Burden

The total mass of a gaseous substance of concern in the atmosphere.

¹³ Stable isotope of carbon having an atomic weight of approximately 13. Measurements of the ratio of ¹³C/¹²C in carbon dioxide molecules are used to infer the importance of different carbon cycle and climate processes and the size of the terrestrial carbon reservoir.

¹⁴ Unstable isotope of carbon having an atomic weight of approximately 14, and a half-life of about 5,700 years. It is often used for dating purposes going back some 40 kyr. Its variation in time is affected by the magnetic fields of the Sun and Earth, which influence its production from cosmic rays (see Cosmogenic isotopes).

Definition courtesy of IPCC AR4.