When was CO 2 last at 400 ppm ? And what was the climate like ?
When was CO 2 last at 400 ppm ? And what was the climate like ?
Descent Into the Icehouse Blog post . 13.04.13 Paul N . Pearson , School of Earth and Ocean Sciences , Cardiff University CF10 3AT , UK . Email : pearsonp @ cardiff . ac . uk
[ Note : This is a quick blog post on a specific current media question , not a review paper so I have not attempted to fully reference everything in it .]
Summary
Atmospheric CO 2 is approaching the 400 ppm mark for the first time in human history which begs the question : when was it last that high ? A recent high profile suggestion is that CO 2 was that high in the Pliocene epoch ( approximately 2.6-5.3 million years ago ) and this is now being repeated in the press and around the internet . Here I point out that this claim is based on a few extreme estimates , mostly from sites that systematically overestimate more recent CO 2 levels , while the majority of published Pliocene CO 2 values are in the 250-400 ppm range . The last time we have consistent evidence for pCO 2 over 400 ppm is in the Early Oligocene epoch more than 26 million years ago . This post presents the key graphs and comments on some of the methods used to calculate past pCO 2 .
1 . Introduction
The famous Keeling curve of atmospheric pCO 2 ( partial pressure of CO 2 ) measurements from Mauna Loa is set to reach 400 ppm ( parts per million ) for the first time in history , possibly this month , an unwelcome milestone for planet Earth . Some hourly average measurements have already exceeded this value ( http :// keelingcurve . ucsd . edu /). The reason for this is a combination of 1 ) the relentless rise in pCO 2 due to anthropogenic emissions and 2 ) the regular small seasonal peak . Annual average atmospheric pCO 2 will exceed 400 ppm within 1-2 years . ( Note : some sources claim that a daily average of 400 ppm was reached on May 9th generating global media reports , but this seems to have been subsequently revised : see http :// www . esrl . noaa . gov / gmd / ccgg / trends / weekly . html ).
Even if we were able to hold pCO 2 at around 400 ppm , we are guaranteed further global warming . This is partly because it takes time for the oceans to warm up in response to the atmosphere and partly because of feedbacks involving , for example , water vapour ( another greenhouse gas ), clouds , methane , ice melt , and changes in the biosphere , all of which take time . These feedbacks and their interactions can be very difficult to model by computer which is one reason why different climate models predict different amounts of warming for the same CO 2 forcing .
Given this , it seems natural to look to the geological record for a time when pCO 2 was as high as it is now . It could be instructive to examine the climate state at a time when all the feedbacks would have had ample time to kick in and respond to each other . We should bear in mind , though , that the past cannot provide a perfect analogue for the future . The further back we go , the more likely it is that other significant factors were different from now ( human civilization and land use for a start , and then