Many chemical compounds present in Earth’s atmosphere are relatively poor absorbers of shortwave radiation but strong absorbers of longwave radiation, thus keeping the lower atmosphere and surface environment warmer than they otherwise would be given Earth’s solar constant. This radiative mechanism has been termed the “greenhouse effect” and responsible atmospheric constituents labelled “greenhouse gases.” According to the IPCC (2007), the most important greenhouse gases are water vapour and carbon dioxide, whilst nitrogen and oxygen—the two most abundant constituents of the atmosphere—have no effect. Many greenhouse gases occur naturally in the atmosphere (e.g. carbon dioxide, methane, water vapour) but others are synthetic, such as chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). Human activity can intensify the greenhouse effect through the emission of greenhouse gases. For instance, the amount of carbon dioxide in the atmosphere has increased by about 35 % in the industrial era, due largely to the combustion of fossil fuels and removal of forests (IPCC 2007).