Dear Mike,
The paper says:
There is a clear seasonal cycle in the e-folding lifetime of
the stratospheric aerosols in the Arctic case ranging from
2 to 4 months. The maximum lifetime occurs during boreal
summer with a minimum during boreal winter with the
formation of the polar vortex and higher rates of tropopause
folding.
So 4 months is the correct number to use if you are looking at a ratio
of impact to mass of sulfur injections.
Alan
[On sabbatical for current academic year. The best way to contact me
is by email, rob...@envsci.rutgers.edu, or at 732-881-1610 (cell).]
Alan Robock, Professor II (Distinguished Professor)
Editor, Reviews of Geophysics
Director, Meteorology Undergraduate Program
Associate Director, Center for Environmental Prediction
Department of Environmental Sciences Phone: +1-732-932-9800 x6222
Rutgers University Fax: +1-732-932-8644
14 College Farm Road E-mail: rob...@envsci.rutgers.edu
New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock
On 3/19/2012 9:42 AM, Mike MacCracken wrote:
Hi Alan---Well, I got the 2 months number from your paper---and used
that. Interesting that a more detailed evaluation indicates that the
lifetime in summer is longer. I think longer times than a week might
well be possible in the troposphere by choosing injection times and
meteorological conditions, so I'll correct to ratio of 10 to 20 to 1
for stratosphere, but noting that there might not be a need for the
aerosols to be there for 4 months, so the longer stratospheric time
might be real, but not necessarily relevant.
On the issue of the amount of pollution, a couple of comments. Aside
from arguments over whether it is the sulfate or things with the
sulfate causing the health effects that have been associated with
sulfate from coal-fired power plants (for any sulfate injection it
would be pure SO2 or whatever without all the other combustion
products---or perhaps one might use sea salt or something else), due
to past coal use in Europe and Soviet Union, we have a reasonable
sense of what the impacts from sulfate might be. With summer only
injections, one would avoid much of the acid deposition problem
(shorter season, and not accumulating on snow and running off all at
once). One would also be choosing emissions times to have air flows
that carry the SO2/sulfate over the Arctic and not over the land. So,
yes, will be some impacts, but can possibly be moderated to be less
than, as your study suggested, the unintended side effects of
stratospheric SO2. I am all for considering and comparing the full
range of possible approaches (stratospheric, tropospheric, surface,
etc.--separately and/or in combination).
With some sense of what might be able to be done and the potential
impacts, the next step is a comparative risk evaluation, as for all
climate engineering. Without doing something, it is hard to see how
the Arctic can be kept from very extensive thawing and loss of the
climate that we have. With it, yes, some different types of impacts
due to the engineering effort, but, assuming it works, a good deal
less, or slowed climate impact on the Arctic, and if loss of
glacier/ice sheet mass can be slowed (or reversed---as Caldeira-Wood
study suggested), then a benefit to the global community.
With some sense of relative risks of various choices, it becomes a
political decision, with its many considerations. I happen to think
that, if any climate engineering is to be considered, having a focused
goal such as limiting polar warming and associated impacts would be
more likely to be considered as a first step than jumping straight to
a global counter-balancing approach, but that is just my opinion. In
any case, rather than saying what is or is not acceptable, it seems to
me our responsibility is to explore and evaluate options and then it
is the governance system that decides about the tradeoffs of pollution
versus un- (or under-) moderated Arctic change (and everything else).
Mike
On 3/19/12 12:03 PM, "Alan Robock" <rob...@envsci.rutgers.edu> wrote:
Dear Mike,
I don't know how you do this 6 to 1 calculation. We found that
the e-folding time for stratospheric aerosols in the Arctic s 2-4
months, with 4 months in the summer, the relevant time. (see
http://climate.envsci.rutgers.edu/pdf/2008JD010050small.pdf ) If
we compare this to the lifetime of tropospheric aerosols, on week,
and add a week to the 4 months for their tropospheric time, the
ratio is 130 days to 7 days, which is *19 to 1, not 6 to 1*.
Furthermore, the health effects of additional tropospheric
pollution are not acceptable, in my opinion.
Alan
[On sabbatical for current academic year. The best way to contact me
is by email, rob...@envsci.rutgers.edu, or at 732-881-1610 (cell).]
Alan Robock, Professor II (Distinguished Professor)
Editor, Reviews of Geophysics
Director, Meteorology Undergraduate Program
Associate Director, Center for Environmental Prediction
Department of Environmental Sciences Phone: +1-732-932-9800
x6222
Rutgers University Fax:
+1-732-932-8644
14 College Farm Road E-mail:
rob...@envsci.rutgers.edu
New Brunswick, NJ 08901-8551 USA
http://envsci.rutgers.edu/~robock
<http://envsci.rutgers.edu/%7Erobock>
On 3/18/2012 5:49 PM, Mike MacCracken wrote:
Hi Stephen--My wording must have been confusing.
For stratospheric injections at low latitudes, the lifetime is
1-2 years.
The aerosols do move poleward and are carried into the
troposphere in mid
and high latitudes. This is one approach to trying to limit
global climate
change, and, as David Keith says, studies indicate that these
cool the polar
regions, though perhaps not in the stratosphere.
