Dedicated to the balanced discussion of global warming
RealClimate – December 22, 2004
This is a very good article that answers the basic scientific question of how we know the source of excess carbon dioxide in the atmosphere. The simple testing is very similar to the way that archeologists can find the age of ancient ruins – they do carbon testing. Since the carbon in fossil fuels has depleted its supply of C14, and the volume of CO2 in the atomosphere is increasing in non-C14 amounts, we can assume that it did not come from recent sources.
carbon dioxide, CO2, forests, fossil fuel, Greenhouse gas, plants, RealClimate
Another, quite independent way that we know that fossil fuel burning and land clearing specifically are responsible for the increase in CO2 in the last 150 years is through the measurement of carbon isotopes. Isotopes are simply different atoms with the same chemical behavior (isotope means “same type”) but with different masses. Carbon is composed of three different isotopes, 14C, 13C and 12C. 12C is the most common. 13C is about 1% of the total. 14C accounts for only about 1 in 1 trillion carbon atoms.
CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.