First, I should have noted that the recent Shindell et al paper makes clear that methane has roles in addition to its own GH effect, so my estimate does not include that.
On the CO2 question, GWP is over a time period. Indeed, as the time is stretched out, the GWPs for other species drop because the lifetime of the CO2 perturbation is so long‹so CO2 certainly has to be controlled. Over the 21st century only, however, the warming influence of emissions of CO2 (ignoring the SO2 cooling influence with which it is associated) and of the non-CO2 gases (plus black carbon) are about equal. I should note that there is also the carryover effect of CO2 perturbation from emissions prior to 2000, but if you want to slow near-term warming, the non-CO2 gases simply have to be addressed (not doing so aggressively is why the temperature rise curves for various stabilization scenarios do not start showing an effect for a several decades). Sharply cutting black carbon, ozone precursor and methane emissions, all of which need to be cut for other reasons‹and developed nations have shown it is possible-- can have a quite rapid effect in reducing radiative forcing (just as volcanic eruptions limiting solar shows there can be a quite strong near-term effect‹on forcing and temperature). But certainly, we also have to reduce CO2 emissions. Best, Mike On 11/16/09 3:22 PM, "Greg Rau" <[email protected]> wrote: > In light of recent modeling results on the lifetime of CO2 in the atmosphere, > I am concerned that the current time-integrated (not instantaneous) GWP > estimate for CO2 has been underestimated and hence GWP's of other gases (esp > short-lived gases) relative to CO2 have been overestimated. E.g., Eby et al., > 2009: > http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2008JCL > I2554.1 > show that 20-30% of excess emissions of CO2 and 60-70% of the subsequent > CO2-caused surface air temp anomally exists 10,000 years after emission. > Isn't this (or is this?) a far larger total time-integrated GW effect than a > mass equivalent emission of CH4? Experts please set me straight. > Thanks, > Greg > >> Agreed, one has to consider a time period, so assume one takes a day that >> when injected there is no decay over this period-so it might as well be a >> second of time one takes-so virtually instantaneous. And I'll assume >> linearity on methane absorption and logarithmic for CO2. >> >> So, for methane, humans have caused an increase of roughly 1000 ppb which >> converts to about 3 GtCH4, and this causes a forcing of about 0.5 W/m**2 (at >> the tropopause) per IPCC. >> >> For CO2, we know that a doubling (so we'll say from 300 to 600 ppm so we are >> in the range of interest) causes a forcing of about 3.6 W/m**2 (at the >> tropopause). So, 300 ppm of CO2 in the atmosphere is roughly 600 GtC or 2200 >> GtCO2 (and global warming potential is done for CO2, I believe. >> >> So, if we take the ratio of forcing to mass for CH4 divided by the ratio of >> forcing to mass for CO2, we get a rough estimate of the instantaneous GWP, so >> >> [0.5/3]/[3.6/2200] equals roughly 100 >> >> for the ratio at t=0 (so allowing for no decay) of the radiative forcing >> caused by a unit mass of CH4 added to the atmosphere to a unit mass of CO2 >> added to the atmosphere. >> >> Not exact, but plausible. >> >> Mike >> >> >> >> >> On 11/16/09 3:42 AM, "Peter Read" <[email protected]> wrote: >>> John, Andrew >>> Re "BTW, does anybody know the _immediate_ warming potential of methane?" >>> Someone will correct me no doubt but my understanding is that warming is a >>> rate process measured in W/m^2 >>> So "instantaneous" [[== "immediate"?]] warming is an incorrect concept >>> Unless it continues for a second, a week, a year, 25 years, for whatever, no >>> warming takes place. >>> So it is necessary to multiply by a duration to get