* The burning of long-buried fossil fuels releases  containing carbon of different isotopic ratios to those of living plants, enabling distinction between natural and human-caused contributions to  concentration.
* There are higher atmospheric  concentrations in the Northern Hemisphere, where most of the world's population lives (and emissions originate from), compared to the southern hemisphere. This difference has increased as anthropogenic emissions have increased.
* Atmospheric O levels are decreasing in Earth's atmosphere as it reacts with the carbon in fossil fuels to form .