Sulphate sunshield dangers

Proposed planetary engineering projects to cool the planet could backfire quite spectacularly. New research shows that a "sulphate sunshade" would punch huge holes through the ozone layer above the Arctic. To make matters worse, it would also delay the full recovery of the Antarctic ozone hole by up to 70 years.

The model is based on the work of Simone Tilmes of the National Center for Atmospheric Research in Colorado, US. Tilmes used computer models to see how a sulphate sunshade (potential + harmful) would affect the ozone layer (useful), which protects us from harmful UV rays. She says it could have "a drastic impact".

Sulphate particles catalyse the breakdown of ozone molecules by chlorine atoms. Western economies have almost entirely stopped using chlorine-based coolants called CFCs, thanks to the Montreal Protocol. However, such substances are increasingly being used in Asia and the atmosphere is still full of CFCs emitted during the 20th century.

To add to our understanding of the situation illustrated in the diagram, Ken Caldeira of the Carnegie Institution of Washington, found that if a sulphate sunshield were deployed and then removed - for instance because of a change in governments - the effects of global warming after the removal would be far worse than before the sunshield.

The model illustrates the salient features of the integrated situation. For example:

• The impact of drought and the impact on global warming of attempts to remove the sunshield are marked with strong effects.
• A special symbol is used to signify that the ozone layer ‘consumes’ UV rays.
  
• The breakdown of ozone molecules has an ‘overloaded’ destructive effect on the ozone layer (thinning).

This process of elaboration aids understanding and provokes creative thinking. For any given model, the reader can then ask questions to improve understanding, or improve the situation directly. For example:

• Is there another way to create a sunshield? (Avoiding harmful sulphates)
• Exactly how does catalysis breakdown the ozone molecules?
• Why is sulphate needed? Can we take sulphate out of the system? Etc.