Back in elementary school, we all learned that trees are awesome. And not only because of climbing and tree forts, but because they make the oxygen you breathe. Your cells need oxygen to convert food into usable energy, so you’d die without it.
You probably also learned that trees and other plants make oxygen using a little chemical reaction called photosynthesis. Using light energy from the Sun, they can combine carbon dioxide and water to make sugar and oxygen, with a little leftover water. But if you take that train of thought one step further: during winter, when it gets colder and darker and those trees drop their leaves, there’s less photosynthesis.
So is there less oxygen too? The answer is actually yes! But only a tiny bit less. First off, you can breathe easy, the Earth has plenty of oxygen, and the amount in our atmosphere is pretty stable. By volume, about 21% of the air we breathe is O2 and almost all of this oxygen is from photosynthesis. But trees and other plants that grow on land aren’t the only source of it.
About half or more of the oxygen in our atmosphere comes from a bunch of microorganisms in the ocean called phytoplankton, which photosynthesize too. Although the oxygen they make gets dissolved in water first, instead of going straight into the air. In fact, the most abundant photosynthesizer on Earth could be a marine cyanobacteria called Prochlorococcus.
Photosynthesis in the ocean also varies seasonally. At higher latitudes in the summer, the Sun is higher in the sky and sunlight penetrates deeper into the ocean. So phytoplankton get more light, photosynthesize more, and make more oxygen.
Warmer water also holds fewer dissolved gas molecules. Because warmer molecules are wriggling around with more energy, oxygen can break away more easily and escape into the atmosphere. Now, it wasn’t until the 1990s that scientists started regularly measuring how much O2 changes seasonally.
At a few different stations around the globe, they collect air samples in flasks and compare them to standardized reference samples. What they actually measure is changes in the ratio of O2 to N2, because nitrogen gas levels stay pretty constant. And they’ve found that O2 decreases by about 24 parts per million by volume during the winter in the Northern Hemisphere.
Since the atmosphere is about 210,000 parts per million of oxygen, that change isn’t very much, only about 0.01%. Now, there is a little bit less seasonal variation in oxygen in the Southern Hemisphere. More of the Earth’s land masses are in the Northern Hemisphere, and all of those land plants release oxygen directly into the air. That means there’s a slightly bigger change in winter when they’re not photosynthesizing as much. But, all in all, you can thank the plants and phytoplankton for keeping you alive.