WASHINGTON — Google has joined forces with the United Nations in the use of satellite imagery and computer overlays to document some of the most pressing cases of environmental degradation around the world.

The partnership between Google Earth and the United Nation’s Environment Programme (UNEP) has produced what Google is calling its UNEP overlay, which is available to any users of the Google Earth software.

After launching Google Earth, users can navigate to the overlay, and click on one of more than 100 environmental hot spots U NEP has identified. The hot spots were chosen because each one has experienced a dramatic amount of environmental change, according to Bruce Pengra, a geographer for UNEP who works out of the U.S. Geological Survey’s Earth Resource Observation and Science data center in Sioux Falls, S.D.

Google Earth users also can click on a particular region to view between two and five different layers of imagery that show how the region has changed over time.

“That’s the real key for the whole concept,” Pengra said. “It shows change rather than just looking at imagery at a static point and time. It’s really remarkable to be able to look at it and see what’s there and say, ‘Wow, it wasn’t that long ago that this was intact.’”

Looking at Las Vegas, for example, the overlays show how the population expanded rapidly between 1973 and 2006, Pengra said. “We have text that accompanies a series of remote sensing images spanning that time period and [it] explains some of the drivers for change and some of the concerns,” Pengra said.

Users view the varying terrain of Las Vegas and can zoom in to see things like how cities sit in the mountains and how geographical features constrain the city’s growth, Pengra said.

Another featured area is South America, Pengra said. The overlays for several different regions of that continent show the considerable degree of deforestation that has occurred, and how roads have cut into the forest in areas like Brazil.

The Aral Sea is another UNEP hot spot featured on the program. A great deal of water in the region has been diverted for cotton farming, and a series of five images demonstrates how the sea itself is shrinking dramatically because of the work that has been done in the area, Pengra explained.

Users of the program can investigate further if a particular area peaks their interest, Pengra said. They can download high-resolution imagery of the areas for themselves, as well as view ground photographs that show what has happened in the area from a different perspective.

The program has its roots in a 2005 document UNEP developed called “One Planet, Many People: Atlas of our Changing Environment,” Pengra said. UNEP had worked together with NASA, the U.S. Geological Survey and the University of Maryland in College Park, to put together a book of imagery showing environmentally troubled areas, along with explanations indicating what damage has been done.

The sites were selected primarily by how visually devastating the damage appeared in the available imagery, he said. At the time, there were only a limited number of sites for which there were multiple sets of satellite imagery to document major changes over a sufficient period of time, Pengra said. For example, in some tropical areas where devastation has occurred, cloud cover limits what satellites can capture of the region, so their problems were not showcased through the document, said Pengra.

Since the document had been released, new imagery of regions not originally included has become available, and that has been added to the Google Earth overlay, he said.

The new partnership with Google, which was announced in September, falls under UNEP’s mission to get environmental science data into the hands of those who make policy, Pengra said. The program also is designed to appeal to international organizations and educational institutions, he added.

“It’s rich enough in content and tools so that even kids in grade school can find an angle for this,” Pengra said. “I like to say that maps are essentially about making lots of information easy for people to access, and this is essentially a leap forward in that same sort of idea.”

The UNEP overlay is one of many overlays that Google Earth is now releasing under its new Featured Content section , which contains a number of data sets that combine information with Google Earth satellite imagery, according to a Sept. 13 press release from Google.

These include a Discovery Networks World Tour that highlights natural wonders, landmarks and other locations. There is also an overlay put together by the National Park Service that gives information to visitors about facilities and more than 16,000 kilometers of trails within the 58 National Parks.

The Jane Goodall Institute also designed an overlay which combines information about chimpanzee research and African deforestation, and Google has launched a program called “Turn Here,” which essentially serves as a city guide for metropolitan areas, the release said.