Sustainability is becoming an increasingly important issue worldwide. More and more people today understand the importance of unsustainable behavior, and the need to urgently transform their lifestyles. Sustainable development is one of the main elements of governments' strategies and policies, owing to a variety of contemporary strategic challenges. These include population growth, energy demand and supply, and water and food security. The rapid rise in the world population, particularly in urban areas, is also a crucial factor affecting climate change. Concentrated populations increase the need to provide energy, water and food, all of which generate carbon dioxide (CO2) emissions (Figure 11.1).

In developing countries, towns and cities are expanding at an unprecedented rate and scale. According to the United Nations “Habitat” study published in 2009, a major shift occurred in 2007–2008, whereby for the first time more than half of the world's population was classified as living in cities. The number of people living in cities, suburbs and towns at the end of 2008 was estimated at 3.3 billion, and this is expected to rise to five billion by 2030. According to the United Nations, 64.1 percent and 85.9 percent of the developing and developed world, respectively, will be urbanized by 2050.