Earth is a system made up of numerous interacting subsystems like the atmosphere, hydrosphere and biosphere. The sun provides the main energy input that drives processes in these subsystems, while Earth's interior is also an energy source. Humans affect Earth's system through activities that cause issues like pollution, acid rain, and global warming. Resources are either renewable over short time spans through natural processes, like plants and solar energy, or nonrenewable over millions of years, such as fossil fuels.