The worlds population has tripled in the last 60 years placing stress on every aspect of the environment. The exponential growth of population and the economic development that must occur in order to sustain it has had many serious implications for the environment because energy generation processes and increase in dwellings (more land is developed every day to accommodate the urban spread). The necessary growth encourages generation of electricity, heating, cooling, or motive force for transportation vehicles and others, which are harmful to the ecosystem.
Without a doubt this is one of the biggest issues facing the environment. We can say that all other major environmental issues flow from the very fact that we are overpopulating the planet, reaching unsustainable levels. The actual maths is that since 1950, the worlds population has increased by almost 3 times from 2,555,982,611 over 7,000,000,000 in 2012.
Check out how it increases even as you read this.