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Patron: Her Majesty the Queen


The Royal Scottish Society of Arts
Showcasing Scotland's Science, Technology and Innovation

7th Meeting of the 187th Session (2007-2008)

"Reversing Global Warming by Increasing Cloud Albedo"

Professor Stephen Salter MBE FRSE FRSSA
School of Engineering and Electronics
The University of Edinburgh

In the Augustine United Church
41 George IV Bridge
Edinburgh, EH1 1EL
On Monday 21st April 2008, at 7pm

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 phytoplankton 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. An alternative solution is 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%. 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. We would need to build about 50 new spray vessels a year, each spraying 10 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. 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.

Stephen Salter is Emeritus Professor of Engineering Design at Edinburgh University and a Vice-President of the RSSA. He works on energy from waves and tidal streams, desalination, traffic congestion, computer-controlled hydraulic pumps and motors, flood prevention, the suppression of explosions, increasing the capacity of road bridges and the reversal of global warming by increasing cloud albedo.

Location of meeting

Society Business

The President, Professor Anthony Busuttil, will be in the Chair

Members of the Public are welcome to attend

Jane Ridder-Patrick, Secretary
Telephone: 0131 556 2161

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