"CO 2 is a trace gas in the atmosphere and insignificant by definition."Of course that's false -- without atmospheric CO2 the Earth's surface would be below the freezing point of water -- but I thought I would compare it to ozone, also a "trace gas."
Atmospheric ozone helps blocks strong ultraviolet light (UV-C) and most of UV-B, which would kill most animals who live on land. (I guess it might depend on how thick their skin is. And turtles? Snails?) Life might never have developed past the stage where the first ocean animal crawled out onto land, whoever's brilliant idea that was.
Wikipedia says atmospheric ozone's concentration is 0.6 ppm, 700 times less than CO2's. But what is its concentration in the "ozone layer?" I couldn't resist a little calculation to figure it out.
To the right is a nice graph of ozone's concentration with altitude. It peaks at about 20 km in height, where its concentration is 36 DU/km.
DU is "Dobson unit" -- "One Dobson unit refers to a layer of gas that would be 10 µm thick under standard temperature and pressure."
One DU is 2.69×1016 ozone molecules per square centimetre, or 2.69×1020 per square metre.So 36 DU/km works out to 9.7×1018 ozone molecules per cubic meter.
What is the density of air at that altitude? 0.088 kg/m3, according to this handy Standard Atmosphere Calculator. Dry air has a molecular weight of 28.97 grams per mole, so the mass of one air molecule is 4.8×10-26 kg.
So at 20 km in altitude, there are 1.8×1024 air molecules per cubic meter.
Dividing one molecule number by the other finds that ozone's concentration at that altitude is 5.3×10-6, or 5 ppmv.
So ozone's maximum concentration is about 80 times less than CO2, and without this "trace gas" we'd be dead. Or never have existed in the first place, unless it developed underground or quickly developed some extremely strong sunscreen lotion.
As Shakespeare wrote, "the dose makes the poison." It's not the amount of something you have, it's its effect that kills you.
After doing this calculation I found this in Wikipedia:
Ozone concentrations are greatest between about 20 and 40 kilometres (66,000 and 131,000 ft), where they range from about 2 to 8 parts per million.---
Aside: If animals never left the ocean, could a technological civilization ever have developed? What might intelligent aquatic species like dolphins and octopi evolve into in another billion years? There