Andrew
I accept that the optics and thermal effects of stratospheric sulphur
and tropospheric sea salt are similar. What is different is their
lifetime, two years for stratospheric sulphur and a few days, depending
on rain and drizzle for tropospheric sea salt. I believe Jon Egil when
he says that tropospheric sea salt will warm the Arctic in winter but
because of its short life we can make sure that all has gone before
winter comes. But because the life time of stratospheric sulphur is so
long it will be evenly spread in a few months. You can do an
experiment with cream in your coffee. David accused us of being patchy
and we accuse him of being promiscuous.
If you look at figures 7.4 and 7.5 on page 135 of Ben Parkes thesis
at http://homepages.see.leeds.ac.uk/~eebjp/thesis/ you will see where
you should spray to control rainfall in either direction in a selection
of places.
All I am asking is that you should distinguish the two technologies for
solar radiation management. However I am also curious about the coffee
cream result relates to non-uniform stratospheric concentrations.
Stephen
Emeritus Professor of Engineering Design School of Engineering
University of Edinburgh Mayfield Road Edinburgh EH9 3JL Scotland
s.sal...@ed.ac.uk Tel +44 (0)131 650 5704 Cell 07795 203 195
WWW.see.ed.ac.uk/~shs
On 30/11/2012 22:26, David Keith wrote:
Andrew
A few comments in response to this and the subsequent comment by
1. This vector representation is useful way to think about trade-offs
when the climate response to CO2 and SRM is reasonably linear. This
stuff is published as: Juan Moreno-Cruz, Katharine Ricke and David W.
Keith. (2011). A simple model to account for regional inequalities in
the effectiveness of solar radiation management. /Climatic Change/,
doi: 10.1007/s10584-011-0103-z. (PDF)
<http://www.keith.seas.harvard.edu/papers/131.Moreno-Cruz.Inequality.SRM.e.pdf>.
Despite the hype about nonlinearity, models are quite linear in the
region of interest, see the appendix to the paper.
2. I was surprised by our results. I expected the trade-offs to be
much stronger. Doing this work pushed me to realize that SRM can do a
substantially better job of compensating CO2-driven climate change
than I had expected. (Of course, it does nothing about the geochemical
impacts of CO2 such as ocean acidification.)
3. Yes, you can consider quantities other than temperature and precip;
and quantities like soil moisture are certainly important.
4. Stephen Salter imply that these results were somehow particular to
stratospheric sulfates, saying: /I think that you must be referring to
geo-engineering with stratospheric sulphur. With tropospheric salt you
can vary precipitation in both directions by choosing the time and
place to spray. /This analysis is applicable to both. It is certainly
true that if sea salt aerosol can be effectively used to alter cloud
albedo over large areas--a proposition which is still quite
uncertain--then it could be used to reduce (them eliminate) the
trade-offs.
We looked at exactly this in a more recent paper examining how
trade-offs can be reduced if you were able to adjust the intensity of
SRM forcing at different locations in seasons: Douglas G. MacMartin,
David W. Keith, Ben Kravitz, and Ken Caldeira. (2012). Managing
tradeoffs in geoengineering through optimal choice of non-uniform
radiative forcing. /Nature Climate Change/, doi: 10.1038/NCLIMATE1722.
(PDF
<http://www.keith.seas.harvard.edu/preprints/158.MacMartin.etal.ManagingTradeoffsThroughNonRadForc.p.pdf>).
Note that this paper explicitly looks at something people in this blog
often ask about which is the ability to tune SRM to focus on restoring
Arctic sea ice.
5. Finally, I do not understand your argument about soil moisture.
Evaporation always equals precipitation the global mean. All else
equal-- and it probably will not be--one expects variability to go
down (not up) as you weaken the hydrological cycle. So my back of the
envelope physics points the opposite way to yours. We look at this in
one of the papers with Kate Ricke and found that at least the one case
we looked at variability did go down.
Of course, model *do not equal* reality.
Yours,
David
*From:*geoengineering@googlegroups.com
[mailto:geoengineering@googlegroups.com] *On Behalf Of *Andrew Lockley
*Sent:* Monday, November 26, 2012 7:08 AM
*To:* David Keith; geoengineering
*Subject:* [geo] Your Vector diagram
David
I remember the excellent vector diagram lecture you gave at Oxford.
In it you represented temperature and precipitation on a
vector.diagram and showed that both cannot be simultaneously corrected
exactly by geoengineering, but that the mismatch was small.
However, that would leave us in a world which was either slightly
drier or slightly warmer than in a non-geoeng world - or a combination
of both.
My concern is that things might be a bit more serious than that. If we
consider a warmer world with the same level of precipitation, the
surface evaporation world be higher and the relative atmospheric
humidity would (I think) be lower. As a result, soil wetness may be
very much lower, as evapotranspiration would be higher. If rainfall
patterns were perturbed, we might additionally get more variability in
both wetness and precipitation.
So we could end up in a world with much drier soils, and possibly
heavier storms, too.
Should your vectors therefore be soil wetness vs temperature, not
precipitation vs temp? Making a bad call on this could really hit
agricultural outputs.
A
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