IN DISCUSSIONS of climate change it is an article of faith that there are no winners, only losers. This is in part an expression of bien-pensant solidarity, but it is also realistic. It recognises the degree to which current human arrangements—farming practices, positioning of cities, etc—are adapted to current climatic conditions, and that shifts in those conditions will impose transition costs even if not in absolute terms dreadful. It also acknowledges the world's ever greater level of interdependency. If the local effects of climate change in Syldavia, say, are pleasing to the residents, those benefits can still be offset by a loss in trade with the much worse affected Ruritania, or through conflict over water resources with now-parched Borduria, or by influxes of refugees from Vulgaria, and so on.

Underlying all this is a concern about uncertainty. There are various places where small shifts in climate might seem locally desirable. But uncertainties in both climate science—how strongly, and with what geographic pattern of effects, does the earth respond to increased greenhouse gases?—and political economy—what levels of greenhouse gas will the earth be subjected to?—make it impossible to guarantee the small shifts that people in those places might like. They might well instead end up with larger changes they liked much less. Better to assume that everyone is a loser, because thanks to the uncertainties it is undoubtedly the case that everyone could be.

In discussing geoengineering schemes, though, talk of winners and losers is rife. Increasingly, the question asked about any scheme to alter the climate in a way that acts to counter greenhouse warming—by scattering sulphates in the stratosphere to cut down incoming sunlight, for example—has been "whose hand will be on the thermostat?" The assumption has been that while geoengineering schemes might help some people and places they will harm others, and that this will lead to inequity and conflict.

The thinking that lies behind this is that though a stratospheric veil of microscopic sulphate particles would undoubtedly cool the earth (they have been clearly seen to do so after large volcanic eruptions) its cooling effect would not be exactly equal and opposite to the warming greenhouse gases provide. Cutting down sunlight delivers less cooling in winter, and none at night. It also has its most direct cooling effect on the surface of the planet, while increasing greenhouse gases warms the surface indirectly by warming the atmosphere above it.

These things can average out. One can imagine setting a system in which the net effect of increased greenhouse gases and increased sulphates in the atmosphere kept the average global surface temperature the same. But that would not mean the climate was unchanged in other ways. In particular, the different effects on the surface and the atmosphere mean that the hydrologic cycle would be changed. Various studies with computer models (and experience with volcanoes) suggest that a greenhouse world kept cool with a sulphate veil will see less precipitation, all things being equal, than a non-greenhouse, non-veiled world with the same average temperature. The pattern of precipitation will change, too, as well as the overall amount.

A new study in the journal Nature Geoscience looks at these issues and what they might mean using the power of a system called Climateprediction.net, one of a number of intriguing attempts to use the spare capacity of a large number of computers to run calculations of scientific interest. Katharine Ricke and her co-authors used its remarkable power to run thousands of simulations of how the climate might develop over this century with different levels of geoengineering, all beginning, in the simulations, in 2005.

They found that for every region of the planet there were geoengineering scenarios in which the projected climate in the 2020s was more like the climate of the 1990s in terms of temperature and, crucially, precipitation than if there were no geoengineering. However there was no scenario that did that for both variables in all regions. And the longer the geoengineering went on, the more striking the effect became; scenarios that kept one region's temperature 1990-like would push the precipitation out of bounds somewhere else, and vice versa.

A particularly salient example of this comes from Asia. There were a number of geoengineering scenarios in which the climate in both India and China in the 2020s looked quite like that of the 1990s, though in all geoengineering scenarios using this particular model India gets a bit wetter than it was before and in most of them China gets a bit drier. Go out to the 2070s, though, and the geoengineering scenarios strong enough to keep China's temperature 1990s-ish cool India below its baseline temperature, while weaker scenarios that keep India's temperature at the levels of the 1990s see China heat up.

That said, in both cases all the geoengineering options gave results for both temperature and precipitation closer to 1990s levels than the models projected for a world without geoengineering. In this sense both countries were "winners"—but maximising the benefits for one would still come at the expense of the other. The same lesson seems to apply quite generally across the world. Few if any regions stand out as certain losers from geoengineering if you accept that it is worth trading off a large change in temperature for a smaller drop in precipitation (in terms of change on the levels seen before the geoengineering). But different levels of geoengineering seem optimal for different regions.

Though the specific regional differences seen in the simulations are interesting, and Ms Ricke intends to explore them further in future work, they should not be over interpreted. Climateprediction.net allowed the researchers to run their model many times, but it was always the same model, and no model of the global climate has as yet any strong claim to getting the details of regional climates right. The core message here is not what level of geoengineering any given region might prefer, but rather the lesson that, though many regions might benefit from geoengineering, they will not all do so to the same extent in any single geoengineering scenario.

Veil of sulphate, meet veil of ignorance

A future programme to compare the effects of geoengineering as simulated by a range of different climate models should reveal any plausibly robust regional predictions that can be made at the current level of scientific understanding. It will not get rid of the fundamental uncertainties, though, and nor would it necessarily be a good thing if it did. Uncertainty about who might do best from what sort of project allows discussions of geoengineering to take place without the parties to the debate knowing in any detail where any nation's specific interests might lie. This introduces what the philosopher John Rawls called a “veil of ignorance”; making decisions as if such a veil existed, Rawls thought, was a good basis for justice. (If regional outcomes could be predicted accurately, a different Rawlsian idea, that of the difference principle, might come into play. This states that just action consist not just of improving things for everyone, but specifically for improving things for the worst off, and would give the effects of geoengineering on the least developed countries a particular importance.)

The continuing uncertainty also emphasises the extent to which discussions of geoengineering do not constitute an alternative to other approaches to climate change: they are instead just an expansion of those efforts. Taking geoengineering seriously is not a question of opting for some clarifying technological fix—of saying, in effect, forget the politics, here come the sulphates. It is a matter of taking the current problems of decision making in conditions of uncertainty to new levels in order to consider possible worlds that aren't quite as bad.