It can be difficult to measure the carbon-footprint of each person. However, since pollution is a community problem, then it makes sense to average out measurements. You could be completely energy efficient, but Paul-the-Polluter next door could cancel out your progress. Because of this, measurements through individual averages are logical.
(Want to know what your individual contribution really is? Grab an emissions calculator and find out!)
So what measurement is best? I believe the most obvious one would be carbon emissions, the largest human-caused greenhouse gas. What's the time frame? A year is way too long. A month is nice, but months vary. A week is almost good. But a day is perfect. I want to know, on average, how much carbon I emit in one day.
Now it's time for some math. According to the Energy Information Administration, the United States emitted 5,814.4 million metric tons of carbon dioxide in 2008 alone. Again, why does this matter? Well grab a calculator and follow along.
If we divide US emissions by the US population, which is currently estimated at 307 million, we get about 19.3 metric tons of carbon emitted per year, per person. That's a lot, but still hard to tangibly quantify. Let's convert this into pounds by multiplying 19.3 by 2,205 (equal to 1 metric ton). And now let's divide this by 365 (for the days in a year).
What's the answer? 116.6 pounds of carbon dioxide emitted per person per day in the United States. Wow. My sister weighs that much. You are probably thinking that there is no way an individual can produce so much carbon, and you're right. This statistic also factors in production from corporations, the government, and other large institutions. But if affects every individual in the same way. This means that 116.6 pounds of carbon dioxide is my [average] daily contribution to carbon emissions. Of course, that is your contribution, too.