CO2 is Just a Trace Gas

One of the talking points favored by deniers like Joe Bastardi is “CO2 is just a trace gas.”  It makes up less than less than four 100ths of 1% of the atmosphere — what harm could it really do?

Skeptical Science has the short and long response of “What the science says” (along with the video above).  Here’s the short answer.

Saying that CO2 is “only a trace gas” is like saying that arsenic is “only” a trace water contaminant.  Small amounts of very active substances can cause large effects.

Here’s the long answer:

CO2 is just a trace gas

by Sarah

CO2 makes up 390 ppm (0.039%)* of the atmosphere, how can such a small amount be important? Saying that CO2 is “only a trace gas” is like saying that arsenic is “only” a trace water contaminant. Small amounts of very active substances can cause large effects.

Some Examples of Important Small Amounts:

  • He wasn’t driving drunk, he just had a trace of blood alcohol; 800 ppm (0.08%) is the limit in all 50 US states, and limits are lower in most other countries).
  • Ireland isn’t important; it’s only 660 ppm (0.066%) of the world population.
  • That ibuprofen pill can’t do you any good; it’s only 3 ppm of your body weight (200 mg in 60 kg person).
  • The Earth is insignificant, it’s only 3 ppm of the mass of the solar system.
  • Your children can drink that water, it only contains a trace of arsenic (0.01 ppm is the WHO and US EPA limit).
  • Ozone is onlytrace gas: 0.1 ppm is the exposure limit established by the US National Institute for Occupational Safety and Health. The World Health Organization (WHO) recommends an ozone limit of 0.051 ppm.
  • A few parts per million of ink can turn a bucket of water blue. The color is caused by the absorption of the yellow/red colors from sunlight, leaving the blue. Twice as much ink causes a much stronger color, even though the total amount is still only a trace relative to water.

“Traces” of CO2

Although percentage is a convenient way to talk about the amount of gas in the atmosphere, it only tells how much is there relative to everything else; percentage doesn’t give an absolute amount.

For example, you have trouble breathing on top of Mount Everest even though the atmosphere still contains 21% oxygen just like at sea level. The percentage isn’t important, you need a certain number of oxygen molecules with each breath, regardless of how much or little they are diluted by inert gases. At an altitude of 8000 m the whole atmosphere is diluted.

The total number of CO2 molecules above our heads in the atmosphere is more important than their percentage in the atmosphere. If the amount of inert nitrogen gas (N2) in the atmosphere were to be cut in half then the percentage of CO2 would jump (to about 600 ppm; 0.06%) without a change in the absolute amount of CO2 and no substantial change in the energy balance of the Earth. Adding a huge number of energy-absorbing CO2 molecules to the atmosphere doesn’t change its percent number very much, only because it’s being added to a vast inert N2 background.


We know the amount of CO2 in the atmosphere has increased because we have measured it. We know the climate has warmed from current and historical data. The link between increasing greenhouse gases and increasing temperature is clear: just as ink makes water more colored, CO2 makes the atmosphere more absorbing. The extra CO2 in our atmosphere is trapping energy that would otherwise escape to space. The measured global warming matches closely with the amount of energy trapped from the greenhouse gases added to the atmosphere.

A doubling of the trace molecule CO2 from 280 ppm to 560 ppm is still a trace, but just like with arsenic, the difference between a small trace and a larger trace is fatal.

* To convert ppm to percentage divide by 10,000.

— Sarah, for Skeptical Science

28 Responses to CO2 is Just a Trace Gas

  1. My favorite answer when someone parrots the trace gas BS (bad science): Take 200µg of lysergic acid diethylamide in 200g of non-chlorinated tap water. That’s 1ppm of LSD. You sure would swallow that. Report back what you’ve seen.

  2. catman306 says:

    Thanks, this is great and to the point.

    Here’s another:
    Just a trace of water in the gasoline can bring down a light plane and kill the occupants.

