History is replete with occasions when an effort to fix one problem creates a worse problem; some examples include the cane toad's human-assisted subjugation of Australia (subject of a highly entertaining documentary), and UNESCO's effort to provide clean drinking water to Bangladesh, which resulted in the biggest mass poisoning in history. Scientists, with great faith in their powerful art and a can-do attitude, sometimes unleash the worst unintended consequences.
While it's hard to predict what good-hearted attempts will backfire, sometimes a person gets an inkling, a premonition, that our species might be about to take a grand misstep. I just got that feeling when I read about some scientists' plan to "seed" the ocean with iron, provoking a bloom of phytoplankton, which will consume a lot of carbon dioxide, thereby lowering the amount of CO2 in the atmosphere. Whatever. I picture the plankton taking over and marauding through the oceans, blinding whales and clogging snorkels—kind of a Jurassic Park-meets-cane toads thing.
In fact, the scientists themselves seem to be hedging their bets about whether this will even work to decrease CO2: "Instead of the carbon sequestered by the phytoplankton sinking to the seabed as planned, it was emitted to the sea and air by the feeding zooplankton. Nevertheless, the Planktos team believe that the Waterbird II mission will raise greater awareness of what the oceans can do in mitigating the effects of climate change."
Oops. But even if the plan blows up in our global face, we can at least celebrate that it'll be a teachable moment.