Additional Facts 3. Water vapor is responsible for clouds, humidity, rain, sleet and snow and up to 90% of the greenhouse effect.
Water vapor also represents about 95% of all greenhouse gasses and is responsible for up to 90% of the greenhouse gas effect. It is in water vapor’s contribution to cloud formation that leaves scientists wondering if water vapor’s contribution to global climate change is currently positive, negative or about neutral. Clouds, which are very bright on top, can reflect back into space a high percentage of the sun’s incoming rays that would otherwise heat the Earth but they can also help trap more heat. While climate models can be “tuned” to give a desired result in either direction, empirical (real) observations of Earth’s climate history indicate that water vapor’s current contribution to global climate change is probably neutral to negative. Otherwise, there would or could be runaway global warming because a temperature rise could cause a CO2 rise and if the CO2 rise caused a positive feedback, the cycle would continue on and on, which it has never done in Earth’s measurable climate history of the last 540 million years.