Atmospheric Scientist Scott Denning Shares Lessons from Dialog with ‘Skeptics’

“I didn’t pull punches” in pointing out the “gravity” of climate challenges, a Colorado State professor says of his exchanges with a Heartland Institute conference of climate doubters. He would do it all over again and hopes his “like-minded” colleagues will do so too, and he offers lessons-learned.

It’s news when a climate scientist convinced of the evidence relied on by a group like IPCC goes head-to-head against climate “contrarians” before a public audience.

Scott Denning photo
Denning: High road?
But High Road
to Where? 

It’s a practice many of the “consensus” scientists studiously avoid, saying they fear elevating an unjust adversary they regard as being often motivated more by political and policy concerns than by a commitment to scientific evidence. Colorado State University atmospheric sciences professor Scott Denning took just such a plunge, however, and with his eyes fully open. And he says he’d do it again and hopes his like-minded colleagues also will.

Denning wrote recently in the UCAR Magazine of his experiences speaking before the annual conference sponsored by the climate-doubtful Heartland Institute in Washington, D.C.

“I was treated with respect and even warmth despite my vehement disagreement with most of the other presenters,” Denning wrote, expressing thanks for prominent platforms he was provided during the conference, including an hour-long keynote debate with contrarian Roy Spencer of the University of Alabama, Huntsville.

Change Minds? Rather, Defuse Rhetoric

Asked if he had accepted the Heartland Institute speech invitation with the idea of “changing some minds,” Denning paused before responding, “Yeh. I guess I did. I hoped to change some minds,” but he added that “a lot of people” at the conference were “not so open-minded.” At the same time, he said he was gratified by the number of conferees who later told him that his remarks “really made me think.” He pointed to a long dinner discussion he had with a former New Zealand environmental minister and “hard-core climate denier” who asked “really insightful questions.”

“These were not a bunch of brain-washed idiots,” Denning said of the conferees, rebutting an impression many in the science community might have. He said his real goal in making his remarks, rather than changing minds, was more along the lines of “diffusing the angry rhetoric on both sides” through, for instance, what he called the “rhetorical tricks” of beginning on “common ground” and of emphasizing facts “that are not in dispute.”

Prior to making his remarks at the Heartland meeting, Denning said he had been getting a ratio of three-to-one comments from fellow scientists that his planned presentation to such an audience would likely backfire. Since posting his UCAR Magazine piece, he said, comments from his “like-minded” science peers have been overwhelmingly favorable and upbeat.

High Road? … High Road to Ruin?

Denning in his article dismissed claims of many scientists that avoiding such head-to-heads with committed climate contrarians amounts to the “high road.” Citing polling showing increased public uncertainties about climate change, he wrote that he fears “the high road to ruin.”

“Strong and persuasive engagement” of contrarians by bona fide experts “articulating the scientific consensus is essential,” he wrote. He urged avoiding jargon, complicated graphics with unreadable titles “and unintelligible lines that look like multicolored spaghetti.”

Denning characterized the Heartland conference audience as having “a strong and obvious identification with libertarian and free-market ideology” and said many in the audience appear to see predictions of risks from climate change as “a stalking horse for intrusive government policies.” He wrote that he countered that “the world needs their ideas to solve one of the most pressing problems humanity has ever faced …. I didn’t pull any punches.” Dozens of participants “told me they’d needed to hear this ‘other side’ of the story of climate change.”

Tips for Being ‘Informative, Persuasive, Productive’

Outlining some lessons-learned from his Heartland conference experience, Denning suggested “informative, persuasive, and productive” approaches such as:

  • Beginning from “common ground,” such as recognizing a need for public policy to be based on facts rather than on political agendas;
  • Engaging a skeptical audience “on a human level,” including using humor and ordinary examples from daily life; and
  • Emphasizing “the basics” and points “not in dispute,” such as basic physics points about CO2 and about combustion of fossil fuels.

“Taken together, these indisputable facts lead to the conclusion that continued burning of fossil fuels will warm the climate,” Denning wrote. “You don’t have to believe experts, and there doesn’t have to be an overwhelming consensus behind them. It just makes sense for the same reason that a teapot placed on a hot stove will get hot.”

Denning’s Views on Pitfalls to Avoid

An example of “what doesn’t work” in speaking with audiences such as those at the Heartland conference, Denning wrote, “is the condescending argument from authority that presumes that the Earth’s climate is too complicated for ordinary people to understand, so that they have to trust the opinions of experts.”

