There is much confusion over whether or not water vapour is a greenhouse gas (GHG). The simple answer is yes, it’s the most abundant GHG responsible for around half of the greenhouse effect, but a half that has kept the planet habitable for millions of life forms. But that is changing.
The confusion over water vapour stems from the fact that it’s not directly man-made. There are no industrial processes churning out vast amounts of water vapour. But there are industrial processes churning out CO2, CH4, N2O, and fluorinated gases which together are:
increasing Earth’s atmospheric temperature...
which increases the rate of evaporation of water from oceans/lakes/wet soil...
which increases the concentration of water vapour in the atmosphere...
which increases water vapour's greenhouse effect...
which increases Earth's atmospheric temperature...and so back to 1, 2, 3, 4, 5 ad infinitum.
A classic case of positive feedback. The only way to escape from this loop is to take other GHGs out of the atmosphere; in other words, stop emissions of CO2, CH4, and NO2 and wait until they spontaneously break down. Under the current energy policies of the largest developed countries, that will not happen in the near future. Meanwhile, Earth's rising temperature will continue to increase water vapour evaporation and the cycle goes on.
For every degree Celsius that Earth’s atmospheric temperature rises, the atmosphere can hold an additional 7% water vapour. The United Nations’ IPCC Sixth Assessment Report states total atmospheric water vapour is increasing 1 to 2% per decade.
If we can’t/won’t reach net zero by 2050, the critical target laid down by the UN’s IPCC, then all hell will break loose. Runaway water vapour concentration, runaway atmospheric warming, runaway climate change.
And nowhere for life on Earth to runaway to.
