What's changed?
While many greenhouse gases occur naturally, the rate humans are adding them to the atmosphere is far from natural. It is estimated that concentrations of carbon dioxide are now more than one third higher than before the industrial revolution, when the large scale burning of fossil fuels and modern industry and agricultural practices began, adding to the atmospheric levels of naturally occurring gases such as carbon dioxide, methane and nitrous oxide. Humans are also creating new, very potent greenhouse gases such as hydrofluorocarbons (HFCs) from industrial activities.