Guest contribution by Mason Inman

Governments are starting to take a look at geoengineering — planetary-scale projects to cool the planet. No longer in the realm of science fiction, this is something that many are calling to regulate — and soon. The technique that has geoengineering proponents most excited, and geoengineering critics most worried, are proposals to shoot sulphate aerosols into the stratosphere. These tiny particles, like those that large volcanoes spew into the air, can cool the planet. ‘Natural experiments’, such as the eruption of Mount Pinatubo in 1991, show that this works.

At a hearing this month of the UK Parliament’s Science and Technology Select Committee, former UK science advisor Sir David King said that for this kind of geoengineering “I would move fairly swiftly, as I have suggested, into a temporary ban, and find the feasible way forward for that. I am not happy about smaller experiments being conducted at this stage in time before the unintended consequences have been fully evaluated.”

As he later explained to Nature Reports Climate Change, “Once we’ve opened the door to field trials, we’ll be slipping into larger-scale operations. This is something that needs to be put in the back drawer for 30 years.”

Others disagree. Banning responsible research into this approach, which is known as solar radiation management, would be a mistake, according to a Policy Forum article in Science this week by Jason Blackstock of the International Institute for Applied Systems Analysis in Laxenburg, Austria and Jane Long of Lawrence Livermore National Laboratory in California.

But this kind of geoengineering, and even research into it, carries side-effects — both physical and political — that are likely to be huge. Geoengineering with sulphate aerosols could disrupt monsoons, which depend on a delicate balance between heating over the oceans and over the land. “The equivalent of one Mount Pinatubo every 4 years could lower global average surface air temperature,” Alan Robock of Rutgers University and colleagues write, in another Policy Forum article in Science this week. “But African and Asian summer precipitation would also be reduced, potentially affecting the water and food supplies of more than 2 billion people.”

So, how do we find out more about the potential side-effects? We could do more climate simulations, start lab tests and do small-scale field trials. But, Robock and colleagues argue that this won’t tell you much about the big questions: how much will it cool the planet? And are the side-effects tolerable?

Current theory suggests that neighbouring sulphate aerosols would tend to clump together, which is one reason for full-scale testing, say Robock and co-authors. And, according to a recent modelling study, these larger particles would cool the planet less than the smaller particles initially injected into the atmosphere. The only way to test the theories and models, they argue, is by injecting a large amount of sulphates regularly into the stratosphere and then seeing how the particles behave. This would also affect the planet’s temperature — the only way to check that the technique is doing its job. “Such an experiment would essentially be implementation of geoengineering," Robock and colleagues write.

But doing such a test would also carry political side-effects. In the politics of geoengineering, perceptions could be as important as realities. “Liability for damages, real or perceived, would become a political challenge,” Blackstock and Long write. If geoengineering tests go ahead, and at the same time the monsoons weaken, how would you tell if it’s an ‘act of God’ or an ‘act of geoengineers’?

Even if some system was set up for compensating the victims, how would you decide when and who to pay, and how would you ensure that the money helps the people hit hardest? Given these difficulties, a temporary ban on research has some appeal. But sulphate aerosol engineering is cheap and easy enough that it’s conceivable that some country could decide to undertake a full-scale trial on its own. Blackstock and Long say that “nations must carefully consider the signals that any unilateral field test sends to the international community.”

When reporting my article Planning for Plan B for Nature Reports Climate Change, I talked with philosopher Dale Jamieson of New York University. This was before the UN negotiations in Copenhagen, and he said, “My worry is that Copenhagen collapses, and then the US government announces a geoengineering research program. And what that looks like to the rest of the world is that geoengineering isn’t ‘plan B’, but ‘plan A’.” Geoengineering “cannot be a substitute for comprehensive mitigation,” Blackstock and Long argue — something almost everyone involved in the debate agrees on.

Testing still presents a Catch-22. To get a real sense of how aerosol geoengineering would work, you need to do a full-scale test. But if you do a full-scale test, then there is a good chance of it having serious side-effects. So it’s not clear how we would get the evidence that King called for, to make sure that “the unintended consequences have been fully evaluated.” Building an international consensus, through a “broadly accessible, transparent, and international political process”, is the way to move forward, Blackstock and Long argue.

But to me, geoengineering with sulphate aerosols still seems like some new, experimental chemotherapy drug that has a chance of saving the patient, but that will probably have severe side-effects. Doctors only test these drugs on the sickest, most desperate of patients, those with few or no other options. How desperate, I wonder, will the world have to be to bring everyone to the table and hash out an agreement on geoengineering — or even just on regulating the trials?

Mason Inman is a science writer based in Karachi, Pakistan.