What if we could build a giant mirror in space to deflect the sun's energy? Or inject sulfur into the stratosphere to cool the earth? Scientists are examining such sci-fi methods as a gigantic Plan B should efforts to end carbon emissions fail. Geoengineering, as the field is called, involves rearranging the environment on a planetary scale. The best-known idea involves the so-called space mirrors. Roger Angel, an astronomer at the University of Arizona, suggests putting trillions of small, ultra-thin lenses into orbit, enough to form a cylindrical cloud with a diameter half the size of the Earth's equator and a length of 60,000 miles. Placed 1.5 million km above the Earth's surface, the massive mirror would reduce the amount of sunlight reaching the planet by about 2%, which Angel believes would be enough to offset a significant amount of warming. Implementing this plan would be no mean feat: the mirrors would collectively weigh 20 million tons and cost trillions of dollars. And to get all those lenses into orbit, we'd have to launch rockets every five minutes for 10 years.
Writing in the journal Climate Change last August, Nobel Prize-winning meteorologist Paul Crutzen theorized that by pumping a huge amount of sulfur dioxide into the stratosphere, we could create a layer of sulfates that would reflect sunlight. Since the Earth itself reflects about 30% of sunlight back into orbit, increasing the reflectivity of the planet just slightly could be enough to counter warming. There's historical evidence that this would work—when Mount Pinatubo in the Philippines erupted in 1991, spewing sulfur into the atmosphere, temperatures around the world dropped for two years. Of course there's a catch: sulfur dioxide is a main cause of acid rain and a respiratory irritant. We'd have a cooler but dirtier Earth.
Such strategies may sound implausible, but the president of the prestigious National Academy of Sciences recommended exploring geoengineering options last year. That these far-out ideas are getting a serious hearing in mainstream science is a measure of how desperate the battle against climate change is becoming.
This is an extended version of the article that originally appeared in TIME Magazine.