Andrew Dessler’s testimony on what we know about climate change


In his recent testimony, Andrew Dessler reviewed what he thinks “are the most important conclusions the climate scientific community has reached in over two centuries of work”. I think that’s a very good choice to focus on, as the basics of what we know is most important, “at least as to the thrust and direction of policy” (Herman Daly). This focus served as a good antidote to the other witness, Judith Curry, who emphasizes (and often exaggerates) uncertainty to the point of conflating it with ignorance.

Dessler mentioned the following “important points that we know with high confidence”:

1.  The climate is warming.

Let’s take this opportunity to show the updated figure by Cowtan and Way, extending their infilling method to the entire instrumental period (pause? which pause?):

Cowtan and Way - Global Avg Temp 1850 - 2012

2. Most of the recent warming is extremely likely due to emissions of carbon dioxide and other greenhouse gases by human activities.

This conclusion is based on several lines of evidence:

- Anthropogenic increase in greenhouse gases

- Physics of greenhouse effect

- Observed warming roughly matches what is expected

- Important role of CO2 in paleoclimate

- No alternative explanation for recent warming

- Fingerprints of enhanced greenhouse effect (e.g. stratospheric warming cooling, which was predicted before it was observed)


Thus, we have a standard model of climate science that is capable of explaining just about everything. Naturally, there are some things that aren’t necessarily explained by the model, just as there’re a few heavy smokers who don’t get lung cancer. But none of these are fundamental challenges to the standard model.

He goes on to explain that the so-called “hiatius” is not a fundamental challenge to our understanding of climate, though it is “an opportunity to refine and improve our understanding of [the interaction of ocean circulation, short-term climate variability, and long-term global warming].”

What about alternative theories? Any theory that wants to compete with the standard model has to explain all of the observations that the standard model can. Is there any model that can even come close to doing that?


And making successful predictions would help convince scientists that the alternative theory should be taken seriously. How many successful predictions have alternative theories made?


3. Future warming could be large 

On this point I always emphasize that the amount of future warming depends both on a combination of factors:

- the climate forcing (i.e. our emissions and other changes to the earth’ radiation budget)

- the climate sensitivity (the climate system’s response to those forcings)

- the climate response time (how fast will the system equilibrates).

Internal (unforced) variability also plays a role, but this usually averages out over long enough timescales.

4. The impacts of this are profound.

In the climate debate, we can argue about what we know or what we don’t know. Arguing about what we don’t know can give the impression that we don’t know much, even though some impacts are virtually certain.

The virtually certain impacts include:

• increasing temperatures

• more frequent extreme heat events

• changes in the distribution of rainfall

• rising seas

• the oceans becoming more acidic

Time is not our friend in this problem.

Nor is uncertainty.

The scientific community has been working on understanding the climate system for nearly 200 years. In that time, a robust understanding of it has emerged. We know the climate is warming. We know that humans are now in the driver’s seat of the climate system. We know that, over the next century, if nothing is done to rein in emissions, temperatures will likely increase enough to profoundly change the planet. I wish this weren’t true, but it is what the science tells us.

Peter Sinclair posted a video of Andrew Dessler’s testimony. Eli Rabett posted Dessler’s testimony in full.

A key distinction in the two senate hearings was that Andrew Dessler focused on what we know, whereas Judith Curry focused on what we don’t know (though “AndThenTheresPhysics” made a good point that Curry goes far beyond that, by e.g. proclaiming confidence in certain benign outcomes (e.g. regarding sensitivity) while claiming ignorance in areas where we have a half-decent, if incomplete, understanding, e.g. regarding the hiatus). I have argued before that emphasizing (let alone exaggerating) uncertainties is not the road to increase people’s understanding of the issue, where what we do know is much more important to convey (if your goal is to increase the public understanding of scientific knowledge). Alongside that I argue that much more attention is needed to explain the nature of science, which is needed to e.g. place scientific uncertainties in a proper context.


Herman Daly said it as follows, in a quote I’ve used regularly over the past few years:

If you jump out of an airplane you need a crude parachute more than an accurate altimeter.

Arguing whether the altimeter might be off by a few inches is interesting from a scientific/technological perspective, but for the people in the plane it’s mostly a distraction.

