In 2009 biological oceanographer Victor Smetacek tried to sink our global warming problem in the sea. The researcher, his scientific team and the crew of the ship RV Polarstern sailed to the Southern Ocean and poured a solution of iron into a small eddy. Iron, a nutrient, triggered a phytoplankton bloom, and the tiny photosynthesizers sucked carbon dioxide from the sky as they grew. When the plankton died, they drifted like snow to the bottom of the ocean, entombing CO 2 in their tiny corpses.

Although the technique, if used widely, could bury a billion metric tons of this greenhouse gas every year, the experiment drew the ire of environmentalists. Such iron fertilization was condemned by organizations such as the World Wide Fund for Nature and the ETC Group, some other scientists, and Germany's environment minister, who worried about unforeseen and toxic side effects, such as plankton growth harming the food chain. Smetacek, who had received prior approval from the governments of Germany and India, eventually stopped pursuing the idea after an international treaty against ocean dumping added cautions about such experiments.

We need to get over the environmentalist skittishness that thwarts these small tests of climate manipulation. Civilization may depend on such geoengineering methods as the planet keeps warming. We need tests to get them right—and stop people from doing them wrong.

Humanity is on pace to raise the planet's thermostat by four degrees Celsius by 2100, according to the Intergovernmental Panel on Climate Change. Its latest report states that technology to pull CO 2 from the air will be needed to avoid that rise.

There are at least two families of geoengineering ideas: those that get rid of CO 2 , the primary greenhouse gas, and those that seek to block sunlight, which buys time. Scientists and engineers have proposed various approaches besides iron fertilization, such as hazing the skies with sulfates to mimic the cooling effects of a volcanic eruption or even launching a fleet of mirrors to deflect sunlight away from the planet. The problem with any of these approaches is that scientists do not know much about potential side effects. Could plants genetically engineered for supercharged photosynthesis kick off another Ice Age by drawing down too much CO 2 ? Would artificial volcanoes shut off crucial Asian monsoon rains by altering cloud and wind patterns? Would any of these world-changing ideas work in the first place, and are some too crazy to pursue?

The only way to find out for sure is to do what Smetacek did: test them, in a contained, rigorous, transparent manner. Not only did the oceanographer obtain government permission, he published the findings and data in a scientific journal so all could see. Yet even small tests like this are taboo. When U.K. researchers announced plans to spray a few tubs of water into the sky in 2011, more than 70 organizations from around the world signed a protest petition. The scientists backed off. These attitudes need to change, and scientific funding agencies need to support such research. The small but discernible effects of a restricted test should do no long-lasting damage. Smetacek's plankton bloom faded quickly. The eruption of Mount Pinatubo in 1991—a large-scale geoengineering “experiment”—did not have lasting climate effects.

Geoengineering experiments do carry risks: setting off artificial volcanoes all over the globe, for instance, might destroy the ozone layer. That is another reason why geoengineering concepts need testing: so people know what not to do.

After all, Smetacek and his crew are not the only people to try out iron fertilization. In 2012 independent entrepreneur Russ George dumped iron overboard with the idea of restoring salmon fisheries and selling carbon credits. That is the kind of rogue geoengineering that we cannot afford.