That's not to say that the speakers were not committed to their research. Just keen to stress the importance of looking into the technology further before deciding whether it's safe or even viable to proceed. To many, it's an option of last resort.

"Neither the members of the working group nor the Royal Society are advocates for geoengineering," said John Shepherd of the UK's National Oceanography Centre in Southampton. "All are serious supporters of 'plan A' – reducing carbon emissions." Shepherd chaired the Royal Society study that published its report "Geoengineering the climate: science, governance and uncertainty" in September 2009. He believes that the document has become like the IPCC report – "people think they know what's in it, but they haven't read it".

According to Shepherd, "one should not wait for an emergency before testing your lifeboat but that is something that needs to be discussed". He hopes that additional research will reveal whether geoengineering techniques are potentially useful or whether they should be "left to one side".

Matt Watson of the University of Bristol, UK, who is working on the recently funded SPICE (Stratospheric Particle Injection for Climate Engineering) project, agreed. "There is a real difference between researching geoengineering and doing geoengineering," he said. "Natural scientists are exceptionally cautious about doing it. But the situation might get to be so bad that we need to think about it."

Sun-shield or clean-up?

To complicate matters, the term geoengineering is a catch-all. Available techniques can be broadly split into two types: solar-radiation management (SRM) methods, which prevent some of the Sun's rays reaching Earth, and carbon dioxide removal (CDR) techniques.

Removing carbon dioxide from the atmosphere has the advantage of addressing the root cause of the problem, returning the climate to its natural state, bringing relatively few uncertainties and risks, and addressing ocean acidification. But there's a snag – it's a very slow process.

Solar-radiation management, in contrast, would take only a few years to have an effect. That means it could be useful in an emergency, such as to avoid a climate tipping point. But such techniques wouldn't recreate pre-warming climate exactly, don't solve the problem of ocean acidification, and are potentially much more risky, for example in some regions they could cause a marked decrease in rainfall.

Who decides?

For many, the meeting was a precursor to the kick-off meeting of the Solar Radiation Management Governance Initiative, a joint initiative between TWAS, the academy of sciences for the developing world, and the Environmental Defense Fund (EDF). "The social and ethical issues are probably much more difficult than the technical issues," said Shepherd. But compared with scientists working on early technology in the fields of nanotechnology and genetic modification, the geoengineering community is relatively far advanced at thinking about governance and societal issues.

That said, some of the very first efforts to regulate geoengineering on an international basis, by the Convention on Biological Diversity (CBD) conference in Nagoya in October, were widely slated at the Royal Society meeting.

The CBD agreed to "ensure that no climate-related geoengineering activities that may affect biodiversity take place with the exception of small-scale scientific research studies that would be conducted in a controlled setting…and are subject to a thorough prior assessment of the potential impacts on the environment".

As Shepherd pointed out, the phrase "a controlled setting" is not well defined. "I think that the CBD got a bit ahead of the game, and tried to regulate before we're ready," he said. "Before we have international decisions, we need to have a much better understanding." Shepherd reckons that since one of the biggest threats to biodiversity is climate change, you could argue that any geoengineering solution that avoids climate change is good for biodiversity.

Public knowledge

Of course it won't just be scientists who decide whether to implement geoengineering – politicians, and hence public opinion, will be involved too. According to psychologist Nick Pidgeon of the University of Cardiff, UK, the public finds risk less acceptable if it's associated with factors such as being a danger to children, unfair, man-made, unfamiliar, poorly understood by science or hidden. And geoengineering has a number of those. But Pidgeon reckons that there will be no single public perception – it will depend on the technology and its deployment.

In a 2009 survey about energy supply answered by 42 people at their homes in Wales and the West of the UK, the one question on geoengineering revealed a lack of knowledge. Those few people who attempted to hazard a guess at what the term meant tended to confuse it, perhaps understandably, with geothermal energy.

When researchers described solar-radiation management technology using mirrors and sulphate particles, reactions ranged from "intrigued to terrified", Pidgeon said. Many people felt that the technology was messing with nature and too sci-fi, and they were concerned about side-effects.

Another survey this year found that just 7% of respondents said that they knew a fair amount or great deal about geoengineering. Around 40% strongly supported or tended to support SRM; intriguingly the don't-knows outnumbered the number of people who were against use of the technology. And 47% of people polled were in favour of CDR. The only factor that appeared to make people more likely to support geoengineering was concern about climate change; factors such as age, gender and income made no difference.

According to Pidgeon, the survey also revealed a tentative result, which needs further investigation, that the more people reported themselves as knowing about geoengineering technology, the less happy they were about any potential implementation of solar-radiation management and the more happy they were about carbon dioxide removal.

"There's always a fear that if you do public engagement, the public will almost immediately turn against the technology," said Pidgeon. "But that doesn't happen." Pidgeon says that the public tends to act more like a jury, really trying to understand the issues and make a fair judgement.

In fact it's possible that talking to people about geoengineering may even help to mitigate climate change. Shepherd found that when he communicates with the public on geoengineering, people tend to make comments such as "good heavens, if you're really considering things like that I'm going to go home and change my behaviour".