THE SUM OF human tinkering with the climate since the beginning of the industrial era is sometimes likened to a planetary science experiment. That captures the magnitude of what is happening and the unpredictability of its results, yet it is also misleading. Global warming is not an experiment, because it is not intentional. Greenhouse-gas emissions are the unfortunate side effects of useful things like modern agriculture, electricity generation and convenient transport. Mankind has not really started experimenting with the climate yet.

But perhaps, given the slow progress in keeping down emissions, it should. A small, underfinanced and somewhat obsessive group of scientists is working on ways of “geoengineering” the Earth to reverse global warming. Some of their proposals are absurdly costly; others are exceedingly dangerous. Still, geoengineering deserves much more serious consideration than it has so far received.

Since climate change is mostly caused by greenhouse gases, the obvious way of reversing it is to remove those gases from the atmosphere. Removing carbon dioxide from the air would also help marine creatures: the oceans are becoming less alkaline as a result of dissolved carbon, which seems to be harming corals. Some scientists are exploring ways of speeding up the natural processes that already do this. Carbon-absorbing minerals like olivine, which is in abundant supply, could be mined, crushed and spread out. Lime or limestone could be tipped into the ocean to react with dissolved carbon dioxide to create bicarbonate ions, allowing the water to absorb more carbon dioxide from the air. Iron and other nutrients could be added to the water to stimulate the growth of algae, which feed on carbon dioxide.

Plants could be grown and then burnt in power stations capable of capturing the carbon that the plants had removed from the air; the gas could then be compressed and buried under the ocean. Carbon dioxide could be filtered out of the smoke that rises from factories and power stations, or even just out of the air. A Canadian firm, Carbon Engineering, has just opened a pilot plant that will do this.

All methods of removing carbon dioxide from the atmosphere are even more challenging than they might seem at first sight. That is because the ocean and the land currently absorb about half of human emissions. If atmospheric concentrations were brought down, some carbon dioxide would naturally “outgas” from the seas and the land, adding to the amount that would have to be removed.

And two fundamental (though contradictory) criticisms are levelled at the carbon-suckers. First, their methods are so costly that they could not possibly be deployed on the scale required to alleviate climate change. And second, if those methods could be made to work, they might introduce moral hazard. If greenhouse gases could magically be removed from the atmosphere tomorrow, why bother with cutting emissions today?

The first objection is a good one. Carbon-removal techniques are indeed extraordinarily costly, and not just in a financial sense. Tim Kruger of the Oxford Martin School estimates that in order to remove just one gigatonne of carbon (roughly one-tenth of current annual emissions) from the atmosphere, 4.5 gigatonnes of lime would have to be dumped into the ocean. That would require 6.5 gigatonnes of limestone, or almost one tonne for every man, woman and child on Earth, and 4,500 factories to make it into lime. Alternatively, growing plants and then capturing their carbon would require enormous quantities of agricultural land to make much difference to the climate.

Still, many of these technologies deserve to be tried out. The costs of some carbon-removal methods might come down in time, though others might turn out to be even more expensive than their proponents think. And at some point in the future one of them, or a combination, will have to be deployed if climate change is to be arrested. It will be impossible to prevent all greenhouse-gas emissions. There will always be individual national holdouts, and there will always be niche uses for gas and oil, such as powering passenger aeroplanes.

The second objection to carbon removal, that it encourages recklessness, would be persuasive only if it could be done cheaply. At the moment it looks so costly and so tricky that it cannot be used to justify putting more greenhouse gases into the atmosphere. Indeed, it would be good to have more research into these technologies if only to see just how costly they would be. Most of the theoretical means of avoiding large-scale global warming assume that tens or even hundreds of millions of hectares of land can be given over to growing energy crops. It would be helpful to know how realistic that might be.

At best, carbon-dioxide removal might turn out to be an expensive way of dealing with the chief cause of climate change. But there is another approach, which is to attack climate change directly. This could work out much cheaper. Indeed, it would almost certainly be cheaper than replacing fossil fuels with renewable sources of power. That is only one reason it is so unnerving.

For all that human activities are perturbing the climate, those actions appear trivial when set beside the enormous heat engines that create the Earth’s weather. Even doubling the concentration of carbon dioxide in the atmosphere would create a radiative forcing of only four watts per square metre, a number dwarfed by the 240 watts per square metre that pours into the Earth from the sun. That suggests a straightforward and highly appealing calculation. Four divided by 240 is 0.017. To offset the warming effect of a doubling of carbon dioxide, then, it might be necessary to intercept only about 1.7% of the solar radiation that currently reaches the Earth.

