Autumn is a season of tranformations. In the eastern United States, leaves tumble through a kaleidoscope of copper and crimson colors. In the west, rivers of golden Aspen trees creep down creek beds and mountain valleys. But in the air over North America, another related change is taking place, albeit an invisible one: the amount of carbon dioxide (CO2) in the atmosphere has reached the lowest point in its seasonal cycle, and has just begun to rise again.
Over the last 200 years, carbon dioxide concentrations have climbed steadily as humans have added more and more of the heat-trapping gas to the atmosphere. However, over the course of a single year, the concentration dips and rises with the seasons. Charles Keeling, a scientist at the Scripps Institution of Oceanography in San Diego, noticed this cycle when he started making regular measurements of atmospheric carbon dioxide at the Mauna Loa Observatory in Hawaii back in 1958.
This pattern actually results from Earth’s vegetation, and the way it “breathes.” To understand why, consider what happens when we breathe. We inhale oxygen and produce CO2 in a process known as respiration. This process allows us to break the bonds in sugar molecules to release the energy we need to live. Plants do this too, especially in their roots and at night. But plants also have the unique ability to carry out the reverse reaction of respiration; they use carbon dioxide during photosynthesis as they store energy from the sun and produce oxygen gas.
There are so many plants that, together, they actually affect atmospheric carbon dioxide concentrations. When plants make new leaves in the spring and use them to grow throughout the summer, they consume more carbon dioxide through photosynthesis than they release through respiration, drawing down CO2. In the fall, after a spectacular display of color, they lose their leaves and slow or stop growing. Now, respiration dominates, releasing CO2. (Algae in the oceans also photosynthesize, but they don’t affect the atmosphere as much.)
But when and how much CO2 concentrations vary throughout the year depends on where you live. In San Diego, carbon dioxide fluctuates by about 10 parts per million (or 2.5%) over the course of the year. In Alaska, the amplitude is twice as great. But at Kiribati, an island in the tropical Pacific, the amplitude is much less, and at the South Pole, carbon dioxide concentrations hardly change at all during the year. When they do, they peak in the southern spring (which is northern fall) and dip in southern autumn (northern spring).
This regional variability stems from three things: the amount of local vegetation, the strength of the seasonal cycle, and how fast carbon dioxide is mixed in the atmosphere. The reason seasonal fluctuations are large in the northern hemisphere is because most of the land on earth lies north of the equator, and it’s covered in vegetation. At higher latitudes, plants also experience a strong seasonal cycle in temperature that greatly affects their ability to photosynthesize and grow.
Mixing of the atmosphere happens quickly within one hemisphere, but it takes longer to mix between hemispheres. Because northern hemisphere carbon dioxide peaks briefly in spring and then quickly begins to decrease, not all of it can be redistributed into the southern hemisphere. This produces local differences in concentration, even though, in the long run, carbon dioxide emitted in one place will contribute to an overall increase everywhere on earth.
In contrast, the topics have loads of plants, but not much of a seasonal cycle. Because of this, plants here grow year round, photosynthesizing and respiring. Air from the tropics also mixes easily into both hemispheres because of local weather patterns, spreading tropical CO2 around. The south pole, on the other hand, lies far from any vegetation, but feels the muted rhythm of the southern hemisphere seasons.
When displayed in a 3-dimensional graph (above), this pattern is called the “flying carpet.” And around the world, throughout the year, we are all along for the ride.
Check out our module on photosynthesis.
Graph carbon dioxide concentrations at different measurement stations around the world to see how the seasonal cycle changes at the Carbon Dioxide Information Analysis Center.
Written by Julia Rosen
Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelors degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her Master's in backpacking around the world and skiing. Julia is a periodic contributor to Oregon States research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.