May 11 2021 I remember sitting as a student in a lecture hall when our teacher began the lesson by stating that the magic limit of 400 parts per million (ppm) atmospheric carbon dioxide (CO 2) had been reached. This happened for the first time in 2013, and by 2016 even the lowest value of the year was over 400 ppm. As these numbers lack any relation to the common life, there's some points of comparison from my family in second picture. So, as you can see, the increase has been relatively rapid since the 1950s. Now, what is the cause of this increase? Of course, atmospheric CO