How to pronounce“gigaton”in English

of you have seen diagrams, like this a a famous wedge diagram. Today we're spewing about 30 gigatons of carbon into theatmosphere. What does that mean? Thirty gigatons naturally occurring carbon in the atmosphere is about 700 gigatons. So it's about 5% a year. In 20 years we're doubling the amount of carbon. Unless it'sabsorbed or unless the earth is adapting. So we don't know what's going to happen. This is the chart that the International Energy Agency where they project