Memorandum 156
Submission from Professor John Latham[22]
1. SUMMARY
- There exists a clear consensus in the geo-engineering
community that although it is strongly hoped that it will never
be necessary to deploy any of the climate mitigation, temperature
stabilisation schemes on which we are working, it is irresponsible
not to examine and test the ones considered to be of significant
promise, to the point at which they could be rapidly made operational,
if viable.
- A crucial requirement of geo-engineering
research is that all significant ramifications associated with
the deployment of the techniques be fully examined, especially
ones that could have adverse consequences: such as rainfall reduction
in agricultural regions where water is already in short supply.
- The geo-engineering idea colleagues and
I are investigating is to increase, in a controlled way, the reflectivity
for incoming sunlight of low-level, shallow oceanic clouds, thus
producing a cooling sufficient to balance global warming.
- This technique, together with assessments
of it from modelling and observational work are summarised below.
The provisional conclusion-subject to satisfactory resolution
of specific problems-is that it could hold the Earth's temperature
constant as CO2 levels continue to rise, for at least several
decades.
- Preliminary indications emanating from
a state-of-the-art fully-coupled atmosphere/ocean global climate
model are that significant restoration of Arctic ice is achievable
via our cloud seeding scheme. This model is also being used to
assess the ramifications associated with the possible deployment
of this technique.
- Hadley Centre scientists have recently
assessed some ramifications of cloud albedo enhancement using
a somewhat simpler model, but since their levels and areal seeding
coverage are significantly different from those we have proposed
to utilise, their results cannot be regarded as applicable, with
accuracy, to our scheme.
- Modelling is of great importance in quantifying
and assessing geo-engineering schemes. However, it would be short-sighted
and counter-productive to exclude observational and field studies
from a programme of geo-engineering research.
2. OUTLINE OF
OUR GLOBAL
TEMPERATURE STABILISATION
SCHEME[23]
1. Atmospheric clouds exercise a significant
influence on climate. They can inhibit the passage through the
atmosphere of both incoming, short-wave, solar radiation, some
of which is reflected back into space from cloud-tops, and they
intercept long-wave radiation flowing outwards from the Earth's
surface: a global cooling, and warming respectively. On balance,
clouds produce a cooling effect, which we propose (Latham, 1990,
2000: Bower et al 2006, Latham et al 2008) to accentuate
by increasing the reflectivity of the shallow, low-level, marine
stratocumulus clouds that cover about a quarter of the oceanic
surface. These clouds characteristically reflect between 30% and
70% of the sunlight that falls upon them. They therefore produce
significant global cooling. A further 10% increase in reflectivity-which
we hope to achieve via cloud seeding-would produce an additional
cooling to roughly balance the warming resulting from atmospheric
CO2 doubling.
2. The reflectivity increase would be achieved
by seeding these clouds with seawater particles sprayed from unmanned,
wind-powered, satellite-guided Flettner-rotor vessels (Salter
et al 2008) sailing underneath the clouds. These particles
would be about one micrometer in diameter at creation and would
shrink as about half of them are carried by turbulence up into
the clouds, where they act as centres for new droplet formation,
thereby increasing the cloud droplet number concentration and
thus the cloud reflectivity (and possibly longevity). In this
way the clouds would reflect more sunlight back into space, possibly
for a longer time, and so planetary cooling occurs.
3. The physics behind this scheme is that
an increase in droplet number concentration (with concomitant
reduction in average droplet size) causes the cloud reflectivity
to increase because the overall droplet surface area is enhanced.
It can also increase cloud longevity (tantamount to increasing
cloudiness) because the coalescence of cloud droplets to form
drizzle-which often initiates cloud dissipation-is impeded, since
the droplets are smaller.
4. Simple calculations indicate that a doubling
of the natural droplet concentration in all suitable marine stratiform
clouds (which corresponds to a reflectivity increase of about
12%) would - roughly-produce a cooling sufficient to balance the
warming associated with CO2 doubling. If the seawater droplets
have a diameter of about 0.8 micrometres the global seawater volumetric
spray-rate required to produce the required doubling of the droplet
number concentration in all suitable clouds is about 30 cubic
metres per second, this modest figure resulting from the small
size of the seeding particles.
