8/8/2019 Whitening the clouds

http://slidepdf.com/reader/full/whitening-the-clouds 1/3

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

http://slidepdf.com/reader/full/whitening-the-clouds 2/310 Planet Earth Wier 2009

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

http://slidepdf.com/reader/full/whitening-the-clouds 3/3Planet Earth Wier 2009  11

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

dr Aa Gaia i eir reearc ciei a e

naia Cere fr Amperic sciece (nCAs), a

e uiveriy f lee.

Prfer Aa By i irecr f e nCAs Faciiy

fr Gr-bae Amperic Meareme a e

uiveriy f lee.Prfer J laam i a amperic pyici a

e naia Ceer fr Amperic Reearc.

sepe saer i prfer f egieerig eig a

e uiveriy f Eibrg.

lara seve i a Pd e a e uiveriy f

lee.

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.