Thanks, Jeff.
I'm copying your answer to the geoengineering group, because it has
enormous implications for the effectiveness of SRM, and on the
positive feedback effects which need to be overcome. SRM may have a
better chance than I thought, if the albedo flip effect is less than
I'd thought. However there may be other feedback effects that now take
more prominence. The water-air interface could be key, if the summer
sea ice retreat leads to more cloud cover in winter. I've not heard of
that positive feedback being discussed - do you know of any papers?
Somebody in the group has suggested the winter clouds are seeded to
clear the skies - would that be practicable?
Cheers,
John
---
Ridley, Jeff wrote:
Hi John,
My albedo flip of 0.23 was based
on the mean summer albedo of sea ice. It is not covered with snow for 2
of the 3 months of summer, even in preindustrial times (from the
climate models - this is because colder winters actually mean less
water transport and less snow. Even through summer temperatures are
colder, less snow needs to be melted before the ice is bare).Clearly
albedo only matters in the summer. We have just done some experiments
with black carbon on snow and found that this increases global
temperatures by about 0.5C through earlier melt of snow cover (similar
experiments have been done by NCAR in the US)..
Cheers
Jeff
Thanks, Jeff. That's most helpful.
I expect there will be some follow up questions, and I'd be grateful if
you could respond to them.
BTW, in one of my emails I gave estimates of the contribution to global
warming from albedo flip, from pre-industrial to complete sea ice
disappearance, as between 0.2 and 0.8 Watts per square metre. Your
estimate had come out 0.25, and mine 0.75. The main difference arose
because of uncertainty in the degree of albedo change: you had taken
0.23 and I'd taken 0.70, if I remember correctly. The context of the
calculation was over requirements for solar radiation management (SRM)
to halt sea ice retreat. Added uncertainty in the calculation arose
from the clouds, but if they reduce the albedo flip (so requiring less
SRM to counter), they also reduce the effectiveness of SRM
correspondingly . I wonder whether black carbon was crucial for our
calculations - because if there is a significant amount, it reduces the
albedo flip. Is this why you took 0.23 for the albedo change? I was
assuming pristine snow pre-industrial!
Cheers,
John
---
Ridley, Jeff wrote:
Hi John,
Global cloud cover is expected
to decrease with increase in CO2. The decrease is in the tropics. This
occurs because increasing CO2 leads to a warming of the middle
troposphere, where the greenhouse gases absorb the outgoing radiation,
and this stabilises the atmosphere against convection. Thus, although
the surface temperatures are warming, a tendency to increase
convection, the tropospheric warming is countering it and less clouds
form. Low cloud such as that in the polar regions is expected to
increase (as confirmed by observations in the Arctic).
As you point out, increased
cloud in the Arctic reduces summer melt but leads to a warming in all
other seasons. In models it is this winter warming that is responsible
for the long term decline in sea ice volume. With more downward long
wave the ice does not thicken so much in winter, preconditioning it for
an earlier melt in summer.
Regards
Jeff
Hi John,
I saw some research somewhere that said that global cloud cover had
decreased over the past decades, contributing slightly to global
warming. But I can't find it now. Do you know about it? Could it be
due to black carbon? Here's an extract from a Hansen essay:
'Black carbon aerosols cause a positive climate forcing that is very
uncertain in magnitude, but we estimate that it is probably of the
order of 0.5 to 1 Watt per square meter on global average. It causes
warming in several ways: by absorbing sunlight, by warming the lower
layer of the atmosphere and thus reducing cloud cover, by making clouds
slightly "dirty", and by darkening snow and sea ice surfaces when it is
deposited there.'
(http://www.giss.nasa.gov/research/features/200111_altscenario/discussion.html)
Perhaps more importantly, do we know what is happening in the Arctic?
I believe it was expected that cloud cover would increase significantly
over coming decades - which would reduce the effectiveness of any
stratospheric aerosol geoengineering. (I'm copying this to Jeff Ridley
who might know the answer.)
Whilst cloud cover has a cooling effect in Arctic summer, I assume it
would have a heating effect in winter. How do these effects balance
out? Has anybody done the calculations?
Cheers,
John
---
John Latham wrote:
Hello Mike et al,
I agree entirely with you Mike that seeding can cause ice fogs to
disperse. Also electrostatic droplet seeding could help dissipate warm
fogs. I should have more carefully indicated that I was focusing on
seeding of clouds containing ice, and particularly mixed-phase clouds.
Cheers, John.
Quoting Mike MacCracken <[email protected]>:
> With one exception, I agree with John. The exception is that I
think it has
> been demonstrated that one can clear an ice fog with seeding, and
this has
> been done to open airports, etc.--not to generate precipitation
(in any
> form).
>
> I would add that the water vapor content of air and clouds above
Greenland
> is likely so low that one could not be likely to get a significant
buildup
> of snow. Whether one could seed storms or change sea surface
temperatures in
> surrounding areas in a way that would lead to greater likelihood
of storms
> depositing snow on Greenland (or Antarctica, etc.) has not, to my
knowledge,
> been looked at at all.
>
> Mike
>
>
> On 12/16/09 4:00 AM, "John Latham" <[email protected]>
wrote:
>
>> Hello Neil, Andrew et al.
