Huge swathes of the ocean are dying and environmental writer Alanna Mitchell says humans are to blame.
Mitchell believes human activity is altering everything about the world's oceans, including their temperature, acidity and the life within them. Most of the earth's oxygen is produced in the sea by single-celled organisms. In addition, our climate is regulated by the ocean's currents, winds and water cycle activity.
A dead zone occurs when water at the sea floor is anoxic - or has very low or no concentrations of dissolved oxygen.
In her new book, "Seasick: Ocean Change and the Extinction of Life on Earth," Mitchell takes her readers to one of the world's nearly 500 dead zones - in the Gulf of Mexico, which is a wide expanse of the ocean that should be alive.
"It's all happening because of the fertilizers that farmers up and down the Mississippi River are putting on their crops," says Mitchell.
According to Mitchell, nutrients from the fertilizer causes phytoplankton to grow and multiply out of control. In addition to oxygen, they produce organic matter, which sinks to the sea floor. There, it's broken down by bacteria and gives off carbon dioxide. That CO2, Mitchell says, and carbon dioxide from the air, is changing the ocean's chemistry.
"We are making the ocean acidic. We're doing that by putting carbon dioxide gas into the atmosphere when we burn fossil fuels. Some of that carbon dioxide is absorbed into the ocean and makes carbonic acid."
Mitchell says increased acidity, along with other forms of pollution and climate change, are killing coral reefs, including the Great Barrier Reef in Australia, a World Heritage Site since 1981.
"I was on the Great Barrier Reef, which is the biggest coral reef in the world and you can also have a strong policy to protect a reef like that, but it's the global problems that are going eventually kill the reef, if we don't do anything."
Mitchell traveled to ocean hot spots worldwide to see firsthand what is happening to the marine ecosystem. She chronicles her journeys in "Seasick," which earned the 2010 Grantham Prize for Excellence in Reporting on the Environment.
"I did 13 journeys in two-and-a-half years to different parts of this big, interconnected system that they call the global ocean, I was on ships with scientists trying to figure out what they are learning because they are the ones who are the doctors of the ocean."
Mitchell also visited the Indian Ocean island of Zanzibar where a local solution has been developed to try to deal with the global problem.
"This is a project set up by a Zanzibari scientist who tries to help the women who live in Zanzibar grow little shell fish so they can either sell them or eat them," says Mitchell. "So maybe this is what our future will be. We'll have fisheries that are much more artisan, much more home grown than these big commercial, industrial fisheries that are harming the population of fish in the ocean."
Mitchell says local initiatives like these, and growing support for more sustainable and environmentally friendly forms of energy, give her hope that there is a way to halt and even reverse the damage we've done to the global ocean.