Recently, I posted the Hawaii record that showed that carbon dioxide has been increasing for the last several decades. To make the plot consistent with the global temperature plot, I showed only annual averages. Now, I show a copy of that same plot with seasonal information included.

Figure 1. Concentration of carbon dioxide at Mauna Loa, Hawaii (inset). NASA graph by Robert Simmon, based on data provided by the NOAA Climate Monitoring and Diagnostics Laboratory. Image from earthobservatory.nasa.gov/Newsroom/NewImages/…

This curve, which may be more familiar to many of you, has lots of wiggles. To look more closely at the wiggles, I obtained some data from the WLEF tower in Wisconsin, taken at 396 meters above the ground. The wiggles in Figure 2 show lots of variation from year to year, but there is a pattern. We can see the pattern easily if we average the data. During the winter, the carbon dioxide values are high. The values fall in the spring, and are smallest in July. By August, carbon dioxide values are increasing again.

What is happening? The WLEF tower is in a forest. During the spring and summer, the trees use up carbon dioxide in photosynthesis. As the trees leaf out, the carbon dioxide decreases. Once summer comes, photosynthesis starts slowing down, and so does carbon dioxide uptake. Like animals, both trees and the soils give off carbon dioxide in respiration. The curve shows the net effect of respiration and photosynthesis.

The carbon dioxide the tower measures does not just come from the forest – it can come from hundreds of kilometers away, and from grasses, shrubs, and crops as well as trees. Like the trees, these plants are also exchanging carbon dioxide with the atmosphere.

Figure 2. Monthly average flask values of CO2 from 396 meters above the surface. The inset shows the average for the ten years shown, to emphasize the change with seasons. Data collected by NOAA ESRL and The Pennsylvania State University and supplied by Ankur Desai (Dept of Atmospheric & Oceanic Sciences, University of Wisconsin-Madison).

Figure 2 is detailed enough to show lots of wiggles that don’t follow a smooth seasonal pattern. As the winds change, air with higher or lower values of carbon dioxide might be brought in. Where would carbon dioxide values be highest? Combustion produces carbon dioxide, so there will be higher values where there are lots of cars, factories, or fires. When trees are leafing out and growing, the carbon dioxide will be taken up. So it is possible that sharp peaks may be for times when the wind was bringing carbon dioxide from an area with lots of cities. Have you ever seen data on how much carbon dioxide is in the air near you?