joules/m^2 >>> It's how many joules get into the low albedo meltwater on top of Greenland's >>> ice that decides how much gets melted each year to fall down crevasses and >>> lubricate the eventual collapse of large areas of ice into the oceans. >>> Meaning that the integral [[roughly]] under the CO2 level curve is what >>> matters [multiplied by the warming potential over that period] when it >>> comes to measuring threats of Greenland's collapse >>> So the key issue is duration - how long elevated greenhouse gas levels last >>> and how to get them down. >>> Think that's right >>> Peter >>>> >>>> ----- Original Message ----- >>>> >>>> From: John Nissen <mailto:[email protected] >>>> <mailto:[email protected]> >>>> > >>>> >>>> To: [email protected] >>>> >>>> Cc: geoengineering <mailto:[email protected] >>>> <mailto:[email protected]> > >>>> >>>> Sent: Monday, November 16, 2009 6:18 PM >>>> >>>> Subject: Re: [geo] Re: Rejected - a simple argument for SRM geoengineering >>>> >>>> >>>> >>>> Hi Andrew, >>>> >>>> You say: "I don't oppose John's argument, but the evidence on the severity >>>> of the problem is far from conclusive." I disagree. >>>> >>>> The methane presents a very real risk - because of the uncertainty on >>>> timing combined with the potential size of methane discharge - perhaps >>>> even enough to cause thermal runaway due to positive feedback, as is >>>> thought to have happened in the past [1]. Risk management involves >>>> identifying events and assessing them in terms of their likelihood and >>>> magnitude of impact [2]. Thus something with a small likelihood (such as >>>> rapid massive methane excursion) can have a high risk, if the magnitude of >>>> impact is sufficiently large (and you can't get much larger than thermal >>>> runaway). >>>> >>>> It is possible that much or most of the methane trapped in frozen >>>> structures has built up over hundreds of thousands of years. There is >>>> little sign of massive methane discharge in the ice record. In fact >>>> methane seems to track the temperature even better than CO2 [3]. >>>> >>>> But of course methane discharge is not the only high risk event - there is >>>> also the Greenland ice sheet disintegration. >>>> >>>> BTW, does anybody know the _immediate_ warming potential of methane, as >>>> opposed to the 20 year value (72), 100 years (25) or 500 years (7.6)? The >>>> lifetime is only 12 +/- 3 years. See [4]. >>>> >>>> Cheers, >>>> >>>> John >>>> >>>> [1] http://*en.wikipedia.org/wiki/Clathrate_gun_hypothesis >>>> <http://*en.wikipedia.org/wiki/Clathrate_gun_hypothesis> >>>> >>>> [2] http://*en.wikipedia.org/wiki/Enterprise_risk_management >>>> <http://*en.wikipedia.org/wiki/Enterprise_risk_management> >>>> >>>> [3] http://*answers.yahoo.com/question/index?qid=20090329215018AAxqYFk >>>> <http://*answers.yahoo.com/question/index?qid=20090329215018AAxqYFk> >>>> >>>> [4] http://*en.wikipedia.org/wiki/Greenhouse_gas >>>> <http://*en.wikipedia.org/wiki/Greenhouse_gas> >>>> >>>> --- >>>> >>>> Andrew Lockley wrote: >>>>> At present the likely methane excursion is far from clear. Further, it >>>>> is also unclear how quickly the total excursion will occur. The >>>>> excursion rate is highly significant due to the short life of methane in >>>>> the atmosphere. The methane ends up as CO2, in itself a major issue. >>>>> However, the CO2's likely effect is nothing compared to the devastating >>>>> temperature spike which may result from a sudden methane excursion. >>>>> >>>>> >>>>> I don't oppose John's argument, but the evidence on the severity of the >>>>> problem is far from conclusive. We need much more research into: >>>>> >>>>> 1) The methane reservoir in clathrates and permafrost >>>>> >>>>> 2) The size of potential methane sources currently frozen as organic >>>>> detritus >>>>> >>>>> 3) The likely changes to the GWP of methane