Climate change is set to empty the ocean of nearly a fifth of all living creatures, measured by mass, by the end of the century, researchers have calculated. In a world that heats up three to four degrees Celsius compared to pre-industrial levels, 17 percent of marine biomass - from minuscule plankton to 100-tonne whales - will be wiped out, they reported in the US Proceedings of the National Academy of Science.
To date, Earth's surface has warmed a full degree (1.8 degrees Fahrenheit). Bigger fish and marine mammals already devastated by overfishing, pollution and ship strikes will see especially sharp declines due to rising temperatures.
Even in a "best-case" scenario of limiting warming to 2C - the cornerstone target of the Paris climate treaty - the ocean's biomass will drop off by five percent. Shallow-water corals, which harbour 30 percent of marine life, are forecast to disappear almost entirely under these conditions. Every additional degree will see the ocean biomass shrink by another five percent. Earth is currently on course to be around 4C hotter by 2100. "The future of marine ecosystems will depend heavily on climate change," said Junne-Jai Shin, a biologist at the French Institute for Development Research and one 35 experts from a dozen countries contributing to the study.
"Measures to protect biodiversity and fisheries management will need to be revisited." Fortunately for life on land, oceans - which cover 70 percent of Earth's surface - consistently absorb more than 20 percent of the greenhouse gases humanity spews into the atmosphere. But the accumulation of all that carbon dioxide has also made ocean water more acidic, threatening to upset the delicate balance of the marine food web.
Some regions will be hit much harder than others, the study found. Climate change will reduce marine biomass by 40 to 50 percent in tropical zones, where more than half-a-billion people depend on the ocean for their livelihood, and two billion use it as their main source of protein.
At the same time, the concentration of life at the poles would likely increase, potentially offering new sources of food. The global population is set to expand from 7.3 billion today to nearly 10 billion in 2050, and to 11 billion by 2100, according to the United Nations.
"Up to now, the biggest threat has been overexploitation and the use of destructive fishing gear," said Callum Roberts, a marine conservation biologist and oceanographer at the University of York in England. "But now, the biggest impact is switching over to climate change, and that is playing out in the sea." The number of damaging marine heatwave days has increased by more than half since the mid-20th century, according to a recent study in Nature Climate Change.