gigatons

A gigaton, abbreviated Gt, is a unit of mass equal to 1,000,000,000 metric tons (tonnes), or 1 × 10^12 kilograms. It is commonly used in earth sciences and climate research to express very large quantities. Because it denotes mass, a gigaton is often qualified by a substance, yielding terms such as GtCO2 (gigatons of carbon dioxide) or GtC (gigatons of carbon). A gigaton equals 1,000 megatons (Mt).

In climate and earth science, Gt and GtCO2 are standard for reporting emissions, removals, and stocks. The global annual emissions from fossil fuels have typically been in the range of roughly 35–40 GtCO2 in recent years. Other uses include measures of biomass and soil carbon stocks, mineral production, and large-scale energy resources. The unit is convenient for communicating large totals without resorting to scientific notation.

Notes: 1 GtCO2 corresponds to about 0.2727 GtC (since carbon has a molar mass of 12 and CO2 44). Also, gigaton is a mass unit, not an energy unit, so energy-related quantities often use different conventions (for example, energy content of emissions is expressed in Mt or PJ, not GtCO2 directly).