IN 1990 John Latham, a cloud physicist, published a short article in Nature under the headline “Control of Global Warming?” It argued that if low-lying maritime clouds were made a bit brighter, the Earth could be cooled enough to make up for the increased warming caused by emissions of greenhouse gases. The brightening was to be achieved by wafting tiny sea-salt particles up into the clouds from below; by acting as “cloud condensation nuclei” (CCN) they would increase the number of water droplets in the clouds, and thus the amount of sunlight they reflect out into space. Latham calculated that a square kilometre of cloud might be kept bright with just 400 grams of spray an hour. And finding out if it was really that easy might be straightforwardly tested. “It seems feasible”, Dr Latham wrote, “to conduct an experiment in which CCN are introduced in a controlled manner into marine stratus.”

A quarter of a century on, such a test may soon be on the cards. For more than ten years Dr Latham’s idea was almost entirely ignored. Then it caught the attention of an enterprising engineer, Stephen Salter of the University of Edinburgh, who looked at ways it might be made practicable, and a small number of researchers started to pay attention. But the question of whether anyone could actually produce ship-borne sprayers that would reliably churn out particles a ten-thousandth of a millimetre in diameter at a rate of 1,000 trillion a second remained open.

Armand Neukermans, a retired Silicon Valley engineer whose achievements include, among other things, the development of the earliest inkjet printers, has with various colleagues (also mostly retired) looked at a range of possible techniques. One that may be up to the job is “effervescent spray atomisation” in which, rather than trying to make truly tiny droplets straight away, you make larger ones in which water mixed with gas subsequently fizzes into particles of the desired size.

Dr Neukermans, Thomas Ackerman and Robert Wood, the latter two both scientists who study clouds at the University of Washington, have with colleagues put together a proposal for field tests to see if such sprayers really work, if their effects can be controlled and measured, and what happens to clouds treated in this way. They are now investigating how to get such a programme financed.

They are not the only people who want to see how ideas from geoengineering studies play out in the real world. David Keith, a professor at Harvard University, has been studying how to reflect sunlight back from an artificial layer of haze in the stratosphere similar to that created by the sulphur thrown up by large volcanic eruptions, which are known to cool the Earth. One of the risks would be that such particles can encourage chemical reactions which deplete the ozone layer. Dr Keith and his colleagues want to study how the rates of such reactions depend on the sizes of the particles and background levels of water vapour; that would help to assess the risks and perhaps find ways to limit them. They have designed a system which would hang below a large balloon 20km up in the sky. It would create a small plume of sulphate particles and then measure the physical and chemical changes.

For both the clouds and the stratosphere, the direct effects of the proposed experiments are tiny. Cloud-brightening on the scale imagined requires less than a litre of seawater a second. The amount of sulphur that might be put into the stratosphere would be about 2% of what a passenger jet crossing the Atlantic emits in an hour. These proposals are not distinguished by the scale of what is envisaged, but by the precision with which they would be carried out and the care with which their effects would be monitored.

The worried ones

Another distinction weighs more heavily. Though these experiments would provide insights useful to scientists in other areas—the physics of clouds and the chemistry of the stratosphere are big topics in their own right—they are being proposed as ways to further research into geoengineering. That concerns many people, and a number of environmental campaign groups oppose all such experiments. Academic critics such as Clive Hamilton of Charles Sturt University in Australia argue that, though the risks to health or the environment may be minor, such experiments pose “political and social risks” that are much more troubling. Experiments could create “lock-in” around a particular research path, forming a constituency that would downplay subsequently uncovered risks and obstacles. And the mere fact of experiments going ahead might lead people to assume that geoengineering could easily be made feasible, and thus to give up on reducing carbon emissions.

Following the money

Perhaps because of such concerns, financing bodies have not yet shown much appetite for geoengineering experiments. In America, most of the relatively little research money spent so far has gone to computer models. In Britain, where three interdisciplinary research programmes in the field are coming to an end, a proposed experiment that would have sprayed water from a balloon was cancelled by the team that had been planning it in 2013 because of worries about the transparency of the process by which the experiment had been set up. At a recent discussion devoted to these British programmes Alan Gadian of the University of Leeds, who works with Dr Latham, Dr Neukermans and their colleagues, made no bones about his belief that the government had a bias against financing experiments like those now proposed for cloud-brightening.

In the absence of government funding, some philanthropists have been helping out. Dr Keith is one of the administrators of the Fund for Innovative Climate and Energy Research, through which Bill Gates and some of his former Microsoft colleagues finance research on geoengineering projects and other things. The fund has supported work by Dr Gadian and Dr Neukermans, among others, as well as by Dr Keith himself. But its position is not to fund field tests of cloud-brightening, stratospheric hazing or anything like them. Other sources of philanthropic money may be available, and the cloud-brighteners may well look into them. Dr Keith and his colleagues, though, want their stratospheric experiments to be funded mostly by the government. “I think we have the best chance to have a healthy dialogue if experiments are publicly approved,” says Dr Keith.

Geoengineering experiments carried out high-handedly or without due respect for sensible concerns would be damaging. But precedent suggests that such experimentation can be regulated. In the 1990s and 2000s there were a number of large experiments aimed at finding out if adding iron to the oceans would spur photosynthesis in such a way as to move carbon from the atmosphere to the abyss. Though they were billed as investigations of climates past—such fertilisation is thought to have contributed to low ice-age carbon-dioxide levels—the possible application of the process as a form of geoengineering aimed at stabilising future carbon dioxide levels was also an inducement.

In the late 2000s such experiments were discussed by the London Convention, which sets rules about pollution at sea. It was decided that research should be allowed, and a fairly impressive set of regulatory requirements was established. At a meeting on the regulation of geoengineering experiments held in Washington, DC at the beginning of December, some oceanographers argued that this new regulatory system was so strict as to discourage worthwhile research. Other participants argued that that was hard to substantiate, since no one had actually tried to get any such experiments approved since the new rules were drafted—and the reason for that is mostly that oceanographers are split over the value of further research. If this is any precedent, it suggests that geoengineering experiments in the atmosphere could go ahead fruitfully, bringing with them new knowledge, new regulatory frameworks and new disagreements—and no obvious risk of lock-in.