Nearly a billion people whose survival and livelihood rely on the ocean will be among the first to experience the impacts of global warming that are already underway, according to a recent study.

Projected changes to the oceans’ biogeochemistry, or the condition and content of the seas, over the 21st century were the subject of the study co-authored by 26 scientists and published in PLOS Biology. The scientists identified key stressors in the ocean that without climate change mitigation will increasingly create hot spots of severe environmental degradation.

About 1.4 billion people live in coastal areas that will experience medium to high ocean biogeochemistry change by 2100. Of those, 690 million live in nations with a medium to high ocean dependence.

“The pattern that emerges is that the temperature is going up and productivity is going down,” said Lisa Levin, a co-author of the study and director of the Center for Marine Biodiversity and Conservation at the Scripps Institution of Oceanography.

Ocean warming due to carbon dioxide emissions, which peaked at 400 parts per million this year, is the main culprit for the decline. By the end of the century, with little or no change in the current emissions scenario, surface ocean temperatures could rise by 2 to 3 degrees Celsius. The subsequent changes would include an increase in acidity, a decrease in oxygen concentration and a decline of ocean productivity by 2 to 20 percent, according to the study.

“In terms of the human element, the hardest hit will be the poorest countries that live on a subsistence matter,” Levin said. “Their food and livelihood comes from the ocean.”

Changes rising up the food chain

In the food web, the first to be affected by a slight or drastic change in the environment are microscopic phytoplankton that serve as a vital food source for fish larvae in shallow and deep ocean floors. A separate study found that cold-water plankton are declining as the waters of the north Atlantic Ocean warm.

The impact on the larger ocean food chain travels from the sea creatures to the communities that are dependent on fisheries and productive coral reefs.

The scientists assembled a global distribution map of 32 marine habitats to assess the potential vulnerability of the ocean’s biological systems.

“A lot of the areas most impacted by stressors have multiple things overlapping because they already are partially dictated by low oxygen and pH levels,” said Andrew Thurber, co-author of the PLOS study and a scientist at the College of Earth, Ocean and Atmospheric Sciences at Oregon State University.

One caveat is that some species will not be harmed. Species that were once rare in a region may expand their numbers.

Cascade of effects

As the ocean’s natural chemistry changes over time, the stressors will create a cascade of effects on species, such as changes in body size and growth, survival and abundance, and range and distribution.

The study does not stop at the science and goes on to expand into socioeconomic risk.

In an effort to measure the impact, the scientists used three different metrics of people’s reliance on the ocean: jobs, revenues and food.

“This highlights the looming vulnerability in developing and low-income countries, and an unfortunate disparity between those who benefit economically from the processes creating climate change and those who will have to pay the most of the environmental and social costs,” the study says.

The solution, the scientists say, is to reduce greenhouse gas emissions. The creation of marine reserves and better management of fisheries could help, but it won’t mitigate the direct impact of climate change, Thurber said.

“If we continue with our business-as-usual scenario and keep on polluting, we’re essentially going to double the amount of people who are least able to adapt to climate change,” he said. “The question now is how willing we are for any sort of strategy we can use for mitigation.”