National and international policies are needed to address global environmental problems because what happens in one area of the world can affect another.
Rivers transport species, nutrients, and pollution, which eventually reach the sea.
Ocean currents transport pollutants to nearby and distant areas.
Damming or pulling too much water out of rivers can cause them to run dry in other regions downstream.
Overexploitation of common resources like ocean fish stocks affects everyone.
Air currents transport pollutants throughout the atmosphere, such that pollution generated in one area can migrate to another. When greenhouse gases are released, they affect global rather than just local climate.
As a consequence of global warming, sea levels rise. Rising sea level caused by ice melt and thermal expansion affects low-lying areas first.
Environmental issues that affect the entire planet require a global response, guided by policies we all agree to follow.