Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Irreversible Climate Change Already?

  1. Oct 19, 2009 #1


    User Avatar

    Here is a news release from the US Geological Survey.
    I've copied some parts of it, but please read the entire release before commenting.


    In brief, there are lots of uncertainties and potentials for climate change to impact
    the role of the arctic as a CO2 sink and CH4 source. The science isn't there yet
    to make much of a prediction and it may be decades before we know for sure.

    However, what is interesting are the current CO2 sink and CH4 source numbers.
    Although the CH4 source is one eighth the CO2 sink (50 vs 400), since CH4 is 23 times
    more powerful as a greenhouse gas, the implication is that the Arctic is
    already providing a positive feedback to climate change. It is providing roughly
    effectively 3 times more greenhouse gases than it is absorbing. In other
    words, even if human CO2 emissions were reduced to near zero, there would
    still be a net positive global warming from sources/sinks inherit in the Arctic.

    If true, that's a huge statement because unless there are significant changes in the
    dynamics of the Arctic, that means climate change is probably irreversible.
    By irreversible, I mean even if Humans were to cease all CO2 emissions
    (which is highly unlikely), there would still be climate change due to the current
    CH4 emissions from the Arctic.

  2. jcsd
  3. Oct 19, 2009 #2


    User Avatar

    Staff: Mentor

    The title is not correct, there has never been any "irreversible" climate change recorded on this planet. Where in this link that you quoted do they say "irreversible". And please link to the study that shows it is "irreversible".

    Personal opinions and theories are not allowed.
  4. Oct 19, 2009 #3
    Last edited by a moderator: May 4, 2017
  5. Oct 19, 2009 #4
    He defined what he means by irreversible. And the he also presented it as a question not a statement.

    The paper is cited in the press release from the USGS. The USGS is a valid source within forum guidelines.

    Here is the http://www.esajournals.org/toc/emon/79/4"

    Here is thehttp://www.esajournals.org/doi/pdf/10.1890/08-2025.1" [Broken]
    Last edited by a moderator: May 4, 2017
  6. Oct 20, 2009 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    One should indeed be clear by what one means by "irreversible". There ARE ways in which climate change can potentially be irreversible, that is when there are several stable states for the same situation (a case of multistability), and a relatively small perturbation flips the climate from one stable state to another one (which could be a snowball earth, or a runaway greenhouse). Taking away the initial "drive" will then not bring climate back to its original stable point. This is a way one could say the change is "irreversible".

    If one just means "it's going to take a much longer time to come back, than to drift away", that's strictly speaking not irreversible, but could be considered "irreversible for all practical purposes".
  7. Oct 20, 2009 #6
    There are four elements I think about the role of CH4 as greenhouse gas. First, the factor 23, second, the residence time of CH4, third the actual trend of CH4 in the atmosphere and fourth, the most stunning, the paleoclimatological evidence of the the role of CH4.

    The outcome, in my opinion, is that the role of CH4 is about 23 times overstated.

    So first, what is this factor "23 times more powerful" about? Let's consult http://geoflop.uchicago.edu/forecast/docs/Projects/modtran.html once more.

    Let's start to check what doubling either greenhouse gas does on the radiation out. If you hit "submit the calculation" from the defaults, you see in the output:

    Now double the value for CO2 (375 -> 750 ppm) and calculate again to get Iout, W/m2 = 284.672 hence a difference of ~3.2 W/m2

    Now reset CO2 to 375 and double the CH4 (1.7 -> 3.4 ppm) and now we get 287.09 W/m2 or a difference of only ~0.76 W/m2. Compared to 3.2 W/m2 not exactly a factor 23, is it?

    Let's try a different way seeing what one unit of greenhouse gas does, setting both CO2 and CH4 to zero to get a basic Iout of 320.28 W/m2. Now If we apply one ppm of CO2 we get 315.57 ppm (delta ~ 4.7 W/m2) an for one ppm of CH4 we get an Iout of 319.024 a mere ~1.3 W/m2. So at unit level it appears that CO2 is about 3.75 stronger for greenhouse effect than CH4

    Maybe it helps if we restore the default values (375 and 1.7 ppm) again and see what happens if we increase each with one ppm individually, Unfortunately CO2 on 376 and 377 gives the same result as 375. Only at 378 ppm we get get Iout, W/m2 = 287.812, or a delta of 0.032, which would be ~0.01 W/m2 per ppm. If we increase CH4 to one ppm to 2.7 we get Iout, W/m2 = 287.373 or ~0.47 W/m2. A whopping 47 times more than the increase of CO2 from 375 to 376 ppm.

