This figure shows the history of <a href="https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_atmosphere" class="extiw" title="w:Carbon dioxide in Earth's atmosphere">atmospheric</a> <a href="https://en.wikipedia.org/wiki/carbon_dioxide" class="extiw" title="w:carbon dioxide">carbon dioxide</a> concentrations as directly measured at <a href="https://en.wikipedia.org/wiki/Mauna_Loa_Observatory" class="extiw" title="w:Mauna Loa Observatory">Mauna Loa</a>, <a href="https://en.wikipedia.org/wiki/Hawaii" class="extiw" title="w:Hawaii">Hawaii</a> since 1958.  This curve is known as the <a href="https://en.wikipedia.org/wiki/Keeling_curve" class="extiw" title="w:Keeling curve">Keeling curve</a>, and is an essential piece of evidence of the man-made increases in <a href="https://en.wikipedia.org/wiki/greenhouse_gas" class="extiw" title="w:greenhouse gas">greenhouse gases</a> that are believed to be the cause of <a href="https://en.wikipedia.org/wiki/global_warming" class="extiw" title="w:global warming">global warming</a>.  The longest such record exists at Mauna Loa, but these measurements have been independently confirmed at many other sites around the world <a rel="nofollow" class="external autonumber" href="http://cdiac.ornl.gov/trends/co2/contents.htm">[1]</a>.
<p>The annual fluctuation in carbon dioxide is caused by seasonal variations in carbon dioxide uptake by land <a href="https://en.wikipedia.org/wiki/plant" class="extiw" title="w:plant">plants</a>.  Since many more forests are concentrated in the <a href="https://en.wikipedia.org/wiki/Northern_Hemisphere" class="extiw" title="w:Northern Hemisphere">Northern Hemisphere</a>, more carbon dioxide is removed from <a href="https://en.wikipedia.org/wiki/Earth%27s_atmosphere" class="extiw" title="w:Earth's atmosphere">the atmosphere</a> during Northern Hemisphere <a href="https://en.wikipedia.org/wiki/summer" class="extiw" title="w:summer">summer</a> than <a href="https://en.wikipedia.org/wiki/Southern_Hemisphere" class="extiw" title="w:Southern Hemisphere">Southern Hemisphere</a> summer.  This annual cycle is shown in the inset figure by taking the average concentration for each month across all measured years.
</p>
The red curve shows the average monthly concentrations, and blue curve is a smoothed trend.
<a href="//commons.wikimedia.org/wiki/User:Delorme" class="mw-redirect" title="User:Delorme">Delorme</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>. Data from Dr. Pieter Tans, <a rel="nofollow" class="external text" href="http://www.esrl.noaa.gov/gmd/ccgg/trends/">NOAA/ESRL</a> and Dr. <a href="https://en.wikipedia.org/wiki/Ralph_Keeling" class="extiw" title="w:Ralph Keeling">Ralph Keeling</a>, <a rel="nofollow" class="external text" href="http://scrippsco2.ucsd.edu/">Scripps Institution of Oceanography</a>.