A recent study explores the limited information on the amount of carbon dioxide produced by human activity in coastal oceans, which are the saltwater environments that connect land and sea. Gathering this data is essential for determining how much emissions reductions are necessary in the future.
Excess carbon dioxide from human actions—including burning fossil fuels, changes in land use, and deforestation—is absorbed by the world’s oceans. This absorption helps reduce global warming, but it can also negatively impact marine organisms, such as fish and plants.
Although the effects of anthropogenic carbon dioxide on open oceans have been thoroughly examined, there has been insufficient observational data concerning its levels and origins in coastal waters. These coastal zones, which span from estuaries to coral reefs, play a vital role in linking land and sea.
A new study led by Wei-Jun Cai’s lab at the University of Delaware, called “The Source and Accumulation of Anthropogenic Carbon in the U.S. East Coast” and published in Science Advances, seeks to fill this information gap.
The primary author, Xinyu Li, received her doctorate from UD’s School of Marine Science and Policy in 2023 and is currently a postdoctoral researcher at the Pacific Marine Environmental Laboratory. Wei-Jun Cai, who serves as associate dean for research and the Mary A.S. Lighthipe Chair Professor of Earth, Ocean, and Environment, advised Li during the study. Co-authors include Zelun Wu, who is pursuing dual doctoral degrees at UD and Xiamen University, as well as Zhangxian Ouyang, another postdoctoral researcher at UD.
The team examined a comprehensive carbonate dataset obtained from five research cruises conducted between 1996 and 2018, focusing on the East Coast of the United States, particularly the Mid-Atlantic Bight, a coastal area that stretches from Massachusetts to North Carolina.
The 1996 dataset was supplied by Doug Wallace, a professor of oceanography at Dalhousie University, which enabled the researchers to monitor changes in carbon dioxide levels over time. Except for the 1996 cruise, data collection was conducted by members of Cai’s group as part of the Ocean Acidification Program led by the National Oceanic and Atmospheric Administration (NOAA).
The researchers utilized this data to explore how much and from where anthropogenic carbon dioxide enters coastal waters, which are essential for understanding the global carbon budget.
The surface waters, which extend to about 200 meters deep, experienced the greatest rise in anthropogenic carbon dioxide due to their direct exposure to the atmosphere, leading to increased absorption of atmospheric CO2.
Cai noted that an interesting finding of the study was the examination of the ratios of natural versus anthropogenic CO2 in the water and how the age of the water influences the accumulation of anthropogenic carbon.
The surface water, which is fresher and comes from the Gulf Stream flowing in from the Gulf of Mexico, showed elevated levels of anthropogenic carbon dioxide alongside lower levels of naturally occurring carbon dioxide.
In contrast, the middle layer of water (below 200 meters) contained higher levels of natural carbon dioxide and lower levels of anthropogenic carbon dioxide.
“The surface water has very high anthropogenic carbon dioxide while the middle layer, known as the Antarctic Intermediate Water that originates from the Southern Ocean, has traveled for a long time, potentially around 100 years from the Southern Ocean to the East Coast,” Cai explained. “This water contains significant natural carbon dioxide due to microbial decomposition, but it has minimal anthropogenic carbon.”
Below these layers resides the North Atlantic Deep Water, which descends during winter and takes over two decades to travel from the Labrador Sea to the East Coast. “This water has slightly elevated levels of anthropogenic carbon dioxide,” Cai added. “Each water mass has documented carbon dioxide levels from the time of its formation, allowing us to trace these changes. It’s fascinating to observe that the newer waters exhibit the highest anthropogenic carbon levels.”
Li characterized this distribution as a “sandwich structure,” featuring high anthropogenic carbon at the surface, low levels in the middle layers, and intermediate levels deeper down.
“This distribution is closely linked to the age of the water, as it interacts with the atmosphere at the surface and absorbs carbon dioxide from the air,” Li stated.
The research also revealed that anthropogenic carbon diminishes from offshore to nearshore waters, which corresponds to decreased salinity. This indicates there is not a net increase in the transport of anthropogenic carbon dioxide from coastal areas, such as estuaries and wetlands, into the open ocean.
“When we apply our findings to low salinity waters, like those exiting the Delaware and Chesapeake Bays, we observed that there is indeed very little increase in anthropogenic carbon dioxide in these low salinity areas,” Cai clarified. “These waters hold a substantial amount of natural carbon dioxide, but very little anthropogenic carbon dioxide.”
This conclusion aligns with previous studies indicating that the transport of anthropogenic carbon dioxide from estuaries and wetlands to the continental shelf is essentially negligible or even negative. Factors contributing to this include their low buffer capacity and short residence times, which hinder their ability to absorb anthropogenic CO2. Additionally, the loss of North American wetlands occurs at three times the rate of their growth, limiting opportunities for carbon capture and transfer to coastal waters.
Cai emphasized the broader impacts of these results on the global carbon cycle. “This study clarifies conflicting conclusions from land-based research,” he stated. “There is ongoing debate about whether there is increased movement of anthropogenic carbon dioxide from land systems to coastal oceans. Our findings indicate that there is no natural transport of anthropogenic carbon, and that carbon found in coastal waters mainly originates from offshore water masses and the atmosphere directly above. The majority of this carbon is then transported offshore into the ocean.”