“Appeals to ‘overwhelming scientific consensus’ are more likely to confirm the audience’s suspicions of some kind of nefarious conspiracy than to change minds,” Denning wrote, and “even the concept of peer review can sound sinister.”

Denning concluded by acknowledging “a huge gap” on what he called “the gravity of the climate problem.”

“It will take the committed and respectful participation of mainstream climate scientists in settings that may lie outside their comfort zones,” and he said he hopes more “like-minded colleagues” will take that plunge.

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32 Responses to Atmospheric Scientist Scott Denning Shares Lessons from Dialog with ‘Skeptics’

  1. Ric Werme says:

    I attended the Heartland Conference, and especially appreciated its theme or respect for the scientific method. While I’m a software engineer, not a scientist, I started out as chemistry major but quickly discovered I was born to program computers.

    Dr. Demming approached the conference with exactly the right frame of mind. He had something he wanted to say, he said it, and he defended it well. That’s how science is supposed to work while we struggle to find information in the very noisy data stream we call weather.

    My personal disagreement with what Dr Demming presented was his concentration on the effects of CO2. While I certainly understand (somewhat!) the the CO2 absorption curve, I suspect that any increased heating on the ground will increase convection and provides another channel for heat transport beyond long wave radiation.

    Who’s right? We’re probably both wrong, or at least don’t know about some key effect, but people will figure it out soon enough. I’d rather work with him than against him. We can make a lot more progress that way.

    • Dear Ric Werme, thanks for your comments.

      You wrote: “I suspect that any increased heating on the ground will increase convection and provides another channel for heat transport beyond long wave radiation.”

      This is certainly true. However, convection or evaporation doesn’t actually remove the extra heat from the Earth system. Because the Earth as a whole is surrounded by vacuum, only radiation can accomplish the needed transfer to space.

      The mechanism for doing this, as for any physical object in the universe, is to increase the temperature of the object so that the longwave emission increases (as the 4th power of temperature).

      Best regards,
      Scott Denning

  2. I was at the conference and had a brief discussion with Scott Denning. He is very sincere and open minded. He is apparently an expert on CO2 sources and sinks and I wish I had more opportunity to discuss that with him. I did not find his defense of global warming alarmism particularly convincing.

    • Dear Norman Rogers, thanks for your comments. I don’t wish to promote “alarmism,” but rather common sense. Like the Heartland organizers, I don’t condone political propaganda that masquerades as scientific “debate.”

      Thanks also for your interest in my work on CO2 sources and sinks. You can read more about it on my website ( I don’t think you’ll find any “alarmism” there!

      Best regards,
      Scott Denning

  3. Dear Messrs Werme and Rogers,

    I don’t “defend alarmism.” Rather I assert common sense:

    1) Emissions from developing countries (10x US population) will double or triple CO2 by 2100 (compared to 18% increase since 1975)

    2) CO2 emits heat (measured fact)

    3) Heat warms things up

    4) Earth’s climate is quite sensitive to imposed heating and cooling (see Little Ice Age and Medieval Warm Period, or countless older climate changes)

    5) Changes in surface longwave radiation (not CO2) in this century will be 4 to 8 times greater than Medieval Warm Period or Little Ice Age

    So while I agree that “something might happen to make the extra heat go away,” it seems pretty irrational to bet my life savings on it!

    A truly skeptical person will be suspicious of blind faith in some unknown effect coming along to bail us out. A responsible skeptic will rather take action to protect his or her interests.

    Scott Denning

    • Jasper Gee says:

      Dr Denning, you say “CO2 emits heat”. How does CO2 emit heat?

      • Dear Jasper Gee,

        Everything in the universe emits heat: you, me, the Earth, the Sun, cosmic dust. The hotter things are the more they emit.

        Gas molecules emit or absorb heat when the energy of the vibration or rotation of their bonds changes.

        Scott Denning

    • John Kannarr says:

      I take issue with a statement you made:

      “Beginning from “common ground,” such as recognizing a need for public policy to be based on facts rather than on political agendas;”

      Your assumption is that somehow political principles, aka “agendas,” are different from or opposed to “facts.” Obviously men disagree on political principles, so they can’t all be in agreement with facts. From my viewpoint, though, men operating in freedom has been demonstrated throughout history to be superior to men being directed by some minority, no matter how much that minority believes it has superior knowledge. So I believe that the free market is indeed in accord with the facts of human welfare.

      “A responsible skeptic will rather take action to protect his or her interests.”

