Our planet would be unrecognizable without the ocean for myriad reasons, not least of which is the key role it plays in keeping our climate in check. A specific ocean layer, known as the twilight zone, can take a lot of the credit for making that happen.
Carbon dioxide levels in our atmosphere "would jump by nearly 50 percent" without the many ecological services the twilight zone provides, according to the Woods Hole Oceanographic Institution based on Cape Cod in Massachusetts. Scientists at that research hub explored in a recent report what we already know about this complex marine ecosystem and the questions that remain.
From housing the vast majority of marine fish by weight, to facilitating the removal of up to 6 billion metric tons of carbon from the upper ocean every year, the twilight zone's daily operations help make life possible — within and beyond its salty waters. And though this fact has long been true, we've taken it for granted as our climate has been changing.
"Some of the impacts we're experiencing [during] the climate crisis would be much worse if it weren't for the ocean as it normally operates," said Ken Buesseler, a biogeochemist at Woods Hole, who contributed to the report.
Yet in the face of indiscriminate fossil fuel use, the planet is struggling to maintain its ecological order. Human-driven climate change is posing a major threat to the ocean's ability to regulate itself and, by extension, the global ecosystem.
You can think of the ocean like a gradient. Sunlight illuminates a small stretch of the surface, then filters down until it completely disappears at the ocean's pitch-dark depths, around 1,000 meters below. Separating these two extremes is a massive, shadowy midwater section that teems with life.
Buesseler thinks of this twilight zone – also called the mesopelagic zone, from the Greek words for 'middle' and 'open sea' – like a gatekeeper between the surface and the deep. It facilitates the transfer of some of the carbon that started out in our atmosphere down to the bottom of the ocean, where it can stay sequestered for hundreds to thousands of years.
But a warmer, more acidic ocean does us no favors when it comes to maintaining its role as one of the biggest carbon sinks on our planet. The ocean stores 50 times more carbon than the atmosphere, and 20 times more than land plants and soil combined, Buesseler said. If these climate trends persist, it's not clear whether its title as carbon champion will be sustainable.
In addition to studying how the ocean takes in carbon and how those processes are at risk, there's another alluring, existential question: Can humans intentionally alter the ocean to be even more efficient at soaking up carbon dioxide in order to stave off ecological catastrophe?
Here's a look at exactly how the ocean helps us get rid of excess carbon, the ways its systems are already shifting due to climate change and researchers' biggest hopes and fears looking to an uncertain future when it comes to our own carbon emissions.
The 'dance' that keeps carbon in check
Where the sun shines at the surface of the water, microscopic plants called phytoplankton and algae are happily soaking up those rays and taking in carbon dioxide while they photosynthesize. In exchange for that CO2, these largely invisible organisms produce around half of the oxygen we land dwellers breathe, Buesseler noted.
Then, when the sun goes down, the countless fish and other sea creatures that dwell in the midwaters embark on a long journey, traveling up hundreds of meters to feed on those tasty aquatic plants (and each other) under the cover of darkness. As they eat, they consume some of the carbon that those tiny plants absorbed from the atmosphere. At sunrise, the creatures make their way back to the twilight zone to hide from predators until repeating the process the next evening.
"It is this huge, coordinated dance that all of these organisms go on every single day," said Morgan Raven, an organic geochemist and geobiologist at the University of California Santa Barbara. "It's happening on time scales we could observe with our eyes, [but] it's invisible to most of us."
When those small fish poop, or when their bigger predators poop, that waste contains some of the carbon captured by the plants. So too do the decaying bodies of any sea creatures.
Meanwhile, some percentage of the microscopic plant life that thrives at the surface dies and sinks fairly quickly through the water column, Raven said. These tiny particles combine with other organic materials, plus little pieces of rocks and shells, to form what researchers call "marine snow," and for good reason: this amalgamation of particulate ocean waste strongly resembles snow falling on a winter night.
All of this matter has the chance of reaching the deep ocean, where it can stay out of the atmosphere, fueled by plain old gravity.
Through the eyes of a robotic explorer on the seafloor, marine snow looks like "primordial soup coming down on you," Raven recalled.
"It's just flecks of carbon from basically the atmosphere maybe a week ago that got turned into something alive that's sinking to the seafloor," she added. "And if it gets into the mud, it could be in a rock for millions of years" – a connection between our present and a much larger geologic time scale.
The daily twilight zone commute — known as the migrant pump in the report — plus the gravitational pump fueled by marine snow are two of the ways the ocean transfers carbon. There's also the mixing pump, which is fueled by specific events like winter storms that help push carbon from the surface to the depths through the process of physical mixing, Buesseler said.
