Not all carbon dioxide is created equal, because the carbon atoms it contains come in slightly different weights. About 99 percent of the carbon in the atmosphere is carbon-12, the lightest kind. The rest is carbon-13 and carbon-14, which are a little heavier. Since humans started adding CO2 to the atmosphere through fossil-fuel burning and deforestation, the percentage of these heavier carbon atoms in the atmosphere has gotten smaller, and that’s a smoking gun that can tell us a lot about where the carbon came from.
Actually, it’s two different kinds of smoking guns. What makes carbon-14 so informative is that it’s radioactive (although there isn’t enough of it to be at all dangerous). Over time it changes into nitrogen through radioactive decay. That means that the amount of carbon-14 stored in plant tissue gradually gets less over time (it takes nearly six thousand years for half the carbon-14 in a given sample to decay into nitrogen) and eventually becomes very small.
Fossil fuels such as coal and oil are mainly the decomposed remains of plants that lived millions of years ago. Because the remains are so old, just about all the carbon-14 has changed into nitrogen. So when we add CO2 to the atmosphere by burning fossil fuels, the total amount of CO2 increases, but the amount of carbon-14 does not. Since humans started burning fossil fuels over two hundred years ago, the fraction of carbon-14 in the atmosphere should be getting smaller, and that’s exactly what air samples show. The exception was during the brief period when nuclear weapons were being tested in the atmosphere, creating more carbon-14. Measuring the shrinking percentage of carbon-14 helps confirm that fossil-fuel burning has been increasing the amount of atmospheric CO2.*
As for carbon-13, it’s not radioactive like carbon-14, but it’s still informative. Carbon-13 molecules are bigger than carbon-12, so plants have a harder time turning them into food. For that reason, plants have a lower fraction of carbon-13 than the atmosphere does.
Because fossil fuels come from plants, they also have less carbon-13 than the atmosphere. So as we’ve added fossil-fuel carbon to the atmosphere, the fraction of carbon-13 should have decreased. And it has.
Tracking these two types of heavier carbon atoms tells us that about 25 percent of the CO2 now in the atmosphere appears to have come from fuel burning or from the destruction of living plants, largely from the clearing of rain forests. That’s about the same amount scientists estimate should be there based on how much fossil fuel we’ve burned. So they’re even more confident that the extra CO2 in the atmosphere comes from humans.
*Carbon-14 in the atmosphere decays the same way carbon-14 in fossil fuels does, but it’s constantly replaced by cosmic rays striking the upper atmosphere from space.