      As a responsible skeptic, I think there is much more to fear from technocrats seeking to impose by force their ideas of how others should be required to live than from any possible climate situation, whether human-modified or not. Therefore I will argue against allowing the U.N. or the U.S. to grab more power to control the economy and man’s access to energy. That is where my true interests lie.

      • Dear John Kanaar,

        I agree with you about the virtues of freedom and free markets.

        But facts are facts. The sun really will come up tomorrow, the Earth’s gravitational acceleration really is 9.8 m/s2, and today really is Tuesday on this side of the Dateline. These things are facts no matter what your political beliefs. Physics is not political.

        A responsible skeptic adjusts his or her beliefs according to experience and facts, rather than rewriting facts to suit his or her beliefs. It’s kind of the definition of “skepticism.”

        Scott Denning

  4. RZ says:

    Speaking as a science writer: This report above underlines how academia have lost touch with reality.

    the complete otherworldly view ‘the scientists’ (warmists, as there are many scientists skeptical of the IPCC, but it is bad for your career to admit) have of themselves as they only read the partyline dictated by journals like Nature (and their highly political stance on global warming and environmental issues, with lax peer review to alarmist papers and a Berlin wall to all papers critical of this position) and ignore overwhelming scientific criticism on climate simplism and the fact that the holy grail lies in questions on climate sensitivity, not CO2-basics. How on earth can you call this criticism names like ‘the contrarians’.

    While conferences like Heartland have a clear political agenda (Well, don’t the IPCC and many on the Global warming train also have?) and there is no great unifying skeptic theory, the scientific community should wake up and realize that their outspoken certainty on high climate sensitivity is not supported by data, and that this causes most of the criticism (together with the most outlandish claims made by environmentalists/politicians, that go uncorrected by scientists in media).

    Speaking of dr Demmings presentation: this is an insult, a childlevel presentation on basic effects of CO2 and IPCC-cliché, the above article states that you should treat ‘the contrarians’ as little children: explain us your certainty on high climate sensitivity while data on climate point the other way. Fact is that apart from some physical basics on CO2 facts grow thin on the warmist side

    Climate science only gains trust if it gets rid of it’s amateurism on data availability, corrects wrongdoers in their own camp and openly corrects outlandish alarmist claims in media by colleagues and politicians.

    But this will never happen. It is no surprise outsiders of climate-science with business-background had to do the dirty work as they are used to higher standards on data and have lived in the real world

    • Dear RZ, Thank you for your comments.

      My point in attending the Heartland Conference was that despite the appearance of what you call “camps,” there is only one real world. Let’s both agree that the physics of the climate system does not depend on politics, and see where that takes us.

      One important point on which I agree strongly with the Heartland organizers is that we need public policy based on the facts of the real world rather than hostile “camps” that make up facts to fit a political agenda.

      You wrote “explain us your certainty on high climate sensitivity while data on climate point the other way.” I will assume that by “climate sensitivity” you mean the amount that the Earth’s surface temperature changes per Watt per square meter of heating or cooling by radiation.

      Consider the historical warming that occurred between about 1000 and 1200 AD, often called the “Medieval Warm Period,” or the cooling that occurred during the “Little Ice Age” from about 1400-1700.

      In each case the Earth’s surface temperature apparently changed by about 1 degree Celsius in response to changes in the brightness of the Sun of about 1 Watt per square meter. [Although some scientists are skeptical that these changes were global, as we don't have very god records except from Eurasia]. The Medieval Warm Period was associated with the expansion of European agriculture and trade, and with Viking colonization of Greenland and possibly North America. The Little Ice Age was associated with glacier advances, crop failure and famine in Europe, and the collapse of those Viking colonies.

      We know from these historical events that
      1) Climate can change quite a lot when radiant heat raching the Earth’s surface changes;
      2) Over the past 1000 years or so, surface temperature changed by about 1 degree Celsius per Watt/m2 of heating/cooling; and
      3) That these changes caused substantial impacts on western economies

      Further back, we know for sure that there have been huge changes in climate due to periodic ice ages every 100,000 years or so. The last Ice Age reached a maximum about 18,000 years ago that included mile-thick ice sheets where New York and Chicago are now. The ice sheets left huge deposits of gravel that mark their former positions, so we can easily map where they were.

      From about 18,000 years ago until about 10,000 years ago the climate of the Earth warmed dramatically, causing the great ice sheets to melt. Global temperatures increased by about 5 degrees Celsius, and the oceans rose between 200 and 400 feet (depending on the amount of land subsidence that had occurred previously).

