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How Bad is CO2?

01 Mar 2020 | Backgrounders

Sensitivity Backgrounder


John Robson

With so many politicians and activists talking about a “climate emergency” you might think scientists have recently discovered something new and ominous about the effects of greenhouse gases. In fact, they’ve done exactly the opposite.

Don’t believe me? Stick around. I’m John Robson, and this is a Climate Discussion Nexus Backgrounder on the Climate Sensitivity Question.


The whole discussion about climate change ultimately comes down to one question: How sensitive is the climate system to increased greenhouse gas levels? How much does adding CO2 to the atmosphere cause temperature to rise, weather to change, or anything else? If climate is very sensitive to CO2, everything we do matters. If it’s not, well, there’s no cause to panic.

What scientists have been trying to estimate is called “Equilibrium Climate Sensitivity,” or ECS. It means the average warming that will happen around the world after doubling the amount of CO2 in the air, once of all Earth’s complex climate systems have fully responded.

John Robson

In our video on The Simple Physics Slogan, we learned why it’s so difficult to make such estimates. In climate models, as in reality, CO2 doesn’t cause much warming directly. Most of the supposed effect comes through feedbacks from whatever warming CO2 does cause, feedback involving clouds, water vapor, wind patterns, ocean currents and so on, all of which are highly uncertain, hard to predict and feed back on one another. So what have scientists found or, at least, tried to find?


Back in the 1890s, Swedish chemist Svante Arrhenius made the first attempts to estimate the Earth’s ECS. He figured it was around 5 degrees Celsius, plus or minus one degree.

It’s a very high number. But the topic attracted relatively little attention for the next 80 years, until it became an issue that fossil fuel use was pushing the atmospheric CO2 content upwards.

Then in 1979 the US National Academy of Sciences published a report written by MIT’s Jule Charney and a group of coauthors. Based on early computer simulations, they suggested ECS was likely around 3 degrees C, plus or minus 1.5 degrees.

John Robson

Given the range of uncertainty, it could be as low as 1.5, which isn’t very serious, or as high as 4.5 degrees, which is serious. And actually, a follow-up study by the National Academy in 1983 leaned toward the low end:


“The climate record of the past hundred years and our estimates of CO2 changes over that period suggest that values in the lower half of this range are more probable.”

John Robson

That 1979 finding, the so-called “Charney range” of 1.5 to 4.5 degrees Celsius, has bounded ECS estimates ever since. It was reaffirmed in the 1983 report of the US National Academy, and then repeatedly reaffirmed over the next 30 years by the UN’s Intergovernmental Panel on Climate Change, or IPCC.

The suggestion that it’s near the low end hasn’t exactly been the main focus, but it’s right there in the report.

So what mainstream climate science actually says is that doubling atmospheric CO2 might well cause a temperature increase of around one and a half degrees, in which case the effects will be small and we shouldn’t worry much. Of course it might be on the high end, above 4.5 degrees, in which case there will be problems. But interestingly, the models also say that if climate sensitivity is high, the effects will take longer to arrive. The higher ECS turns out to be, the slower the warming.

So what it all boils down to is that, according to the conventional wisdom, at least in its fine print, the bigger the problem, the more time we have to adjust. Which in turn means the idea of a climate “emergency,” in which CO2 causes dramatic changes to happen fast, is not mainstream science at all. It’s unscientific alarmist fearmongering.

And that’s not all. As it turns out, science hasn’t told us sensitivity is high, nor have semi-scientific semi-political bodies like the IPCC.


Over the years, the IPCC accepted the broad Charney range for ECS. But they didn’t necessarily accept Charney and his colleague’s 3-degree best estimate.

In 1990 they proposed a slightly lower best estimate of 2.5 degrees. In their 2007 report they raised it to 3 degrees. But in 2013 they dropped the best estimate altogether, saying they could no longer agree on what it was.

John Robson

So the end result of hundreds of scientists developing dozens of large-scale climate models at a cost of tens of billions of dollars over 35 years, beginning in 1979, was an admission that they were less certain at the end than they were when they started. Which is not itself a bad thing. If scientist are getting unclear results they deserve credit for saying so instead of feigning omniscience.

What is a bad thing is that while scientific uncertainty was actually increasing, political rhetoric was going in the opposite direction. By 2013 governments everywhere were feigning omniscience, and displaying genuine bad manners, telling us the science was settled and heaping abuse on anyone who dared suggest that climate change isn’t an evidence-based fast-motion catastrophe. But where is the evidence?


It’s important to remember that these estimates of climate sensitivity didn’t come from observing the actual climate. They came from examining how climate models behave. But the whole point of the vaunted modern scientific method is that you test theories against evidence. So in 2002, a group of scientists proposed a way of doing so, of estimating ECS based on real-world data, not free-floating computer simulations.

These researchers’ way of using observations of temperatures, CO2 levels and other climate variables in a statistical model to estimate ECS directly, has come to be called the Energy Balance method. It isn’t completely independent of climate models, but it has the advantage of requiring the resulting ECS estimate to be consistent with historical data.

