Solar geoengineering – overstated claims

Sir, – Your article “Solar geoengineering to cool the planet: Not if, but when” (Science, May 5th) presents a flawed perspective on the technology of geoengineering.

Assertions such as “the technology is essentially ready to go” and “We know that it will work” give the impression a technology solution is ready to be switched on as soon as certain economic and governance problems can be worked out. This is hard to tally with “solar geoengineering is not happening yet”. Unfortunately, a scientific understanding of solar geoengineering (via computer modeling of the global climate system) is conflicted with the physical existence of a technology that can perform solar geoengineering.

As engineers know, while successful development of technology often starts with scientific understanding, there are many stages of scaling up prototypes before a technology can be said to be “ready to go”. The Technology Readiness Levels (TRL) standard – originally developed by NASA for space technologies in the 1970s – has TRL levels from one to nine. For the European Commission, a TRL of less than six is ​​considered immature. A US government report from a number of years ago rated solar geoengineering as TRL Two.

A major issue for solar geoengineering is that full scale means planetary scale. Other researchers have described this as “collective experimentation” for which the results in advance are “unknown and unknowable”.

At present, solar geoengineering is better described as an idea rather than a technology. Overstated claims about its maturity serve to influence research and policy priorities away from mature technologies and practices that actually reduce greenhouse emissions and (to use a word completely absent from your article) are much more sustainable. – Yours, etc.


Senior Research Fellow,

Environmental Research


University College Cork.

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