Gases in Earth’s atmosphere (carbon dioxide (C0₂), methane (CH₄), nitrous oxide (NO), perfluorocarbons (PFCs), hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), sulphur hexafluoride (SF₆), and water vapour (H₂O)) that absorb part of the solar radiation reflected from the Earth’s surface and trap the heat in the atmosphere; this is referred to as the greenhouse effect. An increase in the concentration of greenhouse gases is likely to result in rising average temperature of the Earth, or global warming. Given uncertainty in how the climate system varies naturally and reacts to emissions of greenhouse gases, it is difficult to determine how much of any observed changes can be attributed to human activity.