Why does 400ppm matter? & what it has to do with the Pliocene

In May 2013, the Mauna Loa Observatory in Hawai’i recorded the first time global carbon dioxide levels surpassed 400ppm (parts per million). As of this Tuesday, we are sitting at 403.98ppm and have been over 400ppm for much of the last year (Scripps). This observatory has been measuring atmospheric carbon dioxide since 1958. So, you might ask, why is the number 400 so important?

Average atmospheric carbon dioxide levels for the last 12 months at the Mauna Loa Observatory in Hawai’i. (From: Scripps Institution of Oceanography)

Basically, the answer is: the earth hasn’t seen such high levels of atmospheric carbon dioxide since the mid-Pliocene warm period. 3.3 to 3 billion years ago. During that time, the continents looked much like they do today, but otherwise, the world was quite different. The climate was 2-3°C warmer than it is today and sea level was 25m higher. The expansive ice-sheets of today’s Antarctica had just begun forming. As atmospheric carbon dioxide continues to climb, our world could start looking more like the Pliocene in the future. (Haywood and others, 2009)

Charles Lyell, an important historical figure in geology, once said “the present is the key to the past.” But it could also be said that the past is the key to the present. That is the case for the mid-Pliocene warm period. The hope is that the more that can be learned and understood about the earth in the past, the better the changes happening today can be understood.

A little more about the Pliocene
The Pliocene epoch covers the time between 5.4 and 2.5 million years before present. On the geological timescale, it precedes the Pleistocene epoch which is known for its repeated glaciations and being the time of the last great ice age. While it was significantly warmer during the Pliocene than it is today, the Pliocene actually encompasses a marked cooling from the preceding warm Miocene epoch. Much of today’s forests were grasslands then, creating ideal conditions for grazing mammals which were plentiful.

One of the biggest changes to occur during the Pliocene was the joining of North and South America by the Panama land bridge. This allowed the spread and mingling of many species which were previously isolated from each other. This probably had a big effect on ocean circulation and because ocean circulation is closely linked with climate, this may have impacted climate as well. (University of California Museum of Paleontology, Encyclopedia Britannica)