Similar to a giant sponge, the ocean absorbs a quarter of the excess CO2 produced every year from human activities (anthropogenic carbon) around the world. Carbon dioxide dissolves in the surface water and through the overturning circulation of ocean currents and mixing processes, is slowly transported into the ocean’s interior—which allows the surface ocean to absorb more CO2. In this cycle, CO2 reacts with the water molecules in the ocean to form carbonic acid in a process known as ocean acidification. Like ocean warming, an increase in ocean acidification can also have a profound impact on marine ecosystems.
University of Hawaiʻi at Mānoa Oceanography Professor Christopher Sabine has devoted his life to understanding the connections between the ocean and anthropogenic carbon. After earning his PhD in chemical oceanography at UH Mānoa in the early 1990s, Sabine spent the next decade conducting high-quality carbon measurements in an effort to better understand where inorganic carbon is stored in the ocean.
“Initially, we were thinking that ocean storage of anthropogenic carbon was a good thing,” said Sabine, who also serves as the associate dean for research at UH Mānoa’s School of Ocean and Earth Science and Technology. “While producing the first robust, global synthesis of anthropogenic carbon based on direct ocean carbon measurements, we in the scientific community came to the realization that the accumulation of more than 100 billion metric tons of anthropogenic carbon in the ocean would negatively impact marine organisms in ways not considered previously.”
With this discovery, the field of ocean acidification research was created. Today, researchers have written tens of thousands of articles on ocean acidification, a term that did not exist 20 years ago.
One of the initial concerns with ocean acidification was the impact it would have on calcifying organisms, that is, creatures that produce calcium carbonate skeletons or shells. Calcium carbonate is what forms the white sand beaches of Hawaiʻi. It is formed when corals, or other calcifying organisms, take a dissolved calcium ion and a carbonate ion and put them together to create a solid, calcium carbonate.
“As the ocean absorbs more CO2 from the atmosphere, the concentration of carbonate ions decreases—nearly 20% so far,” said Sabine. “With less carbonate ion in seawater, it becomes more difficult for corals and other calcifiers to form their critical skeletons and shells.”
Since 2018, Sabine has been monitoring ocean carbon concentrations around Hawaiʻi using autonomous, buoy-based systems he helped develop as a researcher with NOAA. He is also working with colleagues to develop and test new instruments for measuring ocean acidification, as well as methods to better understand the impacts of climate change and ocean acidification on Hawaiian corals.
“Corals are a particularly vulnerable species because they are sensitive to rising ocean temperatures through a phenomenon known as coral bleaching, and they exhibit slower growth rates and increased fragility from ocean acidification,” said Sabine. “These combined stresses, together with an increased risk of damage from a possible hurricane or drowning from rising sea levels, are a grave concern for Hawaiʻi’s coral reefs.”
This effort is an example of UH Mānoa’s goal of Building a Sustainable and Resilient Campus Environment: Within the Global Sustainability and Climate Resilience Movement (PDF) and Excellence in Research: Advancing the Research and Creative Work Enterprise (PDF), two of four goals identified in the 2015–25 Strategic Plan (PDF), updated in December 2020.