      We also know that about 1/3 of all the CO2 in the atmosphere dissolved into the oceans during the Ice Ages, partly as a result of the colder water and partly due to increased algae growth fertilized by iron in continental dust blown into the oceans.

      Between 18,000 years ago and 10,000 years ago the ice melted, making the Earth’s surface much darker so that it could absorb more sunlight. The CO2 that had dissolved into the oceans in the Ice Age also returned to the atmosphere so that the total amount of energy reaching the surface from above increased by about 7.5 Watts/m2.

      So from the very large climate changes since the height of the last Ice Age we can learn that:

      1) When energy inputs change a lot climate can change a lot (a mile of ice over Chicago, for example)!
      2) The sensitivity of the Earth’s surface temperature to changes in radiant energy from above is about (5 degrees Celsius) / (7.5 Watts/m2) = 1.5 degrees C per W/m2
      3) These changes were absolutely central to the development of human society

      You might ask why the climate was apparently more sensitive (1.5 degrees per W/m2) at the end of the Ice Age than is was (1 degree per W/m2) at the end of the Little Ice Age. I suspect the reason might be that the changes that ended the “Big Ice Age lasted longer (many thousands of years) while the changes that ended the Little Ice Age lasted only a few centuries.

      I hope this explains why scientists expect the Earth’s surface temperature to respond as it does to changes in radiant energy. Furthermore I hope you are not “insulted” and no longer feel that I am treating you as a “little child.”

      Best regards,
      Scott Denning

  5. Dr Denning

    I’m impressed you would go into “the belly of the beast.” I feel you are overly dependent on one tiny aspect of warming, what I consider the coincidence of an increase in CO2 since 1950 being credited with causing warming that began 300 years ago. And that half the time we have had cooling since 1950, and that most of the rural contiguous United States have had little or no warming since 1895.

    I agree with Dr Spencer that the Earth has been releasing more heat energy to outer space as needed, which would explain why natural warming has not caused run-away warming for a million years – rather, we have had eight or nine alternating glacial and interglacial periods during that time, each of which has featured an imperious disregard of CO2. This one seems to be doing the same.

    Again, I appreciate your approach to this issue. It’s one I’ve never experienced at RealClimate. Or in another context, in my daily reading of the San Francisco Chronicle, which always has us being inundated by rising sea levels, even as San Francisco’s own century and a half records show little increase. It seems that if the Chronicle had journalists with a “nose for news,” they would check the publicly available information as I do and raise objections too.

    Don’t invest in any carbon offsets. They work as well to combat natural climate change as Papal indulgences did to gain eternal Heavenly peace.

    • Dear Michael Combs, Thanks very much for your comments. Actually I found that the “belly of the beast” is mostly occupied by people much like myself who were friendly and willing to discuss these things rationally without resorting to calling one another names or trading insults.

      I agree with you that there are many things that cause climate to change, and that trying to explain all climate change as a result of changes in CO2 is not very useful. Please read my response to the previous poster for several examples of historic and prehistoric climate changes that were certainly NOT driven by fossil fuel emissions. You might be surprised to learn that climate scientists have never argued that CO2 is the only way climate can change.

      Also, I agree with you that it is impossible to infer global trends in climate by looking at surface temperatures from just one region, like the rural contiguous United States. This is why we try to use global data instead, and Roy Spencer’s satellite data is as global as we get (though unfortunately covers a very short period of time).

      You wrote:

      “the Earth has been releasing more heat energy to outer space as needed, which would explain why natural warming has not caused run-away warming for a million years”

      The Earth radiates energy to space according to its temperature, as do all objects in the universe. The more heat comes in (from the Sun), the more heat is radiated back out. The mechanism by which the Earth and all other objects accomplish this is that the temperature increases a little so that outgoing emission increases (as the fourth power of temperature).

      Regarding “runaway” warming, the Earth has never experienced runaway greenhouse warming like Venus (thank God!), not just for a million years but for 4.5 BILLION years.

      On the other hand the Earth has definitely experienced MUCH warmer climates in the past, with CO2 at least 10 times higher than today. During most of Earth’s long climate history there was no ice at all on this planet and during the Eocene (50 million years ago) the bottom of the ocean was about 15 degrees Celsius (compared to 3 degrees C today). Using the heat capacity of water and the average depth of the oceans (about 4000 meters) try calculating how many Joules must have been removed from the Earth to cool all that water!

