Since there are only 400 ppm in the atmosphere how can a molecule, which doesn't generate heat but absorbs it, possibly have such a great heating effect on the other 999,600 molecules?

During the Jurassic/Mesozoic era the density of CO2 was as high as 7000 ppm and the earth was lush with vegetation and animal life. Aren't we actually in an exceptionally low CO2 density, historically?

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    $\begingroup$ Note that during the jurassic there were also jungles on antarctica and one of the largest deserts in history. The earth does not care what we do, humans who want to keep having farms and the technology and society it supports are who are in danger. $\endgroup$
    – John
    Commented May 4, 2019 at 20:22

2 Answers 2


Yes, carbon dioxide truly is a greenhouse gas.

With regard to the immediate greenhouse effect, 99.96% of the dry atmosphere might as well be vacuum. Nitrogen (N2), oxygen (O2), and argon are extremely weak greenhouse gases at the temperatures present in the atmosphere. The only greenhouse gases in our atmosphere are trace gases. (The most powerful greenhouse gas is water vapor, and that, too is a trace gas in our atmosphere.)

The 99.96% of the atmosphere that doesn't directly participate in the greenhouse effect is however present. At sea level, a molecule of carbon dioxide will travel freely for about a tenth of nanosecond before colliding with another gas molecule. This mean time between collisions increases to a few tens of nanoseconds at the top of the troposphere. A greenhouse gas molecule such as CO2 that has just absorbed a thermal infrared photon will very quickly transfer that absorbed energy to the non-greenhouse gases that comprise the vast majority of the atmosphere.


As long as homo sapiens have existed the rate of carbon dioxide haven't exceed 350 ppm until 1990. All now living species are adapted to this.

There is an enormous amount of energy from the sun that hits earth each second and if the same amount doesn't radiate from earth the temperature will raise (or lower), slow or fast. Carbon dioxide and water vapor are transparent for the high frequent radiation from the sun, but not from the low frequent radiation from the earth. That's why 410 ppm in december 2018 is alarming.

  • $\begingroup$ That's a bit wrong in that the pre-industrial CO2 levels were about 280 ppm; existing species lived through ranges of 180-280 ppm for the past 800,000 years. CO2 levels were at about 310 ppm when David Keeling began his measurements in 1958. $\endgroup$ Commented May 5, 2019 at 20:26
  • $\begingroup$ @jeffronicus: yes, that's right. My formulation may be a little misleading. $\endgroup$
    – Lehs
    Commented May 5, 2019 at 23:49

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