bucket22 a posé la question dans EnvironmentGlobal Warming · il y a 10 ans

What implications are there from a recent study on atmospheric CO2 during ancient times?

"Quantifying atmospheric CO2 concentrations ([CO2]atm) during Earth’s ancient greenhouse episodes is essential for accurately predicting the response of future climate to elevated CO2 levels. Empirical estimates of [CO2]atm during Paleozoic and Mesozoic greenhouse climates are based primarily on the carbon isotope composition of calcium carbonate in fossil soils. We report that greenhouse [CO2]atm have been significantly overestimated because previously assumed soil CO2 concentrations during carbonate formation are too high. More accurate [CO2]atm, resulting from better constraints on soil CO2, indicate that large (1,000s of ppmV) fluctuations in [CO2]atm did not characterize ancient climates and that past greenhouse climates were accompanied by concentrations similar to those projected for A.D. 2100. "

http://www.pnas.org/content/107/2/576

This isn't entirely inconsistent with previous research. Some CO2 reconstructions are in line with this and others showing much larger CO2 concentration have had very large error bars for hundreds of millions of years ago.

As for implications, I can think of a few, namely:

1. The "CO2 is good" argument from so-called "skeptics" is pretty much buried. This is an argument made by Exxon hacks:

http://www.huffingtonpost.com/2009/02/27/co2-famin...

...and by the journalist Lord Monckton. Although comparing the time of dinosaurs and simple animals to our modern human civilization is pretty naive to begin with, certainly, it's impossible to claim very high atmospheric CO2 was good when it wasn't that high.

2. On a related note, this implies that by 2100, on our current path, we will begin to exceed atmospheric CO2 levels not seen in at least 500 million years, back when there were scant evidence of land animals. At the very least, it means we're conducting a tremendous global experiment with no precedent.

3. Very high CO2 levels (many thousands of ppm) during some ancient periods were inconsistent with the global temperature evidence for that time. In other words, one would have expected more warming. While there's plenty of uncertainty in data going back that far, such a study appears to resolve some of the remaining paleoclimatic discrepancies, and perhaps indicates a higher climate sensitivity with less CO2 change, at least from the paleo evidence.

Mise à jour:

Dana says:

" I thought it was fairly well established that geologic CO2 levels were in the several thousands of ppm."

Not all studies indicated that, but many certainly did.

http://en.wikipedia.org/wiki/File:Phanerozoic_Carb...

10 réponses

Pertinence
  • Anonyme
    il y a 10 ans
    Meilleure réponse

    Wow, that's quite a finding if correct. I wish the whole paper were available for free. I thought it was fairly well established that geologic CO2 levels were in the several thousands of ppm.

    If true, it does bury the whole "more CO2 = good for life" argument. Not that the argument had any basis in reality to begin with, comparing species and climates hundreds of millions of years apart.

    It would also be interesting to see how this changes CO2 vs. global temperature data during the periods in question. Another denier argument has been "why wasn't the planet hotter when CO2 levels were in the thousands of ppm?". Well if they weren't, that's not much of an argument anymore either.

    *edit* I see, it looks like Rothman at least has CO2 levels very stable up to 500 million years ago.

    http://www.pnas.org/content/99/7/4167.full

  • il y a 10 ans

    So now they've rewritten the climate history going back not only 1,000 or 10,000 years but hundreds of millions of years......

    Amazing. Absolutely amazing.

    I believe the climate history as it stood before the climate became a political issue. When dinosaurs roamed the Earth, atmospheric CO2 concentrations were 12-15 times higher than they are today. Not double.

    What they're doing is changing the facts to fit the argument. The facts used to be that the MWP was warmer for two centuries than it is today and gave way to the LIA rather than further warming; that the HM was significantly warmer despite lower CO2 concentrations; and that there were some periods tens and hundreds of millions of years ago during which temperatures were about 30% warmer but CO2 levels were 12-15 times higher than present.

    Those facts don't support 'catastrophic runaway warming' and don't lend much support to material warming stemming from a 35% increase in atmospheric CO2 concentration.

    So now the facts have been rewritten.

    The climate history has it has stood for decades is being rewritten and now fits an established agenda. And it is being rewritten by those who support that agenda.

    I will need to read the entire report concerning ancient climate - with respect to the MWP and HM the case for revision is shaky at best (tree rings, ignoring direct evidence of what grew when and where, etc....). I suspect that the same will be true here.

    This is the politicization of science.

  • Anonyme
    il y a 10 ans

    It is irrelevant because the continents were not in their current configuration.

    During the Paleozoic most continents were around the equator. At one point they formed Gondwana (Gondwanaland), a landmass consisting of what now are South America, Africa, Madagascar, Antarctica, India, southern Europe, Florida, and parts of South Asia, and Australia.

    No climate scientist has ever claimed that global geography plays no part in climate - but deniers seem unable to grasp that fact.

  • il y a 10 ans

    Thanks for the link.

    This does not look like good news. It suggests (but see Gary's comment) high sensitivity of climate to CO2 concentration.

    And no, it does not imply greater uncertainty regarding the relative the recent past, where we have the ice core record.

    (Not that great uncertainty would be grounds for complacency; the reverse is true since it would make worst-case scenarios appear less unlikely)

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  • Anonyme
    il y a 10 ans

    oh dear. Hansen mentions this in his new book. it changes the range of possible sensitivity considerably.

    this makes more sense of his 'what's the worst that can happen' scenario of genuine venus style runaway warming, which even i had been thinking was a bit unlikely. <shiver>

  • Anonyme
    il y a 10 ans

    I recall reading recently that there was much more CO2 in the atmosphere a couple of million years ago. Today there is less than .03%! Google "pie chart" and CO2 and see for yourself. Makes ya wonder.

  • Moe
    Lv 6
    il y a 10 ans

    So the MWP was not as warm as first determined and along with the LIA they were only a regional events, and since CO2 levels weren't as high in the past as previously calculated, it's obvious now to me that our current Global Warming is unprecedented and caused by me. Halalua, I see the light, I'm a believer and not a denier anymore, where do I send in my tithing?

  • il y a 10 ans

    PNAS articles go free after 6 months, so at least it will enter the open domain eventually.

  • Nathan
    Lv 4
    il y a 10 ans

    What this tells me is that scientists really don't know as much about carbon dioxide concentrations and temperature in ancient times as they claim that they do. I still think that we don't have enough information to prove AGW.

  • Anonyme
    il y a 10 ans

    It emboldens the notion of uncertainty in the science. That would be the main implication.

    Thanks for posting -- it'll be interesting to watch how that study unfolds.

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