Your cloud brightening approach is also to limit global
warming. I'd also
suggest that we could offset some of the global warming by
sulfate aerosols
out over vast ocean areas instead of sulfate's present
dominance over, now,
southeastern Asia, China, etc.--so keeping or modestly
enhancing the present
cooling offset. [And reducing cirrus may also be a viable
approach.]
A third approach is to cool the poles (and this might be good
for regional
purposes alone), but cooling also pulls heat out of lower
latitudes and
helps to cool them somewhat. The Caldeira-Wood shows it works
conceptually
(they reduced solar constant) and Robock et al. injected SO2 into
stratosphere to do (but the full year injection of SO2/SO4
likely spread
some to lower latitudes and the monsoons were affected). One
thing Robock et
al. found was that the lifetime of sulfate in the polar
stratosphere is
about two months, and so that means that the potential 100 to
1 advantage of
stratospheric sulfate is not valid, and we're down to 6 to 1
compared to
surface-based approaches such as CCN or microbubbles to cool
incoming
waters, sulfate or something similar over Arctic area, surface
brightening
by microbubbles, etc.--noting that such approaches are only
needed (and
effective) for the few months per year when the Sun is well
up in the sky.
As David Keith also says, there is a lot of research to be
done to determine
which approaches or alone or in different variants might work,
or be
effective or ineffective and have unintended consequences,
much less how
such an approach or set of approaches might be integrated with
mitigation,
adaptation, suffering, etc.
Best, Mike MacCracken
On 3/18/12 12:52 PM, "Stephen Salter" <s.sal...@ed.ac.uk>
<mailto:s.sal...@ed.ac.uk> wrote:
Mike
I had thought that the plan was stratospheric aerosol to
be released at
low latitudes and would slowly migrate to the poles where
is would
gracefully descend. If you can be sure that it will all
have gone in 10
days then my concerns vanish. But if the air cannot get
through the
water surface how can the aerosol it carries get there?
It will form a
blanket even if it is a very low one.
A short life would mean that we do not have to worry
about methane
release. But can we do enough to cool the rest of the
planet? Perhaps
Jon Egil can tell us about blanket lifetime.
Stephen
Mike MacCracken wrote:
The Robock et al simulations of an Arctic injection
found that the lifetime
of particles in the lower Arctic stratosphere was only
two months. In that
one would only need particles up during the sunlit
season (say three months,
for only really helps after the sea ice surface has
melted and the sun is
high in the sky). During the relatively calm weather
of Arctic summer, the
lifetime of tropospheric sulfate, for example
CCN--emitted above the inversion is likely 10 days or
so. It is not at all
clear to me that the 6 to 1 or so lifetime advantage
of the lower
stratosphere is really worth the effort to loft the
aerosols.
And on the temperature rise in the polar stratosphere,
I would hope any
calculation of the effects of the sulfate/dust
injection only put it in
during the sunlit season
radiation during the polar night, so, with a two month
lifetime of aerosols
there, it makes absolutely no sense to be lofting
anything for about two
thirds of the year. And so likely no effect on winter
temperatures (although
warming the coldest part of the polar winter
stratosphere might well help to
prevent an ozone hole from forming).
So, I think a tropospheric brightening approach is
likely the better option.
Whether it can be done with just CCN or might also
need sulfate seems to me
worth investigating (what one needs may well be not
just cloud brightening,
but also clear sky aerosol loading).
Best, Mike
*****
On 3/17/12 8:41 PM, "Ken Caldeira"
<kcalde...@carnegie.stanford.edu>
<mailto:kcalde...@carnegie.stanford.edu> wrote:
That is just misleading. The third attachment is
a top-of-atmosphere
radiation balance on the email I am responding to
shows shortwave radiation.
The attached figure shows the corresponding
temperature field from the same
simulation for the same time period. Note Arctic
cooling.
Also, we should not focus on individual regional
blobs of color in an
average
of a single decade from a single simulation.
The paper these figures came from is here:
http://www.atmos-chem-phys.net/10/5999/2010/acp-10-5999-2010.pdf
_______________
Ken Caldeira
Carnegie Institution Dept of Global Ecology
260 Panama Street, Stanford, CA 94305 USA
+1 650 704 7212 kcalde...@carnegie.stanford.edu
http://dge.stanford.edu/labs/caldeiralab @kencaldeira
YouTube:
<http://www.youtube.com/watch?v=a9LaYCbYCxo>
Climate change and the
transition from coal to low-carbon electricity
<http://www.youtube.com/watch?v=a9LaYCbYCxo>
Crop yields in a geoengineered climate
<http://www.youtube.com/watch?v=-0LCXNoIu-c>
On Sat, Mar 17, 2012 at 1:31 PM, Andrew Lockley
<and...@andrewlockley.com>
<mailto:and...@andrewlockley.com>
wrote:
Hi
Here are some model outputs which Stephen sent
me. These appear to show
localized arctic warming in geoengineering
simulations. This could be due
to
winter effects.
I assume this is the source for the
controversial figure in the BBC quote
A
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