      So we know for sure that Earth’s climate is quite capable of massive climate change over long periods. The idea that climate is somehow “locked” or that a “global thermostat” will prevent climate change in the future seems to contradict the geologic record.

      Tell you what, I won’t invest in carbon credits. But I suggest you refrain from betting your life savings on the idea that climate can’t change.

      Best regards,
      Scott Denning

  6. John Nicol says:

    Dear Dr Denning,

    It is so refrreshing to find someone who studies aspects of climate and on balance at least, takes the view that carbon dioxide is responsible for global warming, but still takes the time to listen to the other person’s view and to try to explain why you favour the science results supported by the IPCC.

    I have spent a great deal of time trying to communicate with similar scientists in Australia, most of whom are in well established climate research units while some are simply off-shoots of Geography Departments, in nearly all of our universities, and with our major research Group at CSIRO. None is at all interested in communicating in the way that you have done so I spend most of my time talking to others who agree with my own point of view which is very boring. I have a friend, a retired fellow physicist who is very strongly of the same view as your own and we have a very interesting and stimulating ongoing debate. This is how science shouldbe working where we can listen to others, present our results, perhaps criticise theirs while they do the same with ours. Without this stimulation science would die. Well done. Please help to encourage your fellows to do as you are doing. I do also try to encourage other “sceptics” to listen to the “other side” and make a proper analysis of what they are telling us, as do all my friends, but sometimes it is hard to get that important message across, because people are so bunkered down in their own trenches.
    Best wishes,
    John Nicol
    60/50 Coriander Place, Forest Lake Qld 4078 Australia
    Ph: 07 3879 8848 Mob: 0409 761 503

    • Dear John Nicol, thanks for your comments. It’s great to hear that people appreciate a frank but respectful discussion.

      You wrote: “I spend most of my time talking to others who agree with my own point of view which is very boring. I have a friend, a retired fellow physicist who is very strongly of the same view as your own”

      Fair enough, but I hope we can both agree that the physics of the Earth does not depend on our “views.” A basic tenet of science is that there is an objective world out there whose behavior is revealed through experiment and falsification of hypotheses.

      Regardless of our respective views on the subject, tripling or quadrupling the CO2 content of the atmosphere will either warm the Earth’s surface or it won’t. One of our “views” is just wrong.

      Best regards,
      Scott Denning

  7. Derek Tipp says:

    Your talk at Heartland which I have watched on line asked the audience for solutions to the problem of dealing with a massive increase in CO2 emissions from the likes of China, India (and Africa too) who you assert, quite correctly, are going to industrialise in this century. My hunch is that they will do so using predominantly fossil fuels as they are now, for the obvious reason that they are by far the cheapest option. Wind is expensive and unreliable, nuclear has its own obvious drawbacks, and carbon capture and storage is also expensive and largely untested. The only circumstances in which the West should agree to reduce their own emissions is by having a world-wide agreement. Unilateral action is no more than a futile gesture at great cost to the people and industry of the West. Whether a world wide agreement is possible or even enforceable I rather doubt. At present the weakness of the West is leading to them being taken for mugs.

    • Dear Derek Tipp, Thanks for this comment.

      With roughly 10 times the US population very poor and on the verge of industrialization, CO2 emissions will very likely quadruple in coming decades if fossil energy is used to grow modern economies in China and India. A very simple mass balance calculation shows that this would lead to CO2 concentrations near 1000 ppm by 2100, which would add more energy to the Earth’s surface than the change at the end of the last Ice Age. The last time that much energy was added to the surface it took thousands of years to happen, warmed the climate by 5 degrees and raised sea levels by hundreds of feet. Let’s hope the climate is much less sensitive to heating than it was 18,000 years ago!

      I agree with you that this catastrophic outcome is pretty much independent of US emissions, since we have a tiny population compared to the rest of the world. Nonetheless we stand to suffer the consequences in economic losses, refugee crises, and who-knows-what kinds of political disruption.

      In my own view, a prudent approach would be to take aggressive steps to apply the power of free markets and the creative energy of our entrepreneurs to develop and bring to market other technologies that can provide for future economic growth without quadrupling CO2.

      Currently, the political right seems content to simply argue that heat doesn’t warm things up, which seems rather counter to the observed world around us. My point at Heartland was that somebody had better propose market-based solutions to these problems, or all the solutions will be the rather ineffective ones coming from the political left.

