[greenbean36191]

Yep, CO2 is a trace gas in the atmosphere and not even the most abundant greenhouse gas (which is water vapor). It actually only makes up about .04% of the atmosphere. However, it's very efficient at trapping heat and accounts for as much as 1/4 of the greenhouse effect. We are causing increases in other gases like methane, but they make up even smaller proportions of the atmosphere and are less efficient at trapping heat.

To grasp how such a small constituent of the atmosphere can be so disproportionally important to the system you don't really even need to worry about the physics. Just think of your reef tank. Calcium makes up about the same percentage of seawater that CO2 makes up in the atmosphere. Imagine if you were to increase or decrease your calcium by 35%. Your alkalinity would go nuts, your pH would probably swing, and your corals, snails, and any other life that used Ca would be very unhappy if not dead. If we were talking about the ocean you would expect to see the same problems, plus some very important components of the plankton would be killed off, which would decrease fish populations and in turn even affect terrestrial animals including birds and people.

The same sort of thing can happen with the climate. There are all sorts of feedbacks built in that can magnify small changes. As a result, a small increase in CO2 can indirectly cause much more warming than the heat it can trap alone. As a very cartoonish example, when you heat the planet with CO2 you also increase the amount of water vapor. Some of that forms clouds and helps cool things off, but being a greenhouse gas it also traps heat. The small warming due to H2O+CO2 can cause melting of permafrost which releases more CO2 and methane. It also causes ice to melt, exposing the darker land or ocean underneath, which absorbs heat rather than reflecting it like the ice did.