National Geographic : 2009 Aug
Some call it hubris; others call it cool reason. But the idea that we might combat global warming by deliberately engineering a cooler climate---for instance, by constructing some kind of planetary sunshade---has lately migrated from the fringe to the scientific mainstream. We are already modifying climate by accident, say proponents of geoengineering; why not do something intentional and intelligent to stop it? Hold on, say critics. Global warming shows we understand the Earth too little to engineer it without unintended and possibly disastrous consequences. Both sides worry that facts on the ground---rising seas, melting ice, failing crops---may cut short the geoengineering debate. "If a country starts thinking it's in their vital interests to do this, and they have the power, I find it hard to imagine them not doing it," says Ken Caldeira, a climate expert at the Carnegie Institution. Caldeira is talking about the easiest, cheapest form of geoengineering: building a sunshade in the stratosphere out of millions of tons of tiny reflective particles, such as sulfate. Planes, balloons, battleship guns pointed upward---there is no shortage of possible delivery vehicles. And there is little doubt you could cool Earth ART: BRYAN CHRISTIE DESIGN. NOT TO SCALE 60,000 miles 1 million miles Sunshade Earth 4,500 miles The cloud of disks would dim sunlight by around 2 percent. The shadow wouldn't be as deep as what's shown here, but it would be spread over the whole planet and would cool it by several degrees.