Carbon Dioxide is a gas which is interwoven into the natural cycle of life on Earth. We are all carbon based life forms. The term "organic" refers to any chemical compound with carbon in it. It is what plants inhale, it is what we exhale. Stripping away all of our philosophy about the soul and life after death, and just looking at things in a dry scientific sense, life is a chemical reaction, and CO2 is one of the key ingredients. So to learn a little bit more about this process, let's do a chemical reaction in the lab and draw parallels.
Imagine you put on your lab coat and stand in a laboratory with a beaker, a collection of chemicals, and a stirring rod to mix them with (don't forget your safety goggles!). You take two chemicals, stir them together, and a precipitate forms on the bottom of the beaker. Let's suppose you chose this particular precipitation reaction to observe:
[1] 2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)
You mix the chemicals together in solution, and a precipitate forms: Mg(OH)2. However, when you examine your solution, you will find that there are still small amounts of 2NaOH(aq) and MgCl2(aq) floating around in liquid form. What is going on? Well it's important to realize that chemical reactions are not one way events. That arrow in equation [1] should really point both ways. Even as the chemicals are condensing into a solid, small amounts of the precipitate are turning back into their original reactants. The proportion of chemicals that remain in liquid form, to those that condense into a solid is roughly proportional to the speed of the "forward reaction" compared to the speed of the "reverse reaction" (It also depends on things such as the surface area of the precipitate). In this case, as in most cases, the speed of the forward reaction is much much greater than that of the reverse, leaving you with but a trace amount of your original reactants left suspended in your solution.
Starting again, you pour the two chemicals into your solution, but this time don't mix them in the right proportion. Instead of mixing NaOH(aq) and MgCl2(aq) in a ratio of 2:1, you mix them in a ratio of 1:1. Now at the end of your reaction you test and find only a trace amount of NaOH, but lots and lots of MgCl2. This is because the Sodium Hydroxide is "pinching off" the reaction. Meaning that the reaction completes long before all of the Magnesium Chloride condenses into precipitate.
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Now take your beaker and make it larger. Expand it larger and larger, until it stretches around in a monumental sphere. The bottom of your beaker is the surface of planet, and your "solution" is the atmosphere. Your reactants are all of the molecules in the air. The wind mixes these chemicals together and a precipitate forms. The precipitate is greenish colored, and there are all sorts of little things moving around amongst it. This is the Life Reaction. It is a little more complicated than our previous experiment, but we can figure it out if we use reaction [1] as an example. In this case, the "forward reaction" that involves CO2 is controlled mainly by photosynthesis:
[2] CO2(g) + H2O(aq) + photons → {Plant Matter}(s) + O2(g)
Note: We only care about how this reaction relates to CO2. Since after the Carbon is absorbed, a bunch of chemical reactions follow that transform the specific compound which is produced in photosynthesis into a variety of different forms. Thus I have used "Plant Matter" to simplify.
Remember, the structure of a tree is mostly made out of Carbon, which was absorbed through it's leaves. That's right, trees are mostly air. So what we have is Carbon being sucked out of the air, combining with water, and precipitating as plant matter with an Oxygen byproduct. Just like before, the amount of Carbon in the air is controlled by the proportion of the rate of the forward reaction, to that of the reverse reaction. The reverse reaction is controlled by animals, who gather energy from plant matter and burn it up in mitochondria, exhaling a CO2 byproduct.
[3] {Plant Matter}(s) + O2(g) + H2O → {Animal Matter}(s) + CO2(g)
Note: There is something missing here. The fact that animals process certain minerals which plants need to grow, and are used as fertilizer. We are focusing in on CO2 here, so this is left aside.
So you see, animals get their Carbon from plants. They then breath in Oxygen (O2), slap a Carbon atom on it (CO2) and expel it (losing some mass in the process). Because the amount of Animal Matter is constrained by the amount of Plant Matter, for CO2 the reverse reaction will always be much slower than the forward reaction. Like our first example, this leaves a trace amount of CO2 (our reactant) in the air, with the rest of the Carbon being held as Biomass (our precipitate).
