Understanding the world's thermostat

Nov 9, 2017

Jim Burkitt and Andrew McDonnell pull a oceanographic instrumentation package near the equator.
Credit Courtesy of Andrew McDonnell

Scientists have been trying to find ways to reduce our carbon dioxide emissions for decades, but the scientific community has also been studying natural ways the earth stores carbon, preventing it from being released into the atmosphere. The largest natural storage container for carbon is the ocean, and new details are emerging on how waters near the equator are sending carbon-rich material into the deep ocean and diverting it from the atmosphere.

Scientists have known for some time that the ocean stores a lot of carbon, essentially acting as the world’s thermostat.

“The atmosphere contains 750 petagrams of carbon, but the oceans contain 38,000 petagrams of carbon,” said University of Alaska Fairbanks professor and researcher Andrew McDonnell. “The ocean contains far more carbon than the atmosphere and even the soils and plants combined.”

If you’re wondering just how much carbon that really is, it’s a lot, about 38,000 billion tons.

McDonnell has been working with scientists around the world to get a better idea of how the ocean stores that carbon. He published a study on the topic in Nature Geoscience in October, and he found that waters along the equator are particularly good at sending organic matter, such as phytoplankton blooms that would otherwise decay and release carbon, into deeper waters.

“We found that there was this zone of particles that are sinking down to between 1,000 and 4,000 meters deep into the ocean, and it’s about three times greater than the amount of material that is sinking in places away from the equator,” he explained.

McDonnell and his colleagues used special cameras to examine how the raw or digested particles are descending into those waters, and they found just below the surface, a particular species of zooplankton is helping that happen.

They feed near the surface during the night to avoid predators and return to deeper waters during the day.

“They swim down to the deep ocean, like a 1,000 meters or so, where it’s dark and they can avoid those predators,” McDonnell said. “In doing so, they bring that material that they ate up in the surface ocean and end up defecating at depth and creating fecal pellets and additional particulate matter that ends up staying down deep in the ocean and continues to sink deeper.”

That carbon-dense matter is then carried farther down by deep-water currents, which send it all around the globe, and that material is also rich in nutrients.

“The Gulf of Alaska is one destination for them, where those nutrients and carbon eventually comes to the surface, and that’s part of the reason there’s a lot of nitrate in the Gulf of Alaska, nutrients that can be fuel for fisheries,” McDonnell explained.

So, Alaska’s fishermen have zooplankton along the equator to partially thank for our abundant fisheries, but there is a caveat to Alaska being a major stop for all this carbon.

Because those currents don’t just pick up nutrients or organic matter, they also collect a whole lot of man-made carbon that’s dissolved into the water.

“We’re adding additional dissolved carbon to the subsurface waters, and by the time they get to Alaska, they have a lot of carbon dioxide in them,” McDonnell said. “That makes them more acidic and more prone to cause issues for organisms that are trying to form shells or are operating near their chemical limit in a sense.”

McDonnell notes that there are more areas of the world’s oceans that need be studied and potentially other ecological processes that help them store carbon.

He said understanding those systems is important because just a small change, such as a decline in the zooplankton population, could allow the world’s largest cache of carbon to begin releasing the gas at a faster rate, fundamentally changing how the world’s thermostat works.