The IPCC estimates that there is a 5% increase in atmospheric humidity per 1°C increase in global average temperature
Warmer oceans and air means increasing humidity in the air. Firstly, water vapor is the main GHG. Over 90% of the GHGs is water. More humidity in the air is an enhancing feedback mechanism. Water vapour feedback may double the amount of CO2 warming. Secondly, more water vapor in the air means heavier rainfalls and floodings.
You can feel that water vapor is a greenhouse gas in e.g. a desert. Even if the temperature at midday can be a sweltering 50°C, the same place may approach 0°C at midnight. If there is a heavy cloudcover, temperatures will not fall that much.
- IPCC, 2007
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