Earth's average temperature has warmed by 0.8 degree Celsius over the last 100 years or so. The reason is increasing concentrations of greenhouse gases, particularly carbon dioxide, in the atmosphere. The concentration of CO2 has now reached 394 parts-per-million in the air we breathe—and would be even higher, roughly 450 ppm, if the oceans weren't absorbing a good deal of the CO2 we create by burning fossil fuels, clearing forests and the like.
The basic physics have been understood for 150 years. Global warming has been observed for at least 30 years. International negotiations to restrain greenhouse gas emissions have been ongoing since 1992. And yet, other than during economic recessions, emissions have steadily marched up. If global warming is a problem—one likely already producing weird weather, rising seas and extinctions, among other effects that could be considered dangerous—we are not addressing it.
So is it time to consider something a little more radical? Specifically, the family of ideas for restraining climate change captured under the rubric of geoengineering? Or, as the U.K.'s Royal Society puts it: the deliberate, large-scale manipulation of the planetary environment. As the guest editors of a special issue of Philosophical Transactions of the Royal Society A note: "Geoengineering is no longer the realm of science fiction."
The science fiction-y schemes vary from proposals to block out the sun via mimicking volcanic eruptions to massive machines the size of power-plant cooling towers to strip CO2 from the air at an accelerated rate. Or maybe you prefer creating CO2-storing peatlands by raising water tables, or engineering Sphagnum moss to better fend off microbial decomposition when dead. While we're at it, the crops that cover 11 percent of Earth's continental surface could be engineered to reflect more sunlight, or the ocean near Antarctica could be fertilized with iron to promote diatom blooms that ultimately bury carbon at sea.
In the end, there is a set amount of greenhouse gases that can be dumped into the atmosphere if we want to avoid catastrophic climate change. Scientists' best guess is that we can emit 1,000 petagrams, or 1 trillion metric tons, of carbon if we want to stay below 2 degrees Celsius of warming (less than the amount of warming that characterized the shift from the ice-ridden Pleistocene to the milder epoch that birthed human civilization known as the Holocene). We have already emitted more than half of that and will emit the rest of that limit within a few decades if we continue to burn fossil fuels, clear forests and such at anything like present rates.
As climate modeler Ken Caldeira of Stanford University discusses in the September issue of Scientific American in his article "The Great Climate Experiment," we are now effectively setting the temperature of the planet for the next several millennia.
If the world collectively fails to restrain pollution, then we might need to deploy geoengineering techniques in a hurry to prevent catastrophic climate change. So doesn't it make sense to investigate the promise of various techniques promise and perils? This is not a new idea—geoengineering hit President Lyndon Johnson's desk in the 1960s along with a report on climate change that suggested he might deal with the problem by spreading reflective particles on the oceans—just a relatively unexplored one.
All this points to a more fundamental philosophical question about geoengineering, which, as the name implies, is global in scope: Who controls the thermostat? If greenhouse gas emissions are unlikely to turn Earth into Venus, technical remedies are quite sufficient to induce another Ice Age. In fact, weather control was first explored as a weapon during the Cold War. The barriers to entry are relatively low: an island nation, say, with a battery of big guns could start shooting sulphates into the air to block sunlight and cool the climate until somebody stopped them. Or sulphates could be used regionally to stave off, say, a heat wave. Scientists have already begun the task of assessing which method (existing aircraft or, maybe, tethered balloons) and particles might serve best (it's not sulphate, it's diamonds or, even better, the minerals you find in your sunblock!) Bonus: these other particles might let the sky stay blue rather than the hazy white expected from stratospheric sulphates, though the impacts of such particles falling out of the sky and covering the planet are unknown.
Such schemes have an apocalyptic feel and bring up images of Dr. Strangelove or other mad scientists. As one respondent to a survey of public attitudes toward geoengineering in England, Scotland and Wales in 2010 put it: "I don't think you should mess about with the climate… I think that's very dodgy to be honest." Of course, we already are messing about with the climate. And that means the question that can't be dodged is: What are we going to do about it?