You Asked:
Will space mirrors really be able to cool the planet? — Roxanne, Atlanta, Georgia

Marshall Answered:
Carbon dioxide and methane are two common greenhouse gases. Human consumption of fossil fuels releases billions of pounds of carbon dioxide gas into the atmosphere every day, and a wide variety of natural processes create methane. One of the problems with the rising level of greenhouse gases in the atmosphere is that it causes the atmosphere to trap more of the heat that arrives from the sun. Thus, earth is getting warmer.

One way to solve this problem might be to put mirrors or sunshades in space that would reflect away or block some of the sunlight that would normally reach the planet. Three approaches have been suggested:

1) We could loft a single huge mirror. Depending on its design, it might need to be as large as 600,000 square miles.

2) Use thousands of large mirrors, which might come in the form of space solar power satellites, to block sunlight and generate electricity.

3) Use a swarm of millions of smaller mirrors. One approach is described here and uses lunar materials to create the mirrors that orbit far from earth. In another approach, millions of smaller satellites form a ring, much like the rings of Saturn, in low earth orbit to shade the tropics.