A gas within the atmosphere which absorbs and emits energy radiated by the Earth. Carbon dioxide (CO2) is the most important greenhouse gas being emitted by humans.
The atmosphere is composed of different gaseous constituents but the most important in terms of climate change are the so called greenhouse gases. Radiation from the sun is not absorbed particularly well by greenhouse gases, but it does warm the Earth’s surface, the atmosphere and clouds which in turn emit energy back to space in the form of infrared radiation. Greenhouse gases are quite effecting at absorbing (and re-emitting) radiation at specific wavelengths within the spectrum of infrared radiation and so act like a blanket that restricts loss of this radiation into space. As a result, the more that is trapped, the greater the warming.
The greenhouse effect is natural and without it the Earth would be considerably colder. The primary greenhouse gases are: Water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3).
The is an international attempt that contains legally binding commitments, in addition to those included in the UN Framework Convention on Climate Change, targeted at reducing anthropogenic greenhouse gas emissions.