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Now with any chemical reaction in the natural world, it is very likely that the reactants will not be perfectly balanced. Thus there will almost always be a single chemical which is pinching off the reaction, leaving one trace chemical, and one or more excess chemicals.
Turning our eyes to reaction [3], we see that the amount of Animal Matter is constrained by the amount of Oxygen, and the amount of edible Plant Matter. Considering what we know about nature, it is evident that the amount of edible plants is the thing pinching off this reaction (animals are far more likely to starve to death than to asphyxiate). In many regions it is constrained by the amount of water, but in general it is safe to say that globally the struggle is over food (keep in mind that the majority of Carbon is processed by organisms living in the ocean).
Next, we take a look at reaction [2]. What is the scarce chemical pinching off the reaction? In the deep ocean, and in the rainforest, it's photons. In the desert regions of the planet, it's clearly water. However, in general, for the majority of photosynthetic life it's neither of those things. Let's take a look at the Earths atmospheric composition.
CO2 barely registers on the scale, making up less than .04% of our atmosphere! Such a small, trace amount of CO2 strongly suggests that Carbon Dioxide is the limiting ingredient in reaction [2]. And since we know that the amount of Animal Matter is constrained by the amount of Plant Matter, we can extrapolate that the amount of total Biomass in the environment is being limited by the amount of CO2. Carbon Dioxide is pinching off the Life Reaction!
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Let's turn our eyes back to reaction [1] and take the case where we had added the chemicals in a proportion of 1:1, such that NaOH was pinching off the reaction. What would happen if we were to add a bit more NaOH into the mix? Quite simply, more precipitate would form. The amount of MgCl in solution would decrease in proportion to how much NaOH was added. Also, there would be a slightly greater amount of NaOH in solution, because there is now more precipitate. Meaning that the reverse reaction goes slightly faster. However, it would still be an infinitesimal trace amount, barely able to even be detected in solution.
Translate this knowledge to the Life Reaction. Adding more CO2 to the environment precipitates more Biomass. However, CO2 still remains in the atmosphere in only a trace amount. What does this mean for the environment? It means plants grow faster, and flourish more leaves and seedlings. Grass grows taller, forests grow more densely packed. Also, the environment is able to support a more dense animal population, with more biodiversity. More herbivores means more carnivores, and stronger, larger, healthier apex predators.
There is evidence for this. Anyone who runs a greenhouse will tell you that if you want your plants to grow fast, you pump the room full of extra CO2. This effect is known to be especially profound on plants that grow in desert regions. It is possible that a higher availability of Carbon in the atmosphere could shrink desert regions worldwide (forested areas can precipitate rainfall, possibly solving the issue of the scarcity of water). It helps that Carbon Dioxide is a heavy gas with a high specific gravity, meaning that it tends to hang close to the surface of the Earth, enriching plant life.
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My Personal Conclusions
The take away is, all of this casts strong doubt over whether or not CO2 emissions are a net negative for humans, or nature itself. Remember, oil is compressed plant matter. Those plants originally got that Carbon from the air. Taking a different point of view, you might assert that humans are simply liberating trapped Carbon back into the environment where it can once again be used as biomass, rather than sitting unproductively underground.
Everything has costs and benefits, and you need to weigh them both before deciding whether something is a good thing, or a bad thing. All of the supposed costs of Carbon emissions (the ocean swallowing up cities...etc) seem rather far fetched. There just isn't a lot of evidence to support a catastrophic outcome; peer reviewed science is still fairly timid on the doomsday prophecies we all heard in school. Whereas the benefits to global food production and biodiversity are based in logic, and hard science on the Carbon Cycle.
This is what makes sense to me, and is the logical conclusion I've arrived at based on the evidence I have before me. If you'd like to dispute me, please keep your criticisms confined to the arguments I have made above.
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Originally Posted by Danke
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