In the fall of 1993, a ship sailed into the Pacific Ocean carrying nearly 1,000 pounds of iron crystals packed into barrels, then dumped it all into the waves. The next morning, the water was tinged a subtle green from newly sprouted phytoplankton. The microorganisms, which need iron to grow, draw carbon dioxide out of the air as they metabolize. Scientists already knew that Earth's atmosphere was swelling with the gas, and that the planet would soon be in desperate need of cooling down.
Fertilizing the ocean with iron is a form of geoengineering, a set of technologies that are compelling for their potential to meaningfully alter Earth's systems, and controversial for the same reason. Geoengineering techniques, such as spraying chemicals into the sky to encourage clouds to produce more rain and ease drought, are becoming part of mainstream climate discussions.
As the planet heats, geoengineering is poised to manipulate it in unprecedented ways, whether researchers invent machines to suck greenhouse gases out of the atmosphere or nudge microorganisms to do it for them. Tinkering with home-grown plankton sounds like a less dramatic approach than, say, putting a massive parasol in outer space to shade the planet from the light of its star.
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