Why Does Atmospheric CO2 Peak in May?


Tim Lueker, research scientist in the Scripps CO2 Research Group, only needs one sentence to explain why atmospheric CO2 peaks in May. 

“Springtime comes in May in Siberia,” he says.

Let’s take a look at the details of this statement to reveal the processes that drive this annual May peak:


First, the significance of spring is related to the shift of terrestrial plants from barren winter branches to bountiful spring leaves.  After the leaves on the trees drop in the fall, the leaf litter and other dead plant material break down throughout the winter thanks to the hard work of microbes.  During this decomposition, microbes respire and produce CO2, contributing to atmospheric CO2 levels in the process.  Thus over the course of the winter, there is a steady increase in CO2 in the atmosphere.  In the spring, leaves return to the trees and photosynthesis increases dramatically, drawing down the CO2 in the atmosphere.  This shift between the fall and winter months to the spring and summer results in the sawtooth pattern of the Keeling Curve measurement of atmospheric CO2 such that every year there is a decline in CO2 during months of terrestrial plant photosynthesis and an increase in CO2 in months without large amounts of photosynthesis and with significant decomposition.

… comes in May…

May is the turning point between all the decomposition throughout the winter months and the burst of photosynthesis that occurs with the return of leaves to the trees in spring. CO2 measurements all over the globe reflect this pattern of peak CO2 concentration occurring each May, regardless of the level of that peak.  Atmospheric CO2 has reached daily peaks of 400 parts per million for the first time this year as a result of the upward trend in CO2 overall, and the first monthly peak will likely occur in May.

… in Siberia.

While it is spring and summer in the Northern Hemisphere, it is fall and winter in the Southern Hemisphere, so why don’t these signals of photosynthesis and respiration cancel one another out?  For one thing, the mixing between the hemispheres is too slow for there to be much interaction between their two cycles.   It takes roughly a year for the air to mix between the Northern and Southern hemispheres.   The mixing within each hemisphere, in contrast is only weeks to months.  This is why a similar cycle is seen at all our Northern Hemisphere observing stations regardless of their latitude.   There is a much larger amount of land in the Northern Hemisphere, particularly with huge forested areas in Siberia, while the Southern Hemisphere is dominated by ocean, but because of the slow mixing, even if there were as much land in the south, the Mauna Loa cycle wouldn’t look very different.

Also, while photosynthesis in the ocean is also extremely important to atmospheric chemistry (phytoplankton being responsible for the air we breathe today), this marine photosynthesis does not drive the annual peak in atmospheric CO2 because little of the CO2 goes into the atmosphere.

That’s not to say that the entire peak depends on Siberia alone.  While Siberia is important because it is home to the largest area of boreal and temperate forests that drive the seasonal cycle, carbon dioxide exchange over North America is also very important to the cycle measured at Mauna Loa. The measurement at the observatory there does tend to lag the mainland as it can take a few weeks for seasonal swings to propagate to Mauna Loa’s latitude.

So, with the passing of May and the trees in Siberian forests and forests across the Northern Hemisphere showing their leaves, we’ve reached this year’s atmospheric peak in CO2.

Emily Kelly is a fifth-year student in the laboratory of Scripps marine biologist Jennifer Smith. 

Related photo:

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6 thoughts on “Why Does Atmospheric CO2 Peak in May?”

  1. Better explanation is that CO2 varies seasonally approximated by a sine curve. However there are two hemispheres in opposite seasons, so CO2 is the sum of a positive and a negative sine curve. If the two hemispheres were mirror images, they would cancel out to no variation. They are not, so the May peak represents the result of the two competing hemispheres.

  2. Why dont they stil compare ice core data post 1958?
    It would be interesting to see how this compares to the muana loa data.

    Surely these two cannot be compared as they re from different parts of the earth and why such a sudden increase from 1958

  3. Examining the Keeling curve itself ( the cyclical version, month over month, which I could not find on this site…) it would appear that there is about a ~7ppm CO2 fluctuation (decrease) from May to October. If the Northern Hemisphere is responsible for this 7ppm reduction, which seems quite large, then some how extending this growth cycle, either in size or duration, would seem to be a natural way to adsorb more CO2 than is currently being extracted.

    For instance, the extra 2ppm each year the Keeling curve is growing could theoretically be offset by a 30% increase in the growing cycle of the Northern Hemisphere (.3 * 7 = 2.1). Yeah this is an off the scale radical consideration but theoretically, is it possible?

  4. Much as I would hate to see the microbes go hungry why not gather the dead plant material and store it. Without water microbes would not be able to break it down. It could be stored as either biochar or as is. It would work, easy to prove, and while it may not buy us thousands of years it is a simple inexpensive solution that could be started right away with immediate effects. We are nearly out of time.

  5. On the one month chart the daily averages appear to be at or near the maximum of the hourly averages. I don’t understand how this can be the case. Can you explain the maths?

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