The Pliocene. A period of time 2-5 million years ago hosting carbon dioxide levels ranging from 350 to 405 parts per million and global average temperatures that were 2-3 degrees Celsius hotter than 1880s levels. The great ice sheets of Greenland and West Antarctica were feeble, if they existed at all. And seas were about 25-80 feet higher than today.

(CO2 hit above 401.84 parts per million on March 9, 2015, and above 403 parts per million on March 10 — levels that test the upper boundary of CO2 last seen during the Pliocene and entering a range more similar to the Miocene. Image source: The Keeling Curve.)

In the context of human warming, the amount of heat forcing we’ve added to the global atmosphere from carbon dioxide emissions alone has been hovering in the range of the Pliocene for the past two decades. A heat forcing that, if it remained steady over a substantial period of time, would almost certainly revert the world to a climate state last seen during that time.

But by 2015, the global human heat forcing from carbon dioxide emissions had begun to exit the period of the Pliocene. Now we are entering a period in which atmospheres are more similar to those seen during the Middle Miocene Climate Optimum — the last time CO2 measures exceeded a threshold of roughly 405 parts per million (see here and here)

The Middle Miocene Climate Optimum occurred between about 15 and 17 million years ago. It hosted an atmosphere in which carbon dioxide levels varied wildly from 300 parts per million to 500 parts per million. Temperatures were between 3 to 5 degrees Celsius hotter than the 19th Century. And sea levels were about 120 to 190 feet higher. During this period, the world was still cooling down from the heat of the Paleocene and Eocene epocs. Carbon was being sequestered. And it was the first time the world broke significantly below a 500 part per million CO2 plateau that had been established during the Oligocene 24 to 33 million years ago.

The great glaciers in East Antarctica were mostly well established, even though their scope was a mere shadow of what we see today. The Greenland and West Antarctic glaciers did not exist. They would have to wait for about another 5-10 million years for the Earth to cool further.

(Glaciation since PETM. Image source: Dr James Hansen.)

As of March 9, 2015, atmospheric CO2 levels had reached 401.84 parts per million. Already a level testing the Pliocene-Miocene boundary, this measure will continue to increase through the rest of March, on into April, and keep rising until middle or late May. At that point, global CO2 levels will have reached around 404 parts per million. At least the highest levels seen in the last 3 million years and possibly the highest levels seen in 15 to 18 million years.

If the greater portion of this range is correct, then we are now breathing air that none among our species or even our hominid relatives have ever breathed since their setting foot on this world.

But CO2 alone doesn’t tell the whole story. Equivalent CO2 levels (CO2e) including all human emitted greenhouse gasses — methane and a host of industrial gasses — will reach about 484 ppm CO2e this year (see here and here). And that forcing puts us easily within the range of the warmest periods of the Miocene. A brew of heat trapping gasses including exotic chemicals that no creature has likely ever breathed while living on the Earth.

Links:

The Pliocene

Middle Miocene Climate Instability Associated With CO2 Variability

CO2 and Climate Closely Linked During Middle Miocene

Sea Level and Ice Volume Variations

IPCC 2007 Chapter 6 — Paleoclimate

Dr James Hansen

The Keeling Curve

NOAA ESRL CO2 Measure

CO2 Levels for February Eclipsed Prehistoric Highs

A Faustian Bargain on the Short Road to Hell — Living in a world at 480 CO2e

400 PPM CO2? Add in All other Gasses and it’s 478 CO2e

Hat tip to Aldous