Post on 09-Apr-2018
8/8/2019 Whitening the clouds
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The light grey stratocumulus clouds are visible
off the coast of Chile, and were measured
using the NERC BAE 146 and
Dornier 128 aircraft on the
NERC-funded VOCALS
consortium project.
Planet Earth Wier 2009 9
that warms the Earth. One o these was the
cloud whitening scheme we discuss here.
As these techniques could buy us time to
implement methods to reduce CO2, it would
be very wise to research their viability, in case
we need them in an emergency. Te cloud
whitening scheme has to operate continuously
and produces a one-o eect. But its advantages
Whitening the cloudsCig e emii a cae cimae cage i via. Bwa if we ca’ i qicky eg avi maiaria
caarpe? Aa Gaia a ceage ecribe a iea
a c ep keep e Ear c a by preci ime.
In Te Revenge of Gaia (2009), James Lovelock
argued that catastrophe will happen within
the next 30 years. Severe storms and droughts
will become the norm, carbon osetting is
a joke, and current eorts to promote ethical
behaviour are a scam. Is he right? Here, we
discuss an alternative approach to dealing with
climate change – geoengineering the clouds so
they become whiter and reect more sunlight
back into space beore it reaches the Earth.
Geoengineering is man-made environmental
change. Since the industrial revolution, people
have been geoengineering the planet – cutting
down rainorests, burning ossil uels, and
pumping CO2 and other radiative gases intothe atmosphere. Environmental temperature
change is now accelerating, not only due to
CO2, but also because o the release o other
gases such as methane.
Some o this comes rom agriculture, but
the greater concern is that the Canadian and
Siberian permarost could thaw, allowing the
methane held in underground gas felds to
escape. Although methane is relatively short-
lived in the atmosphere, it is between 20 and
70 times more potent as a greenhouse gas than
CO2 and could cause a runaway heating eect,only mitigated by the large amount o latent
heat needed to melt the ice caps.
Te philosophy of cloud whitening So-called geoengineering schemes are designed
to reverse the harm we have already caused
and to provide a breathing space in which to
cut greenhouse gas emissions. But we need to
understand the science behind them, to avoid
the risk o unintended consequences. Several
possible schemes were analysed and discussed
in the Royal Society report Geoengineering the
climate , published in September 2009.
Te report recommended research into two
plans aimed at managing the solar radiation
lie in its low ecological impacts.
Its only ingredients are seawater and air.
Te energy to run it would come rom the
wind and be relatively cheap. It could be easily
and immediately shut down, with conditions
returning to normal within a ew days. It would
give us precise and rapid control, via satellite
measurements o albedo – how reective the
WhItEnInG thE Clouds
8/8/2019 Whitening the clouds
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clouds are – and cloudiness ed back through a
global model. It would be cheap to implement.
And i current small-scale experiments confrm
that the theory works, we could put it into
action quickly.
Stratocumulus cloudsOceans cover 70 per cent o the globe, and
low-level stratocumulus or ‘layer’ clouds cover30 per cent o the oceans. Tese clouds are
very important parts o the atmospheric and
ocean global heat engine system. In November
2008 a large international feld project, based
in Arica, Chile, with over 200 scientists, fve
aircrat and two ships, measured these clouds in
situ and with remote sensing. NERC unded a
consortium project, VOCALS, with scientists
rom our UK universities. Te image on the
previous page, taken by the Geostationary
Operational Environmental Satellite (GOES)
during the project, shows the extent o these
clouds.
Te water droplets in clouds reect sunlight
back into space. Te numbers o these droplets
As these techniquescould buy us time toimplement methodsto reduce CO2, it
would be very wiseto research their viability, in case weneed them in anemergency.
in clouds depend largely on the number o
Cloud Condensation Nuclei (CCN). Tese
are tiny particles o matter like dust or soot
that orm a seed around which water droplets
can orm. Many CCN are produced over the
land. Tis means land-locked clouds contain
many hundreds o cloud droplets per cubic
centimetre, while clouds that orm over the sea
contain substantially less: typically only a ew hundred per cubic centimetre. Generally, or a
given total amount o water in a cloud, the more
droplets that are present, the smaller these drops
are. And clouds with smaller droplets tend to be
whiter, and hence more reective.
