A new program led by the U.S. seeks to limit amounts of soot, hydrofluorocarbons and methane released into the atmosphere. In this file photo from 2009, a researcher ignites trapped methane from under a pond's ice cap in Alaska. Todd Paris/AP hide caption

itoggle caption Todd Paris/AP

The new version of the map includes 13 zones, with the addition for the first time of zones 12 (50-60 degrees F) and 13 (60-70 degrees F). U.S. Department of Agriculture hide caption

itoggle caption U.S. Department of Agriculture

Altering the upper atmosphere could block enough sunlight to offset the warming effects of climate change and protect food crops. But what are the risks? iStockphoto hide caption

itoggle caption iStockphoto