I have heard this question before and it was answered kind of. The amount of CO2 emitted by man to the atmosphere in 2006 if we were to think about it in terms of a liquid would be one and half drops out of an eye dropper into an Olympic sized swimming pool. Tell that to your liberal friends, let them tell you how reducing it to 1 drop would change the composition of the pool water. See how silly it is, how insane the line of reasoning is "To do something" just to do something. This whole scheme was cooked up by the European Greens in the early 1990's as another way to package socialism, cause it wasn't selling as naked Socialism. I mean who could be against the earth right? They are in retreat on this and scrambling around for the shriveling grant money as this farce dies. Watch the Jesse Ventura show Conspiracy Theory on this subject, the cast of characters at the top of this fraud, its goals, how they bought up all the liberal science departments and how many good scientists sold their soul to this new religion.
My preference in those cases is the political argument. Use the swimming pool and show us shrinking our economy when we take out a few cups of water (CO2 is proportional to economic activity). Now show China dumping gallons in (they are building two coal based power plants a week). The science is hopelessly politicized since the government has bought the research results they want (that calls for more government control). So at this point politics should be the preferred mode of argument.
Earth worship, the new state religion.
Let's see if this is right:
Weight of the atmosphere: 5 x 10^15 metric tons
CO2 emitted in 2007: 2.93 x 10^7 metric tons
So the 2007 CO2 added 1 part in 1.6 x 10^8 to the atmosphere.
An Olympic sized swimming pool contains at least 2.5 x 10^6 liters of water.
There are about 20 drops in a milliliter or 20000 in a liter, so there are 5 x 10^10 drops in an Olympic sized pool.
So as far as relative mass is concerned, the CO2 addition in one year is more like 300 drops from an eyedropper in an OSSP, or about 15 ml or 1 Tablespoon of water in an Olympic sized pool