Tese clouds are maintained by a complex
balance o actors. How ast the water droplets
collide and coalesce aects whether they
precipitate out to orm raindrops, or maintain a
stable system. Tere is still a lot we don’t know
about how these processes interact.
Te technology John Latham has suggested that by increasing
the number o droplets in maritime layer
Simulation of the effects of the cloud cooling scheme using the UK Met Office’s HadGAM climate model, using clouds with 375 droplets per cubic centimetre. The purple areas show the strongest cooling effect, with
green areas representing more limited cooling. The overall impact is global cooling of around 8 watts per square metre. Doubling CO 2 concentrations from present levels would cause warming of around 3.7 watts per
square metre.
(Reproduced with kind permission of The Royal Society, Latham J et al. 2008, Global temperature stabilization via controlled albedo enhancement of low level maritime clouds, Phil. Trans. R. Soc. A, 366, 3969-3987)
8/8/2019 Whitening the clouds
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WhItEnInG thE Clouds
Stephen Salter’s spray ship design.
J o h n M c N e i l l
clouds, known as stratocumulus, we could
signifcantly increase the amount o solar energy
these clouds reect.
Te idea is to inject a fne spray o sea-salt
rom the ocean surace into the clouds. Te
salt particles would act as CCN, artifcially
increasing the number o droplets in the cloud,and so reducing their size and making the cloud
more reective – that is, whiter. Tis would in
turn reect more sunlight beore it reaches the
Earth and so reduce its rate o warming, and
could buy us time – maybe as much as 50 years.
We need urther research, including
numerical modelling and feld experiments, to
determine the ideal size o the sea-salt CCN.
But preliminary results rom climate models
show that a modest increase o CCN in marine
stratocumulus clouds could produce the desired
cooling, and suggest this method would let us
compensate or anything up to a doubling o atmospheric CO2 rom pre-industrial levels.
Tese initial results rom models also suggest
that the biggest cooling rom this scheme –
as opposed to injection o sulphate into the
stratosphere, another proposal entirely – would
occur around the poles. Tis is consistent with
what the theory predicts, and is good news, as
the poles are precisely where cooling is most
needed to stop permarost rom melting. It
uses natural seawater spray and can be turned
o immediately, i it turns out to produce
undesirable consequences.
Scientists, including Stephen Salter o theUniversity o Edinburgh, have suggested a
design or a eet o about 2000 wind-powered,
unmanned yachts which incorporate a
sophisticated spray mechanism. Te design
would release sea-spray with a diameter o
around 0.8 microns, providing CCN or the
clouds.
We propose to perorm detailed research
into the scheme, and to fnd out whether it is
viable within fve years. Tis research has our
elements. More work is needed on modelling
the physics o clouds; there are sti ll questions
about how big the sea-salt CCN should be and
how the clouds will respond as CCN numbers
increase. We are already collaborating with
top US cloud physicists on this. We also need
urther research on climate modelling, and
we need to develop and build Stephen Salter’s
test yachts. Finally, we need small-scale feld
experiments in a region o stratocumulus to test
whether the idea works in practice.
Developing a test spray system and
conducting a feld experiment to assess the
scheme’s viability will cost around £6 mil lion.
Tis is an insignifcant sum compared with thecost o doing nothing. In fve to ten years, we
could have an answer to Lovelock’s question:
‘Could we have done anything to slow down
the warming and the irreversible change in the
Earth system?’
More inforMation
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further reading
J. laam, 1990, Cr f gba warmig?
Nature 347
J. laam et al . 2008, Gba emperare
abiizai via cre abe eaceme f
w eve mariime c, Phil. Trans. R. Soc. A , 366,
3969-3987
J. lveck, 2009, Revenge of Gaia , Pegi
s. saer, et al . 2008, sea-gig arware fr e
c abe me f reverig gba warmig,
Phil. Trans. R. Soc. A (2008) 366
The idea is to inject a fine spray of sea-saltfrom the ocean surface into the clouds.