Mass Units - (n.)
When dealing with the enormous quantities of carbon discussed in global climate change studies, scientists use the following units:
tonne: a unit of weight equal to 1000 kg (103 kg).
kt: kilotonnes; equal to 1000 tonnes or 106 kg.
GtC: gigatonnes of carbon; 1 GtC is equal to 109 tonnes of carbon or 1012 kg. 3.7 Gt carbon dioxide will give 1 GtC. To give you an idea how big this unit is: in 1997 the total supply of petroleum for the USA (including imports) was approximately 1 Gt per year.
TgC: teragrams of carbon or 1012 grams of carbon.
PgC: petagrams of carbon or 1015 grams of carbon; 1 PgC is equal to 1 Gigatonne of carbon.