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CO2 is just a trace gas

Posted on 30 August 2011 by Sarah

ink in water

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.

arsenic-mug

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.

Photo credit: http://www.photographyblogger.net/15-cool-pictures-of-ink-in-water/

Note: This post is the Basic rebuttal to "CO2 is just a trace gas"

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Comments 101 to 104 out of 104:

  1. Eric @ 100, Well I'll have to say I'm sorry for my terse reply to your comment. Sorry. Based on intuition (I'm a tech, not a scientist ;) I would guess that effectively zero of the principle CO2 IR band photons emitted at ground level make it to space directly. I had to look it up, but see the figure here, from wiki: http://en.wikipedia.org/wiki/File:Atmospheric_Transmission.png The grey area along the CO2 spectrum shows that it maxes out at several absorption bands. The following description of the detectors used for water vapor images on GOES satellites says they are tweaked for sensitivity at a water absorption band or channel, (6.5-7.4 micron), which makes sense if you're interested in water vapor. http://cimss.ssec.wisc.edu/goes/misc/wv/wv_intro.html If you check that band against the figure at my first link, you'll see that 6.5-7.4 microns (on the logarithmic x axis) is where water absorbs strongly, and not much else does. (CO2's strongest absorption band is more in the 13-17 micron range.) So the black on the water vapor image, aside from being an assigned translation of the IR signal to a visible representation, and not an innate property, indicates lack of water vapor, not lack of absorbance/emission by CO2. Thanks your considered reply to my previous comments. Cheers.
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  2. There is one problem with this article. It conveniently assumes that the "trace" amounts of CO2 equates to the "trace" amounts of the examples that lie right on the boarder of what is acceptable. For example, arsenic (0.01 ppm is the WHO and US EPA limit) would cause health concerns if doubled via the definition of the EPAs goal of setting limits. However, if arsenic in the water is actually at 0.001 ppm, then doubling would be no problem, in fact you could increase it 10x and still drink the water without a second thought. It is a misleading to use these examples with assumptions that the equivalent "trace" amounts just barely meet the definition of what is acceptable; arsenic at 0.01000001 is no longer a acceptable amount, again by the definition of the EPAs goal of setting limits. With this said, I agree that using the "trace" amount argument without considering anything else is extreemly weak, as it does not include feedback mechanisms. However, to disgard the "trace" amount argument without the hard proof that those feedback mechanisms are as strong as most climate models claim is equally weak.
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  3. Jeff313, The article does not use the words "equate" or "equivalent" once. Instead it points to the similarity of the (false) "trace-only"-argument in giving examples most people can quickly relate to.
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  4. The article simply debunked the idea that "trace" means it cant possibly be a problem. The direct radiative effects of CO2 as well as the feedbacks are dealt with in other articles. If you have problems with this, then please see the article Sensitivity is low
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