John Robson

Initially the Energy Balance estimates were very high—over 6 degrees C, with an uncertainty range going to infinity. Which if true would certainly indicate a looming climate crisis. But the early authors cautioned that they didn’t have all the data they needed, especially for the world’s oceans, and they therefore used creative extrapolation to fill in the gaps. It took another 10 years for the necessary data sets to be assembled.


Once they were, a series of studies began to appear, starting in 2012, from numerous independent teams estimating ECS using the Energy Balance approach and similar methods with more complete data. And what they showed is remarkable.

John Robson

And now a word from our sponsor. And that’s you, because the Climate Discussion Nexus is supported by ordinary Canadians who want to see more common sense, more logic, and more facts in the discussion about climate change… and less yelling. If you want to help us, subscribe to our YouTube channel, go to our Patreon page, make a pledge, become a monthly sponsor. Now back to our regularly scheduled programming.


Here is the Charney best estimate and uncertainty range for ECS that models have stayed within since the 1970s. And here are a dozen [1,2,3,4,5,6,7,8,9,10,11,12] data-driven ECS best estimates published from 2012 to 2018. They are all clustered down at the low end of the range.

John Robson

The clear implication from these newer data-driven studies is that climate sensitivity in the real world is only about half the average level in climate models. And while the Energy Balance model is still a work in progress, with each new study the authors have taken account of various methodological criticisms and shown that revising the approach doesn’t really change the pattern.


This 2018 paper by Nicholas Lewis and Judith Curry, published in the American Meteorological Society’s Journal of Climate, is particularly important because it applies the Energy Balance method to the IPCC’s own data sets while taking account of all known criticisms of that method. And it yields one of the lowest ECS estimates to date: 1.5 degrees, right at the bottom of the Charney range.

John Robson

Looks like the National Academy’s conjecture in 1983 was right: ECS values in the bottom half of the range are more probable.


Another important line of evidence for low sensitivity has emerged from the study of the atmosphere over the tropics. Climate models predict that this is the region where we should see the strongest effects of CO2, yet there’s been very little warming there since records began in the late 1950s. A study published in 2017 examined 40 years of data from weather satellites and once again concluded that historical warming is consistent with a climate sensitivity only about half as big as the climate model average.

John Robson

Back in 2007 the IPCC claimed, based on their models, that ECS was “very unlikely” to be less than 1.5 degrees. But ten years later, using the IPCC’s own data, Lewis and Curry found that there’s a fifty percent probability that ECS is below 1.5 degrees. They concluded: “High estimates of ECS… derived from a majority of… climate models are inconsistent with observed warming during the historical period.”

So the next time anyone asks you whether you “believe the science”, tell them the real question is whether you believe the models or the evidence. You can believe one or the other but not both because they contradict each other. Or at least, most models contradict the evidence. And the ones that don’t are also the ones that show very little warming from CO2.

And alarmist claims about a “climate emergency” aren’t based on data or science. At best they’re based on models known to be inconsistent with reality and at worst they’re made up out of thin, hot air.

Join the conversation online at climatediscussionnexus.com. Subscribe to our YouTube channel, follow us on Twitter and like us on Facebook and, if you want to see more content like this video, visit our Patreon page and become a supporter.

For the Climate Discussion Nexus, I’m John Robson.

11 comments on “How Bad is CO2?”

  1. This is another clear, concise explanation delivered flawlessly! How could anyone watching this not get it?
    In Ontario, with pressure from alarmists like Al Gore, David Suzuki, Gerald Butts and the German politician Herman Scheer, the Liberal government signed 20 year contracts for industrial scale wind turbines. The ratepayers of Ontario were convinced that rural residents, who began reporting harm from audible noise, LFN/acoustic pulsations/infrasound trespassing onto their properties and into their homes, ought to be ignored by the government agents and the wind companies because rural residents were just 'collateral damage' to the higher cause of 'saving the planet', considering the emergency of man-made global warming.
    Given the information we now have, thanks to the dedication of clear headed, brilliant people in the scientific community, is it fair that rural peoples' homes will remain unsafe for the duration of these contracts? Or has this incursion of industrial wind turbines in rural communities been about money, greed and an agenda to make rural communities uninhabitable?

  2. So it seems that the many scepics that have been saying "There is no emergency" may be correct and that Greta Thunberg must stop telling our children that "We are on fire!".

  3. CO2 is central to the climate emergency hypothesis and its insane offshoot, the war on carbon.
    Dr. Tim Ball in his book "Human Caused Global Warming : the biggest deception in history" states that CO2 is only 3% of total Greenhouse gases, water vapor represents 95%.