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35 Responses to “Andrew Dessler’s testimony on what we know about climate change”

  1. Bobby Says:

    Ha! Love the quantum uncertainty cartoon! Great post.

  2. Pieter Zijlstra Says:

    Dessler qualifies current climate science as ‘standard model’.
    Such qualification is probably more fruitfull in discussions then the ‘concensus model’.
    In a standard model one can accept unresolved part of the puzzles being worked on. Consensus hints on that the work is done.

  3. gaia.sailboat Says:

    This is a GREAT piece. For sure, you can tell, many, including those with training who should know better, just love to debate minutia of climate. They hate conclusions – because they might stop the debate which is so much fun.

    The debate about how many climate change angels can stand on the head of a pin should cease and we should start talking about positive things – like the replacement of fossil fuel with feasible, clean energy sources.

    New, safer nuclear technology seems the answer – like LFTR Thorium reactors. Let’s put our energy into building one as a demonstration. That will end the silly, endless debate. Let’s get to work.

  4. jpratt27 Says:

    Reblogged this on jpratt27 and commented:
    Great post. Puts it all so simply. We are running out of time. The Nero’s are fiddling while the planet burns.

  5. gaia.sailboat Says:

    One more thing..We should also prepare for when it really starts hitting the fan (California now in Emergency Drought Conditions). What do we do? Where exactly do we run? If our govt was responsible it would develop Emission Failure Emergency Plan. Migrate North?

    Or when food and water run low do we just start another debate cycle about what to do, how many should die, who should die and who should live, how hot it will finally get, etc.?

  6. andthentheresphysics Says:

    Reblogged this on And Then There's Physics and commented:
    Given that I’ve had a couple of posts about Judith Curry’s testimony to the EPW, I thought it might be good to also have something discussing Andrew Dessler’s testimony. However, I’m feeling lazy (well busy) and Bart’s post covers most of what I would have said, so – I hope Bart doesn’t mind – I thought I would simply reblog this, rather than writing something of my own.

  7. uknowispeaksense Says:

    Reblogged this on uknowispeaksense and commented:
    An excellent piece, however, the reference to Judith Curry’s testimony is too kind. She is deliberately misleading and therefore dishonest. Call a spade a spade.

  8. andthentheresphysics Says:


    Call a spade a spade.

    I think this is an interesting issue and one that’s worth debating. My view – a few months ago – was that being civil and just trying to discuss the science are clearly and honestly as possible, would be the best strategy.

    The problem with “calling a spade a spade” is how do you actually prove that someone is being deliberately misleading and dishonest. Often I see people doing this and they end up defending their supposed lack of civility and noone considers that what they were trying to say was fundamentally correct.

    On the other hand, the being civil and simply sticking to the science doesn’t appear to be working very well. It’s fairly clear that real dialogue is virtually impossible (in my view at least) and that this is essentially a conflict. So maybe the solution is to get more direct and to “call a spade a spade”.

    To be clear, I’m yet to form a real opinion about this, but I do think this is a topic worth discussing because it does seem like a major problem today is how one counters the mis-information.

  9. Bart Verheggen Says:

    Thanks for all the feedback and the “reblogs”. My intent was to focus on Dessler’s excellent exposition. I just mentioned Curry’s approach, one that I clearly find wanting in terms of science communication, in passing. Moreover, judging the validity of what is said is one thing, but judging the intent behind it is vastly more tricky and usually a great start for a non-constructive flame war. But it is always a dilemma, as theresphysics (ATTP?) points out. There’s room for multiple communication styles in terms of how to deal with skeptical smokescreens.

  10. Paul Kelly Says:

    The perception of an inflated sense of certainty in some climate communicators is one of the roots of skepticism. Dessler speaks of predictions. Right now there is an almost perfect storm of unfulfilled climate predictions about sea ice, temperature and cyclonic activity. Does this disprove any of the science? Of course not, but it does make persuading about the science more difficult.

  11. Tom Curtis Says:

    Paul Kelly, that claim is false. If it were true, a “perception of an inflated sense of certainty” among climate “skeptics” would lead to acceptance of the mainstream – but the same “skeptics” who point out the supposed over confidence of climate science communicators swallow whole outrageously over confident statements by “skeptics”.

