Part 5 of my AGW argument was attempting to demonstrate that the rise in carbon dioxide levels over the past 50-100 years is feasible just from the measurements of our emissions.
The numbers weren't coming out the way I excepted, so I ditched the attempt. Let's look at these numbers again (I compel thee).
I came up with a conservative estimate of 1250 megagrams output from our motor vehicles. I mentioned before that I was worried I might be equivocating between multiple definitions of "car" or "motor vehicle", applying the wrong figures. That was estiamted to be about a forth of total human CO2 output - coming out to 5 gigagrams.
Looking at government studies, the 2008 U.S. output (note, not the world-wide rate that I was trying to estimate) was 5,839.3 million metric tons , or converted to the awesome metric system, 5839300 gigagrams.
The heckWait, seriously? Now, the U.S. makes up a big chunk of the total output, but let's say that's the whole world. I was seriously off by 6 orders of magnitude?
Try it - enter into Google "convert 5,839.3 million metric tons into gigagrams" - maybe it doesn't understand the conversion.
I tried "convert 5839300000 metric tons into gigagrams", and came out to the same answer, in case it was tripping up on the wordage.
Nope, trying some other website. It says 5,839,000,000,000 kg - or 5839 gigagrams - okay, now I'm only off by 3 orders of magnitude.
Nope, trying another website. It says 5,839,300,000,000 kg - or 5,839.3 gigagrams.
Pffft. Right. A "metric ton" is 1000kg. I could have done this in my head. I think Google is off here.
So, the U.S. output of CO2 is 5.84 tetragrams in 2008I was way off. Looking at my estimation of how much CO2 it would take to obtain 120 ppm CO2 in the atmosphere that has a total mass of about 5.1480*10^18 kg ± 1.5*10^15 kg, it's much easier now. 
If we're assuming we're releasing 2008 amounts of carbon dioxide, in one year, that would account for a ratio of (yearly CO2 mass output / total atmosphere mass), correct?
Plugging "(5.84*10^12) / (5.1480*10^18)" into Google, and I get the result about - 0.00000113442
That's about 1.13 ppm. That's more what I would have expected. It would take about 106 years to account for the 120 ppm increase since the early 1800s. Then again, we haven't been pumping out 2008 levels straight along.
Let me find a corroborating source for the atmospheric mass. The almighty Wikipedia says 5×*10^18 kg, which agrees with my other source .
If it was smaller, that would account for the difference more, but no luck there.
The U.S. outputs roughly one-forth of the total human output, so basically multiply these figures by 4 - meaning, it would take a little over 26 years of 2008 CO2 output levels to account for the 120 ppm increase - which is a lot more reasonable.
Next, I'll see if there's any other contributing factors, such as things humans might be doing that aren't directly contributing to the levels, but rather degrading CO2 sinks - such as deforestation.
I didn't want to just throw a figure from the big bad government at the audience. I wanted to provide some figures from varying sources that they could better understand how those numbers came to be. I still plan to take that route.
Works Cited http://www.eia.gov/oiaf/1605/ggrpt/carbon.html