    So where is this 23 times coming from? You can draw your own conclusions. But as the last example shows, it’s more like 23 times almost nothing.
    Back later, there is plenty more.
    Last edited by a moderator: Apr 24, 2017
  8. Oct 20, 2009 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think the 23 more comes from equal QUANTITIES of gas added:

    375 ppm CO2 and 1.7 ppm CH4 gives you (MODTRAN) 287.844 W/m2

    add 10 ppm CO2: this gives you 287.75 W/m2 or a forcing of 0.094 W/m2 for 10 ppm CO2

    add 10 ppm CH4: you get 285.143 W/m2 or a forcing of 2.7 W/m2

    so here we find a factor of 28 more forcing for 10 ppm of CH4 added, than for 10 ppm of CO2 added.

    Of course this factor changes as a function of the exact delta.

    It gives you the comparative effects of releasing 1 Gton of CO2 or 1 Gton of CH4, starting from our current situation, and for small effects.
  9. Oct 20, 2009 #8
    Note that I did the same for one ppm for each with the 47 times result in my previous post.

    Anyway the second factor was resident time of CH4 in the amosphere. That's estimated to be about a decade. This simply implies that CH4 would disappear rather quickly as it is oxydized in the atmosphere, so you'd need a big continuous source to keep the levels up whereas there must be limits to the stored amounts in the Arctic.

    Third factor was the actual CH4 levels of the last few years which can be seen to be stabilizing http://www.mfe.govt.nz/environmental-reporting/atmosphere/greenhouse-gases/atmospheric-levels.html at Barring Head for instance:

    http://www.mfe.govt.nz/environmental-reporting/atmosphere/greenhouse-gases/images/ch4-baring.jpg [Broken]

    ... giving no support for an additonal large CH4 source so far.

    Fourth factor is worth a seperate thread I think, back for that later.
    Last edited by a moderator: May 4, 2017
  10. Oct 20, 2009 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, I saw that, but given the very small difference in power, we might have numerical roundoff errors (although, on the other hand, 10 ppm is not a "small change" for the CH4, I agree).

    I didn't know this. Then indeed, there isn't any reason to consider this as an important greenhouse gas, so I wonder why people DO consider it important.

    About the evolution of the CH4 concentration, what happened around 1999 that made for the kink ?
  11. Oct 20, 2009 #10
    You also must consider the fact that when CH4 decays in the atmosphere it recombines with oxygen, the end result is one CO2 molecule and four H2O molecules. Since this process usually takes place in the stratosphere where UV energies are higher, methane is the primary source of water vapor above the tropopause.
  12. Oct 20, 2009 #11
    Apart from that it's not a linear effect so the effect of 1 ppm is not the same as 1/10 of 10 ppm.

    Because that's how it appeared in the Greenland Ice cores. I'll try and start a thread on that in a while.

    The answer has not risen beyond any speculation. It's simply unknown.
  13. Oct 20, 2009 #12
    True, but per 1 ppm CH4 on 1.7 ppm, is a whole lot different than 1 ppm CO2 on -say- 385 ppm. Furthermore the greenhouse effect of the stratosphere is supposed to be cooling, not warming.
  14. Oct 20, 2009 #13


    User Avatar

    Evo; Please notice that I posed question.

    WeatherRusty; Thanks for the links. It looks like the "reversible" question has been answered already for human causes. My question/surprise was that the Arctic Ecosystem itself was becoming a climate driver all by itself.

    Skyhunter; Thanks for coming to my defense by injecting some reason.

    Andre; is vanesch correct about the number 23? I think he is.

    However, your point about what happened to CH4 atmospheric concentrations since 1999 is relevant. Not clear if the USGS factored that in with their emission numbers.
    I haven't seen much of an timeline accounting for the changes in CH4 emissions and sinks (if that is what we call them). Something seems to have changed for the better, but what?