in future atmospheres, as the >>>>> levels of hydroxyl radicals etc. shift over time and as a direct result >>>>> of methane excursion. Recent research on this asks more questions than >>>>> it answers. >>>>> >>>>> >>>>> >>>>> My personal feeling is that the range of likely outcomes is from a >>>>> significant amplification of AGW, right up to a 'Great Dying' sized >>>>> mass-extinction event when we come within a sneeze of sterilising the >>>>> entire planet. I'd be tempted to bet on the latter, but I wouldn't be >>>>> around to collect the winnings, so I'll keep my wallet in my pocket. >>>>> >>>>> >>>>> >>>>> My suggestions is that geoengineers look at ways to clean up the methane >>>>> from the atmosphere, and then lock down the resulting carbon. My guess >>>>> is we've got about 50 years to do this, but virtually no time at all if >>>>> the methane can't be dealt with once it's in the atmosphere. Therefore, >>>>> I'd support John out of precautionary principle-based reasoning. >>>>> >>>>> >>>>> >>>>> A >>>>> >>>>> >>>>> 2009/11/15 jim woolridge <[email protected]> >>>>> >>>>>> Nice one, John; the train of argument is clear (of limpid clarity, in >>>>>> fact!) The problem is that the people and institutions addressed are >>>>>> in the business of politics, the art of the possible, rather than in >>>>>> the business of logical evaluation. They hear what you are saying and >>>>>> must see the validity of it. But politically what is true and what is >>>>>> doable do not always coincide, as we all know from as many examples as >>>>>> one cares to ennumerate. >>>>>> >>>>>> We have to keep hammering away at the arguments, to the point at which >>>>>> they are generally understood and accepted, and also keep on >>>>>> politicking in the sure and certain hope that eventually the >>>>>> acceptance of the arguments and the cowardice/caution/horse sense/ >>>>>> opportunistic careerism of the politicos will achieve the right kind >>>>>> of intersection. In the next year or so (& wouldn't it be a great >>>>>> help to have the environmental NGOs on board.) >>>>>> >>>>>> >>>>>> >>>>>> On Nov 12, 10:51 pm, John Nissen <[email protected]> wrote: >>>>>>> > It is incredible. It is so obvious. >>>>>>> > >>>>>>> > 1. Global warming is driven largely by atmospheric CO2 according to >>>>>>> the >>>>>>> > concentration above its pre-industrial level; and >>>>>>> > >>>>>>> > 2. After emissions are stopped it could take millenia for the >>>>>>> > concentration to fall back to that level, because the effective >>>>>>> lifetime >>>>>>> > of some of that excess CO2 is many thousands of years. >>>>>>> > >>>>>>> > Therefore: >>>>>>> > 3. Drastic emissions reduction, even to zero overnight, cannot and >>>>>>> will >>>>>>> > not stop the Arctic continuing to warm for decades. >>>>>>> > >>>>>>> > Therefore: >>>>>>> > 4. The Arctic sea ice will continue to retreat, accelerating the >>>>>>> warming >>>>>>> > due to the albedo effect. >>>>>>> > >>>>>>> > Therefore: >>>>>>> > 5. The permafrost will continue to thaw releasing increasing >>>>>>> quantities >>>>>>> > of methane, a potent greenhouse gas, potentially adding many degrees >>>>>>> to >>>>>>> > global warming; and >>>>>>> > >>>>>>> > 6. The Greenland ice sheet will become increasingly unstable, >>>>>>> > potentially contributing to an eventual sea level rise of 7 metres. >>>>>>> > >>>>>>> > Therefore: >>>>>>> > 7. To avoid these two catastrophes, we need to cool the Arctic >>>>>>> quickly >>>>>>> > enough to save the Arctic sea ice; and >>>>>>> > >>>>>>> > 8. Probably the only feasible way to do this is through solar >>>>>>> radiation >>>>>>> > management (SRM) geoengineering. >>>>>>> > >>>>>>> > 9. SRM is not a last resort, it is needed now to cool the Arctic. >>>>>>> > >>>>>>> > It is incredible that people do not seem to follow this train of >>>>>>> logic - >>>>>>> > it is so obvious. >>>>>>> > >>>>>>> > Yet when I challenged a panel of geoengineering experts to refute >>>>>>> this >>>>>>> > argument, the response was that geoengineering (even just to cool the >>>>>>> > Arctic) was too dangerous - not that the argument was false! [1] >>>>>>> > >>>>>>> > So we continue to hear politicians and their advisers claiming that >>>>>>> > emissions reduction alone can be sufficient to keep the planet safe. >>>>>>> [2] >>>>>>> > >>>>>>> > And we continue to hear geoengineering experts saying that >>>>>>> > geoengineering should only be used as a last resort. [3] >>>>>>> > >>>>>>> > How can this mindset be changed quickly, to avoid leaving >>>>>>> geoengineering >>>>>>> > too late? >>>>>>> > >>>>>>> > John >>>>>>> > >>>>>>> > P.S. Apologies to those who have heard this all before and accept the >>>>>>> > logic as self-evident. >>>>>>> > >>>>>>> > [1] This challenge was put to the panel at the launch of the Royal >>>>>>> > Society geoengineering report, on September 1st, with response from >>>>>>> the >>>>>>> > team leader and panel chairman, Professor John Shepherd. >>>>>>> > >>>>>>> > [2] For example at the geoengineering hearing at the House of >>>>>>> Commons, >>>>>>> > November 2008. >>>>>>> > >>>>>>> > [3] For example at the congressional hearing on geoengineering, >>>>>>> November >>>>>>> > 2009. >>>>>> >>>>>> -- >>>>>> >>>>>> You received this message because you are subscribed to the Google >>>>>> Groups "geoengineering" group. >>>>>> To post to this group, send email to [email protected]. >>>>>> To unsubscribe from this group, send email to >>>>>> [email protected] >>>>>> <mailto:geoengineering%[email protected] >>>>>> <mailto:geoengineering%[email protected]> > . >>>>>> For more options, visit this group at >>>>>> http://*groups.google.com/group/geoengineering?hl= >>>>>> <http://*groups.google.com/group/geoengineering?hl=> . >>>>> >>>>> >>>>> >>>>> -- >>>>> You received this message because you are subscribed to the Google Groups >>>>> "geoengineering" group. >>>>> To post to this group, send email to [email protected]. >>>>> To unsubscribe from this group, send email to >>>>> [email protected]. >>>>> For more options, visit this group at >>>>> http://*groups.google.com/group/geoengineering?hl= >>>>> <http://*groups.google.com/group/geoengineering?hl=> . >>>> >>>> >>>> >>>> >>>> -- >>>> You received this message because you are subscribed to the Google Groups >>>> "geoengineering" group. >>>> To post to this group, send email to [email protected]. >>>> To unsubscribe from this group, send email to >>>> [email protected]. >>>> For more options, visit this group at >>>> http://*groups.google.com/group/geoengineering?hl= >>>> <http://*groups.google.com/group/geoengineering?hl=> . >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> No virus found in this incoming message. >>>> Checked by AVG - www.*avg.com >>>> Version: 8.5.425 / Virus Database: 270.14.67/2505 - Release Date: 11/15/09 >>>> 19:50:00 >>>> >>>> -- >>>> You received this message because you are subscribed to the Google Groups >>>> "geoengineering" group. >>>> To post to this group, send email to [email protected]. >>>> To unsubscribe from this group, send email to >>>> [email protected]. >>>> For more options, visit this group at >>>> http://*groups.google.com/group/geoengineering?hl= >>>> <http://*groups.google.com/group/geoengineering?hl=> . >> -- >> You received this message because you are subscribed to the Google Groups >> "geoengineering" group. >> To post to this group, send email to [email protected]. >> To unsubscribe from this group, send email to >> [email protected]. >> For more options, visit this group at >> http://*groups.google.com/group/geoengineering?hl=. > > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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