There are two components in the greenhouse effect — a natural one caused by the amounts of greenhouse gases naturally found in the atmosphere — and a man-made one caused by the amounts of greenhouse gases that man adds to the atmosphere as a result of his activities.
The contribution that a greenhouse gas makes to the greenhouse effect depends on the amount of it that it is the atmosphere and how much heat that particular gas absorbs and re-radiates.
In descending order, the gases that contribute most to the Earth’s greenhouse effect are: water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and ozone (O3).
In terms of the amount of heat they can absorb and re-radiate (known as their global warming potential or GWP), methane is 23 times more effective and nitrous oxide is 296 times more effective than carbon dioxide.
However, there is much more carbon dioxide in the Earth’s atmosphere than there is methane or nitrous oxide.
Not all the greenhouse gas that we emit to the atmosphere stays there indefinitely. For example the amount of CO2 in the atmosphere and the amount of CO2 dissolved in surface waters of the oceans stay in equilibrium, because the air and water mix well at the sea surface. Thus when we add more CO2 to the atmosphere, a proportion of it dissolves in the ocean.
We know that if there was no natural greenhouse effect, the heat emitted by the Earth would simply pass outwards from the Earth’s surface into space.
Physics tells us that without the natural greenhouse effect, the Earth's average surface temperature of 14 °C would be as low as -23 °C, the so-called ‘black body’ temperature of the Earth.
We also know that in our solar system, Mars, Venus, and the moon Titan also have surface and lower atmospheric temperatures that are consistent with the predicted greenhouse effects of their atmospheres.