    It may be that over confidence in expression by climate communicators is a pretext seized onto “post hoc” by “skeptics” to justify to themselves their rejection of climate science – but it is no reason.

  12. Tom Curtis Says:

    Bart Verheggen, intentions are undetectable from single claims. However, a person with honest intentions will:
    1) Acknowledge their errors when pointed out;
    2) Notify and acknowledge errors their errors which they discover before others; and
    3) Advise of errors by members of their own side so that they can be acknowledged and corrected.

    In my experience, these sorts of behaviour are almost entirely absent from climate “skeptics”. Absent these behaviours, you lose the right to the charitable interpretation of gross errors as simply being mistakes. People who do not self correct may not in fact be guilty of first degree vericide (truth killing); but they are guilty, at least, of second degree vericide. Calling them honest would be misleading.

    The real question is, is something gained by pointing that out. If it was a private debate, no I don’t think anything would be gained. This debate, however, is public. By not notifying people of a reasonable assessment that people are less than honest, you leave it open that people will be deceived – thinking them honest brokers when they are in fact advocates, and advocates without integrity.

    Further, your reticence to point out the lack of integrity of so-called “skeptics” is not returned. Those “skeptics” are busy tarring and feathering climate scientists as dishonest, fraudulent, and even genocidal on patently stitched up, out of context quotation.

    Consequently, my personal view is that you should give any critic of AGW a chance to show whether or not they are honest by their willingness to acknowledge errors, and criticize errors from their side. If they are not, you should only give them the benefit of the doubt if they do not slander climate scientists; or if their criticisms are in fact reasonable (which is not the same thing as being correct).

  13. Eli Rabett Says:

    Nice piece. Andy Dessler did a good job in his statement.

  14. Paul Kelly Says:

    I call it a root. Tom Curtis calls it a pretext, Maybe underpinning is a better descriptive. We are all greatly influenced by our socio – psycho – cultural filters. Whether any opinion is pre or post hoc is probably irrelevant to successful persuasion. And yes, inflated certainty works both ways.

    It is important to understand those you wish to persuade. How about some examples of outrageously over confident statements by “skeptics”?

  15. Tom Curtis Says:

    Paul Kelly, you did not feel it necessary to provide examples from the limited range of over confident statements from climate communicators, but expect examples of over confident statements from deniers. It is as though you expect no proof for the existence of atoms, but require proof to believe the sun is in the sky in daylight hours. For now I content myself with the statement by Judith Curry that the recent IPCC report weakened the case for attribution.

    And, no, “underpinning” is not a better word. The characteristic feature of reason is that it applies regardless. If A is a reason for B, and C is not relevantly different from A, then C is a reason for B – always. Claiming that a perception of over confidence “underpins” rejection of climate science only claims that it is the reason obliquely. Even treated as a pychological “reason”, ie, a cause rather than a rational basis, only diverts from the more important underpinning factors without which that perception would be irrelevant; and with which most of those who reject the climate science would do so anyway.

    Finally, as regards to successful persuasion, we will never persuade people who allow themselves to be persuaded by over confident statements by science communicators because we will never be able to stop some science communicators from making such claims. The only way to do so would be by denying freedom of speech, which is anathema both science and democracy. Therefore, the correct approach to those whose rejection of science is “underpinned” by the existence of such statements is to point out that it is no basis for their opinion.

  16. deminthon Says:

    “The perception of an inflated sense of certainty in some climate communicators is one of the roots of skepticism.”

    No, it isn’t, and that is not consistent with actual empirical skepticism, which rejects such ad hominem arguments. Rather, this is just one of many forms of rejection apologetics … a reason invented to support a position already held.

    “It is important to understand those you wish to persuade.”

    We do. They are motivated, for the most part, by ideological views about economics and about policies they are opposed to, rather than by following the evidence wherever it happens to lead. See, for instance, the work of sociologist Stephan Lewandowsky.

  17. deminthon Says:

    “How about some examples of outrageously over confident statements by “skeptics”?”

    How about “Global warming is a hoax”, “Global warming violates the second law of thermodynamics”, “Humans are too puny to change the climate”, “Climate is always changing current change is just part of the cycle”, “The major cause of climate change is the sun”, and so on ad infinitum?

