Argument: Geoengineering may result in unexpected consequences
More examples: the atomic bomb was never predicted to be as powerful as it turned out to be, as a result many thousands of people died due to its use in war. These deaths are irreversible and cannot be brought back. [...] There have been numerous failed experiments that have resulted in no better of an environment previous to the experiment. Ex. Algae blooms and iron. [...] We cannot predict the future, if people had known the consequences of burning fossil fuels, the current global climate change crisis could have been avoided. [...] We have the same amount of knowledge about geo-engineering as we had known about fossil fuels."
The Earths biosphere is unpredictable. The world is a complex place. Humanity so far can barely predict weather patterns, so how can even the most competent scientists be expected to changes systems that they don't even fully understand in a safe manner.
Alan Robock, associate director of the Center for Environmental Prediction at Rutgers University: "We need to rethink our actions, not invent new ways to engineer the planet itself. Scientists may never have enough confidence that their theories will predict how well geoengineering systems can work. With so much at stake, there is reason to worry about what we don’t know."