Nonetheless, if the planet starts heating up rapidly, and droughts are causing mass death, it’s very possible that we’ll become desperate enough to try solar management. The planet would rapidly cool a few degrees and give crops a chance to thrive again. What would it be like to live through a geoengineering project like that? “People say we’ll have white skies—blue skies will be a thing of the past,” Cascio said. Plus, solar management is only “a tourniquet,” he warned. The greater injury would still need treating. We might cut the heat, but we’d still be coping with elevated levels of carbon in our atmosphere, interacting with sunlight to raise temperatures. When the reflective particles precipitated out of the stratosphere, the planet would once again undergo rapid, intense heating. “You could make things significantly worse if you’re not pulling carbon down at the same time,” Cascio said.

That’s why we need a way of removing carbon from the atmosphere while we’re blocking the sun. One of the only geoengineering efforts ever tried was aimed at pulling carbon out of the atmosphere using one of the earth’s most adaptable organisms: algae called diatoms. In several experiments, geoengineers fertilized patches of the Southern Sea with powdered iron, creating a feast for local algae. This resulted in enormous algae blooms. The scientists’ hope was that the single-celled organisms could pull carbon out of the air as part of their natural life cycle, sequestering the unwanted molecules in their bodies and releasing oxygen in its place. As the algae died, they would fall to the ocean bottom, taking the carbon with them. During many of the experiments, however, the diatoms released carbon back into the atmosphere when they died instead of transporting it into the deep ocean. While a few experiments have suggested that carbon-saturated algae can sink to the ocean floor under the right conditions, the jury on this option is still out.

Another possibility would be to enlist the aid of rocks. One of the most intriguing theories about how we’d manipulate the earth into pulling down carbon was dreamed up by Tim Kruger, who heads the Oxford Martin School’s geoengineering efforts. I met with him across campus from Driscoll’s office, in an enormous stone building once called the Indian Institute and devoted to training British civil servants for jobs in India. It was erected at the height of British imperialism, long before anyone imagined that burning coal might change the planet as profoundly as colonialism did…

However we do it, we need to begin to maintain the climate at a temperature that’s ideal for human survival. Instead of allowing the planet’s carbon cycle to control us, we would control it. We would adapt the planet to our needs by using methods learned from the earth’s history of extraordinary climate changes and geological transformations. We’ll also need to adapt the climate to serve the creatures that share the world’s ecosystems with us. If we want our species to be around for another million years, we have no choice. We must take control of the earth. We must do it in the most responsible and cautious way possible, but we cannot shy away from the task if we are to survive.