Simple Climate readers are alongside some of the world’s finest minds in asking questions that trouble even leading climate scientists. That’s the conclusion that I’ve reached after trying to help reader Marc Piore answer a question based on research covered here back in July.
The particular study that troubled him was an assessment of ocean phytoplankton levels from 1899 to today published in top journal Nature. In it, Boris Worm and his colleagues at Dalhousie University, Canada, found that the populations of phytoplankton in the oceans had fallen 40% since 1950, crediting climate change as the cause of the decline. At the time Worm was widely quoted as saying that phytoplankton is responsible for producing “half the oxygen we breathe”.
On that basis, Mark Piore asked: “If phytoplankton produces half the oxygen we breathe and populations have fallen by 40%, then has total oxygen production fallen 20%?” He was understandably concerned, as a fall in the amount of oxygen we need to breathe sounds even scarier than global warming. I put this question to Worm, and he answered: “Superficially the argument you present is correct.” However he points out that there are several complicating factors to be taken into account, based on what phytoplankton does with oxygen, and how.
Plants – including phytoplankton – recycle CO2 back into oxygen through the process known as photosynthesis, with the help of energy in the form of light, which is absorbed by proteins containing chlorophyll. Plants remove carbon from CO2 as they turn it into oxygen, creating chemicals that help the plant to grow. One way to estimate how much photosynthesis has happened is to assess its “primary production”, which measures the total amount of chemicals the plant has made and added to its biomass. Phytoplankton is responsible for half of all primary production – which is what the “half the oxygen we breathe” quote refers to.
Worm’s study pulls together data on the level of chlorophyll in the oceans, and there are limitations in assessing how changes in this figure affect photosynthesis. “We report a 40% drop in surface chlorophyll,” he says. “We cannot say very much about deeper phytoplankton, and we can also not rule out that primary production has changed at a different rate than chlorophyll. The two are definitely correlated, but may not be in a 1:1 relationship globally.”
Also, because plants and phytoplankton need light to perform photosynthesis, when they do not have it they must respire, much like humans breathe, changing oxygen back into CO2. “It is important to keep in mind that the balance of oxygen in the ocean is driven by both production and consumption,” Worm explained. While production has likely declined due to the lower levels of phytoplankton, so has consumption. “Thus the net effect of phytoplankton decline on the ocean and planet’s oxygen budget is quite unclear,” Worm said.
This point is backed up by University of Georgia marine scientist Wei-Jun Cai, who points out that this balance means that phytoplankton normally consumes most of the oxygen that it produces. “Around 90% of the plankton production is respired in the surface ocean and a small fraction moves into the deep ocean,” he notes. He also points out that there is a massive amount of oxygen in the atmosphere – around a billion, billion (1015) tonnes. By contrast total annual primary production of oxygen is around 300 billion (3 x 1011) tonnes. If there were a 20% fall in this figure, and consumption stayed the same, it would take nearly 5,000 years to use up the Earth’s supply of oxygen. This is a longer timespan even than the decades and centuries that climate change has emerged and will emerge over.
As Cai and Worm note, it seems reasonable to assume that the link between plant consumption and production will help maintain the balance between CO2 and O2. “A similar question would be: “Do we need to worry about the oxygen level in the air because of fossil fuel burning?” Cai says. “Not at all. Yes, oxygen levels are decreasing when we burn fossil fuel, but way before you can feel that, all fossil fuel will be depleted.”
However, it’s interesting to note that Marc Piore is not the only person to have raised questions about the amount of oxygen in the atmosphere. Leading physicist Freeman Dyson has also expressed his frustration that atmospheric oxygen levels are not monitored more closely. “The reservoir of oxygen in the atmosphere is large but not infinite,” he wrote in his book “From Eros to Gaia” in 1990. “Since eight tons of oxygen are used up for every three tons of carbon burned, and we are burning six gigatons of carbon per year, we might expect that the oxygen is being used up at the rate of about 13 parts per million per year. Thirteen parts per million should be measurable. Whether or not the general public is concerned, there are important scientific reasons for measuring the oxygen.”