The letter writer, whom I support, speaks to confusion between variability in CO2 levels and the steadily rising average level. He addressed this topic in response to letters from my community (that I missed) . My experience tells me that if a letter writer denounces global warming and cites something like natural flucuations exceding manmade, then there is a "think tank" out there providing these talking points. So I thought I'd resurrect these illustrations.
Below is a redrawn version of atmospheric CO2 levels and a closeup of the yearly cycle.
Here's a blow-up of one year:
I emphasize that this is an illustration that I drew from a plot of the data, but I didn't plot the data (another project). It is my impression from the graph I used as my source that during a year, atmospheric CO2 level fluctuates by 6-7 parts per million while the average, largely a result of human activities, rises 1.5-2 parts per million. I believe the talking points I refered to are using the size of the yearly fluctuation to show that it overwhelms the anthropogenic increase. (I'm keeping an eye out for this talking point, and welcome any pointers to a reference making this argument.)
What causes the yearly fluctuation?
Here is the yearly cycle shown with an illustration of the seasons.
The top panel is the yearly CO2 level and the bottom panel shows the seasons at northern latitudes. From left two right: Bambi eating bark from dormant trees, converting the carbon into CO2. Spring comes and though Bambi still eats and exhales CO2, the trees and grass are absorbing CO2 and converting it to plant matter (as mostly cellulose?). By late summer, many forests dry out and taper off their uptake of CO2. Autumn marks a great reduction in photosynthesis accompanied by decay (which I believe also releases carbon to where it can be devoured by microbes that release CO2). And last vast areas are frozen, allowing the animal life to exert a dominate influence on atmospheric CO2 levels.
It's reasonable to ask whether the southern hemisphere offers some cyclic behavior that compensates. (I suspect that it complicates measurement rather than compensates--yet another topic to explore.)
Below is a comparison of the two hemispheres:
If I understand correctly, the disparate distribution of land masses at latitudes that respond to seasonal change with significant changes in vegetation are what create the yearly cycle in atmospheric CO2. I do not know what happens to the southern hemisphere's oceans as summer comes and goes there.
The long-term CO2 levels have fluctuated without humanity's help. Climate modes, like El Nino can spur the growth of vegetation to pull the long-term trend downward or melting of permafrost can push it upward:
There are many mechanisms that can act regionally or globally, but what I find interesting is the effort to examine how global warming will affect this yearly cycle. A review article and research paper in Nature 3 January 2008 addressed this question by examining the points where the blue curve crosses the yellow, long term line.
I'll be adding an illustration on that when I can. (Update: Illustration is part of this postSeasons, Sinks, and Sources)