The ocean is vital for sustaining life on Earth. By providing oxygen to the atmosphere and absorbing carbon dioxide emissions, the ocean regulates climate through carbon sequestration. Carbon sequestration is the natural or artificial process by which carbon dioxide is removed from the atmosphere and held in solid or liquid form. The ocean is the largest carbon sequester on the planet, absorbing one-third of all anthropogenic emissions. This process mirrors the forests’ ability to capture carbon from the atmosphere. However, instead of plants intaking CO2, the oceans depend on another important primary producer: microscopic marine algae called phytoplankton. Phytoplankton absorb CO2 and convert it into oxygen through photosynthesis. Phytoplankton form the base of the ocean food web, relying on sunlight and carbon dioxide to grow. These microscopic organisms produce 50% of the oxygen we breathe.
Phytoplankton in the ocean consume carbon dioxide as they photosynthesize. When they are eaten or decompose, some of the carbon they contain is released into the ocean depths; this process is called the biological pump. According to a recent study by Woods Hole Oceanographic Institute (WHOI), the biological carbon pump is twice as efficient as previously estimated. Scientists measured the depth of the sunlit surface layer, or euphotic zone, to determine how efficiently the biological pump captures carbon from particles known as “marine snow.” The scientists determined that the euphotic zone extends to greater depths in some regions of the ocean. Therefore, they estimate that the biological carbon pump carries two times the amount of carbon down from the ocean’s surface than previously thought. This study can help scientists further incorporate biological processes to create more accurate global climate models.
The ocean acts as a huge carbon sink. The carbon not consumed by marine organisms (about 10%) is stored in sediment before it is transformed into hydrocarbons. The current concentration of atmospheric carbon as of November 29, 2021, is 416 parts per million (ppm). Without the biological pump, it is estimated that atmospheric carbon would have reached 600 ppm. Mauna Loa Observatory provides a graphical representation of the change of atmospheric carbon over time. The biological mechanism for capturing and storing carbon interacts with an important physical mechanism known as the physical or solubility pump. The solubility pump transports dissolved inorganic carbon through the water column. The solubility of CO2 in the ocean is temperature dependent. Cold, polar regions of the ocean act as carbon sinks, whereas warm, equatorial waters are sources of carbon.
“While the ability of the ocean to capture and store carbon has helped to slow the accumulation of atmospheric CO2, it has come at a cost,” states Dr. Jamie Shutler, a professor of earth observation at the University of Exeter. Since the Industrial Revolution, scientists have observed a 26% increase in the ocean’s acidity. The United Nations Educational, Scientific and Cultural Organization (UNESCO) states that “as CO2 dissolves in seawater, it forms carbonic acid, decreasing the ocean’s pH [potential of hydrogen].” This results in ocean acidification, or the decrease in the ocean’s pH value. Ocean acidification changes the chemistry of seawater and has detrimental impacts on marine life. The ocean’s surface pH has dropped by 0.1 pH units in the last 200 years and is projected to further decrease by 0.3-0.4 pH units by the end of the 21st century. Ocean acidification is measured on a logarithmic scale, meaning that a change of one unit represents a tenfold change in acidity.
Ocean acidification has far-reaching implications on the health of the ocean and all who depend on it. This change in the ocean’s chemistry impacts marine organisms and ecosystems. Coral reefs are one of the most biodiverse ecosystems on the planet but, the effects of ocean acidification threaten their extinction within the next 50 years. Coral ecosystems, which an estimated 25% of all marine life depend on, suffer from the influx of CO2 emissions and warming ocean temperatures. Weakened coral along the coastlines also leaves coastal communities vulnerable to severe weather events. Ocean acidification has also been shown to endanger fisheries and aquaculture. UNESCO states that “Regular observations and measurements of ocean acidification in open oceans and coastal areas are necessary to improve our understanding of the effects, enable modelling and predictions and help inform mitigation and adaptation strategies.”
Scientists argue that unless we significantly reduce anthropogenic CO2 emissions, both the levels of carbon dioxide and ocean acidity will continue to rise. However, there are a few proposed ocean carbon removal approaches to consider. First is the idea of coastal blue carbon. Coastal ecosystems, that is mangroves, saltmarshes, and seagrass meadows, are among the most productive ecosystems on the planet, and they are incredibly efficient at capturing and storing atmospheric carbon. Current studies from the National Oceanic and Atmospheric Administration (NOAA) suggest that “mangroves and coastal wetlands annually sequester carbon at a rate ten times greater than mature tropical forests.” They also store three to five times more carbon per equivalent area than tropical forests. Coastal habitats capture significant amounts of atmospheric carbon and store this carbon in the soil. However, when these habitats are destroyed the stored carbon is released back into the atmosphere. Unfortunately, the loss of coastal habitats is occurring at an alarming rate, mainly due to coastal infrastructure, ports, and extreme weather events. Still, these habitats offer a sustainable approach to climate change mitigation. By protecting coastal habitats, we protect coastal communities, provide habitats for an abundance of species, and create a space for shared collaborative efforts towards climate change mitigation.
Another proposed biological approach is the cultivation of large-scale seaweed, or macroalgae. This approach relies on the seaweed’s ability to photosynthesize. Scientists already know that seaweed captures carbon, but they can also leverage this potential by cultivating and harvesting seaweed for use in a variety of products such as biofuel, animal feed, and fertilizer. In addition to reducing emissions, seaweed cultivation may also reduce acidification, as found in a 2020 study by the World Resource Institute. Seaweed is already being used in the shellfish industry to improve the growing conditions for oysters, mussels, and clams and reduce saltwater acidity which damages the calcium carbonate in their shells. Emerging research on the benefits of seaweed cultivation is promising, but more research is needed to understand if this approach is economically viable. For the shellfish industry, whose stocks rely on the health of their shellfish, it certainly seems to be the best option.
According to a 2014 study published in Frontiers in Ecology and the Environment, Marine biologists have recently discovered that whales play a significant role in capturing carbon from the atmosphere. Whales essentially act as a biological pump, accumulating carbon through their diet of phytoplankton. The carbon remains in the whale’s body during its lifetime. According to Chris Johnson, a marine scientist working with the World Wide Fund For Nature (WWF), “Whales accumulate carbon in their bodies during their long lives. When they die, they sink to the bottom of the ocean. Each great whale sequesters 33 tons of CO2 on average, taking that carbon out of the atmosphere for centuries. A tree, meanwhile, absorbs only up to 48 pounds of CO2 a year.”
Scientists understand that to prevent both the levels of CO2 in the atmosphere and ocean acidity from continuing the rise, we need to significantly reduce CO2 emissions into the atmosphere. It is important to note that there are limitations to approaching climate change mitigation strategies from the carbon solutions standpoint. We do not want to counterbalance the carbon flux homeostasis of the ocean. The ocean has proven to be extremely resilient, but it is our responsibility to act as stewards and protect vital marine and coastal ecosystems. As with the carbon sequestration of terrestrial ecosystems, more research is needed to better understand the ocean’s carbon sequestration processes. Understanding the ocean’s essential role in the carbon cycle can better inform scientists, policymakers, and the public as we all work together to mitigate the effects of anthropogenic climate change.
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Featured image from pmel.noaa.gov/co2/story/Ocean+Carbon+Uptake(opens in a new tab)