Hi All
In the recent Phil. Trans. Roy. Soc. special issue on Solar
Geoengineering to meet Paris the Paris target at
http://rsta.royalsocietypublishing.org/content/376/2119/20160454table 1
lists, as a disadvantage relative to stratospheric sulphur, that marine
stratospheric clouds cover 10% of the Earth which means that marine
cloud brightening is ‘patchy’.No reference to this number is given and
it contradicts the 18% ‘low not high clouds’ mentioned by Charlson and
Lovelock in their 1987 paper about the CLAW hypothesis and the effects
of dimethyl-sulphide from phytoplankton.There is also the Jones Hayward
Boucher paper of 2009 which concluded that marine cloud brightening over
the best 3% of the oceans would offset about half the thermal damage
since preindustrial times.
However as clouds move the patchiness is smoothed out. Furthermore the
life of condensation nuclei will be approximately half the mean time
between rain showers so clear skies mean longer nuclei lifetimes.The
Twomey effect is logarithmic so it is better to have half the dose over
double the area and spraying under clear skies gives nuclei a chance to
spread. I argue that some short-term patchiness is much less serious if
the patches are */our/* patches. Finally the recent paper by Ahlm et
al.at doi:10.5194/aco-2107-484 suggests that marine cloud brightening
works much better than I would have expected with no clouds.
The Royal Society table did not have any space for the disadvantages of
stratospheric sulphur relative to marine cloud brightening and I am
reluctant to knock any technology about which I am not an expert.However
if I was forced to suggest entries for the contents of a disadvantage
table they would be as follows:
Nasty acid everywhere compared with medicinally beneficial salt mainly
over sea.
Very little control over the areas affected.
Very long shut-down times in the event of a volcanic eruption leading to
over-cooling.
Reflection of outgoing infra-red during polar winters leading to warming.
Use of fossil fuel from aircraft release rather than the use of energy
from the wind.
Even though recent climate modelling has not yet used monodisperse spray
with sizes of both liquid drops and dry salt residues both in the
Greenfield gap and has not varied spray patterns with seasons or surface
temperature anomalies, the majority of ensemble results show that as
well as cooling there is a trend for dry places to become wetter and wet
ones to become drier.We can hope that we can learn how to improve on
this trend.
I suggest that the choice of time and place to do marine cloud
brightening will confer advantages similar to the use of an accelerator,
steering and brakes on road vehicles.
On 02-Apr-18 9:16 PM, Andrew Lockley wrote:
https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target
Solar geoengineering as part of an overall strategy for meeting the
1.5°C Paris target
Citation:
Douglas G. MacMartin, Katharine L. Ricke, and David W. Keith.
4/2/2018. “Solar geoengineering as part of an overall strategy for
meeting the 1.5°C Paris target
<https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target>.”
Philosophical Transactions of the Royal Society, 376, 2119.
Download Citation
<https://keith.seas.harvard.edu/publications/solar-geoengineering-part-overall-strategy-meeting-15%C2%B0c-paris-target#>
Download
macmartin_ricke_keith_ptrs.pdf
<https://keith.seas.harvard.edu/files/tkg/files/macmartin_ricke_keith_ptrs.pdf>
1.03 MB
Abstract:
Solar geoengineering refers to deliberately reducing net radiative
forcing by reflecting some sunlight back to space, in order to reduce
anthropogenic climate changes; a possible such approach would be
adding aerosols to the stratosphere. If future mitigation proves
insufficient to limit the rise in global mean temperature to less than
1.5°C above preindustrial, it is plausible that some additional and
limited deployment of solar geoengineering could reduce climate
damages. That is, these approaches could eventually be considered as
part of an overall strategy to manage the risks of climate change,
combining emissions reduction, net-negative emissions technologies and
solar geoengineering to meet climate goals. We first provide a
physical science review of current research, research trends and some
of the key gaps in knowledge that would need to be addressed to
support informed decisions. Next, since few climate model simulations
have considered these limited-deployment scenarios, we synthesize
prior results to assess the projected response if solar geoengineering
were used to limit global mean temperature to 1.5°C above
preindustrial in an overshoot scenario that would otherwise peak near
3°C. While there are some important differences, the resulting climate
is closer in many respects to a climate where the 1.5°C target is
achieved through mitigation alone than either is to the 3◦C climate
with no geoengineering. This holds for both regional temperature and
precipitation changes; indeed, there are no regions where a majority
of models project that this moderate level of geoengineering would
produce a statistically significant shift in precipitation further
away from preindustrial levels. This article is part of the theme
issue ‘The Paris Agreement: understanding the physical and social
challenges for a warming world of 1.5°C above pre-industrial levels’.
See also: Solar Geoengineering
<https://keith.seas.harvard.edu/researchareas/solar-geoengineering>,
David Keith
<https://keith.seas.harvard.edu/people/david-keith-0>,Academic
Publication <https://keith.seas.harvard.edu/publications-type/academic>
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