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I'm doing some homework on the increase of $\mathrm{CO}_2$ in the atmosphere. On one of the graphs it shows the amount of $\mathrm{CO}_2$ peaks in May and has a low in November. Why is this? Shouldn't it just be constantly going up?

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Overall, the measured amount of $\mathrm{CO}_2$ increases, but in a cyclic pattern, as can be seen in observations shown below:

enter image description here

Image source: NOAA: Trends in Atmospheric Carbon Dioxide (numerous observation data sets and graphs are available from that website)

The fluctuations are seasonal, which is explained in the web page Seasonal carbon dioxide range expanding as more is added to Earth's atmosphere (NSF, 2013) summarises the seasonality as being

Levels of carbon dioxide in the atmosphere rise and fall each year as plants, through photosynthesis and respiration, take up the gas in spring and summer, and release it in fall and winter. Now the range of that cycle is expanding as more carbon dioxide is emitted from burning fossil fuels and other human activities.

Specifically, the effect is more pronounced in the northern hemisphere, with

more carbon is accumulating in forests and other vegetation and soils in the Northern Hemisphere during the summer, and more carbon is being released in the fall and winter

Daily as well as seasonal variations were observed in northern Japan, reported in the article Diurnal and seasonal variations in atmospheric $\mathrm{CO}_2$ in Sapporo, Japan: Anthropogenic sources and biogenic sinks (Miyaoka et al. 2007).

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  • $\begingroup$ I wonder why this variation would occur at all, since it is summer in the northern hemisphere when it is winter in the southern. So why wouldn't they balance each other out? $\endgroup$ – Cyberherbalist Sep 24 '14 at 22:34
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    $\begingroup$ Table 3 in Chapter 2 of Sverdrup, Johnson and Fleming (1942): The Oceans, Their Physics, Chemistry, and General Biology provides an overview on the land/water distribution with latitude. Based on these data the land area on the southern hemisphere extents to only 48 % of the land area on the northern hemisphere. $\endgroup$ – BHF Sep 25 '14 at 1:33
  • $\begingroup$ Something that I just learned was CO2 is released whenever we till the soil. My impression was that this is a huge proportion of CO2 we find early in the year. Check those statistics... $\endgroup$ – stormy Sep 25 '14 at 20:57
  • $\begingroup$ You might want to add the hemispheric difference into the actual answer, considering it is the key concept of the answer. $\endgroup$ – John Apr 22 '17 at 13:29
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The respiration of plants is not evenly distributed through seasons since the northern and southern hemisphere have a different amount of land and plants. The southern hemisphere has far less land. In the northern winter, the carbon released increases and then by ~May the northern hemisphere has enough carbon uptake (due to increased photosynthesis) to cause a dip in the $\ce{CO2}$ record. Then summer in the north ends... autumn commences, and net $\ce{CO2}$ starts to rise again.

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  • $\begingroup$ yes, I have mentioned this factor in my answer (quoted) $\endgroup$ – user889 Dec 20 '14 at 4:08
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The magnitude of these seasonal variations differ from location to location. The graph below portrays variations in CO2 levels at Point Barrow Alaska (PTB), La Jolla California (LJO), Mauna Loa Observatory (MLO), Christmas Island (CHR), Samoa (SAM), and the South Pole (SPO) over the last 60 years.

Graph portraying variations in CO2 levels at six sites over the last 60 years. All of the sites show a general increase in CO2 levels. The northern hemisphere sites also show a marked seasonal variation.
Source: Scripps $\text{CO}_2$ Program, Global Stations $\text{CO}_2$ Concentration Trends.

Note that all sites show a general increase in $\text{CO}_2$ concentrations over those 60 years. The sites in the Northern Hemisphere (Point Barrow, La Jolla, and Mauna Loa) also show seasonal variations, with the variations being the greatest at Point Barrow. The sites in the Southern Hemisphere (Christmas Island, Samoa, and the South Pole) show a much reduced seasonal variation, essentially vanishing at Samoa and the South Pole.

One reason for these variations is the relatively slow mixing between the northern hemisphere and southern hemisphere atmospheres. It takes about a year for changes in the northern hemisphere atmosphere to make their way into the southern hemisphere atmosphere (and vice versa), and even longer for those northern hemisphere changes to migrate to the South Pole. This slow mixing time means that seasonal variations are confined to the hemisphere in which they occur.

There's a large difference between the land to ocean ratio in the northern hemisphere versus the southern hemisphere, and an even greater disparity outside the tropics. The large temperate land mass in the northern hemisphere is the primary cause of those seasonal variations in CO2 levels in the northern hemisphere. Plants absorb CO2 from the atmosphere in the spring and summer, and release CO2 back to the atmosphere in the fall and winter.

References:

"Why are Seasonal CO2 fluctuations strongest at Northern Latitudes?" Scripps Institution of Oceanography

Daniel James Jacob, et al., "Atmospheric distribution of 85Kr simulated with a general circulation model," Journal of Geophysical Research 92:D6 (1987).

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