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-funde d VOCALS consortium project. Planet Earth Wier 20099 that warms t he Earth. One othese was the cloud whitening scheme we discuss here. As these techniques could buy us time to implement methods to reduce CO 2 , it would be very wise to research their viability, in case we need them in an emergency. Te cloud whitening scheme has to operate continuouslyand produces a one-oeect. But its advantages Whitening t h e c l o ud s Cig e emiiacae cimae cage ivia. Bwaif we ca’iqicky egavimaiariacaarpe? AaGaiaaceageecribe aiea acep keep e Earcaby preciime. I n Te Revenge of Gaia (2009), James Lovelockargued that catastrophe will happen within the next 30 years. Se vere 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, peopl e have been geoengineering the planet – cutting down rainorests, burning ossil uels, and pumping CO 2 and other radiative gases into the atmosphere. Environmental temperature change is now accelerating, not only due to CO 2 , but also because othe release oot her gases such as methane. Some othis 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 metha ne is relatively short- lived in the atmosphere, it is between 20 and 70 times more poten t as a g reenhouse gas than CO 2 and could cause a runaway heating eect, only mitigated by the large a mount olatent heat needed to melt the ice c aps. Te philosophy of cloud whiteningSo-cal led 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 ou nintended conseq uences. Several possible schemes were ana lysed and discussed in the Royal Society report Geoengineering theclimate, published in September 2009. Te report recommended research into two plans aimed at managing t he 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 easilyand immediately shut down, w ith conditions returning to normal within a ew days. It would give us precise and rapid control, via satellite measurements oalbedo – how reective t he WhItEnInG thE Clouds
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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)