Why is the sea water salty, and not the water of the big lakes? Is the salt concentration changing over time?
It is thought that the salt in the oceans stems from erosion of bedrock on continents, where the minerals from these rocks are eventually carried out by rivers to the oceans. Over time, the oceans, which act as the final sink for almost all rivers, become more salty. Rivers that don't make it to the ocean are trapped in basins known as "endoheric" basins, instead ending in a lake (like in the case of the Dead Sea) that also gets saltier over time. Most big lakes with outlets are fed by fresh water, either from the atmosphere as rain, from the melting of glaciers, or from underground aquifers, leaving them less salty.
People don't really know what happens to the salt concentration (or salinity) of the oceans over time; Accurate salinity maps of oceans are hard to produce since oceans are so big. As the Earth's temperature rises in the future (both from natural and human effects) - two things will happen to the salinity. First, increased evaporation over oceans will tend to make the salinity rise. Second, increased melting at the poles will bring more freshwater into the oceans, which will decrease the salinity. Which of these two effects dominates the water cycle in the future will determine the change in salt concentration.
For more information about the salinity data that is available (in North America), check out the National Oceanographic Data Center's database.
This page was last updated on June 27, 2015.