On Thursday May 10th 2013 the Mauna Lao Observatory in Hawaii recorded the concentration of carbon dioxide in the Earth’s atmosphere passing the 400 parts per million mark for the first time since records began in 1958. What the records (known as the Keeling Curve after the scientist who devoted his life to collecting them) show is that concentrations of carbon dioxide in the Earth’s atmosphere have been climbing steadily in that time.
Ice core data, obtained from analysis of the gases trapped bubbles in polar ice, go back 800,000 years, and in this time the planet has not experienced this level of carbon dioxide. Analysis of carbon isotopes present in compounds made by tiny marine phytoplankton preserved in ancient ocean sediments suggests that the last time the planet experienced 400 parts per million was in the Pliocene, between 5.3 and 2.6 million years ago.
In the mid-Pliocene warm period, 3.3 to 3 million years ago, the Earth had several other key similarities with the contemporary Earth – a similar intensity of sunlight reaching the Earth; similar global geography; similar parameters of the Earth’s orbit; and the fact that many mid-Pliocene species are still extant.
There are also some key differences between these two Earth phases. Sea levels were higher (estimates range from 5 to 40 meters higher than today); the Greenland and Antarctic ice sheets were smaller than today; the jet stream was sluggish, if it existed at all – the planet existed in a permanent El Nino. Average temperatures were 3 or 4 C higher than today, and possibly 10 C higher at the poles.
For these key reasons, many scientists consider the mid-Pliocene to be a key climate analog for the Earth’s future. However, there is one key reason that makes this analogy so particularly significant for climate science: in the mid-Pliocene, the levels of carbon dioxide in the atmosphere were decreasing. Today they are increasing at an unprecedented rate. During ancient climate change, an increase of 10 parts per million might have taken 1,000 years or more to happen. Now, if emissions continue to increase as predicted, the planet is set to reach the 1,000 ppm level in only 100 years.