LONDON — The world's population of seven billion is set to rise to at least 10 billion by 2100, but could top 15 billion if birth rates are just slightly higher than expected, the United Nations said on Wednesday.

In a report ahead of ceremonies on October 31 to mark the seven billionth human alive today, the UN Population Fund (UNFPA) warned demographic pressure posed mighty challenges for easing poverty and conserving the environment.

The world is becoming a hungrier place - consuming more food, fuel and material resources - as its population bulges. But the increase in consumption is not even and not driven by the same factors. China and India are consuming more because their populations have vastly increased in the past 40 years. The United States, Japan and many countries in Europe are having much less population growth, but the people are using more resources. Meanwhile, many in South American and African countries have had relatively small population growth and much less increase in consumption.