Climate scientists began to notice something wasn’t quite adding up around 2003. The levels of greenhouse gases continued to rise. The sea level continued to rise. The ice continued to retreat. However, it appeared as though the planet had momentarily stopped warming as surface temperatures plateaued. As the climate debate grew louder, some pundits used that opportunity to raise concerns about the overall situation. They failed to take into consideration what was going on deep underwater, which at the time hardly anyone was taking into consideration.
It turns out that the heat that the atmosphere was not displaying had been quietly absorbed by the ocean. Not at the surface, where measurements are most convenient, but far below, in waters that most instruments could hardly reach, between 100 and 300 meters below the surface. The surface told a completely different story, but that’s where the warmth was going, tucking itself into the Indian Ocean and the western Pacific.
In 2015, researchers at NASA’s Jet Propulsion Laboratory finally verified this, revealing that during the same decade that surface temperatures appeared to stagnate, the ocean’s subsurface layers had been steadily warming. Climate change did not stop. Heat was shifting to a less obvious location. Most people are unaware of how important that distinction is.
The current work being done by researchers at the Rosenstiel School at the University of Miami builds directly upon that understanding and advances it in ways that seem truly novel. Instead of merely monitoring temperature, they are examining the chemistry of the ocean, particularly in areas where oxygen has been reduced and microbial life thrives in environments that are uncommon for most of the surface world.
Mariana Bif, an assistant professor in the Department of Ocean Sciences and the work’s principal investigator, created a technique to extract chemical signals from data that was never intended to do so. The devices in question are autonomous robotic sensors called BGC-Argo floats, which drift through the ocean and gather data about every ten days. They were designed to quantify nitrate. Bif’s group discovered a way to extract measurements of thiosulfate and nitrite from the same UV spectra that those sensors were already gathering. To be honest, it’s the kind of methodological innovation that isn’t given enough credit outside of scientific circles.
They were taken aback by what they discovered in the Eastern Tropical North Pacific. The microbial processes that control how much nitrogen escapes from the ocean into the atmosphere, known as nitrogen cycling in low-oxygen zones, proved to be far more dynamic and variable than previous models had predicted. These processes are dynamic. They change in response to shifting ocean conditions in ways that eventually affect the atmosphere above, the carbon cycle, and marine food webs.
All of this is part of a larger narrative. In a different study, Bif and researchers from the Monterey Bay Aquarium Research Institute looked at two significant marine heatwaves that occurred in the Gulf of Alaska between 2013 and 2020. They discovered that warming events impede the ocean’s biological carbon pump in addition to raising temperatures. Under normal circumstances, carbon dioxide is absorbed by phytoplankton close to the surface, which then dies and sinks, trapping the carbon for thousands of years. That conveyor belt stalled during the heat waves. Rather than falling, carbon accumulated close to the surface. Although the pump didn’t completely stop, it did slow down and change in ways that might have long-term effects on how much carbon the ocean can truly absorb in the future.
It’s still unclear how long-lasting these disruptions are and whether there is complete or partial recovery in between heatwaves. Continuous ocean monitoring is crucial today, in part because of this uncertainty. Over the past few decades, the size and intensity of marine heatwaves have increased. The climate projections we are using may need to be revised if the ocean’s ability to absorb carbon is diminishing, even in part, during these occurrences.
This is where the University of Miami team’s method comes in handy: it allows for the extraction of more data from instruments that are already in the water without the need for a completely new fleet of sensors or years of extra deployment. That has a certain grace to it. Science doesn’t always advance by creating something significantly bigger. Sometimes it advances by learning to read what is already there with greater care, thoroughness, and honesty than before.
The majority of this planet is covered by the ocean. Compared to the land above it, the majority of it is still much less understood. The chemical, thermal, and biological processes occurring in those deeper layers are not incidental. It could be the main one.