>>
>> Cloud seeding (principally to make rain) has had a long and
highly chequered
>> history in the 60 years since Vonnegut, Schaefer &
Langmuir did their
>> pioneering work. Many studies since then were contaminated by
commercial
>> interests. There is no essentially no consensus as to whether
and under what
>> circumstances it will work. This is especially true for clouds
that contain
>> ice. Whether seeding will enhance, reduce or have negligible
effect on such
>> clouds depends on: atmospheric stability, cloud-base
temperature, updraught
>> speed, presence or absence of conditions in which natural
secondary can
>> function, level and location etc etc. I think there is good
reason to feel
>> sceptical of studies reporting quantitative estimates of
changes induced by
>> seeding. My view is that - as with several geoengineering
schemes - what is
>> urgently required is well-controlled , comprehensive field
experiments. Only
>> then will it be possible to establish whether cloud seeding
might, on a
>> regional scale, be important vis-a-vis climate change.
>>
>> Cheers, John.
>>
>>
>> ********************************
>>
>> Quoting Neil Farbstein <[email protected]>:
>>
>>> > I'm glad we agree. Small cloud seeding experiments
over Greenland
>>> > should be practical. We should give this some
thought and modeling.
>>> > Can you do that at your lab? Google satellite
pictures and weather
>>> > satellites can locate clouds that are likely targets
to seed; The
>>> > bigger clouds in the places that are most strategic.
>>> > An international organization or groups of
industrialized nations can
>>> > pay the local residents or the greenland government
subsidies for
>>> > participating in the cloud seeding program.
>>> >
>>> > On Dec 15, 8:27 pm, Andrew Lockley <[email protected]> wrote:
>>>> >> For clarity, the main reason my for
suggesting cloud seeding is that it
>>>> can
>>>> >> be used to build up greenland to a height
where the air temperature is
>>>> low
>>>> >> enough to sustain the ice sheet. The could
potentially reverse the
>>>> >> catastrophic Greenland tipping point - which
will unleash
>>>> several metres
> of
>>>> >> sea level rise over a few hundred years.
Game over for London, Venice,
>>>> >> Florida, New York, etc. if that happens.
Worth a bit of jet fuel or a
>>>> few
>>>> >> rockets to prevent that, I'd argue.
>>>> >>
>>>> >> A
>>>> >>
>>>> >> 2009/12/15 Neil Farbstein <[email protected]>
>>>> >>
>>>> >>
>>>> >>
>>>>> >> > I suggested the same cloud seeding
strategy a month ago. Somebody
>>>>> >> > authoritative said that the biggest
snowfall recorded over the
>>>>> >> > alaskan arctic caused the biggest
melt water recorded during the
>>>>> >> > spring. That's anecdotal evidence
but I dropped the idea of
>>>>> working on
>>>>> >> > that anyway.
>>>> >>
>>>>> >> > There wont be a melt water problem
if clouds are seeded over large
>>>>> >> > parts of the gulfstream to cool it
and the winds that blow off it.
>>>> >>
>>>>> >> > On Dec 15, 5:47 am, Andrew Lockley <[email protected]> wrote:
>>>>>> >> > > I note the use of cloud
seeding by the Chinese, and its
>>>> >> unexpected effect
>>>>> >> > in
>>>>>> >> > > causing huge snowfalls in
Beijing. It seems that there may be
>>>> >> two useful
>>>>>> >> > > geoengineering approaches
with this technique, and I'd be
>>>>>> interested in
>>>>>> >> > > hearing comments.
>>>> >>
>>>>>> >> > > 1) Rebuild ice - by
inducing snowfalls over Greenland,
>>>> >> Antarctica and the
>>>>>> >> > > Arctic, it would perhaps be
possible to maintain ice. In
>>>>>> Greenland,
>>>>> >> > where
>>>>>> >> > > the height of the ice cap
is critical, this would seem a
>>>>>> particularly
>>>>>> >> > > appealing prospect.
>>>>>> >> > > 2) Albedo modification -
Fresh snow is whiter than old snow,
>>>>>> especially
>>>>> >> > in
>>>>>> >> > > polluted areas. Is the
albedo change worth pursuing? My
>>>>>> guess is
> not.
>>>>>> >> > > HOWEVER, I suspect that
triggering significant autumn and spring
>>>>> >> > snowfalls
>>>>>> >> > > in permafrost regions, we
could potentially significantly modify
>>>>>> albedo.
>>>> >>
>>>>>> >> > > I invite comments.
>>>> >>
>>>>>> >> > > A
>>>> >>
>>>>> >> > --
>>>> >>
>>>>> >> > You received this message because
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>>>>> >> > "geoengineering" group.
>>>>> >> > To post to this group, send email to
>>>>> [email protected].
>>>>> >> > To unsubscribe from this group, send
email to
>>>>> >> >
>>>> >>
>>>> [email protected]<geoengineering%2bunsubscr...@go
>>>> oglegroups.com>
>>>>> >> > .
>>>>> >> > For more options, visit this group at
>>>>> >> >http://groups.google.com/group/geoengineering?hl=en.-
Hide
>>>>> quoted text
> -
>>>> >>
>>>> >> - Show quoted text -
>>> >
>>> > --
>>> >
>>> > You received this message because you are subscribed
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>>> > Groups "geoengineering" group.
>>> > To post to this group, send email to [email protected].
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>>> >
>>> >
>>> >
>>
>>
>>
>>
>
>
--
John Latham
[email protected] & [email protected]
Tel. 303-444-2429 (H) & 303-497-8182 (W)
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