What is geoengineering—and why should you care?（1） As the threats of climate change grow, we’re all likely to hear more and more about the possibilities, and dangers, of geoengineering. Here’s what it means. It’s becoming clear that we won’t cut carbon emissions soon enough to prevent catastrophic climate change. But there may be ways to cool the planet more quickly and buy us a little more time to shift away from fossil fuels. They’re known collectively as geoengineering, and though it was once a scientific taboo, a growing number of researchers are running computer simulations and proposing small-scale outdoor experiments. Even some legislators have begun discussing what role these technologies could play (see “The growing case for geoengineering”). But what is geoengineering exactly? Traditionally, geoengineering has encompassed two very different things: sucking carbon dioxide out of the sky so the atmosphere will trap less heat, and reflecting more sunlight away from the planet so less heat is absorbed in the first place. The first of these, known as “carbon removal” or “negative emissions technologies,” is something that scholars now largely agree we’ll need to do in order to avoid dangerous levels of warming (see “One man’s two-decade quest to suck greenhouse gas out of the sky”). Most no longer call it “geoengineering”—to avoid associating it with the second, more contentious branch, known as solar geoengineering. This is a blanket term that includes ideas like setting up sun shields in space or dispersing microscopic particles in the air in various ways to make coastal clouds more reflective, dissipate heat-trapping cirrus clouds, or scatter sunlight in the stratosphere. The word geoengineering suggests a planetary-scale technology. But some researchers have looked at the possibility of conducting it in localized ways as well, exploring various methods that might protect coral reefs, coastal redwoods, and ice sheets. Where did the idea come from? It’s not a particularly new idea. In 1965, President Lyndon Johnson’s Science Advisory Committee warned it might be necessary to increase the reflectivity of the Earth to offset rising greenhouse-gas emissions. The committee went so far as to suggest sprinkling reflective particles across the oceans. (It’s revealing that in this, the first ever presidential report on the threat of climate change, the idea of cutting emissions didn’t seem worth mentioning, as author Jeff Goodell notes in How to Cool the Planet.) But the best-known form of solar geoengineering involves spraying particles into the stratosphere, sometimes known as “stratospheric injection” or “stratospheric aerosol scattering.” (Sorry, we don’t come up with the names.) That’s in part because nature has already demonstrated it’s possible. Most famously, the massive eruption of Mt. Pinatubo in the summer of 1991 spewed some 20 million tons of sulfur dioxide into the sky. By reflecting sunlight back into space, the particles in the stratosphere helped push global temperatures down about 0.5 °C over the next two years. And while we don’t have precise data, huge volcanic eruptions in the distant past had similar effects. The explosion of Mount Tambora in Indonesia in 1815 was famously followed by the “Year Without a Summer” in 1816, a gloomy period that may have helped inspire the creation of two of literature’s most enduring horror creatures, vampires and Frankenstein’s monster. Soviet climatologist Mikhail Budyko is generally credited as the first to suggest we could counteract climate change by mimicking this volcanic phenomenon. He raised the possibility of burning sulfur in the stratosphere in a 1974 book. In the following decades, the concept occasionally popped up in research papers and at scientific conferences, but it didn’t gain much attention until the late summer of 2006, when Paul Crutzen, a Nobel Prize–winning atmospheric chemist, called for geoengineering research in an article in Climatic Change. That was particularly significant because Crutzen had won his Nobel for research on the dangers of the growing ozone hole, and one of the known effects of sulfur dioxide is ozone depletion. In other words, he thought climate change was such a threat that it was worth exploring a remedy he knew could pose other serious dangers. So could geoengineering be the solution to climate change, relieving us of the hassle of cutting back on fossil fuels? No—although the idea that it does is surely why some energy executives and Republican legislators have taken an interest. But even if it works (on which more below), it’s at best a temporary stay of execution. It does little to address other climate dangers, notably including ocean acidification, or the considerable environmental damage from extracting and burning finite fossil fuels. And greater levels of geoengineering may increase other disruptions in the climate system, so we can’t just keep doing more and more of it to offset ever rising emissions.