      Of course I’m no expert on international policy, just on climate science, so take my reply for what it’s worth. Scientists don’t get to decide what to do. Our role is simply to remind everybody that the physical world behaves in certain ways regardless of political ideology!

      Best regards,
      Scott Denning

  8. Charles Higley says:

    I love the idea that you can pretend to argue with climate skeptics by appealing to “the human level.” In other words, do not present the real science (because there is none) that supports global warming by man.

    ““articulating the scientific consensus is essential,”
    Consensus among those paid to have a common opinion is meaningless. If there was a population of unrelated and unconnected scientists who agreed, it might be considered meaningful, but they could ALL still be wrong.

    Talking up daily “what ifs” and trying to tout the “gravity” of climate change will never change the fact that science, real science, will always prove them wrong. AND policy decisions made based on junk science will always fail and more likely will be disastrous failures, hurting multitudes of people in multitudes of ways.

    Science is falsifiable and the AGW claims are not falsifiable and thus not science. AGW is junk science based on two hockeystick graphs for temperature and CO2, which are both bogus, and on climate computer models that are so flawed it’s laughable that we actually have paid these clowns billions for such crappy work and products.

    Recently it was discovered that the models actually start with the planet as a low temperature star, both emitting and absorbing energy. What boggles the imagination is that the idiot programmers, pretending (in their dreams) to be “scientists,” forgot that this star only absorbs energy on one side at any given time and emits energy all over 24/7. This completely screws up everything that follows in the model.

    This is such an egregious oversight that they should ALL BE FIRED and then we recover every dollar they stole in this fraud. Of course, this is ignoring the multitudinous other failings and omissions that abound in the rest of the models.
    In grade school, we all learned of the water cycle and the water vapor convection component—it is essentially missing from the models—responsible for 85% of the energy transfer to altitude. The models only consider radiative processes. No wonder they have trouble locating the missing heat; it has left the planet already.

    • Dear Charles Higley, Thanks for your thoughtful comments. It’s nice of you to say “you love my ideas,” but I think you were being sarcastic!

      The basic science of global warming due to anthropogenic CO2 was discovered and published in the scientific literature more than 100 years before the invention of climate models. It’s not hard to understand and it has nothing to do with hockey sticks.

      Here’s the argument:

      1) Burning stuff made of carbon emits CO2 into the air
      2) CO2 emits heat, in the form of thermal radiation
      3) Heat warms things up
      4) Therefore, burning stuff made of carbon will warm things up

      I understand that you don’t like climate models, but climate models are totally unnecessary to understand the reason that people have been predicting global warming from burning fossil fuels for the past 150 years.

      Best regards,
      Scott Denning

  9. ProgContra says:

    Dear Dr Denning,

    I know this is strictly off topic, but as a sceptic I would be interested to hear your view of the recent work of Dr Murry Salby on the carbon cycle, as high-lighted by Judith Curry, Jo Nova, Bishop Hill etc.


    • Dear ProgContra,

      The Salby talk is a surprisingly naive “rediscovery” of carbon cycle science from the early-mid 1980′s.

      The claim that “natural sources of CO2 are 150 GtC/yr while fossil fuel emissions are only 5 GtC/yr” is very misleading. The total amount of CO2 in the atmosphere is only about 800 GtC. So at 150 GtC/yr, Salby would seem to expect doubled CO2 about every 5 years due to natural sources.

      Obviously the fallacy here is that the 150 GtC coming mostly from death and decomposition is nearly balanced by life (photosynthesis).

      More to the point, fossil fume emissions are known quite well because billions of dollars of shareholder value depend on knowing them precisely. The rate of growth of CO2 in the atmosphere (about 4 GtC/yr) is only about half the rate of fuel combustion (about 9 GtC/yr, not 5).

      So the question is not “where is all the CO2 coming from?” as Salby seems to think, but rather “where is it going?” I’ve spent most of my career working on that and we are getting better answers than before, but it’s silly to pretend we don’t know the sources — perhaps Salby can help find the sinks!

      Finally, what Salby has “discovered” is that the *anomalies* or changes in CO2 growth rate from year to year are modulated by climate fluctuations (especially El Nino). This has been known for 30 years, and there are dozens of papers in the literature that would help him sort it out, some written when Salby was quite young.

      Thanks for asking!
      Scott Denning

  10. John Eggert says:

    Dr. Denning.

