Much of the ocean we see may seem as desolate as a desert and many of the animals that live here are fighting an eternal struggle to find food and preserve valuable energy where they swim through the "The Big Blue Seas", on wide-ranging migrations that can take them across oceans and back.
The world's oceans produce 70 percent of our oxygen and absorb huge amounts of greenhouse gases. Sea water is salty and it has an average salinity of 35 parts per thousand. The salinity is constantly changing and it is influenced by the discharge from large rivers, evaporation and various temperatures. The temperature of the sea is different depending on where on earth and on how deep you measure it. From the warm tropical sea surfaces 25 degrees C. to the ocean depths, or the seas around the poles that reach only a few degrees above freezing at the most. The water in the oceans move across the earth in so called ocean currents. Ocean currents can be thousands of kilometers long and they have a major impact on the climate and ecosystems around the globe.
Threats to the oceans:
Global warming and the acidification of ocean water.
Overfishing and the destruction of natural protection like coral reefs.
Garbage dumping, especially plastic.
International shipping and their release of ballast water transporting alien species around the world.