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The deeper layers of the ocean are warming at a slower pace than the surface but animals living in the deep ocean are more exposed to climate warming and will face increasing challenges to maintain their preferred thermal habitats in the future, according to a new model in Nature Climate Change which analyzed contemporary and future global patterns of the velocity of climate change across the depths of the ocean. 

Despite rapid surface warming, the team found that global mean climate velocities in the deepest layers of the ocean, greater than half a mile below the surface, have been 2X to nearly 4X faster than at the surface during the second half of the 20th century. The larger velocities are due to greater thermal homogeneity of the deep ocean environment and their metric describes the temporal rate and direction of temperature changes, as a proxy for potential shifts of marine biota in response to climate warming.

While climate velocities are projected to slow down under scenarios contemplating strong mitigation of greenhouse gas emissions (RCP2.6), they will continue to accelerate in the deep ocean.

Climate velocities in the mesopelagic layer of the ocean (200-1000 m) are projected to be between 4 to 11 times higher than current velocities at the surface by the end of this century. Marine life in the mesopelagic layer includes great abundance of small fish that are food for larger animals, including tuna and squid. This could present additional challenges for commercial fisheries if predators and their prey further down the water column do not follow similar range shifts.

The authors also compared resulting spatial patterns of contemporary climate velocity with those of marine biodiversity for over 20,000 marine species to show potential areas of risk, where high biodiversity and velocity overlap. They found that, while risk areas for surface and intermediate layers dominate in tropical and subtropical latitudes, those of the deepest layers are widespread across all latitudes except for polar regions.

"Our results suggest that deep sea biodiversity is likely to be at greater risk because they are adapted to much more stable thermal environments," says Jorge Garci?a Molinos, a climate ecologist at Hokkaido University's Arctic Research Center, who contributed to the study. "The acceleration of climate velocity for the deep ocean is consistent through all tested greenhouse gas concentration scenarios. This provides strong motivation to consider the future impacts of ocean warming to deep ocean biodiversity, which remains worryingly understudied."

The scientists caution that while uncertainty of the results increases with depth, life in the deep ocean is also limited by many factors other than temperature, such as pressure, light or oxygen concentrations. 

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