than 2011. I guess there's a possibility  the report is for 2013 but the link indicates that it's 2018 (No explicit mention of the date of the report in the text. Why?)

    If in fact it's 2018, why the delay in examining research?

  7. Cozumelito:

     From the title page of the report you cite:

    "This chapter should be cited as:
    Seneviratne, S.I., N. Nicholls, D. Easterling, C.M. Goodess, S. Kanae, J. Kossin, Y. Luo, J. Marengo, K. McInnes, M. Rahimi,
    M. Reichstein, A. Sorteberg, C. Vera, and X. Zhang, 2012:" my emphasis

  8. Take note that these are very large reports that take years to compile, write, and publish. Thus, the latest research will be several years prior to the published date of the final report.

  9. I believe the IPCC reports are probably the most used information to help set climate policy around the globe. However after reading this,

    and part of their summary :

    " Climate policymaking and the public narrative are significantly informed by the important work of the IPCC. However, IPCC reports also tend toward reticence and caution, erring on the side of “least drama”, and downplaying the more extreme and more damaging outcomes. Whilst this has been understandable historically, given the pressure exerted upon the IPCC by political and vested interests, it is now becoming dangerously misleading with the acceleration of climate impacts globally. What were lower-probability, higher-impact events are now becoming more likely. This is a particular concern with potential climatic tipping points — passing critical thresholds which result in step changes in the climate system — such as the polar ice sheets (and hence sea levels), and permafrost and other carbon stores, where the impacts of global warming are non-linear and difficult to model with current scientific knowledge.However the extreme risks to humanity, which these tipping points represent, justify strong precautionary management. Under-reporting on these issues is irresponsible, contributing to the failure of imagination that is occurring today in our understanding of, and response to, climate change. If climate policymaking is to be soundly based, a reframing of scientific research within an existential risk-management framework is now urgently required. This must be taken up not just in the work of the IPCC, but also in the UNFCCC negotiations if we are to address the real climate challenge. Current processes will not deliver either the speed or the scale of change required."


    So, i was wondering just how correct is this need to modify the scope of the IPCC reports. 

    " What Lies Beneath" put into words my feelings on our current  Aussie state - pretend we care but open more coal mines and do nothing..

    This video was also enlightening , a new documentary series , pt 1.!

  10. prove we are smart @134,

    With one exception, I have only read the summary of the paper Spratt & Dunlop (2018) 'What Lies Beneath: The understanding of existential climate risk' which you refer to. In short, the need for action on AGW was demonstrated by the science over a quarter of a century ago yet globally our emissions increased as though no such science existed. There is now, policy-wise, an agreement that radical action is needed to prevent a +1.5ºC increase in global temperature, radical as such a goal requires global CO2 emissions in future years to be restricted to something like 120Gt(C). The general message from the science is that such a goal will prevent the very bad AGW effects from happening. Yet in terms of policy, there is still little urgency. The enthusiasm for declaring a Climate Emergency has not yet resulted in any useful policy but let us hope that it will, soon. Spratt & Dunlop (2018) suggest the 120Gt(C) is flawed but it seems to me that its argument would need to be far better presented to be a useful contribution.

    Their main criticism is that the science includes those "fat tails" which may make a +1.5ºC world a bit too hot to handle. There has been quite a bit of criticism of the IPCC for failing to make such potential outcomes better known. Most recently, Oppenheimer et al (2019) 'Discerning Experts: The Practices of Scientific Assessment for Environmental Policy' has described well instances of the IPCC's failings (three of the authors have an account of the book HERE). But while there are systemic problems within the IPCC assessments, they do not systematically underestimate AGW. There is perhaps need for the failings of the IPCC assessments to be better understood and in addressing that, I have no criticism of Spratt & Dunlop (2018).

    But Spratt & Dunlop (2018) goes further than this. There is a feel of this in their assessment of the impacts of AGW. Their Summary begins:-

    "Human-induced climate change is an existential risk to human civilisation: an adverse outcome that will either annihilate intelligent life or permanently and drastically curtail its potential, unless carbon emissions are rapidly reduced."

    The word "existential" is usually, in my view, very poorly wielded in regard to AGW. Here, the use "an existential risk to human civilisation" is refreshing to see. But then it is followed by "annihilate intelligent life" which is something I find very difficult to envision.

    So this got me delving into one further section of Spratt & Dunlop (2018). The section Carbon Budgets makes a number of points (♣ Not working other GHGs into the budget, ♣ Not evaluating "fat tails" which would reduce carbon budgets to zero, ♣ Relying on models that potentially run cool, ♣ Not defining pre-industrial temperature adequately, ♣ Ignoring actual peak temperatures) but I only see one of these issues that is germane to setting carbon budgets (the cool models). So for me, Spratt & Dunlop (2018) seems to be 'firing from the hip' a bit too much and needs to tighten up its message if they want to make a serious contribution.

  11. M A Rodger @ 135, i think shooting from the hip is exactly what the IPCC should be doing. With CO2 levels still increasing from the last three decades, taking slow aim and firing is just missing the target. Just how unprepared / misinformed are we in Australia? Look at this,

  12. Why has the IPCC and other international bodies attempting to address the problem stopped talking about the cumulative nature of CO2 and who has historically contributed more to the problem?

    AFter all, those of us old enough to remember the Kyoto Protocol recognize this:

    "The Kyoto Protocol is based on the principle of common but differentiated responsibilities: it acknowledges that individual countries have different capabilities in combating climate change, owing to economic development, and therefore puts the obligation to reduce current emissions on developed countries on the basis that they are historically responsible for the current levels of greenhouse gases in the atmosphere."

    This was almost universally recognized before, ignored today.  Does the IPCC have a poltical agenda?  I know political topics are not allowed, but here there is talk about IPCC as if it is just a scientific body, when it clearly has taken political positions throughout its history.  

  13. Gzzm @137 ,

    your language is rather vague wrt  "address the problem".

    The various IPCC reports, over the years, are intended to summarize the overall climate science.  The IPCC also produces a condensed version for Policymakers i.e. politicians.

    Gzzm, best if you clarify what you mean by politics  and political positions.   The term politics  covers a vast range of meaning.

    As example, is it politics  to say about a wildfire: "Quick, send in the fire-fighters"  or on the other hand to say:  "Nah just let it burn, come what may" ?   Are such decisions practical decisions, or are they political decisions?

    Can be hard to say what, if any, is politics.   Probably best if we simply use common sense wrt  "the problem".

    Gzzm, please clarify any points you wish to make.

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