There may come a time where it would be immoral not to intervene to stop the Earth's temperature from rising. But engineering the climate by injecting particles into the stratosphere to reflect the sun's light is ambitious and risky. Before we do this, it would be important to know that the cure is not worse than the disease.
How to test the unknowns: The Earth system is complex. A path forward might include a series of experiments, with appropriate governance, that start at a very small (meter, kilogram) scale. Early work should not perturb the climate but rather teach us about, for example, potential chemical effects of injection.