  3. Leif says:

    A little off topic but important. What happens when capitalism works for social well being instead of making the rich richer?

  4. Nicolas Müller says:

    Fluoride in your toothpaste isn´t important – it´s just around 1000 ppm of the content

    My favourite: the mercury in CFL cannot be harmful. If you swallow the whole CFL, the 0.004 g mercury compared to your 80,000 g body are just 0.05 PPM of your body mass!!!

  5. BBHY says:

    The important part is that CO2 is a different substance with different physical properties than the other gases, mostly nitrogen and oxygen, in the atmosphere.

    Take a ten foot long glass rod and shine a light through it and you can easily see the light at the other end. Glass is transparent.

    Now place a sheet of aluminum foil over the end of the glass rod. No light goes through, even though the aluminum is only 0.003% of the glass. The aluminum is opaque while the glass is transparent and that makes all the difference, not the length of the glass or the thickness of the aluminum.

  6. Mike says:

    The same nuts who say CO2 is a trace gas and so cannot have a large effect on climate will say, in the next sentence, that life on Earth could not exist without it. You cannot get dumber than that. I am sure some can dig up some quotes.

  7. cervantes says:

    Bastardi must know perfectly well that his act is bogus. What’s his exit strategy, I wonder?

  8. MA Rodger says:

    (Hope nobody minds me cuting & pasting my SkS comment in here. I thought it worked quite well.)

    At 390 parts per million by volume, there is still a lot more CO2 floating about, more than there is some other significant stuff. There is about 3,000 billion tons of CO2 in the atmosphere. We complaint of this poor old planet being over-crowded by today’s human population yet the total mass of all us humans is way less than a single billion tons. It’s probably still less than 1 billion tons if you count everybody who ever lived since we first evolved as homo sapiens 200,000 years ago.

    Still, that billion ton’s worth of humans has proved enough to convert a third of the planet’s land area (indeed the majority of the fertile bits)into monocultures of pasture & arable crop. And just for good measure we’ve kicked off the sixth mass extinction event in the planet’s entire 4.6 billion year history. So no one should be dismissing us as some insignificant trace substance!

  9. Gary says:

    I think the “sometimes a trace is important” argument is a specious one. A better analogy than 1 part per million LSD in a quart of water having a big impact would be to say what is the impact of that much LSD on the time it takes to bring that water to a boil. We are talking about physical properties like energy transfer and absorption, not chemical or biological activity.

    I am curious if anyone here could explain the physics of how such a small component of the atmosphere could have that big an impact on the atmospheric temperature, without resorting to an analogy.

  10. mark says:

    Let’s not forget homeopathic medicines–they have strong effects, even when so dilute they contain no molecules of the active ingredient! (Somehow I suspect some people will believe this while simultaneously believing the CO2 is harmless because it’s just a trace argument.)

  11. Tom Gray says:

    Sorry, I don’t understand why the analogy is specious. Sometimes a trace amount IS important, witness the several examples. This is one of those times. Here’s more on the physics: . Also, the comment above about coating the end of a glass rod with aluminum foil is on point–it’s the property of the “trace amount” that is important, not the … amount.

  12. Edith Wiethorn says:

    @Gary Good point & good question. I do think the ppm analogy is a good one for starting thinking about dynamic, sensitive systems. And I think it is critical to continue clarifying how climate science is explained. I think the target audience should be as Einstein described: “If you really know what you’re doing you can explain it to an intelligent ten-year-old. This isn’t dumbing down. ITOs are hardwired to want to know how the world works. Before we drop the analogy of ppms I want to add one I noticed but failed to capture the miniscule % – it’s a perfect example for denier pundits: Viagra.

  13. Anne van der Bom says:


    You don’t understand. The ‘CO2 is a trace gas’ is presented as EVIDENCE that stands on itself. Just one example of another trace substance with significant effect, albeit biological, is enough to throw that evidence out.

