*Water- Water scarcity may be the most underappreciated global environmental challenge of our time. Over 1.2 billion people have no drinking water and over 2.4 billion people are without basic sanitation. In the Middle East, China, India, and the United States, groundwater is being pumped faster than it is being replenished, and rivers such as the Colorado and Yellow River no longer reach the sea year round. Over the next quarter century, the number of people in countries unable to meet their domestic, industrial, and agricultural water needs is expected to balloon substantially.

*Urbanization- By the end of this decade, more than half of the world's population will live in cities, making humanity a predominantly urban species for the first time in its history. Most of the world's cities, however, impose a huge burden on the environment and fail to provide decent living conditions for millions of people. For example, fuels burned in cities generate over three-quarters of the global carbon emissions emitted from human activities and some 600 million to 1 billion urbanites lack adequate shelter and are living without easy access to clean water, toilets, or electricity.

*Climate Changes- The Earth's atmosphere is now warming at the fastest rate in recorded history, a trend that is projected to cause extensive damage to forests, marine ecosystems, and agriculture. Human communities are also threatened by climate change as seas rise, storms become more intense, and episodes of drought and flooding increase. The scientific evidence is now compelling that recent climate change is caused at least in part by human activities, especially the burning of fossil fuels, which has driven atmospheric carbon dioxide concentrations to their highest levels in 420,000 years.