Final answer:
The primary source of saltwater on Earth is the oceans, which hold most of the Earth's saltwater. Freshwater is mostly contained in glaciers and ice caps, with only a small percentage available as liquid freshwater. Estuaries are dynamic environments with variable salinity, and coal was the main contributor to CO2 levels during the Industrial Revolution.
Step-by-step explanation:
The source of salt water on Earth is primarily the oceans. They contain a significant amount of salt, making up most of the planet's saltwater reservoirs. Glaciers and ice caps, also mentioned, are large bodies of freshwater, not saltwater. The majority of freshwater, accessible or not, is found in glaciers and ice caps, with a minor portion being available as liquid freshwater.
When considering aquatic habitats, an estuary is characterized by its varied salinity; it is an area where freshwater from rivers mixes with salty seawater, creating a dynamic environment with fluctuating salinity levels. Finally, during the Industrial Revolution, coal was the primary fuel used, which contributed to increased levels of carbon dioxide and subsequent global warming.