What do you think the world will look like in 20 years? Or in 50 years? Will the world your children and grandchildren inherit resemble anything you've known? Will the future of humanity be one of hope and opportunity, or scarcity and destruction?
Take a moment to read this project:

World population is currently growing by 80 to 90 million people each year, and will exceed six billion in early 1999. This rate of growth is projected to continue for roughly the next 30 years, leading to a global population in 2030 of around 8.5 billion. These levels of increase and of total population are unprecedented in human history, and create challenges to the natural environment, and to human quality of life, previously unimagined.

The issue of population, and its relationship to the condition of humanity, and the condition of this planet, is often subtle and complex. But it impacts nearly every aspect of our lives, from education and employment to the environment. Human numbers and lifestyles are behind resource depletion and pollution. They drive migration and immigration, war and political instability, scarcity and starvation.
The larger the Earth’s human population, the more food, water, and energy are needed to support that population. To produce more food, more land must be cultivated, and more fertilizers and pesticides applied. To provide more water for human needs, more lakes and aquifers must be depleted, and more rivers dammed or diverted. To generate more energy, more power plants must be built and more fuels must be consumed. All these processes result in loss of habitat and biodiversity, more soil depletion and erosion, and greater pollution. Followed to its conclusion, this ultimately means a decline of the natural world, lower food production capacity, increased health problems, and a generally lower quality of life for all of us.
To accommodate more people, we require more schools, more cars, more highways, houses, and hospitals. To construct these we need more land, more lumber, more metals, and energy. This means more open space and habitat paved over, more logging, more mining, more pollution and increased greenhouse gas emissions. Infrastructure, environmental remediation, and the increased levels of service necessary to support growing populations are all expensive.