    The deniers using that argument will have to prove with hard physical evidence why 390 ppm of CO2 in the atmosphere can not have any effect. They never presented such evidence.

    I can however take up your challenge and point to the ozone layer protecting our planet from UV radiation. Stratospheric ozone is less than 0.07 ppm of our atmosphere, yet its UV blocking properties are denied by none.

  14. Tom Gray says:

    Ha, I like it! Agree, though, need to keep simplifying to degree that can be done without introducing inaccuracy. Still more important: engaging (not necessarily “winning”) the conversation with deniers. I think, BTW, the video is an excellent demo, very simple, very easy to understand.

  15. Mike says:

    You are assuming there is some sort of sharp line that divides physical processes from chemical and biological ones. There is not. But I am glad you made your comment because it may show those of us interested in science education and communication a type of misconception we weren’t aware of.

    As for your question, trapping a small amount of heat year after year leads to significant temperature change over time. You can find more here:

  16. Leif says:

    I would think that another experiment that might be worth trying.. Take two clear plastic bottles and put some CO2, mix vinegar and baking soda, and pour the gas in one. Leave the tops off, CO2 will stay put. Place in the sun and measure the temperature of each. No attempt need be made as to the exact proportions as we are just looking at the big picture here.

  17. Pangolin says:

    What? In english? That might take a few days; ok a week. Possibly two weeks.

  18. Dan Miller says:

    While I agree with other replies to your question, I’ll try to give you another answer that explains how CO2 can have a large effect while being such a small percentage of the atmosphere. Think about the light passing through the water beakers in my YouTube video above. Each photon passes though billions of transparent H2O molecules, but along the way there is a chance that a photon will hit an ink molecule. When it does, the ink molecule stops the photon dead in its tracks. That is why ink turns the water dark at such incredibly small concentrations. In the YouTube video, I diluted the ink from the bottle by a factor of 10:1 before doing the demonstration, so the first dilution was actually 28 ppm, not 280 ppm. If I had used 280 ppm ink, the water would have been all black and you would not have been able to see the impact of increasing concentrations! Of course, the ink manufacturer diluted the ink molecules (carbon!) in a liquid to make the liquid ink, so the actual concentration may have been 2.8 ppm!

    CO2 is very effective at stopping infrared radiation. When an infrared photon is emitted from the earth and travels up to space, it passes by billions++ of transparent nitrogen and oxygen molecules, but 0.039% (390 ppm) of the time, the photon hits a CO2 molecule, which does a good job of stopping it and then re-emitting it in some other direction.

  19. Dan Miller says:

    Bill Nye the Science Guy did exactly that and it worked! For another cool demonstration directly showing CO2 blocking infrared radiation, see:

  20. Neal J. King says:

    “I am curious if anyone here could explain the physics of how such a small component of the atmosphere could have that big an impact on the atmospheric temperature, without resorting to an analogy.”

    Gary, an important point to understand is that the proportion of CO2 in the atmosphere is TOTALLY IRRELEVANT. What matters is the absolute numbers of CO2 molecules, not what % they are of the total gas; because those other non-CO2 molecules are not “blocking” the same infrared photons that the CO2 is “blocking”.

    If you were to add 100 times as much N2 or O2 as exists already in the atmosphere, the % of CO2 would drop by another factor of 100. But it wouldn’t change the impact of CO2 on infrared photons, because those N2 and O2 molecules are not competing with the CO2 molecules, they don’t have any impact on the infrared “blocking”.

    Another analogy: If you have a flock of ducks in a hunting park, the number of ducks that get shot depends strongly on the number of hunters in the park. It doesn’t make any difference how many non-hunters there are. So the % of hunters among the humans in the park is totally irrelevant: It could be 1 in a million, it could be 95%. What matters is how many hunters there are: 1, 5, 100 …

    What we know from the basic atmospheric radiation studies is that there is plenty of CO2 to affect the infrared photons, and that each time you double the amount, you increase the “blocking” effect by 3.7 Watts/m^2. Over the last 150 years, we’ve increased it by 35% or so; if we do it again, we’ll be just about at that doubling point.

