The Earth absorbs radiation from the Sun, mainly visible and ultraviolet, and warms up. The warm surface then re-emits energy as infrared radiation.Some gases in the atmosphere, called greenhouse gases, contain bonds that absorb this infrared radiation. The main ones are carbon dioxide (C=O bonds), methane (C-H bonds) and water vapour (O-H bonds). This is the same process used in IR spectroscopy: the bonds absorb IR radiation at frequencies that match their vibrations.The absorbed energy makes the bonds vibrate more. The molecules then re-emit infrared radiation in all directions, including back towards the Earth's surface. This keeps the lower atmosphere warmer than it would otherwise be. This is the greenhouse effect; without it the Earth would be too cold for most life.Nitrogen (N2) and oxygen (O2), which make up about 99% of dry air, do not absorb IR radiation. Their bonds join identical atoms, so their vibrations cause no change in dipole.Global warming
Human activities have increased the concentrations of greenhouse gases: burning fossil fuels releases carbon dioxide, and farming and landfill sites release methane. More IR radiation is absorbed and re-emitted towards the Earth, so the average temperature rises. This is global warming, which leads to climate change.Worked example: explain why methane is a greenhouse gas but nitrogen is not.
Methane has C-H bonds whose vibrations change the dipole, so they absorb IR radiation and re-emit it towards the Earth. Nitrogen has an N≡N bond between identical atoms, with no dipole to change, so it cannot absorb IR radiation.
Human activities have increased the concentrations of greenhouse gases: burning fossil fuels releases carbon dioxide, and farming and landfill sites release methane. More IR radiation is absorbed and re-emitted towards the Earth, so the average temperature rises. This is global warming, which leads to climate change.Worked example: explain why methane is a greenhouse gas but nitrogen is not.
Methane has C-H bonds whose vibrations change the dipole, so they absorb IR radiation and re-emit it towards the Earth. Nitrogen has an N≡N bond between identical atoms, with no dipole to change, so it cannot absorb IR radiation.