By Richard Proudlove
An Australian Government advisory panel have reported that almost 50% of the reef’s coral died due to mass bleaching in 2016 and 2017. They have also advised that the reef will decline further unless global warming is maintained below 1.2 o C above pre-industrial levels.
Recent analysis suggests a 1% chance of limiting the global temperature rise to 1.5 o C. Thus, it appears that The Great Barrier Reef is destined to be a big, fat, tragic canary in the global warming coalmine (fossil fuel allusion intended).
In the absence of a sudden and rapid reduction in global greenhouse gas emissions, recovery options are few and tenuous. In fact, that’s a generous assessment. We have basically reached the point where anything that provides a glimmer of hope is “worth a crack”.
One such proposal is to cool the ocean around the reef by “brightening” clouds overhead. Modelling and measurements indicate that increasing cloud cover and density would reduce water temperatures around the reef, although whether the effect is sufficient to arrest coral bleaching is uncertain.
How can we “engineer” the climate?
Cloud “brightening” is a form of Solar Radiation Management (SRM). SRM aims to reduce surface solar irradiance by deflecting or reflecting sunlight. Techniques include covering large areas of land or ocean with reflective materials, positioning reflectors in space and injecting particles into the stratosphere.
The last of these, known as Stratospheric Aerosol Injection, offers a relatively cheap way to rapidly reduce global temperatures. Its effectiveness has been proved by volcanic eruptions. For example, sulphate aerosols thrown into the stratosphere by the 1991 eruption of Mount Pinatubo reduced average global temperatures by an estimated 0.5oC.
In fact, atmospheric aerosols from pollution are already helping to slow temperature rise due to global warming. Could this be the answer to our climate change woes? Well, there is (of course) a catch.
So what’s the catch?
Modelling indicates that spraying sulphates into the stratosphere could significantly reduce rainfall in Asia, Africa and South America and deplete the Ozone Layer. Since sulphate aerosols remain in the atmosphere for up to two years, these impacts could not be quickly reversed.
There is also concern that using SRM to slow global warming could detract from efforts to reduce greenhouse gas emissions and lock future generations into continuing with this approach or face rapid catastrophic temperature rises if they stop.
This is why SRM techniques are generally considered to be an option of last resort and field tests have been few and frowned upon. Worryingly however, there are no legal obstacles to stop anyone giving it a go, resulting in growing pressure for an international climate engineering governance framework.
What are the implications for cloud brightening over the Great Barrier Reef
Cloud brightening works by “seeding” stratocumulus clouds with seawater droplets to increase the abundance of cloud condensation nuclei, increasing cloud droplet concentration. Salt particles from this process leave the atmosphere within ten days, so consequences can be reversed within a short period.
This sounds relatively benign, so why not go ahead?
The risk is that going ahead will legitimise using SRM techniques to tackle climate change consequences, encouraging others to proceed with potentially larger scale, less benign SRM deployments, regardless of the impacts.
To engineer the climate or not...when will that be the question?
The idea of “climate engineering” is gaining momentum as the prospect of dangerous climate change becomes more imminent. This case highlights the urgent need for international guidelines governing the circumstances under which deployment of SRM techniques like Stratospheric Aerosol Injection might be justified.
So, should cloud brightening be tried over the Great Barrier Reef? Clearly those behind the proposal must be mindful of the implications and proceed with caution.
This post originally appeared on the University of Melbourne's Scientific Scribbles blog
Banner image: Used under Creative Commons