The Last Time Atmospheric CO2 was at 400 parts per million Humans Didn't Exist

The planet has passed a disturbing landmark, a marker on a continuing highway to climate disruption. On May 9th, the NOAA and the Mauna Loa observatory reported that atmospheric CO2 levels touched 400 parts per million. Before humans started burning fossil fuels, they were around 280 parts per million.

Mauna Loa measurements of carbon dioxide. From Mauna Loa measurements of carbon dioxide. From


The last time atmospheric CO2 was at 400 parts per million was during the ancient Pliocene Era, three to five million years ago, and humans didn’t exist.

  • Global average temperatures were 3 to 4 degrees C warmer than today (5.4 to 7.2 degrees F).
  • Polar temperatures were as much as 10 degrees C warmer than today (18 degrees F).
  • The Arctic was ice free.
  • Sea level was between five and 40 meters higher (16 to 130 feet) than today.
  • Coral reefs suffered mass die-offs.

And much more: As Robert Monroe notes: “The extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events. Now the planet is poised to reach the 1,000 ppm level in only 100 years if emissions trajectories remain at their present level.”

Here are some scientific links for those wanting to know more:


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Another perspective on historical CO2 levels:

According to this, plant stomata is a better indicator of historic CO2 levels and there are reasons to question ice core CO2 readings, especially the low and "stable" reading of 280 ppm we are told existed prior to the Industrial Revolution.

I'm not sure the extent to which peer-reviewed science supports this claim (I reviewed some of the pieces cited in the linked comment site above), but leave this for others to address. My understanding of the current scientific evidence on paleo levels of greenhouse gases suggests it is very clear that current increases are both human caused and far outside the levels seen for hundreds of thousands, if not millions, of years.

The latest climate change denier that I must suffer arguing with claims that A) the insanely high CO2 levels are even greater evidence that we are on the precipice of a massive decline i global temperatures;and B) not only is there no evidence that CO2 is a greenhouse gas at all, but the scant evidence which does exist suggests the opposite - that rising temps cause a rise in CO2, and when the CO2 levels hit a certain balance with global temps, global temps plummet, as is seen in the ice core records. Considering we've been teetering along for the last ~10,000 years, as seen i the ice core records, we should be declining, and he claims we are, as average temps for the last 12-15 years have been level, not increasing.

By DrStrangepork (not verified) on 14 May 2013 #permalink

Up is down; down is up. Not much you can do when someone believes something so directly contradicted by evidence, physics, observations, and generally well-understood science. Good luck!

If there is any hope of reasoning, you might point them to the good rebuttals of all of the major denier points at, or some of the other good science websites.