In the past 50 years, the amount of water in the open ocean with zero oxygen has gone up more than fourfold. In coastal waters, including river mouths and seas, low-oxygen sites have increased more than tenfold since 1950. Scientists expect oxygen to continue decreasing even outside these zones as Earth warms.

"Oxygen is fundamental to life in the oceans," said Denise Breitburg, a marine ecologist from the Smithsonian Environmental Research Center. "The decline in ocean oxygen ranks among the most serious effects of human activities on the Earth's environment. Actually, it's a mass loss to all the support services that rely on recreation and tourism, including hotels and restaurants and taxi drivers and everything else. The reverberations(影响) of unhealthy ecosystems in the ocean can be extensive. "

In areas traditionally called "dead zones", like those in the Chesapeake Bay and the Gulf of Mexico, oxygen plummets(速降) to such low levels that many animals die. As fish avoid these zones, their habitats become smaller and they're easier to be attacked or caught. But the problem goes far beyond "dead zones". Even the smaller oxygen decline can prevent growth in animals, hinder reproduction and lead to disease or even death. It can also cause the release of dangerous chemicals such as nitrous oxide, a greenhouse gas up to 300 times more powerful than carbon dioxide. While some animals can boom in "dead zones", overall biodiversity falls.

Climate change is the key criminal in the open ocean. Warming surface waters make it harder for oxygen to reach the inside of the ocean. Furthermore, as the ocean as a whole gets warmer, it holds less oxygen. In coastal waters, too much nutrient pollution from land creates algal blooms, which use up oxygen as they die and break down.

People's livelihoods are also on the line, the scientists reported, especially in developing nations. Smaller fisheries(渔场) may be unable to relocate when low oxygen destroys their harvests or forces fish to move elsewhere. So we humans have to carry on a war now and win the war.