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CO2 is not the only greenhouse gas that contributes to the greenhouse effect.

Water vapour and methane are two other important contributors.

The different greenhouse gases affect each other in different ways. For CO2, the following relationships are of special importance:

 The more CO2 in the atmosphere, the higher the temperature.

 The higher the temperature, the more water vapour and methane will accumulate in the atmosphere.

Parts of the greenhouse effect attributed to CO2 in this book will therefore be secondary effects caused by increased levels of water vapour and methane, caused by increased levels of CO2.

Since the root cause for these secondary effects is the change in CO2 content, the contributions from these secondary effects are included in the term "CO2 related" greenhouse effect used in this book.

The total amount of methane and water vapour in the atmosphere is determined not only by the amount of CO2, but also a number of other conditions and processes. Discussion of these conditions and processes is outside the scope of this book.

The CO2 content of the atmosphere is normally indicated in ppm (parts per million). This unit of measurement is also used in this book.

One ppm is equivalent to 1/000 of one thousandth (‰). (1,000 ppm equals 1‰). Today's CO2 content in the atmosphere of 400 ppm can therefore also be set as 0.4‰.

This is a very low concentration; however, since CO2 is a very efficient greenhouse gas, even a small increase in the amount of CO2 will result in significant climate changes.