To comment further on the upper bound for aerosol forcing: the
following abstract may be of interest. Consideration of such aerosol
indirect effects, coupled with the potential methane feedbacks,
suggests a range for future warming scenarios much wider than IPCC
ranges. As such, clarity on the upper bounds of radiative forcing
available under geoengineering programmes would be very helpful. In
my personal opinion, it's crucial to establish whether or not a single
geoengineering programme, or a combination, would be capable of
controlling global mean temperatures under conditions of both
high-feedback and high global warming potential for methane. I fear
that they may not.
Reference/abstract below
A
Atmos. Chem. Phys., 11, 6961-6969, 2011
www.atmos-chem-phys.net/11/6961/2011/
doi:10.5194/acp-11-6961-2011
Large methane releases lead to strong aerosol forcing and reduced cloudiness
T. Kurtén1,2, L. Zhou1, R. Makkonen1, J. Merikanto1, P. Räisänen3, M.
Boy1, N. Richards4, A. Rap4, S. Smolander1, A. Sogachev5, A.
Guenther6,
Abstract. The release of vast quantities of methane into the
atmosphere as a result of clathrate destabilization is a potential
mechanism for rapid amplification of global warming. Previous studies
have calculated the enhanced warming based mainly on the radiative
effect of the methane itself, with smaller contributions from the
associated carbon dioxide or ozone increases. Here, we study the
effect of strongly elevated methane (CH4) levels on oxidant and
aerosol particle concentrations using a combination of
chemistry-transport and general circulation models. A 10-fold increase
in methane concentrations is predicted to significantly decrease
hydroxyl radical (OH) concentrations, while moderately increasing
ozone (O3). These changes lead to a 70 % increase in the atmospheric
lifetime of methane, and an 18 % decrease in global mean cloud droplet
number concentrations (CDNC). The CDNC change causes a radiative
forcing that is comparable in magnitude to the longwave radiative
forcing ("enhanced greenhouse effect") of the added methane. Together,
the indirect CH4-O3 and CH4-OH-aerosol forcings could more than double
the warming effect of large methane increases. Our findings may help
explain the anomalously large temperature changes associated with
historic methane releases.
> ---------- Forwarded message ----------
> From: "Simone Tilmes" <[email protected]>
> Date: Jun 29, 2012 4:33 PM
> Subject: Re: [geo] interview with me in Neue Zurcher Zeitung (translated
> into German)
> To: "Ken Caldeira" <[email protected]>
> Cc: <[email protected]>
>
> Dear Ken,
>
> thanks a lot to Matthias, to translate the article!
>
> I think it is important to point out that there is very likely a limit on
> how much the Earth's surface could be cooled using sulfate aerosols, due to
> coagulation processes and fall out of aerosols. Only less than 2 W/m2
> reduction of global net surface SW flux was achieved in the study by
> Heckendorn et al., 2009, using a micro-physical model to consider size
> distributions of the aerosols. Niemeier et al., 2010, achieved a stronger
> forcing if injecting particles at 30hPa, which allow them to stay longer in
> the stratosphere. Though it will be hard to inject particles that high.
>
> Cheers, Simone
>
> References:
> Niemeier, U., H. Schmidt and C. Timmreck, The dependency of geoengineered
> sulfate aerosol on the emission strategy, Atmos. Sci. Let., DOI:
> 10.1002/asl.304, 2010.
>
> Heckendorn P, Weisenstein D, Fueglistaler S, Luo BP, Rozanov E, Schraner M,
> Thomason LW, Peter T. 2009. The impact of geoengineering aerosols on
> stratospheric temperature and ozone.
> Environmental Research Letters 4: 045108. DOI: 10.1088/1748-9326/4/4/045108.
>
>
> Simone,
>
> Not reading German, I don't know precisely what is in the story.
>
> What I said to the reporter is that if the aerosol layer similar to that
> of Mt Pinatubo were sustained, it would produce a cooling of about 3 K.
>
> This is based on estimates of Mt Pinatubo producing around 4 W / m2 of
> radiative forcing (cf. Crutzen, 2006?), which is similar to a CO2
> doubling -- and 3 C per CO2 doubling is in the middle of the range of
> estimates for climate sensitivity. So, this is just a back-of-envelope
> calculation.
>
> A question of course, given particle aggregation and so on, is whether
> such a layer could be sustained.
>
> Best,
>
> Ken
>
>
> On Thu, Jun 28, 2012 at 6:37 PM, Simone Tilmes <[email protected]
>
> <mailto:[email protected]>> wrote:
>
> Dear Ken,
>
> in the article you stated that Mt Pinatubo is assumed to result in a
> global cooling of about 0.5 degree. It is also stated in the text of
> the article that the amount of aerosols emitted, if they would stay
> in the stratosphere for a longer time period, would result in a 3
> degrees global cooling. Could you point me to the study you are
> referring to that calculates this amount of cooling if injecting
> volcanic aerosols of the amount of Mt Pinatubo?
>
> Cheers, Simone
>
>
> Ken
>
> It should follow from your argument about the land-sea temperature
> difference reducing precipitation on land that a technique which
> had the
> initial effect of cooling the sea would be more attractive. If
> it were
> also possible to have a frequency response shorter than the monsoon
> cycle we could play useful tricks about the phase of operations
> relative
> to the monsoon season.
>
> Stephen
>
> Emeritus Professor of Engineering Design
> Institute for Energy Systems
> School of Engineering
> Mayfield Road
> University of Edinburgh EH9 3JL
> Scotland
>
> Tel +44 131 650 5704 <tel:%2B44%20131%20650%205704>
> Mobile 07795 203 195
> www.see.ed.ac.uk/~shs <http://www.see.ed.ac.uk/~shs>
>
>
>
>
> On 28/06/2012 06:27, Ken Caldeira wrote:
>
> pdf attached.
>
>
> http://www.nzz.ch/wissen/__wissenschaft/sonnenschutz-__fuer-die-erde-1.17282213
>
>
>
> <http://www.nzz.ch/wissen/wissenschaft/sonnenschutz-fuer-die-erde-1.17282213>
>
>
> _______________
> Ken Caldeira
>
> Carnegie Institution for Science
>
> Dept of Global Ecology
> 260 Panama Street, Stanford, CA 94305 USA
>
> +1 650 704 7212 <tel:%2B1%20650%20704%207212>
> [email protected]
> <mailto:[email protected]>
> <mailto:kcaldeira@__carnegiescience.edu
> <mailto:[email protected]>>
> http://dge.stanford.edu/labs/__caldeiralab
>
>
> <http://dge.stanford.edu/labs/caldeiralab> @kencaldeira
>
> *Currently visiting * Institute for Advanced Sustainability
> Studies
> (IASS) <http://www.iass-potsdam.de/>
> *and *Potsdam Institute for Climate Impact Research (PIK)
>
> <http://www.pik-potsdam.de/>*__in Potsdam, Germany.*
>
>
>
>
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