Pollution, food shortages/distribution problems, unemployment, poverty.
Couldn't a decrease in population make it a lot easier to solve these problems?
For instance, the amount of farmland it takes to maintain livestock is much larger than the area it would take to feed the human race directly. However, this would put millions of people out of a job where there is already too many unemployed. If we had less people, then we could feed the population without putting people out of jobs.