    I perform radiant heat calculations in a field other than climate research. In that field, the impact of CO2 diminishes to 0. The “path length” at which this occurs is about 500 The path length at which the impact begins to deviate from the simplified logarithmic model of Ramanathan and Hansen is about 100 It is only in climate models that this saturation is not acknowledged. If you are interested, you can find the method in Bejan, Adrian; Kraus, Allan D. Heat Transfer Handbook. John Wiley & Sons., 2003 Page 618, which I assume Yale will have in the engineering library.

    • John Eggert says:

      Should have said the incremental impact of increasing CO2 diminishes to 0.

      • Dear John Eggert, Thanks for your comments.

        The saturation effect of long wave radiation in an absorbing and emitting medium is in deed well known. I’m not sure why you state that this effect is “not acknowledged” in climate models. That’s just not true.

        Infrared gas spectroscopy is very much alive in the 21st century, and in particular the spectroscopy of CO2 in the atmosphere is making huge gains due to the new CO2 instruments now in orbit and planned for launch over the next few years. The field has come a long way in the past decade, but nobody has yet discovered a sudden loss of emission at hundreds of parts per million.

        This question was brought up by Fred Singer at the Heartland Institute “debate” I conducted last month. My “opponent” in the debate, Roy Spencer (a prominent climate contrarian) kindly corrected Fred by explaining that in fact the CO2 emission lines are the basis for satellite temperature profiling, and are not saturated.

        Best regards,
        Scott Denning

        • John Eggert says:

          Dear Scott Denning:

          Thank you for your response. Yours is the first acknowledgment of this that I have seen from the side of ‘non-deniers’ (for want of a better or less charged term). Indeed, at a blog called scienceofdoom (where there is an excellent description of the science of CO2 absorbance), I was told explicitly that saturation will never occur. I’ve an estimate of atmospheric CO2 path length that puts it today, somewhere between 200 to 300 Full saturation occurs at around 500 So yes, I agree, the atmosphere is not yet saturated. If my method of estimation is correct, that will occur somewhere around 800 ppm The impact from 300 to 400 bar cm is less than the impact from 400 to 800 though because the effect as saturation approaches no longer follows the previous logarithmic model. As saturation approaches, the rate of change of effect of increased CO2 approaches 0.

          • GP Alldredge says:

            John Eggert:

            You are to be commended for trying to relate your knowledgebase on IR radiation to that used in climate science. I too came from an IR work domain removed from climate science, but dealing with IR propagation in the Earth atmosphere (aircraft as targets with IR signatures, IR missiles as threats, and the atmosphere between them, for example). And if my experience is any guide, you must be prepared to expand your knowledgebase somewhat to make a firm connection with climate science.

            I think you got short-shrift from scienceofdoom in part because you used the word “saturation”, where optical scientists might use “extinction” for what you meant. In climate science, what “saturation” is used for is related to an objection (by K. Angstrom) to S. Arrhenius’s greenhouse effect calculation that arose soon after Arrhenius’s paper appeared. It has been soundly refuted many times over since; so sciencofdoom is correct on that point.

            I recommend to you a little review “Infrared radiation and planetary temperature” by R. T. Pierrehumbert that appeared in the January 2011 issue of Physics Today (pp 33-38; freely available pdf at URL:

            This review addresses many issues that may be helpful in extending your knowledgebase further into climate science. (The author also sometimes blogs at under the handle “raypierre”, and he addresses some of these issues–including K. Angstrom’s objection–there; just search for postings by raypierre.) The review shows detailed top-of-atmosphere IR spectra observed by spacecraft for Venus, Earth, and Mars in the range 200 to 1600 (1/cm) [50 to 6.25 um] which includes the CO2 667 (1/cm) [15um] band, with blackbody curves ranging from 285 to 210 K for comparison; the contrasts between the planets are very dramatic and informative.

            If you want to get more deeply involved in IR radiation computations as they arise in climate science, a good start is raypierre’s UChicago website, where he has Python computational machinery, including links to detailed IR spectra databases, developed for use in his course on Principles of Planetary Climate (also the name of his advanced textbook published in 2010 by Cambridge Univ. Press).

            Scott Denning is quite correct in stating that climate modelers do IR propagation correctly. (If you were relying on someone else’s assertion to contrary, then I would advise to be very wary of what they tell you with regard to climate science!) The one thing they cannot do in the clear sky parts of their models is do the line-by-line integrations, because those are too compute-intensitive for climate model runs; they must rely on doing the line-by-line integrations offline, and then building look-up tables or parametric interpolating functions to represent the line-by-line effects during the model runs. But they are very careful about this. And then, there is “how to handle cloud effects…”. (Another story for another time, but they are very careful about this too.)

