Sure, James, but forgive me - I really hate math, so this isn't gonna look like math.
I started out with a sample plot that I was using for soil data (irrelevant project I was doing). The area of that quadrant was 1734.83 sq meters. The carbon content of that area was 300.5 Mg, which is fairly easy to calculate. The estimated tree mass in the quardrant was 601 Mg (lots of calculations went into this and I don't remember the exact methodology, so I assume I was right; just roll with it) and the wet-weight to carbon content ratio in most wood is about 0.5 (constant). Multiply those numbers, you get 300.5 - dramatically higher than the soil of the quadrant, which we estimated to hold about 12 Mg of carbon. I thought that was kinda cool to quantify.
The EIA puts out data on each state's emissions. Illinois emitted 234 million Mg of carbon in 2014. I gave you the area - 1734.83 sq meters - and how much carbon the trees in that plot held - 300.5 Mg - so you can figure out the rest: 778,702 of our quadrants in order to neutralize Illinois' emissions. Keep on mathing and eventually you get to 1.3 million square kilometers of forest area.
As for whether or not the sample is useful and how much error there is in a clearly not statistically significant data plot, you can make the argument that I would need to take a bunch more samples with identical stem densities and take the harmonic mean of the basal areas of like 800 different plots in order to say that I actually found a constant amount of carbon that a certain amount of trees can sequester at any given time, but I'm not that invested, so you can either take the math with that grain of salt or you can assume that I did my due diligence in finding a relatively normal quadrant of edge forest along the southwestern shores of Lake Michigan, one of the most polluted areas in the country, to take data from. You could also say I skipped a bunch of steps and am therefore hiding something, which is fair, but I don't really care to explain the ins and outs of a project that I don't even have the design to anymore, so I'm just gonna trust that I did it well at the time. I got an A on the project I did regarding woodland carbon sequestration using this data, if that is any consolation. You could also say that you can't use this data to say how much carbon that woodland could sequester in a year's time, which is fair; in fact, that is something that I said to my professor. He told me not to worry about that, which meant less work for me, so I didn't.
So, again, are the numbers exactly right? Probably not. Would I use these numbers in a research paper? Probably not. Was my quadrant fairly normal as far as the average forest goes in the United States? Yep. Therefore, am I fairly close? In all likelihood, yes. Maybe it's only 1.8 Texases instead of 2.0. Whatever. Dramatic numbers are dramatic numbers.