On some level, this isn't all that surprising; more people live in the cities and suburbs than rural areas, and that means more cars and more CO2. But what was interesting was that the data didn't line up with previous estimates that assumed the relationship between population density and the amount of driving in an area is linear. It turns out that isn't exactly the case. Instead, the amount of driving increases more rapidly than expected until the population hits 1,650 people per square kilometer. After that it increases more slowly. As a result, some city centers' CO2 emissions were overestimated by almost 500 percent, while suburban emissions across the board were underestimated.