  21. Robert In New Orleans says:


    Just a little off topic, but do you for see doing a sequel or revision to your web video “A Really Inconvenient Truth” in light of all the new information from climate science and how things are changing faster than what the models suggested?

  22. Dan Miller says:

    Yes. The new version of my talk is called “Boom or Bust?” and I talk about how bad things are getting as well as the enormous economic opportunity of addressing climate change (I’m a clean tech VC). I’ll be giving the talk at the Commonwealth Club in San Francisco on November 18.

    I’ve been telling friends recently that even though I give talks about how things will be much worse, much sooner than most people think, even I am a little taken aback by how fast things are happening.

    To understand a bit how society can ignore such a clear and present danger, see George Marshall’s talk at I’ve been using some of George’s slides in my talk for several years. In some ways, the psychological issues are more important than the science of climate change. We understand the problems and the solutions, but we still fail to act.

  23. prokaryotes says:

    Really, great educational video, thanks. Posted this on my blog too.

  24. otter17 says:

    “One of the talking points favored by deniers like Joe Bastardi is “CO2 is just a trace gas.” It makes up less than less than four 100ths of 1% of the atmosphere — what harm could it really do?”

    Joe Bastardi says that? That is so bush league.

  25. Leland Palmer says:

    Well, methane is worse.

    We’re running about 1.8 ppm of methane, and it’s responsible for something like a quarter as much warming as CO2.

    When it suits them, Deniers use the “greenhouse gas absorption of infrared radiation is logarithmic to concentration” talking point- ignoring that this means the first little bit is the most potent.

  26. Leland Palmer says:

    The long term problem is that we appear to be setting up conditions in which CO2 is not the only problem- and perhaps not even the main problem.

    CO2 absorption of infrared will tail off- not the way that Denier websites say it will- but the way the IPCC report says it will. Deniers often use a specific logarithmic curve- I kid you not- which was put together by an amateur scientist and construction manager for a hotel in Fiji.

    But by the time CO2 absorption starts truly saturating, we will likely be dealing with three other huge problems- the methane from hydrates problem, the indirect atmospheric chemistry effects of methane problem, and the water vapor feedback to forcing from any source. The sum of all four including CO2 could be truly devastating.

    Isaksen- Strong Atmospheric Chemistry Feedback to Arctic Methane Emissions

    The water vapor feedback to forcing from all sources could end up being the biggest greenhouse problem of all.

    [JR: The water vapor feedback has always been well known and large — BUT CO2 remains the biggest issue by far and will for a long, long time.]

  27. Leland Palmer says:

    Hi Joe-

    I really hope you are right. CO2 is certainly what seems to be driving all the rest, and appears to be the key to avoiding most or all of the other forcings and feedbacks.

    With any luck, we’ll never find out.

  28. Gnobuddy says:

    Another small ppm number to think about – the murder rate for large cities in the USA is of the order of magnitude of one murder per 10,000 citizens. If we assume each murder was committed by one person, who never murders a second time, that would mean there is only one murderer in every 10,000 people.

    In other words, *all* murders are committed by a mere 100 parts per million of murderers in the population!

    Note that in reality many murderers kill more than once, meaning there are even fewer of them – less than 100 ppm in the USA. Also murder rates are substantially lower in some other countries, sometimes as much as ten times lower. In Germany, following the same reasoning as above, only 10 ppm of murderers are responsible for committing all murders in the country!

    The analogy is fairly direct, if you think about it – one atom of C02 can “murder” a photon (absorb it) even though that photon has made it past 100,000 nitrogen or oxygen atoms with no “harm”. Think of the C02 atoms in the air as murderers in the general population of humans, and you get a graphic example of small numbers having great significance.