Third Cuise of the U.K. Ocean Acidification Programme in the Southern Seas

When was CO2 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 CO2 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 CO2 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 CO2 levels, while the majority of published Pliocene CO2 values are in the 250-400 ppm range. The last time we have consistent evidence for pCO2 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 pCO2. 1. Introduction The famous Keeling curve of atmospheric pCO2 (partial pressure of CO2) 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 pCO2 due to anthropogenic emissions and 2) the regular small seasonal peak. Annual average atmospheric pCO2 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 pCO2 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 CO2 forcing. Given this, it seems natural to look to the geological record for a time when pCO2 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