The number of humans on Earth has increased from about 1 billion in 1800 to over 7 billion today, and is projected to increase to about 10 billion by 2100. The "UN World Population Report 2001" estimated the sustainable carrying capacity of Earth as between 4-16 billion. While the exact number is uncertain (and depends on standards of living, industrial efficiency, and other factors), eventually we will run out of space and resources.
This problem is farther in the future than climate change and economic difficulties, but that also means we have more time to think about it and do something about it.
China's efforts at population control have led to human rights abuses. A sudden drop in births could crash the economy. However there must be a better way - please come up with it.