Over the Thanksgiving holiday I read a very interesting paper in the journal Nature by J.M Gray and six co-authors (citation at end of article). The article was about the seasonal changes in carbon dioxide in the atmosphere. The iconic curve of increasing CO2 concentrations in the atmosphere is a common figure in many discussions about human-induced climate change. But instead of revisiting the relationship between burning fossil fuels (red line in Figure 1 below), let’s look at another interesting aspect of the CO2 curve that Gray et al. published on in Nature–”seasonality”. (I will wait to talk about what Gray and his colleagues found in another post.)
The black squiggly curve in Figure 1 above shows atmospheric CO2 concentrations averaged over each month. The amplitude of the “squiggles” are controlled by latitude (Figure 2), with higher northern latitudes showing the largest amplitude, and higher southern latitudes showing the lowest amplitude. What would cause such a change? Let’s look in more detail.
If we zoom in on the Mauna Loa data at a higher resolution (daily) and just for one year (2015), we can see that the “squiggles” are actually seasonal cycles (Figure 3). The highest values are in May and the lowest in September. Now we have to think about what would cause the CO2 concentrations to change during the year. Humans add CO2 to the atmosphere at roughly constant rates throughout the year (red line in Fig. 1). That continuous addition causes the upward trend in the CO2 time series. This relatively smooth increase matches the fossil fuel emission curve (Figure 1, red line), but not the seasonal cycle. It turns out that, the seasonal squiggle is caused mostly by land plant growth and senescence during the year (ocean processes also play a role, but not as strong as terrestrial processes). During the growing season, photosynthesis uses CO2 in the atmosphere, decreasing concentration. When plants slow/stop photosynthesizing in the fall, respiration takes over (mostly by microbes in the soil eating plant detritus–“rotting”) producing CO2. Atmospheric CO2 concentrations are controlled by these seasonal processes, in Mauna Loa (and on average globally) peaking in about May and falling to a nadir in about September (Figure 3). Now, let’s return to Figure 2, and have a closer look.
Note again in Figure 2 that the strength of the seasonality of CO2 decreases with more southernly stations: The farther south the station is, the smaller the swing in CO2 over a year. This results from the larger proportion of land in the northern hemisphere. In the northern hemisphere, there are more plants to photosynthesize and remove CO2 from the atmosphere and more plant detritus to rot to add CO2 to the atmosphere (Figure 4). But, if we look closely at different sites we can see some subtleties to these controls.
Let’s compare Pt. Barrow, Alaska, (way up on the North Slope) and the South Pole (as far south as you can get)(Figure 5). Note first that the seasonality is reversed–the two curves are mirror images of one another! So, although the northern hemisphere has the dominant control on global averaged CO2 seasonality, latitude still controls seasonality at a particular site. Pt. Barrow CO2 is maximum Dec-Jun (NH winter and spring), then falls to a minimum Jun-Sep (NH summer ), rising again Sep-Dec (NH fall) to the NH winter maximum. The South Pole is the opposite, controlled by the SH seasons, not the NH seasons.
So, every winter, which ever hemisphere you are in, you will be greeted by higher concentrations of CO2 in the atmosphere. And each year the values climb higher than previous years because of the continuous addition from humans. But, what about that Nature paper I started this post with? Well ran out of space for that in this post and we had to understand carbon dioxide seasonality before we can delve into it. It is really interesting, so I will continue that discussion in the next post–hopefully before the end of the year! In the next post, I will look at if this seasonality is changing over time along with the concentrations. What do you think? Citation: Direct human influence on atmospheric CO2 seasonality from increased cropland productivity, Josh M. Gray, Steve Frolking, Eric A. Kort, Deepak K. Ray, Christopher J. Kucharik, Navin Ramankutty & Mark A. Friedl, Nature 515, 398–401 (20 November 2014) doi:10.1038/nature13957.