When compared to 1990, the number of people worldwide experiencing moderate to severe water shortages is expected to _______ by 2025.

The statement suggests a projection regarding changes in the number of people facing water shortages worldwide by 2025 compared to 1990. Although the specific word is missing in your question, the most suitable word to fill in the blank would likely be "increase" or "rise." This reflects the projected trend indicating that, due to factors such as population growth, climate change, and increasing demand for water, more people are expected to face moderate to severe water shortages in future years.

For a concrete understanding:

1. Historical Context: In 1990, particularly due to less severe climate issues and smaller populations, water scarcity was less prevalent.

2. Current Trends: Data from various global studies indicate that because of global population growth, urbanization, and climatic changes, the challenges surrounding freshwater access are likely to worsen.

3. Projection for 2025: Prognostics by organizations such as the UN have suggested significant increases in water scarcity in various regions worldwide.

To summarize, we would expect that compared to 1990, the number of people experiencing moderate to severe water shortages is expected to increase by 2025.