Image caption Injection of sulphate aerosols is designed to mimic the cooling effect of volcanic eruptions

A pioneering test of a climate "tech fix" planned for October faces a six-month delay as scientists discuss the issues it raises with their critics.

The test is part of the UK-based Stratospheric Particle Injection for Climate Engineering (Spice) project.

It would use a balloon and a kilometre-long hose to spray water into the upper atmosphere - a prelude to spraying climate-cooling sulphate particles.

But the funders believe that more talks about the social aspects are needed.

The project is supported to the tune of £1.6m by UK research councils, including the Engineering and Physical Sciences Research Council (EPSRC), whose independent advisory panel recommended the delay last week.

The test would have put the UK at the forefront of practical climate engineering research.

Dr Matt Watson of the UK's Bristol University, who leads the overall project, said he endorsed the decision, although his team had been "taken aback" when they first heard the news.

"We're talking about a pressure washer you could buy in a hardware shop, a long hose, and two bathloads of water, so you couldn't have a more benign experiment," he told BBC News.

There is a very big difference between being keen to research geoengineering and being an advocate for deployment Matt Watson, Bristol University

"But in the end it's the social context that's important - and we realise there's no point in having the (ESPRC independent panel) process unless we're going to work with it."

The initial deployment, due to take place from an abandoned airfield in Sculthorpe, Norfolk, will almost certainly not take place before April.

If and when it does happen, the balloon will be allowed to rise to an altitude of 1km, tethered to the ground with reinforced hosepipe.

The pressure washer will pump water from the ground and spray it from the end of the hosepipe. Researchers will use the set-up to investigate practicalities such as how the balloon and the pipe react to high winds.

A planned series of further trials is envisaged, eventually answering the question of whether it would ever be practical to put large quantities of sulphate aerosols into the stratosphere this way.

The principle behind the idea is that high-altitude aerosols would cool the planet's surface by reflecting solar energy back into space, mimicking the effect of huge volcanic eruptions.

The 1991 eruption of Mount Pinatubo in the Philippines, for example, ejected at least five cubic kilometres of ash and gas which rapidly spread around the globe, decreasing the average global temperature by 0.5C.

Image caption The postponed test would pump water droplets 1km above the ground

Climate engineering - or geoengineering, as it is often known - is a highly controversial subject.

As well as aerosol injection, ideas include devices to absorb carbon dioxide from the air, giant sunshields in space, and changing the reflectivity of land through planting different crop strains.

Proponents say research is needed into these technologies because humanity will probably need them one day, as society is unlikely to keep greenhouse gas emissions low enough to avoid dangerous impacts of climate change.

Image caption Deployment of a giant sunshield in space to reflect solar energy is another geoengineering idea

The Spice team - drawn from a number of universities as well as Marshall Aerospace - calculates that 10 or 20 giant balloons at a 20km altitude could release enough particles into the atmosphere to reduce the global temperature by around 2C.

But opponents argue that even testing could have harmful impacts, that there are questions of ethics and international law that remain unanswered, and that even raising the prospect of geoengineering distracts from initiatives to curb emissions.

Helena Paul, co-director of environment group EcoNexus, said she was "really pleased" at the latest news.

"We are certainly not ready to carry out experiments, and this project should not just be delayed, but should be cancelled immediately," she told BBC News.

"This is particularly important because while the scientists involved keep saying that reducing emissions is the primary necessity, they risk distracting attention from that necessity at a crucial moment."

At last year's UN Convention on Biological Diversity meeting, governments agreed that geoengineering projects should not have an adverse impact on biodiversity.

But that was one of very few attempts to regulate the issue internationally, which opponents argue is a big missing ingredient given that large-scale deployment of technologies in one country could have significant impacts in others.

Research shows that the UK public share some of these concerns; in surveys, very few people were unconditionally positive about the concept of geoengineering.

Over the next six months, the Spice team will engage with stakeholder groups, discussing the ethical, social and legal issues surrounding their project.

The details have yet to be worked out, but discussions are sure to involve opponents such as EcoNexus.

However, Dr Watson said there was a need to divorce the concept of researching these technologies from their actual deployment as a climate "fix".

"My personal framing of this is that there is a very big difference between being keen to research geoengineering and being an advocate for deployment," he said.

"I am not in any way an advocate for deployment."

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