The U.S. government tracks total emissions of greenhouse gases, in millions of metric tons from 2000 to 2010. After 4.4 years, greenhouse gas emissions reached its highest level for that period of time, at 7060 million metric tons. After 8.4 years, the greenhouse gas emissions had dropped to 6957.6 million metric tons. The relationship between years since 2000, , and greenhouse gas emissions over this time period, , can be modeled with a quadratic equation.