To:-
[kcalde...@carnegiescience.edu];[s.sal...@ed.ac.uk];[geoengineering@googlegroups.com];[sgub...@annualreviews.org];[mfitzsimm...@annualreviews.org]
From:-
[lat...@ucar.edu]
Hello Ken et al.,
Following on from yr exchange (Ken) with Stephen Salter re your recent, very
interesting paper
The Science of Geoengineering
Annual Review of Earth and Planetary Sciences
Vol. 41: 231-256 (Volume publication date May 2013)
DOI: 10.1146/annurev-earth-042711-105548
Ken Caldeira,1 Govindasamy Bala,2 and Long Cao3
I write to make one correction and one or two comments regarding the possible
value of patchiness and s points regarding Marine Cloud Brightening,(MCB),
which some readers may not be familiar with.
1. There seems little doubt that, if any SRM geoengineering technique were
deployed,
it would produce some changes in rainfall patterns and amounts. A crucial
question
surrounding all SRM techniques is whether such deployment would produce
a reduction in rainfall, in any cultivated regions, which would result in a
significant reduction in agricultural yield. If so, this SRM technique should
be abandoned, unless some safe way is found of modifying the technique or
operational procedures to redress the situation in this same region.
There have been several published studies which address the effect of Marine
Cloud
Brightening (MCB) on global rainfall. In an influential paper by Jones et
al.(2009),in which
three large patches of marine stratocumulus were seeded, they found a
significant reduction in precipitation for the whole-averaged Amazon basin. This
finding has been confirmed in our own, more recent studies. Rasch (2009) et al.
on the other
hand, who seeded over significantly larger cloudy areas, ranging from 20 to 70
per
cent of the total area covered by suitable clouds, found no reduction in
rainfall
in this region. Bala et al., (2010), who seeded all suitable clouds, found a
smaller but discernible rainfall reduction over a small fraction of this
Amazonian
region. When Jones et al.(2011) repeated their earlier studies, except that
they did
not seed the Southern Atlantic patch of stratocumulus cloud, they found that
there was no reduction in rainfall in the Amazonian region. A likely explanation
of the disparities between these separate studies is that the rainfall changes
are
sensitive to the location and amount of seeding (Latham et al., 2012). If so,
it might
be relevant to note thatfor several decades not all regions would need to be
seeded t
o produce the required amount of seeding., so we might have a range of
flexibility
of seeding location that could be helpful. Much more work is required on this
topic.
2.If MCB proves to be viable, and deployment of an SRM scheme necessary,
optimal beneficial cooling might be produced if it was used in concert with
another
possibly viable technique (e.g. stratospheric sulphur seeding) In the former
case,
for example, the primary cooling could be supplied by the stratospheric scheme,
with beneficial adjustments being made by MCB, which can function in a more
localized manner. It may even prove possible and useful to create localized
warming via seeding, to optimize this fine tuning. Although computations
indicate
(subject to various caveats) that either technique might provide independently
adequate temperature stabilization for a few decades, the two acting in concert
could provide additional flexibility and safety.
3.Other issues that might be addressed by exploiting the initially localized
cooling of oceanic surface waters that we hope would be produced by MCB
(and/or the microbubble technique) are coral reef protection and hurricane
weakening. In the latter case, it may prove possible to cool oceanic waters in
the regions where hurricanes spawn. This would probably require continuous
seeding over several months, culminating in the hurricane season. Also, it may
prove possible to produce sufficient polar cooling to maintain existing sea-ice
cover by seeding specially selected cloudy regions of much smaller total area
than
considered in earlier studies. Details of our recently published paper on
hurricane
weakening are given below. A paper on the preservation of coral reefs via MCB
is to be published within the next few weeks.
4.Salter et al. (2008) have focused attention on wind-powered, unmanned,
satellite-guided
Flettner ships, and it was estimated that about 1500 of these spray vessels,
each consuming
about150kW (derived from the wind), would be required to produce the globally
averaged negative forcing of -3.7Wm-2 required to balance carbon dioxide
doubling. Flettner ships have the advantages of low cost, high manoeuvrability
and low carbon footprint. A conventionally, powered ship might consume about
1MW. For both types of vessel, the ratio of the rate of planetary radiative
loss to required operational power is very large (in the range from 10**5 to
10**7)
It follows that considerations of energy efficiency, desirable though that is,
need
not dictate the selection of type of spray vessel. Latham et al. [2008] pointed
out that
the main reason that this ratio is so high for MCB is that Nature provides the
energy required for the increase of surface area of newly activated cloud
droplets,
by four or five orders of magnitude as they ascend to cloud top and reflect
sunlight.
5.Other issues that might be addressed by exploiting the initially localized
cooling of oceanic surface waters that we believe could be produced by MCB
are coral reef protection and hurricane weakening. In the latter case, it may
prove
possible to cool oceanic waters in the regions where hurricanes spawn. This
would probably require continuous seeding over several months, culminating
in the hurricane season. Also, it may prove possible to produce sufficient
polar
cooling to maintain existing sea-ice cover by seeding specially selected cloudy
regions of much smaller total area than considered in earlier papers.
Details of a recently published paper on hurricane weakening via MCB are
Presented below. A paper on the utilization of MCB for the preservation of
coral
reefs will be published in 2 or 3 weeks time.
Cheers, John.
TWO RECENT PAPERS.
John Latham, Keith Bower, Tom Choularton, Hugh Coe, Paul Connolly, Gary
Cooper,Tim Craft, Jack Foster, Alan Gadian, Lee Galbraith, Hector Iacovides,
David Johnston, Brian Launder, Brian Leslie, John Meyer, Armand Neukermans,
Bob Ormond, Ben Parkes, Philip Rasch, John Rush, Stephen Salter, Tom Stevenson,
Hailong Wang, Qin Wang & Rob Wood, 2012, Marine Cloud Brightening,
Phil.Trans.Roy. Soc. A . 2012, 370, 4217-4262. doi: 10.1098/rsta.2012.0086
John Latham, Ben Parkes, Alan Gadian,Stephen Salter, 2012. Weakening of
Hurricanes via Marine Cloud Brightening (MCB), Atmospheric Science Letters,
DOI: 10.1002/asl.402
John Latham
Address: P.O. Box 3000,MMM,NCAR,Boulder,CO 80307-3000
Email: lat...@ucar.edu or john.latha...@manchester.ac.uk
Tel: (US-Work) 303-497-8182 or (US-Home) 303-444-2429
or (US-Cell) 303-882-0724 or (UK) 01928-730-002
http://www.mmm.ucar.edu/people/latham
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