Geoengineering has always been the wacky, mad-scientist climate scheme no one wants to discuss. Sure, the idea sounds simple enough: If the world can’t wean itself off fossil fuels quickly enough, then maybe there’s a way to artificially cool the planet to avoid the worst effects of global warming. But the practical concerns are far from simple. What if there are unforeseen side effects? Who gets to control the sun-blocking technology? It’s no wonder that most experts who fret about climate change would rather just sidestep the topic and focus on Plan A, reducing greenhouse-gas emissions the old-fashioned way.

(Associated Press)

Geoengineering comes in two basic flavors. The first, simpler approach would entail sucking carbon dioxide out of the atmosphere. We could do that by planting more trees, although there’s only so much land to go around (especially as demand for cropland grows). We could try to fertilize the ocean with iron to promote the growth of plankton, which absorb CO2. We could try to speed up natural processes that take CO2 out of the air, such as rock weathering. Or someone could just invent a machine to scoop up the carbon directly.

The second type of geoengineering is much more controversial. This would entail cooling the Earth directly by reducing the amount of solar energy that reaches us. Blocking the sun, in other words. We could put fine particles or liquid droplets into the air to reflect incoming sunlight back into space. (This is more or less what volcanic eruptions do; when Mount Pinatubo erupted in 1991, belching out dust and sulfur, it temporarily lowered global temperatures by nearly 1 degree.) Or we could try to increase the reflectivity of clouds by, say, spraying seawater into the atmosphere.

The chief concern with these sun-blocking ideas is that the Earth’s atmosphere is a complex system, and mucking with it could produce unpleasant side effects. Spraying aerosols into the atmosphere might, for instance, alter wind and rainfall patterns in unexpected ways. What’s more, simply blocking sunlight while going about burning fossil fuels would mean that ocean acidification — which creates problems for coral reefs and other marine life — would proceed apace. And, of course, there are the geopolitical headaches. What if different countries disagree on how much sunlight to block? What if some countries benefit and others lose out from the resulting shifts in rainfall?

Ideally, scientists could conduct more research on these methods to figure out what works. That’s not as easy as it sounds, though. Right now, most geoengineering research is conducted with computer models that try to predict the effects of, say, lacing the atmosphere with aerosols. Many scientists have argued that tinkering with models is no substitute for field experiments. But how do you safely conduct those experiments in the real world, especially when there’s the risk that something might go wrong? Indeed, the Bipartisan Policy Center task force report concluded that, for the time being, “the field deployment of [schemes to reflect sunlight] by the U.S. government or by any other government or entity would be inappropriate.”

Still, as a recent report in New Scientist notes, researchers in several countries are getting underway with a variety of small-scale geoengineering trials. In Britain, researchers are planning to use a balloon to hoist up a kilometer-long hose and spray water into the atmosphere, as a possible precusor to other geoengineering tests (the project has been delayed over environmental concerns). Other scientists have begun seeding small parts of the ocean with iron to promote plankton growth. That’s why the BPC report includes a host of recommendations for rules to govern these research projects, to make sure they’re as safe as possible. There’s no use, after all, pretending we can just avoid the subject. It’s far too late for that.