Marine heat waves clog ocean's carbon sequestration mechanism
Two major marine heat waves in the northeastern Pacific have disrupted the ocean's natural process of sinking carbon to the seafloor via zooplankton fecal pellets. A new study reveals shifts in phytoplankton and zooplankton communities that reduced carbon export during these events. This malfunction could impair the ocean's ability to absorb a quarter of human CO2 emissions.
The ocean acts as a vital carbon sink, where phytoplankton absorb CO2 and are consumed by zooplankton, producing fecal pellets that sink to the depths, locking away the gas. However, marine heat waves are interfering with this process. In the northeastern Pacific off Alaska, two prolonged heat waves—one from 2013 to 2015 and another from 2019 to 2020—altered the marine ecosystem, leading to a buildup of carbon particles near the surface instead of their transport downward.
The study, published on October 6, 2025, in Nature Communications, analyzed a decade of data from Biogeochemical Argo floats in the northeastern subarctic Pacific. These autonomous devices measure ocean chemistry from surface to depths, providing insights without constant ship-based sampling. Lead author Mariana Bif, a marine biogeochemist at the University of Miami, explained how heat waves create a warm, low-density surface layer: “Then in the subsequent spring and summer, that water is even warmer, because it didn’t cool the winter before.” The first event coincided with an El Niño, exacerbating warming, while the second featured reduced salinity that further stabilized the warm surface cap.
Lack of winter winds prevented mixing of cooler deep waters, cutting off nutrients for phytoplankton. This favored smaller phytoplankton species that require fewer nutrients, which in turn supported smaller zooplankton. Co-author Colleen Kellogg noted, “Those guys are going to make smaller fecal pellets, which would kind of float in the water more than sink.” As a result, carbon particles accumulated at 660 feet during the first heat wave and between 660 and 1,320 feet in the second, where zooplankton fragmented them into non-sinking bits. Bacterial breakdown of this organic matter released more CO2 back into the water.
Oceans have absorbed 90% of excess human-generated heat, with extreme temperatures now affecting over 50% of the surface—up from 2% in the 19th century. Anya Štajner, a PhD candidate at Scripps Institution of Oceanography, emphasized the broader implications: “The ocean is changing. And not only is it going to affect the ocean — it’s going to affect the life in the ocean. And eventually that’s going to affect us.” While regional variations exist, such disruptions threaten global carbon sequestration, especially as heat waves intensify without rapid emissions cuts.