Gigatonne

Giga is the prefix for "1000 million (of something)". A gigatonne is then 1000 million tonnes, or 1000 million metric tons, or 1,000,000,000 tonnes. It is abbreviated as Gt.

In the US, 1000 million is written as "one billion", so one gigatonne could be called "one billion tonnes" or "one billion metric tons". In the UK "one billion" means "one million million", or 1,000,000,000,000, so the term "billion" should be avoided as it can be confusing.

To visualize one gigatonne, remember that one cubic meter of water has a mass of one metric ton, or tonne. One gigatonne is 1000 million metric tons. A cubic kilometer is 1000 million cubic meters, or a 'giga-cubic-meter'. A square area of water one kilometer on a side is one square kilometer. If the water is 1000 meters deep, it is one cubic kilometer of water and it has a mass of 1000 million tonnes, or 1 Gt. One cubic kilometer of water has a mass of 1 Gt.

A gigatonne of Carbon is abbreviated as GtC. This unit is used in measuring carbon in environmental science and in the areas of global warming and climate change. Pure carbon has a density of about 2.27 grams per cubic centimeter (a Specific gravity of 2.27), so one cubic kilometer of pure carbon has a mass of about 2.27 GtC, where GtC means gigatonnes of Carbon.

A gigaton is either 1000 million short tons or 1000 million imperial tons.