evidence: Carbon dioxide, water vapor, and several other important greenhouse gases contribute to global warming via the so-called greenhouse effect. [...] Let's consider the principal GHGs one at a time, starting with water vapor, the most abundant greenhouse gas in the atmosphere according to NOAA's Climate.gov. To skip to a specific section, click on its link below. [...] On the one hand, because water vapor is a greenhouse gas, more water vapor could amplify the greenhouse effect and lead to further warming. On the other hand, increasing the humidity may result in more widespread cloudiness, thus reflecting more sunlight and cooling the Earth’s surface. Understanding and quantifying these competing effects is critically important to projecting future climate change.