Today, city governments need to protect citizens from terrorist attacks, outbreaks of disease, and natural disasters. They are also responsible for protecting the environment. Urban planners are making cities more sustainable through solar energy, alternative transportation, increased green spaces, and innovative waste removal or recycling programs. Cities, where people share resources like public transportation, are more efficient than sprawling communities. In fact, some say that cities preserve natural spaces and rural ways of life by concentrating dense populations in urban centers rather than contributing to suburban sprawl.