Preventing Global Warming by Increasing Cloud Albedo.
John Latham, NCAR, Boulder, Colorado 303-497-8182 latham@ucar.edu
Stephen Salter, Edinburgh University, Scotland +44 131 650 5704 S.Salter@ed.ac.uk
www.see.ed.ac.uk/~shs
Key points

Many informed people now accept that increases of carbon dioxide really are damaging the
climate and that damage may suddenly become faster and irreversible because of burning
rain forests, melting ice caps, fewer algae or the release of methane from permafrost.
Despite political wishful thinking, the rate of carbon dioxide release is accelerating and
further increases at higher rates seem very likely. Alternative solutions are needed.

We could tolerate twice present carbon dioxide levels with no temperature change if we
could increase the amount of solar energy reflected back out to space by about 1.5%.

Latham has pointed out that low level clouds are quite good reflectors and their reflectivity
can be increased by exploiting the well-known Twomey effect. This says that clouds with
large numbers of small drops have a higher albedo than ones with a smaller number of large
drops. The clouds can also have greater longevity.

Even if the relative humidity is above 100%, cloud drops cannot form without a small
starting seed known as a cloud condensation nucleus. In clean marine air masses there are
fewer of these nuclei, somewhere between 10 and 100 per cubic centimetre, compared with
1000 to 5000 over land. The albedo of marine stratocumulus clouds would be increased by
greater numbers of nuclei. Doubling the number would increase the amount of solar energy
reflected from cloud tops by over 10%.

A very small drop of sea water is an ideal cloud condensation nucleus, and will therefore
readily grow to become a cloud droplet if it rises into a marine stratocumulus cloud. The
new cloud droplets have an atmospheric lifetime of about one day and so must be
continuously supplied.

We calculate that distributing 1018 drops a second over oceans with the right cloud
conditions would neutralize the thermal effects of a one-year increase of carbon dioxide. In
one micron drops this would be about half a cubic metre of seawater per second.

Spray can be produced from remotely-controlled sailing vessels drawing energy from the
wind. Design work is in progress at Edinburgh University. We would need to build about
50 new spray vessels a year, each spraying 30 kg a second and costing a few million dollars
apiece to keep world temperatures steady despite present levels of carbon dioxide release.
The ratio of solar energy reflected to wind energy needed for spray production is many
orders of magnitude.

All the remaining technical uncertainties would be removed by spending about $5 million.
The full development programme would cost about $50 million.

The only inputs are sea water and wind so the technique is ecologically benign. Introduction
is progressive and locally variable. If necessary, switch-off is almost immediate. Fast,
tactical control may allow us to increase rain in dry places and reduce it in wet ones.
An artist’s impression of the spray vessel. Flettner rotors with Thom fences spin about a vertical
axis and act as very high performance sails. They drive the vessel against the drag of underwater
turbines which generate the energy for spraying. ©John MacNiell.
The block diagram of the spray system. More from www.see.ed.ac.uk/~shs/Climate change.