            A few points might help you get started in wrapping your mind around IR propagation as it appears in climate science:
            –Consider what happens to the IR energy that gets absorbed in what you call a “saturation path”? (It goes into warming the atmosphere that absorbs it, and the CO2 in that part of the atmosphere will in turn re-radiate in all directions at its local temperature.)
            –The three planets mentioned have lapse rates where the atmosphere temperature drops as the pressure decreases with altitude. So the higher in the atmosphere that CO2 molecules re-radiate, the lower their intensity in their radiation bands.
            –Eventually, the atmosphere gets optically thin enough in CO2 where more and more radiation escapes to space without further absorption.
            –All these points are very likely somewhat different than what you encountered in the “saturation path” context brought up in your original comment.
            –Then, raypierre’s review should take you far down the road.

            I hope this helps.

            –GP Alldredge

  11. grypo says:

    Professor Denning,

    In your “Bad & Ineffective Solutions” slide you listed “State & National level penalties” but you said ‘for things’. I’m curious what you meant by that. Did you mean taxes, ie revenue neutral Pigvogian taxes? Or penalties for exceeding ‘caps’ perhaps? Thanks for a great talk, this could be an effective way to get the usually evasive right-wing to the bargaining table. I have a draft up for a post on this presentation at scheduled for later this week.

  12. renewable guy says:

    Dr. Denning,

    I saw a preview that you were talking to the heartland institute in DC. I wondered how that would go. Seeing your talk at SKS I was impressed with your frankness and direct communication. Good job.

  13. Tom S. says:

    This is one of the more thoughtful exchanges I’ve seen. I’m impressed that people on both sides participated.

    Kudos to the open-minded skeptics who took the time to write, to Scott Denning for taking the time to respond, and the Yale Forum for hosting it.

    True skepticism requires staying open to considering new information, and subjecting one’s own beliefs — as well as arguments from all sides — to an equal level of questioning and challenge. It takes a lot more mental effort than mindlessly repeating familiar slogans.

  14. RW says:


    I’m a little late on this, but I recently watched the debate. You say (and Roy agrees) that doubling CO2 will add about 4 W/m^2 to the Earth’s surface.

    Do you agree that the 4 W/m^2 is the reduction in the direct surface to space transmittance or the incremental atmospheric absorption? If yes, then on what physical basis are you assuming that all of this will be incident on the surface or be the same as post albedo solar power all coming down in at the TOA? I presume you are aware that not all the power absorbed by the atmosphere returns to the surface? That a portion of it (about half) escapes to space as part of the flux leaving at the TOA?

    • RW says:


      Let me elaborate in more detail what I’m talking about. In general, the 4 W/m^2 of ‘forcing’ from 2xCO2 is supposed to be equal to post albedo solar power entering at the TOA (i.e. the same as an increase from about 240 W/m^2 to 244 W/m^2), correct?

      This is the origin of the so-called ‘zero feedback’ warming from 2xCO2 of about 1.2C (390/240 = 1.625; 4 W/m^2 x 1.625 = 6.5 W/m^2 = 1.2C from S-B), correct?

      Using some rough numbers, the surface emits a radiative flux of about 390 W/m^2, and about 90 W/m^2 of this passes straight into space and about 300 W/m^2 is absorbed by the atmosphere. Of the 300 W/m^2 absorbed by the atmosphere, about 150 W/m^2 is ultimately emitted to to space as part of the flux leaving at TOA (150 + 90 = 240 W/m^2) and 150 W/m^2 is ultimately returned or re-circulated back to the surface (240 + 150 = 390 W/m^2). Or about half of what’s absorbed by the atmosphere from the surface LW flux escapes to space without being incident on the surface.

      If CO2 doubles, the 90 W/m^2 direct surface transmittance reduces to 86 W/m^2 and the atmosphere absorbs an additional 4 W/m^2 (an increase from about 300 W/m^2 to 304 W/m^2). How can the fraction (half) of the 4 W/m^2 of incremental absorption that escapes to spaced without ever being incident on the surface be considered the same as post albedo solar power all coming down in at the TOA?

      • RW says:


        In other words, what I’m saying the net surface forcing from the 4 W/m^2 of additional absorption from 2xCO2 is only equal to about half or about 2 W/m^2, and the so-called ‘zero-feedback’ warming is only about 0.6C instead of 1.2C.