Without a doubt the biggest issue facing the environment is over population of humans. All other major environmental issues flow from the very fact that we are over populating the planet.
The worlds population has tripled in the last 60 years placing stress on every aspect of the environment. More land is developed every day to accommodate the urban spread.
In 1950 the population stood at 2,555,982,611 compared to 2012 which it now stands at over 7,000,000,000. The actual maths is that the worlds population has increased by almost 3 times. That is staggering when you think about it. That figure is increasing even as we speak!