What does it really mean to have today’s CO2 level of 405-410 ppm?
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It means we would like to somehow find out what levels were like far back in the past, long before any direct measurements could be undertaken. We have good ice core measurements that go back 800,000 years, over eight full ice-age cycles, which place a lid of no higher than the 300, and that was barely reached just once. (You can see this at https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/co2_800k.png) There is also a still deeper core from Antarctica, recently announced, that found a level of 300 from a time at the very beginning of the intensified early Northern Hemisphere glaciation period 2.7 million years ago, which was still within the Pliocene era. Otherwise we only have interpretations based on isotopes and other phenomena that are found in sediments on the ocean floor that are capable of being well-dated.
Fortunately, I have found a major scientific study from 2009 that provides a thorough review of what looks like the best possible source of broader information that can be gathered about these indirect methods. In addition the study serves as a kind of tour guide for many of the unusual and often erratic events that took place during different stages of the Pliocene leading up to and into the glaciation period. Here is a link to that study, which is open to all readers and is worth spending some time on: http://onlinelibrary.wiley.com/doi/10.1029/2010PA002055/full
One thing that should get your attention is that CO2 levels most likely did not go above 420, or maybe 450 at the utmost, at any time during the Pliocene. You can see this graphically in the lower part of one of the charts within the study: http://onlinelibrary.wiley.com/enhanced/figures/doi/10.1029/2010PA002055/#figure-viewer-palo1705-fig-0004 That same chart, in the center, provides an isotope-based indicator from which the pace and degree of temperature changes can be derived, but this study is not about temperatures and does not give that kind of readings.
The point is that today’s CO2 level is about the same as that which existed before the northern glaciation even started. Indeed the way things are going we will soon be completely out of Pliocene territory and well into the Miocene, which makes me want to learn more about what temperatures were actually like in both of those eras.
In yesterday’s letter there was quite a bit reported about the “newly-revised carbon budget” of 240 billion tons (as of early 2015, and designated in just carbon, not CO2) which is supposed to keep us within the 1.5C limit if we can stay on budget. That much tonnage, when released, is enough to add about 60 ppm to the CO2 level if the regular sinks keep working as they are now. On top of that there is always the possibility of the soil carbon effect, including releases from lands now frozen, which some people think will add even larger amounts of carbon to the atmosphere once it gets rolling.
–Note: Regular sinks pick off about 50% of any CO2 released to the atmosphere, adding it to either vegetation or sea water. Of the remainder, each 2.13 billion tons of carbon formed into CO2 means 1 ppm is added to the total that we measure.
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Here is some further commentary on the story about the new carbon budget:
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One of the best new ideas for storing large quantities of CO2 that could potentially be captured some day, once we get desperate enough to go that route:
Carl