  18. deminthon Says:

    “Right now there is an almost perfect storm of unfulfilled climate predictions about sea ice, temperature and cyclonic activity.”

    This is an extreme misrepresentation of the state of affairs. There is massive evidence, in thousands upon thousands of peer-reviewed articles, that is consistent with the basic premises of AGW and only with AGW. Just looking at sea ice there is the ongoing decline of Arctic ice volume, and just looking at global average temperature there is the strong multi-decadal upward trend. As for cyclonic activity, what failed prediction do you have in mind? Please provide a citation, such as or

  19. Bonza Says:

    Bart, when discussing fingerprints I think you mean stratospheric cooling rather than warming?


  20. Bart Verheggen Says:

    Bonza, thanks for pointing out that blooper, I corrected it with strike through.

  21. Bart Verheggen Says:

    Tom Curtis,

    You make some cogent points. Still, I think there’s room for different approaches. For some audiences a certain approach may be more persuasive, while other approaches resonate better with others. I generally try to be very careful with impugning motives (though I’m not immune to the temptation to do so occasionally), but I have no trouble pointing out where arguments seem to be logically defunct or ideologically motivated. I rather keep it to the reader to come up with a fitting adjective for that behavior though; that doesn’t need spelling out. My approach is not for everyone and is indeed less adversarial than that taken by many other mainstream communicators.

  22. Shelama Leesen Says:

  23. Paul Kelly Says:

    deminthon offers mostly good examples of inflated certainty among skeptics. A lot of the skeptic memes are incorrect. Some are downright silly. The storm I describe doesn’t impact the science itself, but it very much impacts the state of affairs in the communication of the science.

    Tom Curtis uses the word reason, which is probably better than root or underpinning. I think it is unproductive to say that those who give inflated certainty (exaggeration) as a reason for their skepticism are dishonest.

  24. Paul Kelly Says:

    Tom Curtis wrote: “… the correct approach to those whose rejection of science is “underpinned” by the existence of such statements is to point out that it is no basis for their opinion.” That’s a good start. Let’s try it with some roll playing.

    Most people get their climate science information from sources other than climate scientists. In the US, for many years, climate communication was dominated by Al Gore. He shone brightly for a time, but the recent sale of his business to an oil state has dimmed his lights. That’s probably a good thing. I think he had become a net negative.


    Those are mostly good examples, The storm doesn’t impact the science itself, but it very much impacts the state of affairs of communication about the science.

  25. Paul Kelly Says:

    Some role playing. How might a skeptic respond to Dessler’s bullet points.

    1. The climate is warming. The skeptic says the climate has warmed but is not warming right now. The climate communicator answers that you have to look at a longer time frame and besides, the climate is warming in places and ways not reflected in the official temperature record.

    2. Most of the recent warming is extremely likely due to emissions of carbon dioxide and other greenhouse gases by human activities.

    I think “extremely” is an example of exaggerate language. That said, I don’t think skeptics have any good argument against “very likely”. They do ask, “If CO2 causes the temperature to rise and CO2 rises every year, why doesn’t the temperature go up every year? Those in the US might note that 2013 was only the 42nd warmest year on record there. .

    3. Future warming could be large.

    This is where the uncertainty widens. Could is not a word of certainty. Bart’s comments on how to approach this are very good. Skeptics say that a warming above 5C is no more likely than a warming below 1C. Regardless of start date, there is no temperature trend that rises to the 0.2C/decade necessary to more than moderate warming in this century.

    I’ll have to get to 4. The impacts of this are profound another time.

  26. gaia.sailboat Says:

    “After all is said and done, more is said than done.”
    The Climate Debate Club needs to convene in California for a stern dose of reality.

    The grazing land is bone dry in mid coast California and large cracks are opening in the ground. Cattle must have feed brought to them since there’s no grass. Aquifer and imported water is all that goes on crops with zero rainfall. We are in “Extreme Drought” according to the USDA. The officials seem to be assuming it will eventually rain like last drought (1990).

    The fallacy is we have a different earth atmosphere now: 400 PPM CO2 vs 350PPM in during the last drought.

    To Debaters: Please give us your solution for more water and hurry. We have only a 24 month reserve.

  27. Bart Verheggen Says:

    Hi Paul, I’ll play along.

