By Karyn Simpson
Scientists are taking a serious look at ocean biological systems that temper carbon levels in the atmosphere and trap them in the ocean depths, a way to slow global warming and put off the 2° C temperature rise that would trigger disastrous levels of sea level rise, extreme temperatures, rainfall and drought.
Climate scientist Jennifer Middleton calls these systems the ocean’s biological carbon pump and explained how it works at the annual Comer Climate Conference in southwest Wisconsin this fall.
Middleton, a post-doctoral research scientist at Columbia University’s Lamont-Doherty Earth Observatory, is studying these systems in the hope that scientists can find ways to use them to help mitigate the effects of climate change related to fossil fuel emissions.
As outlined in the Intergovernmental Panel on Climate Change’s 2018 special report released in October, human-forced climate change is reaching a critical level as fossil fuel emissions of carbon dioxide drive global warming.
We know that the ocean absorbs some 25 percent of the human-produced carbon dioxide in the atmosphere, mitigating climate change at the expense of ocean acidification that is already threatening marine life. But the ocean’s biological carbon pump is different. The natural oceanic system relies on organisms such as phytoplankton and beneficial algae to draw carbon dioxide from the air, trap it as organic carbon and send it into the deep ocean, where it gets buried and is unlikely to rise to the surface for at least a millennium. This results overall in less carbon dioxide in the atmosphere – just what we need as we head toward the global warming tipping point of 2° C, the warning cry of the recently released IPCC report.
“There are these regions of the ocean where circulation causes the water to come up or down,” said Middleton, who is studying ancient climate variability and iron fertilization in the South Pacific Ocean. “In these regions, the exchange of gases and heat between the ocean and the atmosphere are really important because they kind of set the scene for what gets pushed back down in the ocean and circulated through the whole ocean system for a while.”
These systems set the stage for the biological carbon pump, Middleton said. In areas where there is significant upwelling of nutrient-rich water to the surface of the ocean, you get a higher opportunity for ocean productivity, meaning it can take more carbon from the air.
“In these regions, if you can get the ecosystem to generate a lot of primary production – so a lot of photosynthesis, turning carbon dioxide into organic matter – you can sort of suck carbon dioxide out of the atmosphere and turn it into something that is fundamentally different from inorganic carbon, which then chemically behaves quite differently in the ocean,” Middleton said.
This organic matter is typically algae, Middleton said, which may sink to the deep ocean after it dies and – along with the carbon it contains – get buried on the ocean floor.
“If it gets buried as organic carbon in the sea floor, then it’s kind of trapped there and you don’t have to worry about it anymore,” Middleton said.
It doesn’t all get buried, though. Some of the carbon-containing algae will get eaten or will decay before it reaches the ocean floor, which oxidizes the carbon and ultimately releases it back into the air as carbon dioxide the next time the surrounding water returns to the surface, Middleton said.
Ocean productivity – judged by how much carbon it biologically removes from the atmosphere – is highly variable across the globe and depends on a number of factors including water temperature, available nutrients and water density. By studying historical productivity patterns, scientists hope to learn what made certain areas and time periods more productive in order to explore possible ways to duplicate that in our present-day oceans more productive.
“Right now, [productivity] is higher than it was during the last ice age,” said Kassandra Costa, a post-doctoral research scientist at Woods Hole Oceanographic Institution. Costa spoke at the Comer Climate Conference about her research into productivity patterns across the North Pacific Ocean during the last glacial maximum. “It’s a little bit tricky as far as forecasting what’s going to happen under modern climate change because in addition to the nutrients that are reaching the surface, there are other factors that might influence how productive it could be. So I could imagine how productivity might increase with an increase in climate change because the surface water is getting warmer and it has more access to nutrients, but there are other factors that could actually reduce the changes in productivity.”
The biggest limiting factor, Costa said, is the availability of nutrients and micronutrients needed to transform carbon dioxide in the air to organic carbon that can be buried on the ocean floor. When significant upwelling brings nutrient-rich water to the surface of the ocean, the key missing ingredient is iron.
“Today, they’re quite productive, but if they’re too productive, they run out of iron, they run out of that vitamin that they need,” Costa said. “So that could be something that could basically just slow them down from taking off and being hyper-productive as a result of anthropogenic [human-forced] climate change.”
Scientists have hypothesized about a process called “iron seeding,” which could artificially force the ocean to be more productive and bury more carbon dioxide. This would involve adding iron to parts of the ocean that have a history of high productivity for the carbon pump system in hopes that doing so would eliminate that limiting factor and allow productivity to blossom.
“People have said, ‘If we throw a bunch of iron into the Southern Ocean, that will cause a bunch of algae blooms that will then remove carbon from the atmosphere and maybe, hopefully, push it all the way down to the deep ocean and bury it. And then it’s not our problem.,’” Middleton said.
But there are several unknowns with this theory that cause researchers to hesitate.
“We don’t know the efficiency of what fraction of the carbon would stays down there versus coming back up again later, from a geoengineering perspective,” Middleton said. Even if iron seeding increases productivity in an area, that means there’s no way to say exactly what percentage of that would be buried long-term. “And also, just every time humans try to do major-scale interventions of the earth system, it kind of backfires.”
There is also no conclusive data on what effects iron seeding would have on the ocean as a whole, Costa said. While the process might help remove carbon dioxide from the atmosphere, there may be consequences for the ocean system that scientists haven’t yet predicted.
“I think it’s a little bit complicated because we don’t fully know the repercussions that it might have for the whole ocean system,” she said. “I think people are working on it, but I think at the same time we’re trying to be cautious because we don’t fully understand what the full repercussions of experimenting with the ocean like that would do.”
While scientists are interested in continuing to research the possibility of using the ocean to help delay the IPCC’s predictions, it isn’t the end-all solution, and other actions will need to be taken to avoid dramatic impacts from climate change across the globe.
“The ocean’s not going to save us, not going to save the planet from warming, but it will potentially help,” said Aaron Putnam, assistant professor at the University of Maine’s School of Earth and Climate Science. “I mean, half the carbon that goes in the atmosphere goes straight to the ocean. And a good portion of the heat that goes to the atmosphere, that’s in the atmosphere, gets mixed out in the deep ocean. But you know what the net balance is. You can see it in the CO2 charts. You can see it in the temperature records. It’s still warming, so that’s not enough.”
Atmospheric carbon dioxide has now topped 400 parts per million due to fossil fuel emissions, while natural levels have never topped 300 parts per million in the 1 million years leading up to the Industrial Revolution.