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 [email protected] <javascript:>
> http://dge.stanford.edu/labs/caldeiralab
> https://twitter.com/KenCaldeira
>
> Assistant: Dawn Ross <[email protected] <javascript:>>
>
>
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