Spray and pray

Some early satellite pictures contained what appeared to be scratches, says John Latham, a scientist who lives in Colorado. These turned out to be ship tracks—linear clouds that grow on aerosols emitted by ships as they traverse the seas (see image above). What has been done inadvertently could be done better deliberately. If ships were to create tiny salt particles from seawater in just the right places, water droplets would form on them. That would alter a type of cloud called a marine stratocumulus. With more droplets of a smaller size, the cloud would become lighter and thus more reflective. Seawater is innocuous; “it’s benign and it’s infinite,” explains Mr Latham. And marine stratocumulus clouds are so common that the Earth might be cooled substantially.

Ship tracks are linear clouds that grow on aerosols emitted by ships. What has been done inadvertently could be done better deliberately

The big technical problem so far has been to produce nozzles capable of consistently producing tiny droplets. Many ships would be needed to trundle up and down the best cloud-lightening corridors (the west coast of Africa is especially good). But these difficulties hardly seem insurmountable. A report published in 2012 for the Copenhagen Consensus Centre estimated that marine-cloud brightening would prevent global warming even more effectively than a carbon tax.

If spraying seawater into the air would probably cool the Earth, spraying sulphur into the stratosphere would be almost certain to do so. It has been done, after all. Volcanoes spew out sulphur that creates particles which reflect sunlight back into space; those particles also bounce light around the atmosphere, producing wonderful sunsets. These can cool the Earth significantly, albeit briefly (see chart): within a year or so the particles are washed out of the atmosphere.

Sulphur could be sprayed at precisely the right height and in very fine droplets, which would reflect more light for longer. It might take only a small fleet of high-altitude aircraft flying in relays to put enough in the stratosphere to cancel out the entire temperature rise resulting from human greenhouse-gas emissions. The sulphur would eventually fall as acid rain, but not in alarming quantities: the amount of sulphur required would be much less than is currently thrown up into the air by vehicles and factories.

Both marine-cloud brightening and stratospheric aerosols carry risks. One is that cooling the Earth without removing carbon dioxide does not quite return the climate to normal. The more carbon dioxide that is present in the air, the less plants perspire, affecting the water cycle. And the heat-trapping greenhouse effect would still operate, just with less heat in the system. With temperatures more evenly distributed in the atmosphere, there would be less convection and, presumably, less precipitation. So a cooler world with lots of greenhouse gases would probably be a drier world. Any country that suffered a drought would surely blame the geoengineers.

But the biggest problem is what would happen if the engineering stopped. Assuming that greenhouse-gas emissions continued while the ships or aeroplanes were doing their work, abruptly ending the artificial shielding would lead to a sudden jump in temperatures, which would be disastrous for people and the natural world alike. Ken Caldeira of the Carnegie Institution points out that an abrupt “termination shock” could be avoided if geoengineering were used only to slow global warming and then gradually wound down. But that assumes the nations of the world can agree on how to manage the climate. The history of United Nations climate talks suggests they can’t.

Still, these methods ought to be developed and even—very carefully—tested. The Earth might need a drastic intervention, particularly if it became clear that something alarming was about to happen, such as a breakdown of the Indian monsoon. Marine-cloud brightening could be deployed on a small scale to avert specific disasters. Mr Latham suggests that cooling just a few hundred square kilometres of ocean in the right place could make a hurricane less severe. If the climate-modellers are right that hurricanes will become more intense as the ocean warms, this will become increasingly tempting.

Better the devil you know

The most persuasive reason for investigating geoengineering further is that somebody is likely to try it. Countries will have different ideas about when global warming becomes truly dangerous: Britain, for instance, is a lot more sanguine than the Maldives. Some of the more skittish states might start injecting aerosols into the lower stratosphere, perhaps in a clumsy way. If no formal experiments had been carried out and thus scientists in other countries did not know what to look for, it might not be obvious for some time that this was going on.

David Victor, who studies the politics of climate change at the University of California, San Diego, doubts that nations would ever formally agree to engineer the Earth’s climate: their interests are too diverse. He thinks it much more likely that a country would just go ahead and try it. That would put the others in a quandary. Should they forcibly stop that country from acting, or should they step in with superior geoengineering techniques? Before long, Mr Victor says, they could find themselves acting as zookeepers to the planet.