Hi, Ken –
Thanks much for your response. Of course I see how you meant this, and I
don’t really disagree as long as it’s clearly stated that, given the
successful achievement of the same radiative forcing impacts with the two
approaches, then there’s not much difference between the SRM and turning
down the sun. What I was saying – and, as I said, it was more question than
comment – grows out of a general sense that we should all still be looking
for any possible ways that one might not successfully achieve the forcing
impacts currently assumed, for some reason or another.
So, while you write that the surface temperature is not “super sensitive to
changes in the stratosphere,” I was referencing one of the possible ways in
which the surface apparently can be quite sensitive to such changes: for
me, at least, when I first heard of the Solomon et al paper a few years
back, what first popped out at me was how, with so little H2O up in the
stratosphere, changes in it could have such huge impacts on surface
temperature, comparable with changes in CO2 forcing, I think, over the same
period. Then later I began wondering whether stratospheric SRM techniques
could impact this stratospheric hydrology, as opposed to the much more
frequently discussed surface hydrology SRM impacts.
So, that’s why I brought up the methane issue, since there clearly is an
interaction at the surface between sulfur chemistry and methane oxidation,
and methane is very clearly an important source of H2O to the stratosphere.
Luckily, the methane doesn’t seem to be getting oxidized in the right place
to have such a big impact, but could *that* somehow or another get shifted
if you add a bunch of sulfur to the stratosphere? Again, it was really a
question, where I was asking: have such possibilities been explored?
Cheers,
Nathan
On Saturday, July 26, 2014 3:01:47 PM UTC-4, kcaldeira wrote:
>
> Nathan,
>
> Your questions are interesting but your initial statement is not true when
> it comes to first order effects on surface temperature and hydrology. ["..
> the
> similarity ("turning down the sun" and the SRM) depends on the ability of
> the models to reproduce, among other things, the stratospheric chemistry
> correctly. "]
>
> Surface temperature and hydrology is not super sensitive to changes in the
> stratosphere. Key is getting the effective radiative forcing right both in
> terms of total amount and latitudinal distribution.
>
> To repeat what I said in the previous email:
>
> *Of course, what tool you use depends on your goals in using a tool. The
> attached paper shows results indicating that for many purposes, "turning
> down the sun" is a clear and efficient way of simulating many aspects of
> solar geoengineering.*
>
> *If you are looking at effects on the stratosphere or looking at effects
> of diffuse radiation, then you would need to simulate aerosols, but if you
> are just trying to get an idea of temperature and hydrological changes,
> then it seems that "turning down the sun" does a pretty good job.*
>
> Best,
>
> Ken
>
> _______________
> Ken Caldeira
>
> Carnegie Institution for Science
> Dept of Global Ecology
> 260 Panama Street, Stanford, CA 94305 USA
> +1 650 704 7212 kcal...@carnegiescience.edu <javascript:>
> http://dge.stanford.edu/labs/caldeiralab
> https://twitter.com/KenCaldeira
>
> Assistant: Dawn Ross <dr...@carnegiescience.edu <javascript:>>
>
>
>
> On Sat, Jul 26, 2014 at 10:57 AM, Nathan Currier <natcu...@gmail.com
> <javascript:>> wrote:
>
>> This is more question than comment, but it seem to go without saying that
>> the similarity ("turning down the sun" and the SRM) depends on the ability
>> of the models to reproduce, among other things, the stratospheric chemistry
>> correctly.
>>
>> Solomon et al, 2010* (see below) concerned the hugely under-represented
>> impacts of stratospheric water vapor on surfacing warming, and suggested
>> increased stratospheric water vapor might have accounted for 30% of global
>> warming in the 90s. They also noted:
>>
>> Current global climate models simulate lower-stratospheric temperature
>> trends poorly, and even up-to-date stratospheric chemistry-climate models
>> do not consistently reproduce tropical tropopause minimum temperatures or
>> recently observed changes in stratospheric water vapor.
>>
>>
>>
>> Their paper only dealt with changes in one of the two ways that the
>> stratosphere gets its water vapor, but I’ve often wondered about the
>> potential impacts of sulfur SRM on the other one – that is, stratospheric
>> methane. Solomon et al, while suggesting that methane’s role is relatively
>> weak in H2O creation near the tropopause (where it apparently counts most
>> for surface warming), noted:
>>
>> Estimates of the forcing due to (stratospheric) methane oxidation have
>> varied widely among different studies, perhaps because of different shapes
>> of the water profile in the region of greatest sensitivity.
>>
>>
>> One of my questions has been whether prolonged use of stratospheric
>> sulfur SRM, unlike the sudden pulse of a volcano, could potentially give
>> rise to analogous “competitive” reactions to those seen in the troposphere
>> involving sulfur chemistry and methane hydroxylation (i.e., Shindell et al
>> 2007, 2009, 2012, etc), one of the factors that have driven our assumed
>> increase in the indirect forcing effects of methane over the last half
>> decade.
>>
>> I wonder, if additions of sulfur did actually lead, at the decadal scale
>> of the methane lifetime, to increases in stratospheric H2O, what could its
>> maximum impact be? What percentage of all stratospheric H2O comes from
>> methane, in the first place? Does current SRM modeling account for such
>> possible interactions?
>>
>> Best,
>>
>> Nathan
>>
>>
>>
>> *Solomon et al 2010:
>>
>>
>>
>> abstract
>> http://www.sciencemag.org/content/327/5970/1219.abstract
>>
>> full paper
>> http://www.climate.unibe.ch/~plattner/papers/solomon10sci.pdf
>>
>> discussions of, from NOAA & RealClimate:
>> http://www.noaanews.noaa.gov/stories2010/20100128_watervapor.html
>>
>>
>> http://www.realclimate.org/index.php/archives/2010/01/the-wisdom-of-solomon/
>>
>>
>>
>>
>> On Friday, July 25, 2014 11:39:52 AM UTC-4, kcaldeira wrote:
>>
>>> Folks,
>>>
>>> Andrew wrote something the other day about "turning down the sun"
>>> experiments NOT being a good analogue for stratospheric aerosol
>>> geoengineering.
>>>
>>> Of course, what tool you use depends on your goals in using a tool. The
>>> attached paper shows results indicating that for many purposes, "turning
>>> down the sun" is a clear and efficient way of simulating many aspects of
>>> solar geoengineering.
>>>
>>> If you are looking at effects on the stratosphere or looking at effects
>>> of diffuse radiation, then you would need to simulate aerosols, but if you
>>> are just trying to get an idea of temperature and hydrological changes,
>>> then it seems that "turning down the sun" does a pretty good job.
>>>
>>> Enjoy,
>>>
>>> Ken
>>>
>>>
>>> http://link.springer.com/article/10.1007/s00382-014-2240-3
>>>
>>>
>>>
>>> *Modeling of solar radiation management: a comparisonof simulations
>>> using reduced solar constant and stratospheric sulphate aerosols*
>>>
>>> Sirisha Kalidindi · Govindasamy Bala ·
>>> Angshuman Modak · Ken Caldeira
>>>
>>>
>>> Abstract The climatic effects of Solar Radiation Management
>>> (SRM) geoengineering have been often modeled
>>> by simply reducing the solar constant. This is most likely
>>> valid only for space sunshades and not for atmosphere and
>>> surface based SRM methods. In this study, a global climate
>>> model is used to evaluate the differences in the climate
>>> response to SRM by uniform solar constant reduction and
>>> stratospheric aerosols. Our analysis shows that when global
>>> mean warming from a doubling of CO2 is nearly cancelled
>>> by both these methods, they are similar when important
>>> surface and tropospheric climate variables are considered.
>>> However, a difference of 1 K in the global mean stratospheric
>>> (61–9.8 hPa) temperature is simulated between the
>>> two SRM methods. Further, while the global mean surface
>>> diffuse radiation increases by ~23 % and direct radiation
>>> decreases by about 9 % in the case of sulphate aerosol
>>> SRM method, both direct and diffuse radiation decrease by
>>> similar fractional amounts (~1.0 %) when solar constant
>>> is reduced. When CO2 fertilization effects from elevated
>>> CO2 concentration levels are removed, the contribution
>>> from shaded leaves to gross primary productivity (GPP)
>>> increases by 1.8 % in aerosol SRM because of increased
>>> diffuse light. However, this increase is almost offset by
>>> a 15.2 % decline in sunlit contribution due to reduced
>>> direct light. Overall both the SRM simulations show similar
>>> decrease in GPP (~8 %) and net primary productivity
>>> (~3 %). Based on our results we conclude that the climate
>>> states produced by a reduction in solar constant and addition
>>> of aerosols into the stratosphere can be considered
>>> almost similar except for two important aspects: stratospheric
>>> temperature change and the consequent implications
>>> for the dynamics and the chemistry of the stratosphere
>>> and the partitioning of direct versus diffuse radiation reaching
>>> the surface. Further, the likely dependence of global
>>> hydrological cycle response on aerosol particle size and the
>>> latitudinal and height distribution of aerosols is discussed.
>>> _______________
>>> Ken Caldeira
>>>
>>> Carnegie Institution for Science
>>> Dept of Global Ecology
>>> 260 Panama Street, Stanford, CA 94305 USA
>>> +1 650 704 7212 kcal...@carnegiescience.edu
>>> http://dge.stanford.edu/labs/caldeiralab
>>> https://twitter.com/KenCaldeira
>>>
>>> Assistant: Dawn Ross <dr...@carnegiescience.edu>
>>>
>>> --
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