    One of the things about the press releases is that it appears to form the basis for grant request as much as anything.
  15. Oct 20, 2009 #14
    Not the greenhouse effect in general but CO2 specifically. CO2, because of it's quantum shape will radiate more energy in the stratosphere than is available there for absorption. Water vapor in the stratosphere will lead to stratospheric warming.

    I don't know the net result, just pointing out that there is more to CH4 emission forcing than it's absorption spectra.
  16. Oct 20, 2009 #15

    From Post#6:

    Global Warming Potential (GWP) adds one more factor into the consideration of the relative importance or "effectiveness" of the various GHGs. Some gases linger in the atmosphere far longer than others before natural processes remove them. In the case of a GHG, the Global Warming Potential (GWP) takes into account the fact that a GHG that lingers longer has a greater cumulative contribution to the greenhouse effect over its "lifetime" than does a gas that is quickly removed. Recall that water vapor tends to cycle out of the atmosphere in a matter of days; water vapor, therefore, has a negligibly small GWP. Methane takes, on average, about 12 years to disappear from the atmosphere. Carbon dioxide takes centuries. Some gases take even longer periods of time! GWP is stated in terms of a given time interval, such as "the GWP for a 20 year time horizon" or "the GWP for a 100 year time horizon"; this latter is the most commonly stated time period for GWP. Carbon dioxide is used as the reference gas, and therefore, by definition, has a GWP of precisely 1. A definition of GWP could be stated something like: the total radiative forcing produced by a given amount (such as one kilogram) of a particular GHG over the entire course of a specified time period (most commonly a century) as compared with the same amount of carbon dioxide. Methane, which is "pound-for-pound" a much more "potent" GHG than CO2 has a GWP of 62 over a 20 year period. Over a 100 year period, the GWP of methane is a much-reduced 23; though methane is more "potent" than carbon dioxide, it is also "shorter-lived", which tends to offset its total contribution to the greenhouse effect. Nitrous oxide, which is both a very "potent" GHG and a long-lived one (atmospheric lifetime of 114 years), has a GWP of 296 on a 100 year scale. Some fluorocarbons have GWP values of more than 1,000 or even more than 10,000; we would have some really big problems if we released large quantities of them into the atmosphere!


    More near the end of this EPA article including tables of GWP:

    Last edited by a moderator: Apr 24, 2017
  17. Oct 20, 2009 #16


    User Avatar

    So, this is interesting.

    According to the EPA document, the 100 year Global Warming Potential of Methane (CH4) is 25 times that of CO2.

    In the SAR it was 21
    In the TAR is was 23
    In AR4 it is 25.

    So, the USGS quoted an out of date value or is using a differant time horizon.

    100 years is the conventional metric used by most countries.
  18. Oct 20, 2009 #17


    User Avatar

    More from the EPA/IPCC.

    In other words, almost half the CH4 flux are from "natural" sources, which presumably would be things like the Arctic regions reaction to warming already in place.
  19. Oct 20, 2009 #18


    User Avatar

    From the UCAR link.... droughts are good for mitigating methane releases. But the long term predictions of global warming are for globally higher precipition levels.
  20. Oct 20, 2009 #19


    User Avatar
    Gold Member

    ?? The paper referenced by USGS in EM is within the guidelines, not every statement made by the USGS - per my understanding.

    Title, authors:
    Sensitivity of the Carbon Cycle in the Arctic to Climate Change
    A. David McGuire, Leif G. Anderson, Torben R. Christensen, Scott Dallimore, Laodong Guo, Daniel J. Hayes, Martin Heimann, Thomas D. Lorenson, Robie W. Macdonald, Nigel Roulet
  21. Oct 20, 2009 #20


    User Avatar
    Gold Member

    IPCC AR4's list of forcings below. CH4 is 1/3 that of CO2, slightly more than ozone. Perhaps that's due simply to CH4's small concentration in the atmosphere relative to CO2, or some other factor discussed above. For whatever reason, it's a distant second to CO2 in forcing, and thus I think it can't lead irreversibility arguments.

    http://www.jaxa.jp/article/special/eco/img/kimura_photo04_be.jpg [Broken]
    Last edited by a moderator: May 4, 2017
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook