Editor’s note: This article is the third in a three-part series that investigates the connections among increasing atmospheric carbon dioxide concentrations, the ocean’s role in absorbing carbon dioxide and the effect on marine ecosystems, and what happens to atmospheric oxygen levels when the base of the food chain—phytoplankton—dies off.

Not many people realize that without blue-green algae life as we know it would not exist. From its formation 4.6 billion years ago up until about 2.45 billion years ago, Earth's atmosphere contained little or no oxygen. Blue-green algae originated 2.7 billion years ago and were the first organisms to produce oxygen through photosynthesis. The oxygen produced by the algae accumulated in the atmosphere, where some of it interacted with high-energy solar radiation, forming the ozone layer. The ozone layer formed a protective shield against damaging ultraviolet (UV) radiation and allowed the development of the first life on land.

These days, oxygen makes up 21% of our atmosphere, where 98% originates from photosynthesis. Half of the photosynthesis is from ocean plants (mainly phytoplankton) and half is from plants and trees on land. The other two percent of oxygen comes from water vapor getting split apart by UV radiation.

Through photosynthesis, plants convert carbon dioxide and water to oxygen and organic matter, helping them grow and providing us with breathable air. Respiration by humans and other animals reverses this process, taking in oxygen and releasing CO₂. Apart from the natural cycling of these gases, combustion of fossil fuels also releases CO₂ and removes oxygen from the atmosphere.

Monitoring Atmospheric Oxygen

The previous article on Fossil Fuel Emissions and Atmospheric Carbon Dioxide discussed Dr. Charles Keeling's famous observations of the rise in atmospheric CO₂ resulting from fossil fuel combustion. His son, Dr. Ralph Keeling, became interested in the other half of the combustion equation: quantifying the depletion of atmospheric oxygen. He began monitoring atmospheric oxygen in 1989 at stations in La Jolla, California and Cape Grim, Tasmania and observed a loss of 0.0317% from 1990 to 2008 (see graph below).

For each CO₂ molecule created by fossil fuel combustion, Keeling found that nearly three molecules of oxygen are destroyed. As frightening as this may sound, the scientist behind this finding is not worried. "Oxygen depletion from burning fossil fuels is not a serious environmental concern because the changes are miniscule compared with the total O₂ abundance in the air," Keeling explains. (Note: increases in CO₂ from fossil fuel combustion are significant and of concern because the resulting change in percentage of CO₂ in the atmosphere is large.)

CO₂, O₂ and Phytoplankton

Although fossil fuel combustion is unlikely to directly remove enough oxygen from the atmosphere to be of concern, it has the potential to cause a more significant drop in oxygen levels indirectly, through the emission of CO₂. As explained in The Ocean as Carbon Sink: A Double Edged Sword, increased CO₂ emissions are leading to ocean warming and acidification that endanger phytoplankton. So far, researchers have observed a decline in phytoplankton population of 40% since 1950. Since phytoplankton are responsible for the producing half of the oxygen in the atmosphere, it is reasonable to wonder how further declines in their population could affect the concentration of oxygen in the atmosphere in the future.

In a computer modeling study, quadrupling CO₂ concentrations in the atmosphere from preindustrial times reduced the population of diatoms (a major component of marine phytoplankton) by more than 10% at the global scale and by up to 60% in the North Atlantic and sub-Antarctic Pacific (see graph below). However, the study also pointed out that smaller species of phytoplankton may fare better than diatoms in the nutrient-poor conditions created by climate change.

Another modeling study projected that we can expect warm-water phytoplankton species to expand their populations at the expense of cold-water species, which may be driven toward colder waters near the north and south poles. This study emphasized the uncertainty of predicting the responses of phytoplankton to climate change, because of our limited knowledge of how the different species may adapt genetically and phenotypically.

Questions Remain

According to Adrienne Sutton, from the National Oceanic and Atmospheric Administration's Pacific Marine Environmental laboratory, "there is not a scientific consensus on how climate change and ocean acidification will impact phytoplankton. There have been studies that show phytoplankton abundance may decrease with climate change and ocean acidification and there have been studies that show the opposite."

Until we can confidently project how phytoplankton will respond to climate change, we cannot predict how future oxygen concentrations will change. This makes phytoplankton a wild card in the debate over whether or not atmospheric oxygen levels will experience a significant decline. Given that phytoplankton make up the base of the marine food chain as well as produce oxygen, this is an important subject that deserves more research.

Written by :

Elizabeth Klusinske