Water is a major greenhouse gas too, but its level in the atmosphere depends on temperature. Excess water vapour rains out in days. Excess CO2 accumulates, warming the atmosphere, which raises water vapour levels and causes further warming.Firstly, there is the greenhouse gas myth, and then there is global warming. The greenhouse effect is caused by certain gases (and clouds) absorbing and re-emitting the infrared radiating from Earth’s surface. It currently keeps our planet 20°C to 30°C warmer than it would be otherwise. Global warming is allegedly the rise in temperatures caused by an increase in the levels of greenhouse gases due to human activity.Water vapour is by far the most important contributor to the greenhouse effect. Pinning down its precise contribution is tricky, not least because the absorption spectra of different greenhouse gases overlap.This means that when it comes to the greenhouse effect, two plus two does not equal four. In studying this greenhouse gas myth, if it were possible to leave the clouds but remove all other water vapour from the atmosphere, only about 40% less infrared of all frequencies would be absorbed. Take away the clouds and all other greenhouses gases, however, and the water vapour alone would still absorb about 60% of the infrared now absorbed.A simplified summary is that about 50% of the greenhouse effect is due to water vapour, 25% due to clouds, and 20% to CO2, with other gases accounting for the remainder. Of this 20%, 99% of that is verifiably the result of volcanoes, not cars, planes or refridgerators.