Today, Earth's population stands at around seven billion, and it is still growing faster. As a result, the imbalance between what nature replenishes and humans consume will probably continue to grow. So how will so many people live on Earth without exhausting the planet? The key is sustainability—finding new and efficient ways of conserving more and consuming less—so that we do not ultimately exhaust our most valuable resources. Sustainable communities are cities and towns that encourage residents to protect their local environment in ways that also reduce their impact on the larger global environment. Residents in Mbam, Senegal, for example, use solar ovens to cook food. By using solar energy instead of cutting down trees, people are saving forests for future generations. Communities in other places are using improved public transportation systems to reduce the need for cars. In Curitiba, Brazil, city buses are frequent, convenient, and efficient—so 70 percent of Curitiba's commuters use them. As a result, the city has little traffic jam and cleaner air. Another way to prevent depletion of natural resources is through proper forest management. Forest logging—the cutting down of trees—can contribute to water pollution, lead to the destruction of animal habitats, and have other negative effects on the environment. In an effort to preserve forests, several countries have begun creating certified forests. When a forest is certified, the logging is regulated and carried out in a sustainable way. In West Virginia (USA), for example, loggers in certified forests must receive special training to avoid causing soil erosion. Roughly seven percent of the world's forests are certified. Canada has the largest areas, with almost 300 million acres of certified forests.