Which phenomenon is primarily caused by increased water vapor in the atmosphere?

The phenomenon primarily caused by increased water vapor in the atmosphere is the enhanced greenhouse effect. Water vapor is a potent greenhouse gas that contributes significantly to the Earth's natural greenhouse effect, which keeps our planet warm enough to support life. When the amount of water vapor in the atmosphere increases, usually due to higher temperatures, it leads to more heat being trapped. This amplifies the warming that results from other greenhouse gases, such as carbon dioxide and methane.

As the Earth's surface temperature rises, more water evaporates, increasing atmospheric humidity. This creates a feedback loop where the additional water vapor causes even more warming, hence enhancing the overall greenhouse effect. This process is important for understanding climate change because it demonstrates how interactions in the climate system can exacerbate the effects of global warming.

In contrast, options such as global cooling and lower average global temperatures are inconsistent with the implications of increased water vapor, which is more associated with warming. Similarly, increased droughts can be influenced by various climatic factors, not solely the presence of water vapor; hence they do not directly correlate with the primary impact of increased water vapor.