  4. Dear Dr. Robson,
    You keep afloat two ideas in your weekly reports, they are that the climate is changing and that temperature has increased since the mid-19th century. The climate is not changing, full stop. And it is a moot point whether we can have any certainty that the mean global temperature has increased by the alleged 1 C since the mid-19th century, let alone that this is due due increased CO2 emissions. Finally, as I told you before in a private correspondence stop using the words 'climate change' you're not helping quell the now well accepted, but false, claims of the alarmists. Every time the term 'climate change' is used it tells people that there is indeed a problem. The climate is not changing, all we experience is weather variability and nothing is going to happen to the planet due to the weather. ...

  5. "How Bad is CO2" John Robson? CO2 is one of two basic ingredient molecules to life on earth. ALL LIFE DIES WITHOUT CO2! That is two hundred year old science. And CO2 driven climate science desperately avoids an awareness of CO2's bountiful contribution to the environment. CO2's recent increase is greening the planet, shrinking deserts, and bringing a string of world record crop yields. Why? Because the environment is starving for it. Multicellular evolution began 600 million years ago at 7,000ppm. The biosphere has been naturally declining to near LETHAL LOWS ever since. Life begins to die at 100-150ppm. During glacial periods of our ongoing Pleistocene/Holocene Ice Age, CO2 naturally declines to 180ppm. Our fossil fuel recycling program of CO2 back to the environment has been a Godsend.

  6. Actually the climate is changing Dr. Charles Capaday. What you imply correctly is that the climate is not changing outside of our natural normal changes of climate. The climate always changes. And that change is NEVER caused by the life essential trace gas of life CO2. Not today. Not at any time in our current Holocene interglacial last twelve thousand years. Not at any time in the current ongoing three million year Pleistocene/Holocene Ice Age. And not at any time since the beginning of multicellular life 600 million years ago in life bountiful 7000ppm CO2. While CO2 has risen to levels not seen in twelve million years, our temperatures remain right on the healthly natural average of our last twelve thousand year Holocene interglacial - perfectly natural. And far closer to lethal lows than anything remotely harmful. The left is displaying their ignorance of climate by implying it began recently. And that it began with an increase in CO2. Neither is true. The only real effect of our recent increase in CO2 is beneficial and ongoing. Because the environment is starving for CO2. The increase we've already had has greened the planet, shrunk deserts, and brought a string of world record crop yields. A little more CO2 has been a Godsend.

  7. I am not sure why you are arguing with me, or that you are. The way you put it "the climate is not changing outside of our natural normal changes of climate", is basically what I have said. According to the the Köppen climate classification in southern Canada, we live in a Dfb climate = "Warm-summer humid continental climate; coldest month averaging below −0 °C (32 °F) (or −3 °C (27 °F)), all months with average temperatures below 22 °C (71.6 °F), and at least four months averaging above 10 °C (50 °F). No significant precipitation difference between seasons ... ". Our climate has been this way since the European explorers landed here, with yes variations from year to year and decade to decade, but our climate is Dfb and coconut trees are not likely to grow here anytime soon. The alleged 1 C of warming since the mid 19th century, even if true, does not change the fact that our we live in a Dfb climate . As I said, every time the term 'climate change' is used it tells people that there is indeed a problem and consequently ask what is the cause. If the climate is not changing there is no issue to deal with and no need to waste money and resources. Should you be interested in my views on this issue please read this post https://climatediscussionnexus.com/2019/10/16/dont-tell-greta/

  8. I'm not arguing with you at all Dr. Capaday. What I'm hoping to do is make it clear to anyone interested that today's climate is not at all unusual in our current ongoing Holocene interglacial. I'm a farmer who earned an advanced B.Sc. (biology) degree. I also grew up in a beautiful little glacial meltwater valley. I see the evidence of our natural climate every time I walk out of my front door. My belief is that people are quite capable of understanding that the climate always changes and that today's changes are well within the normal and natural ranges. Particularly people who listen to Dr. Robson. I wish fervently to help debunk the false claim of a CO2 driven climate. And I believe that the strongest way to do that today is to understand that in spite of an increase of CO2 to levels not seen in twelve million years, temperatures remain well within Holocene normal and natural levels. That makes the significant rise in CO2 since that mid last century an entirely beneficial and bountiful contribution to the environment.

  9. Dear John,

    At one time you said that CO2 levels are declining. Prof Ian Plimer and others wrote that CO2 increases follow, not precede, warming periods. Now apparently you say that CO 2 increased while global temperature increases between 0% and 1.5% since the mid 1800s.

    Having read considerably into the subject, my inclination is to believe that CO2 increases follow global warming and have no effect whatever on either climate or weather, also that the global temperature - said to have declined slightly since 1995 - is almost not measurable due to the wide variations across the globe.

    My hope is that actual temperature measurements from stable and consistent locations (eg, a remote north-facing mountain valley) will some day prove the fallacy of "global warming" and "climate chenge" caused by human activity, and that this senseless waste of money and natural resources will end.

  10. Thank you Mr. Bateman. I am happy that you understand my point viz. "that every time the term 'climate change' is used it tells people that there is indeed a problem and consequently ask what is the cause. If the climate is not changing there is no issue to deal with and no need to waste money and resources."
    Cheers, Barry.

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