Most important, studies like these help us to prepare for the future by providing an idea of what to expect. In the words of study co-author Toby Ault, “The time to act is now. The time to start planning for adaptation is now. We need to assess what the rest of this century will look like for our children and grandchildren.” Because most fresh water is tied up in glaciers, ice caps, and underground aquifers, a little more than 1 part in 10,000 of Earth’s water is fresh water that is easily accessible for human use.
Water is renewed and recycled as it moves through the water cycle. Precipitation falling from the sky either sinks into the ground or acts as runoff to form rivers, which carry water to the oceans or large inland lakes. As they flow, rivers can interact with ponds, wetlands, and coastal aquatic ecosystems. Underground aquifers exchange water with rivers, ponds, lakes, and the ocean through the sediments on the bottoms of these water bodies. The movement of water in the water cycle creates a web of interconnected freshwater and marine aquatic systems that exchange water, organisms, sediments, pollutants, and other dissolved substances.
What happens in one system therefore affects other systems—even those that are far away. Let’s examine the freshwater components of the interconnected system, beginning with groundwater. Marine and coastal components of the system will be examined subsequently, but note that all these systems interact extensively.