    1. Long term trend vs short term variability. There’s nothing surprising about decadal periods of above average and decadal periods of below average trend. See also this figure from Rahmstorf.

    2. “extremely likely” is just the word chosen by IPCC to denote >95% probability. Again, long term trend vs short term variability. Plus, there are other factors besides CO2. See also

    3. As I wrote, future warming depends on many things, but with a contuing lack of mitigative action, keeping warming below 1 degree is just not gonna happen. Yes there are periods with above 0.2 deg/decade trend.

  28. David Young Says:

    This is all well and good, Bart, but I think it is not very likely to be effective, based on the tract record so far. Rebutting Judith Curry may make you feel virtuous, and reading her mind (as others try to do) is even more satisfying, but tells us more about the mind reader than about reality.

    The climate blog “debate” is likewise largely irrelevant I think. There are some real problems with models as shown for example in Lucia’s recent re-evaluation of models vs. reality over the last 26 years, but even that may not be of anything but historical interest in a few years. The models are almost certainly fundamentally flawed and do not perform consistently across a wide range of conditions.
    But even that is probably a secondary issue, and mainly about the integrity of science.

    Those who want real progress I think should look for solutions that actually can be implemented in the current political climate. I can think of a couple such as substituting natural gas for coal, eliminating the truly stupid corn ethanol subsidies, and fast tracking nuclear power plant applications. Strong mitigation is dead on arrival and I think everyone who is at all connected to reality knows it. And then there is the other problem, effective mitigation will require strong action by the developing world, which has more immediate concerns, like food, shelter, and cheap energy.

  29. Bart Verheggen Says:


    It has nothing to do with feeling virtuous or mindreading. I just stronlgy disagree with Curry’s approach, which imho creates more confusion than clarity and gives a very skewed picture of the science. Not even to mention the many factual and logical errors she makes.

  30. Paul Kelly Says:

    IMHO, a focus of climate communication should be finding a mesh point between the maximum amount of uncertainty the climate communicator can entertain and the minimum amount of certainty the listener requires to favor an energy transformation. After all, the goal is not to correctly educate everyone about climate science. It is to substantially replace fossil fuels in this century.

    Everyone will have a different certainty requirement for favoring energy transformation. Mine, for example, is zero. I’m old enough to have favored transformation long before I’d heard of climate concerns. I don’t discount those concerns; they – even at only a 30-40% certainty – are another acceptable reason for me.

    So far climate communicators have been remarkably ineffective in convincing governments to enact the mitigation policies they desire. Yet, too many are locked into the same old communication methods.

  31. Bart Verheggen Says:


    For many climate scientists-communicators, including myself, the goal is “to correctly educate everyone about climate science.” What people chose to do based on the scientific information (and other factors) is up to themselves. But even then, there’s still a delicate balancec in the communication of uncertainty, namely the balance between understandability and accuracy.

  32. Paul Kelly Says:


    You communicate climate science very well. More should take your approach. Let me try to justify my position. Yes, you and many others’ seek only to educate so people can do as they will, but not telling people what to do is not the same as not wanting that something be done. In the public square, climate scientists-communicators like Hansen, Mann and Tobis certainly put replacing fossil fuels in the forefront. So do a majority of climate scientists on TV, radio and in the press.

  33. Paul Kelly Says:

    Mike Hulme has an on point article at The Conversation.

    “So politics, not science, must take centre stage. As Amanda Machin shows in her recent book, asking climate scientists to forge a consensus around facts with the expectation that decisive political action will naturally follow misunderstands science and politics in equal measure. If democratic politics is to be effective we need more disagreement, not more consensus, about what climate change is really about.”

  34. Bart Verheggen Says:


    See my reflections on Hulme (and Lew and Cook) at

  35. Bill G Says:

    Warming is not the chief villain. That villain is dying precipitation. Society’s Achilles Heel is water. Ancient societies all saw that.

    We built Rube Goldberg water systems (espec California) and stretched things out past their logical limit with clever engineering. It was a house of cards vulnerable to even slight drought. Now we have a major and very likely permanent one. Game over.

    Soon mass migration will be the only alternative. Unless there’s something I’m failing to see. Any solutions out there? Anybody?

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