A graph depicting CO2 levels. The cause for the yearly rise and fall is the annual cycle of plant respiration. Since there is more landmass in the northern hemisphere, the cycle is dominated by CO2 drawdown by <a href="https://en.wikipedia.org/wiki/photosynthesis" class="extiw" title="en:photosynthesis">photosynthesis</a> during the northern hemisphere summer; in winter the plants become a net source.
<p>Image created by Shimmin and released into the PD.
</p><p>Data from C.D. Keeling, T.P. Whorf, 
Scripps Institution of Oceanography (SIO).
</p>
Raw data is available from <a rel="nofollow" class="external free" href="http://cdiac.esd.ornl.gov/ftp/trends/co2/maunaloa.co2">http://cdiac.esd.ornl.gov/ftp/trends/co2/maunaloa.co2</a>.