Final answer:
The United Nations believes that the greatest future global problem after population growth will be a lack of usable freshwater.
Step-by-step explanation:
The United Nations believes that the greatest future global problem after population growth will be a lack of usable freshwater.
This is because water scarcity is already a serious resource problem around the world, and population growth will only exacerbate this issue. Increased demand for water, coupled with its uneven distribution and contamination, will pose significant challenges for sustaining the global population.