Protecting the built environment from the forces of the natural world with dams and seawalls is important work (see "Saving Holland"), as is protecting the natural environment from the engineered world. But the 21st-century engineer should also look to the natural world as a powerful design partner and a source of sustainable solutions. A good place to begin is by studying the way natural materials are constructed at the nano­scale and drawing inspiration from them as we engineer our own materials. Take, for example, the civil engineer's construction material of choice: concrete, the oldest engineered building material and one of the most widely consumed materials on earth, second only to water.

Each year, 1.89 billion tons of cement--the primary component of concrete--are manufactured, enough to produce one cubic meter of concrete for every person alive. Unfortunately, cement is a major source of atmospheric carbon dioxide--largely because it's made by burning fossil fuel to heat a limestone and clay powder to 1,500 °C, which changes its molecular structure. When the cement powder is later mixed with water and gravel, the invested energy is released into chemical bonds that form calcium silicate hydrates--the glue that binds the gravel to make concrete. The production of cement accounts for an estimated 7 to 8 percent of all human-generated carbon dioxide emissions.