5. Major technological components of our
geo-engineering scheme are discussed in detail in Salter et
al, 2008.
6. Ship-tracks are bright streaks crossing
photographs of marine stratocumulus clouds observed from satellites,
resulting from the release into the clouds of droplet-forming
particles in the exhausts of ships sailing beneath them. They
can be adduced as evidence supporting our contention that the
seeding of clouds can enhance their reflectivity.
3. GLOBAL CLIMATE
MODELLING COMPUTATIONS
7. The calculations mentioned above were
simplistic, and although they were useful in providing first estimates
of the viability and requirements of our scheme, a definitive
quantitative assessment of it requires high-quality global climate
modelling. Two separate models were utilized for this work:
(i) the HadGAM numerical model, which is the
atmospheric component of the Hadley Centre Global Model, based
on the Meteorological Office Unified Model (UM), version 6.1;
and
(ii) a developmental version of the NCAR Community
Atmosphere Model (CAM).
8. Both models reveal that the imposed increase
in cloud droplet number concentration resulting from seeding causes
an overall significant global cooling. The largest effects are
apparent in the three regions of persistent marine stratocumulus
off the west coasts of Africa and North & South America, which
together cover about 3% of the global surface. Lower but appreciable
amounts of cooling were found throughout much more extensive regions
of the southern oceans. The five-year mean globally averaged cooling
resulting from marine low-level cloud seeding, with the cloud
droplet number concentration approximately quadrupled, produced
a cooling sufficient to balance the warming resulting from a quadrupling
of the atmospheric CO2 concentration in the case of the UK model,
and a doubling in the case of the NCAR model. If such levels of
cooling could be produced in practice by the proposed cloud seeding
technique, the Earth's temperature could be held constant for
many decades. It follows that the areal fraction of suitable cloud-cover
seeded, in order to maintain global temperature stabilization,
could, for much of this period, be appreciably lower than unity,
rendering less daunting the practical problem of achieving adequate
geographical dispersal of disseminated CCN. Thus there exists,
in principle, latitude to:
(a) avoid seeding in regions where deleterious
effects (such as rainfall reduction over adjacent land) are predicted;
and
(b) seed preferentially in unpolluted regions,
where the reflectivity-changes for a fixed increase in droplet
number concentration are a maximum.
9. The computations showed strong seasonal
variations in the global distribution of cooling, with a maximum
in the Southern Hemisphere summer. This finding underlines the
desirability of a high degree of mobility in the seawater aerosol
dissemination system.
4. DISCUSSION
10. Further work is required on technological
issues and the complexities of marine stratocumulus clouds. Most
importantly, perhaps, we need to make a detailed assessment of
ramifications associated with the possible deployment of our geo-engineering
scheme-for which there would be no justification unless these
effects were found to be acceptable.
11. Deployment of our scheme would result
in global changes in the distributions and magnitudes of ocean
currents, temperature, rainfall, etc. Even if it were possible
to seed clouds relatively evenly over the Earth's oceans, these
effects would not be eliminated. Also, the technique would still
alter the land-ocean temperature contrast, since the initial cooling
would be only over the oceans. In addition, we would be attempting
to neutralise the warming effect of vertically distributed greenhouse
gases with a surface-based cooling effect, which could have consequences
such as changes in static stability which would need careful evaluation.
Thus it is vital to engage in a prior assessment of the ramifications
mentioned above, which might involve currently unforeseen feedback
processes. This work requires requires a fully coupled ocean/atmosphere
climate system model. Such a model has been utilised over the
past few months, at NCAR. Results to date are only provisional,
but one feature that seems increasingly to be valid is that seeding
of marine stratocumulus clouds with seawater particles can cause
significant global cooling, which is a maximum in the Arctic regions,
causing significant restoration of ice cover.
