On the verge of 400 ppm Carbon Dioxide: a symbolic threshold

According to the Scripps Institution of Oceanography, it appears that February 2015 will be the month in which the concentration of carbon dioxide in the atmosphere exceeds 400 part per million:

Various sources have been predicting that we would soon reach this symbolic milestone, and it appears that this will be the month when it happens.

Like any other numerical threshold, there is nothing special about 400 parts per million. What makes this number scary is that it is 50 parts per million above the threshold that most scientists believe represents the maximum sustainable “planetary boundary” for atmospheric carbon dioxide. It is also a scary number because it is uncharted territory: the last time the earth had a concentration of atmospheric carbon dioxide this high was in the Pliocene, three-to-five million years ago and well before the evolution of our species.

We are in uncharted territory, and heading deeper into the unknown and unprecedented.