A new study suggests that the deep ocean isn’t able to hold anywhere near as much carbon as previously thought.

Scientists looked at the cycle of carbon as it’s sucked up by microscopic plants living near the surface of the water that then drift down to the seabed. Based on new particle tracking models, it turns out that this process is ‘leakier’ and retains less carbon long-term than earlier estimates.

“The ocean is an important carbon sink, and the depth to which biological carbon sinks affects how much atmospheric carbon dioxide the ocean stores,” says Chelsey Baker, an Ocean Biogeochemical Model Analyst from the National Oceanography Centre in the UK.

“In this study, we show that the longevity of carbon storage in the deep ocean may be considerably less than what’s generally assumed.”

Carbon needs to be locked away for 100 years to be on a climate-relevant timescale.

Up until now, it was thought that the circulation pathways of the deep ocean would keep every bit of captured carbon that reached a depth of 1,000 meters (3,250 feet) tucked away from the world for several millennia.

Here, the simulations used by the researchers found that only an average 66 percent of the carbon reaching a depth of 1,000 meters (3,250 feet) in the North Atlantic Ocean would be stored for a century or more.

While the efficiency of CO₂ capture varied based on factors including ocean currents and temperature, carbon needed to reach a depth of 2,000 meters (6,500 feet) to be almost certain of staying stored for more than 100 years – at that depth, 94 percent of carbon stayed put for a century or more, the simulations showed.

“These findings have implications for estimates of future predictions of carbon sequestration by global biogeochemical models, which may be overstated, as well as for carbon management strategies,” write the researchers in their published paper.

The news piece can be read here and the published paper is here.