12. It follows from the preceding discussion
that although two separate sets of global climate computations
agree in concluding that this cloud seeding scheme is in principle
powerful enough to be important in global temperature stabilization,
there are defined gaps in our knowledge which force us to conclude
that we cannot state categorically at this stage whether the scheme
is capable of producing significant global cooling. However, if
resolution of these extant issues makes little change to our modelling
results, we may conclude that our scheme could stabilize the Earth's
average temperature beyond the point at which the atmospheric
CO2 concentration reached 550 ppm (the CO2-doubling limit) but
probably not beyond the 1,000 ppm value. The amount of time for
which the Earth's average temperature could be stabilised depends,
of course, on the rate at which the CO2 concentration increases.
Simple calculations show that if it continued to increase at the
current level, and if the maximum amount of cooling that the scheme
could produce is as predicted by the models, the Earth's average
temperature could be held constant for between about 50 and 100
years. At the beginning of this period the required global seawater
spray rate-if all suitable clouds were seeded-would be about 0.15
m3 s-1 initially, increasing each year to a final value of approximately
25 m3 s-1.
13. Recent experimental work involving data
from the MODIS and CERES satellites led to a study by Quaas and
Feichter (2008) of the quantitative viability of our global temperature
stabilization technique. They concluded that enhancement (via
seeding) of the droplet number concentration in marine boundary-layer
clouds to a uniform value of 400 cm3 over the world oceans (from
60°S-60°N) would produce a global cooling close to that
required to balance the warming resulting from CO2-doubling. They
also found that the sensitivity of cloud droplet number concentration
to a change in aerosol concentration is virtually always positive,
with larger sensitivities over the oceans. These experimental
results are clearly supportive of our proposed geo-engineering
idea, as is the work of Oreopoulos and Platnick (2008), which
also involves MODIS satellite measurements.
14. Further encouraging support for the
quantitative validity of our scheme is provided by the field research
of Roberts et al (2008) in which the enhancement of reflectivity
was measured on a cloud-by-cloud basis, and linked to increasing
aerosol concentrations by using multiple, autonomous, unmanned
aerial vehicles to simultaneously observe the cloud microphysics,
vertical aerosol distribution and associated solar radiative fluxes.
In the presence of long-range transport of dust and anthropogenic
pollution the trade cumuli have higher droplet concentrations,
and are on average brighter, the observations indicating a high
sensitivity of cooling by trade cumuli to increases in cloud droplet
concentrations. The results obtained are in reasonable agreement
with our modelling.
15. Our view regarding priorities for work
in the near future is that we should focus attention on outstanding
unresolved issues (scientific and technological) outlined earlier.
The major focus should be on assessment of ramifications associated
with the proposed seeding scheme. At the same time we should develop
plans for executing a limited-area field experiment in which selected
clouds are inoculated with seawater aerosol, and airborne, ship-borne
and satellite measurements are made to establish, quantitatively,
the concomitant microphysical and radiative differences between
seeded and unseeded adjacent clouds: thus, hopefully, to determine
whether or not this temperature-stabilization scheme is viable.
Such further field observational assessment of our technique is
of major importance.
16. A positive feature of our proposed technique
is revealed by comparing the power required to produce and disseminate
the seawater particles with that associated with the additional
reflection of incoming sunlight. A simple calculation shows that
the ratio of reflected power to required dissemination power is
about 10 million. This extremely high "efficiency" is
largely a consequence of the fact that the energy required to
increase the seawater droplet surface area by four or five orders
of magnitude-from that existing on entry to the clouds to that
possessed by the cloud droplets when reflecting sunlight from
cloud-top-is provided by nature.
18. Further advantages of this scheme, if
satisfactorily deployed, are that:
(i) the amount of cooling could be controlled-by
measuring cloud reflectivity from satellites and turning disseminators
on or off (or up and down) remotely as required
(ii) if any unforeseen adverse effect occurred,
the entire system could be switched off instantaneously, with
cloud properties returning to normal within a few days;
(iii) it is relatively benign ecologically, the
only raw materials required being wind and seawater; and
(iv) there exists flexibility to choose where
local cooling occurs, since not all suitable clouds need be seeded.
