Thank you. But keep in mind this is only a possibility. It shouldn't be construed as being definitive in any way. Most climatologists believe most of the rise in atmospheric CO2 since the industrial revolution is due to Man. And it's probably a fairly safe bet - I mean, we're pumping out tons of it. However, there are natural processes that could cause it. We just don't have the enough understanding, or the proper tools, to make any definitive statements yet.
That's a good question. It depends on when you take your measurements, where you take those measurements, and what you're using to take those measurements with. Most estimates peg the carbon content of the atmosphere between 600 and 700 gigatons (billion tons). That would equate to between 2202 and 2569 gigatons of CO2 (it's heavier). These are big numbers, to be sure, but it's a big atmosphere. These values are going to change throughout the year because plants grow in the spring and summer (absorbing CO2), and die off in the fall and winter (releasing CO2). Where you take measurements will change the reading because you need to be in a carbon neutral place, which is kind of hard to find in an open system like ours. Different measuring tools will also give you different results. And when you're dealing with such a minute gas (relative to the entire atmosphere), small measurement variances make for huge variances in the estimates you arrive at.