Because oceans are so vast — covering more than 70 percent of the Earth's surface — many people once thought of them as an unlimited resource. They believed that oceans could provide never-ending supplies of fish and other foods, and absorb infinite quantities of sewage, runoff, and other pollution. We now know that this is not the case.
Human activities are causing dramatic changes to the ocean; the most serious are diminished fish stocks, pollution, and climate change.
When not done sustainably, fishing can cause sharp declines in fish populations. Through overfishing, New England cod are now nearly gone, although their schools were once so abundant that fishing boats had difficulty moving through them. Many species of sharks, bluefin tuna, and west coast rockfish are also overfished (Monterey Bay Aquarium).
Water pollution from runoff and agricultural areas can also harm or kill ocean life. About 80 percent of ocean pollution begins on land, most from "nonpoint" or diffuse sources. For instance, small drops of oil from millions of vehicles make their way to oceans. Farms, ranches, and forestlands also produce nonpoint source pollution through erosion, and can deposit organic matter, fertilizers, and pesticides in oceans.
Even a lazy day at the beach may have long-term consequences for oceans. Scientists recently discovered that 10 percent of the world’s coral reefs are being killed by sunscreen worn by swimmers. Researchers believe that chemicals commonly found in sunscreen make coral reefs more susceptible to certain viruses, causing the coral to bleach out and eventually die (Than).
Pollution may be threatening the oceans' ability to sustain Earth's systems. More than half the oxygen we breathe comes from the photosynthesizing algae that live in the ocean. Air pollution — from burning fossil fuels, dirt, and other sources — is settling into oceans and reducing their ability to produce oxygen (see-the-sea.org). Scientists have also discovered that the oceans have absorbed nearly half of all the carbon dioxide (CO2) created by humans since the industrial revolution. That extra CO2 is changing ocean chemistry, and slowing the growth of plankton, coral, and other organisms that are the foundation of the ocean food chain (Pickrell).
Climate change — and its effect on oceans — is another serious threat to sustainability. Higher air and water temperatures are already causing loss of sea ice, sea level rise, and extreme weather events — all of which affect humans, as well as marine fish and wildlife populations (Ocean Conservancy).
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