Climate change may soon transform oceans teeming with complex life into simple ecosystems that can no longer support the billions of people relying on them as a primary source of food, according to a new study.

Unabated warming and ocean acidification will devastate numbers of zooplankton and smaller fish with a knock-on effect for larger carnivorous fish, according to the meta-analysis, which combines data from more than 600 experiments measuring the effects of climate change on ecosystems. Only microorganisms are expected to increase in number and diversity.

"Our analysis projects decreases in species abundance across many types of organisms. There will be a species collapse from the top of the food chain down," said Ivan Nagelkerken, lead author on the paper and ASC Future Fellow at the University of Adelaide’s Environment Institute in South Australia.

Warmer waters and acidity are also killing coral and oyster reefs. In the US, oyster farms are already spending large amounts of money in an effort to mitigate the negative effects, said Nagelkerken.

Oceans are becoming increasingly acidic as they absorb more and more carbon dioxide. Absorbed carbon dioxide dissolves water and forms carbonic acid, changing the ocean's PH value from slightly alkaline.

Taking action

Global carbon dioxide emissions will likely continue to rise into the near feature, with UN climate chief Christiana Figueres recently warning that the upcoming climate talks in Paris will not limit global warming to 2 degrees Celsius above pre-industrial temperatures. That's the level of warming agreed by countries as the threshold for avoiding dangerous climate change.

But the study authors suggest, there are actions than can slow down climate change's impact on ocean biodiversity.

"We may achieve this by reducing overfishing, thereby maintaining healthy populations and a diverse gene pool in our fishes, and providing them with the chance to possibly adapt," said Nagelkerken. Reducing pollution would also help.

The study "Global alteration of ocean ecosystem functioning due to increasing human CO2 emissions" was published in the journal Proceedings of the National Academy of Sciences (PNAS) and is the "most comprehensive forecast" to-date of the impact of climate change on our oceans, according to the authors.