National and international policies are needed to address global environmental problems because what happens in one area can affect another. Human impact, such as the transport of non-native species, can have far-reaching consequences. In addition, the atmosphere, rivers, and oceans can very effectively deliver pollution from one area to the next. For example, greenhouse gases released in any part of the world impact global climate. Environmental issues that affect the entire planet require a global response, guided by policies we all agree to follow.