"Overpopulation is the world’s top environmental issue, followed closely by climate change and the need to develop renewable energy resources to replace fossil fuels, according to a survey of the faculty at the SUNY College of Environmental Science and Forestry (ESF)."- 
Overpopulation means there are too many people and the resources of the world or the resources of the area where they live can't support them all. Overpopulation leads to poverty, misery and depletion of resources as well as overcrowding and urban sprawl, destroying ecological and biological diversity. Overpopulation also adds to Global warming when it occurs in sync with the technological development of previous undeveloped countries, because more people are using more fossil fuel and thus more people are pumping out Carbon dioxide into the atmosphere.
While Green politics has now become mainstream the fact that overpopulation is the biggest cause of pollution hasn't stopped it becoming a taboo subject to many politicians.
The world's population is increasing at a rate of 200,000 a day, roughly the population of Montgomery, Alabama, and Earth obviously can't sustain this growth indefinitely: even if it could, the world would eventually become an enormous, overcrowded city.