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Dr. Michael E. Mann recounts a story in his recent book (the Hockey Stick and the Climate Wars) about an encounter with Hillary Clinton during a hostile congressional hearing. While he was fending off hostile questions from senators like Senator James Inhofe (who described climate change as "the single greatest hoax ever perpetuated on the American public") Hillary Clinton asked a (friendly) question along the lines of "what was the world like the last time carbon dioxide levels were this high?" (the story is on pages 119-120 of the hardback version of Mann's book).

Mann responded thus:

We have to go back to the time of the dinosaurs probably to find CO2 levels that we know were significantly higher than the CO2 levels today.

This struck me as being something of a spur of the moment exaggeration (I still remember some of my training in geology), a view at least supported by the Wikipedia summary. Mann neither gives a source nor a comment on his response.

So my question is: has current thinking about historic CO2 levels changed? Do we have more CO2 than at any time since the Cretaceous when dinosaurs still roamed the earth? Or was Mann just using hyperbole for effect?

NB: This isn't a question about the effects of CO2 or about the broader issues of climate change. It is just about the specific topic of the history of the atmospheric level of CO2.

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    According to a graph on page 9 of A Global Warming Primer (pdf), the last time CO2 levels were as low as they are today was about 300 million years ago--much longer ago than the Jurasic period. The graph implies that CO2 levels have been considerably higher than today for the entirety of the past ~250 million years. – oosterwal Jun 14 '12 at 5:49
  • @oosterwal - you may want to take a peek at page 13 of the same source, and note that all of human evolution took place within the last millimeter or so of the graph you referenced. – Mark Jan 2 '16 at 17:44
  • @oosterwal It would be better to get information from the scientists (e.g. the IPCC WG1 report) rather than publications of political think-tanks. Note the very next graph in the report gives presents the (rather dated) Scotese temperature reconstruction, without mentioning that it is a schematic rather than actual (proxy) data (for something more realistic, see realclimate.org/index.php/archives/2014/03/…). Also page 7 also shows a rather poor understanding of the carbon cycle, and page 8 is also "questionable". – Dikran Marsupial Jan 5 '16 at 17:13
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Yes, the Cretaceous was the last time atmospheric carbon dioxide was significantly higher than it is today, (and this, of course, depends to a degree on what you interpret by significantly higher)

It's very hard to be definite, because the error-bars on the estimates become large, as we look back in time.

However, as the first graph below, which aggregates the results of several studies, shows, the last time it was significantly higher what it is today, was around the Cretaceous period, denoted by K.

As far as we can tell, current levels are higher than they've been for 15 million years. Now "significantly higher" isn't very specific. Current levels are around 390ppm, up from around 280ppm before the industrial age started. So what's significantly higher? Let's say 800-1000ppm would be significantly higher (2-2.5 times current levels), and as far as we can tell, the last time atmospheric CO2 concentrations were that high, was indeed the Cretaceous period (which ended 65 million years ago).

(note that the present day is on the left in this graph).

enter image description here

The Rothman study is: Atmospheric carbon dioxide levels for the last 500 million years, PNAS April 2, 2002 vol. 99 no. 7 4167-4171, DOI:10.1073/pnas.022055499 , from which the following graph comes, of historic atmospheric CO2 levels expressed as a ratio relative to 2001 levels: so a value on the y-axis of 2 means double the level of 2001: (note that the present day is on the right in this graph)

enter image description here

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    These charts are fine for giving a perspective over the last billion years, but don't have much resolution in the last 100m (ie far enough back to cover the cretaceous). And even Rothman's study, which according to the first chart shows by far the lowest estimates of any study, seems to show that CO2 has been mostly double its current level for the majority of the last 100m years and still about double 50m years ago, well after the end of the cretaceous. – matt_black Oct 7 '12 at 14:06
  • Can we find a graph for the last 1 million years? Does CO2 go up and down with the glaciations? – GEdgar Jan 2 '16 at 21:16
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    @matt_black - Yes, CO2 goes up and down with the glaciations. See here for the last 800,000 years: washingtonpost.com/news/wonk/wp/2013/05/08/… – Mark Jan 4 '16 at 10:55
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    @EnergyNumbers and see Fig. 2 here: es.ucsc.edu/~jzachos/pubs/Zachos_Dickens_Zeebe_08.pdf – DavePhD Jan 4 '16 at 15:58
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According to Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years Science 4 December 2009: Vol. 326, pp. 1394-1397, looking especially at Fig. 2, 15 million years ago CO2 levels were 430 ppm, which is 8% higher than currently (2016).

The article summarizes as follows:

The highest estimates of pCO2 occur during the Mid-Miocene Climatic Optimum (MMCO), ~16 to 14 Ma, the only interval in our record with levels higher than the 2009 value of 387 ppmv.

Where "Ma" means million years.

Additionally, Atmospheric carbon dioxide concentrations over the past 60 million years Nature 406, pp. 695-699 shows CO2 levels of 1000ppm 40 million years ago, 2500ppm 46 million years ago, and consistently above 1800 ppm in the 52-60 million year time period.

So, no, the Cretaceous period was not the last time, significantly higher than (more than double) present CO2 levels occurred more recently in the Paleogene period and somewhat higher levels occurred during the Neogene period.

A more complete quote of Mann's testimony shows it is reasonably accurate:

So Dr. Mann let me ask you, what was the Earth’s climate like the last time that there was atmospheric concentration of carbon dioxide at today’s levels of 370 parts per million?

Dr. MANN. Thank you, Senator, that is an excellent question. We have to go back fairly far into the past to find CO2 levels approaching the CO2 levels today. Ice core studies that have been done over the past decade or so have told us that today’s CO2 level is unprecedented now in at least four glacial or inter-glacial cycles. That is more than 400,000 years. In fact, now as we look back from other evidence that is a bit more tentative, it appears that modern CO2 levels probably have not been observed in 10 million to 20 million years. So we have to go back to the time of the dinosaurs, probably, to find CO2 levels that we know were significantly higher than CO2 levels today.

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    There isn't 100% agreement on what the history of CO2 levels has been. But levels have been falling from ~10 times pre-industrial levels about 50m years ago. I would say that double pre-industrial levels is significantly higher, so Mann is stretching english and the definition of the cretaceous to make his statement. Given that catastrophe is predicted with a doubling from pre-industrial levels, perhaps that would be a sensible base to count as significant. – matt_black Jan 4 '16 at 20:43
  • @matt_black does Mann actually use the word "cretaceous"? – DavePhD Jan 4 '16 at 20:54
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    "time of the dinosaurs"=definition of cretaceous (or at least its end since they existed before it). – matt_black Jan 4 '16 at 20:56
  • @matt_black he should have said "30-55 million years ago" instead of "the time of the dinosaurs" based on Fig. 2 here: es.ucsc.edu/~jzachos/pubs/Zachos_Dickens_Zeebe_08.pdf – DavePhD Jan 4 '16 at 21:10
  • Yes, then at least some of the estimates would agree with him. And it is worth noting that some CO2 estimates have been much higher that that source. – matt_black Jan 4 '16 at 21:21

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