This flexibility might help subdue or eliminate
adverse ramifications of the deployment of our scheme.
5. REFERENCES
CITED IN
TEXT
Bower, K N, Choularton, T W, Latham, J, Sahraei,
J and Salter, S H 2006 Computational assessment of a proposed
technique for global warming mitigation via albedo-enhancement
of marine stratocumulus clouds. Atmos Res 82, 328-336.
Latham, J 1990 Control of global warming?
Nature 347, 339-340.
Latham, J 2002 Amelioration of global warming
by controlled enhancement of the albedo and longevity of low-level
maritime clouds. Atmos Sci Lett. 3, 52-58. (doi:10.1006/Asle.2002.0048).
Latham, J, PJ Rasch, CC Chen, L Kettles, A Gadian,
A Gettelman, H Morrison, K Bower., 2008. Global Temperature
Stabilization via Controlled Albedo Enhancement of Low-level Maritime
Clouds. Phil Trans Roy Soc A, doi:10.1098/rsta.2008.0137.
Oreopoulos, L and Platnick, S 2008 Radiative susceptibility
of cloudy atmospheres to droplet number perturbations: 2.
Global analysis from MODIS J Geophys Res 113. (doi:10.1029/2007JD009655).
Quaas, J and Feichter, J 2008 Climate change mitigation
by seeding marine boundary layer clouds. Poster paper presented
at the session Consequences of Geo-engineering and Mitigation
as strategies for responding to anthropogenic greenhouse gas emissions
at the EGU General Assembly, Vienna, Austria, 13-18 April 2008.
Roberts, GC, Ramana, MV, Corrigan, C, Kim, D and
Ramanathan, V 2008 Simultaneous observations of aerosol-cloud-albedo
interactions with three stacked unmanned aerial vehicles.
Proc Natl Acad Sci USA 105, 7370-7375.
Salter, S, G Sortino and J Latham, (2008). Sea-going
hardware for the cloud albedo method of reversing global warming,
Phil Trans R Soc A, doi:10.1098/rsta.2008.0136.
6. RECOMMENDATIONS
- That in view of the potentially serious
ramifications of unbridled climate change, and the increasing
urgency of this problem, the UK government should provide adequate
funding for the pursuance of research into geo-engineering ideas
which hold significant promise of holding the Earth's average
temperature constant for some decades, in the face of increasing
atmospheric CO2 concentrations. This would provide time for the
development of clean energy sources to replace fossil fuels. It
is not suggested that such schemes be deployed, but that the research
be pursued to the point at which the technique be deemed to be
either unworkable or feasible, in the latter case with all scientific
and technological aspects resolved, and all possible ramifications
of the adoption of such a scheme identified and quantified. The
costs of such a proposition are trivial in comparison with those
of the likely damage accompanying unrestrained temperature increase-a
view unanimously expressed by the participants in the climate-change
workshop, involving economists, scientists and geo-engineers,
held at Harvard University in November, 2007.
- That a committee be appointed to oversee
the planning of a research programme in geo-engineering, and disbursement
of the governmental funding provided for it.
- That though DEFRA and the Hadley Centre
would be major contributors to the proposed committee and geo-engineering
research, there should be funded contributions also from UK universities
and other research institutions.
- That although climate modelling would play
a very important role in the programme of work, it should not
be the only component of the effort. Observational research is
of great importance, as is field research and technological development.
Without these latter components a reliable assessment of geo-engineering
ideas would not be achievable, in my view.
October 2008
22 John Latham is Emeritus Professor of Physics, University
of Manchester, and Senior Research Associate, National Centre
for Atmospheric Research (NCAR), Boulder, Colorado. Back
23
Please note that in the interests of communicating as clearly
as possible with readers who are not climate experts I have tried
to write the following notes without using specialised terminology.
This introduces a "looseness of expression" which I
believe does not destroy the sense of the information I am trying
to convey. Back
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