<human>:  When did the United States become the world’s largest emitter of greenhouse gases?
<bot>: The United States became the world’s largest emitter of greenhouse gases in the early 1980s. The reason for this is that the US economy grew rapidly during the 1970s and 1980s. The growth in the economy led to a rapid increase in the use of fossil fuels, such as coal and oil, which emit greenhouse gases.