The oceans are one of our planet's most important carbon sinks, with currently around 39,000 gigatons of carbon dioxide locked away – that's around 50 times more than what's circulating in the atmosphere right now.
However, we can't rely on this carbon capture and storage to solve our climate crisis problem, because we're producing too much excess CO2 too quickly.
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 Center 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.
As the climate and the ocean shift, models will need to be updated. Experts think the oceans will become more stratified in the future as they warm up, meaning less mixing between layers – and less carbon sinking down to the bottom.
And the more accurate our models, the better we can work out what's coming and how we might be able to prevent it if necessary. Scientists need to know with as much precision as possible how much CO2 we're producing, how much the ocean is capable of storing, and how long it is likely to be locked away for.
It's possible that by augmenting the natural carbon cycle in various ways, more carbon could be taken out of atmospheric circulation - but to do that, we need to know just how effective and efficient the deep ocean is as a carbon sink.
"Our findings could be important because artificially enhanced carbon storage by the ocean is one avenue being explored to help us achieve net zero by 2050. For example, by ocean schemes for carbon dioxide removal, such as iron fertilization," says Baker.
"The effectiveness of such nature-based solutions is often dependent on the assumption that carbon reaching the deep ocean will be stored for hundreds of years, something our work suggests may not be so simple."
The research has been published in Global Biogeochemical Cycles.