South Africa dairy farmers eye carbon credits while curbing emissions
Given the vastness of the global ocean, it's difficult to get a clear idea of what's going on across its depths. Buesseler emphasized that scientists have a lot of outstanding questions in terms of how exactly these pumps operate, and just what portion of carbon actually makes it to the bottom of the ocean without getting eaten or soaked back into the water as carbon dioxide expelled by respirating fish along the way. An estimated 90 percent of particulate carbon gets consumed on its trip through the water column, depending on location and time of year.
Carbon dioxide gets locked and stored in marine waters another way in the form of dissolved inorganic carbon. But that capability may diminish due to the very thing this process has helped protect against.
A changing ocean in a warming world
The key to dissolving carbon dioxide is temperature. Cold water is better at dissolving and absorbing gasses like CO2 compared to warmer water, which is why a large amount of it gets dissolved in the ocean's chilliest waters, according to the report. When that heavy water sinks to the deep sea, large portions of that CO2 can be stored for a long time.
But as the ocean continues to warm like the rest of the planet, its waters are projected to become less efficient at taking in carbon dioxide, and can even release it back into the atmosphere more rapidly.
The more CO2 the ocean takes up, the more acidic its waters become. After years of working overtime to take in some of the excess CO2 we've put in the atmosphere, ocean acidification has already increased 30 percent compared to preindustrial levels, according to the report, and could increase an additional 120 percent by 2100. High-acid environments are bad news for marine life like corals, clams and mussels that build shells out of calcium carbonate. A lower PH in the ocean makes their shells more difficult to grow, more vulnerable to breaking and, in extreme conditions, at risk of dissolving faster than they can grow.
Sea creatures rise to the surface waters to feed at night, then sink back down to the shadowy twilight zone when the sun comes up — a phenomenon called diurnal migration. Video via the Woods Hole Oceanographic Institution
Researchers are working to answer how warming and acidification affect the biological carbon pump. Raven said that changing ocean chemistry will inevitably influence "who's thriving and who's struggling" in the marine ecosystem, which could affect which creatures are making the journey through the water column, or eating marine snow falling to the deep. The report also warns that commercial fishing of the twilight zone without proper management and examination of environmental impacts could "have a substantial impact on the marine carbon cycle."
"We've kind of had it relatively good for a while here, in that the ocean has been forgiving. It's been soaking up more in the past than it will be able to in the future," Raven said. "We're basically filling it up."
Doing more than curbing our emissions
Researchers have a lot more to learn about the twilight zone, and certain developments could offer them more flexibility and opportunity. The rise of robotic, autonomous marine explorers mean that scientists like Buesseler no longer have to go on a ship for 30-day missions in order to gather data, he noted.
But the process of scouting the midwaters is still complicated, requiring a huge variation in sampling in terms of depth, location and time of year. Researchers also need to establish a baseline of how the twilight zone works – but how do you do that while it's actively changing as the planet warms?
"We're trying to figure out what it would have looked like before all of these changes, at the same time that we're just even trying to figure out how to monitor or observe it at all," Raven said.
Across the globe, scientists are also studying how to remove more CO2 from the atmosphere by harnessing the power of the ocean's carbon pumps. These interventions range from distributing nutrients to support more plant growth at the surface, to physically injecting CO2 into the twilight zone or the seabed.
The idea of humans intervening in ecological processes to address a problem – one that we've created – is controversial, raising concern over the potential for unforeseen consequences. But Buesseler argues that while emissions reduction is crucial to addressing climate change, it won't be enough to limit warming to a key 1.5 degrees Celsius above pre-industrial levels.
"You're not going to [limit warming] unless you can also increase storage somewhere on land or in the ocean, and here's this ocean that already is moving billions of tons of carbon," Buesseler said. "Why can't we change that?"
Determining how and where these carbon removal technologies work best, as well as any risks they pose and how to protect the ocean in the meantime, will require steady funding and international cooperation.
The twilight zone is one of several facets of the marine ecosystem that's invaluable to all life on Earth, and researchers say we need to do everything we can to preserve it. But industrialization itself is a kind of multi-century, global human experiment, and we're watching its consequences play out right now. Preventing the worst outcomes while conserving natural ecosystems in the ocean and beyond will require multi-faceted action on the part of humans — some of which might have sounded like science fiction decades ago.
"It's not like there's an alternative, which is to sit back and do no harm," Raven said. "That door has closed."
