When users choose to look at the atlas layer in Google Earth, the digital globe is populated with markers for each location being covered.
A flat map based on the Google design can also be viewed and navigated online.
Clicking on a marker calls up a recently taken satellite photograph alongside an image of the same region taken decades earlier. Accompanying text explains differences between the two photographs.
"The format is highly effective," said Ashbindu Singh, the regional coordinator for UNEP who oversaw the project. "Anyone looking at the pair of images online will be able to identify the changes."
One entry shows images of Iguazú National Park—which stretches over Paraguay, Brazil, and Argentina—taken in 1973 and in 2003.
The dramatic differences between the images illustrate how much rain forest has been lost to farmland and other human uses in a span of 20 years.
Another pairing shows the shrinking of Africa's Lake Chad between 1972 and 2001 due to lack of rainfall and upstream diversion of water by humans.
Once the sixth-largest lake in the world, Lake Chad has shrunk from 8,843 square miles (22,903 square kilometers) in 1963 to less than 117 square miles (303 square kilometers) today.
(Related news: "Shrinking African Lake Offers Lesson on Finite Resources" [April 26, 2001].)
The program also exposes some lesser-known cases, such as the depletion of date palm trees in the Shatt al Arab estuary bordering Iraq and Iran and the fire damage that has charred much of Wyperfeld National Park in southeastern Australia.
"I think it is a great education tool," said Timothy Boucher, a scientist for the Arlington, Virginia-based nonprofit the Nature Conservancy.
Such visual juxtapositions could raise "public awareness of the changes people are making on this planet."
Maps With Many Layers
UNEP originally presented the compare-and-contrast material in a book and on CD earlier this year.
But a flood of additional requests for the information prompted officials to approach Google to develop a way to further disseminate the information, Singh says.
Incorporating the atlas into Google Earth was possible thanks to a new feature in the latest version of the software that was released last month, says Chikai Ohazama, who manages development of the Google Earth program.
The feature allows Google program managers to add content on top of specific geographic regions.
Other batches of featured content that take advantage of the new feature include a compendium of U.S. trails offered by the U.S. National Parks and a serial of the adventures of Tanzanian chimpanzees by the Jane Goodall Institute.
And a layer created by the National Geographic Society offers a collection of society-owned photographs and their geographic origins. (National Geographic News is part of the National Geographic Society.)
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