40 years of photos show our changing planet

<b>Sept. 13, 1972 -- Las Vegas, Nevada</b><br> These images show the rapid growth of Las Vegas, Nevada. (USGS)

<b>Sept. 10, 1986 -- Las Vegas, Nevada<br></b>The tip of Lake Mead is visible east of the city (dark area), where Hoover Dam impounds the Colorado River. (USGS)

<b>Sept. 10, 1992 -- Las Vegas, Nevada<br></b>Las Vegas is one of the fastest growing cities in the United States. (USGS)

<b>Sept. 24, 2000 -- Las Vegas, Nevada<br></b>Las Vegas grew from a population of 1,375,765 in 2000 to 1,951,269 in 2010, a 41.8 percent increase -- the third highest in the country for that decade. (USGS)

<b>Sept. 15, 2011 -- Las Vegas, Nevada</b> (USGS)

<b>June 19, 1975 -- Ariquemes, Brazil<br></b>The conversion of tropical rain forests to pasture and cropland is having dramatic effects on the environment. Particularly intense and rapid deforestation is taking place in the state of Rondonia, Brazil, part of which is shown in this series of Landsat images. (USGS)

<b>July 16, 1986 -- Ariquemes, Brazil</b><br> Systematic cutting of forest vegetation starts along roads and then fans out to create the "feather" or "fishbone" pattern, which begins to show in the eastern half of this image. (USGS)

<b>June 22, 1992 -- Ariquemes, Brazil</b><br> The deforested land and urban areas appear lavender; healthy vegetation appears green. (USGS)

<b>Aug. 10, 2001 -- Ariquemes, Brazil<br></b>About 30 percent (3,562,800 square kilometers) of the world's tropical forests are in Brazil. The estimated average deforestation rate from 1978 to 1988 was 15,000 square kilometers per year.

<b>Aug. 6, 2011 -- Ariquemes, Brazil<br></b>In Rondonia, 67,764 square kilometers of rain forest had been cleared through 2003 -- an area larger than West Virginia. (USGS)

<b>Sept. 24, 1972 -- Riyadh, Saudi Arabia<br></b>The growth of Riyadh, the national capital, is dramatic between 1972 and 2011. (USGS)

<b>Aug. 31, 1990 -- Riyadh, Saudi Arabia<br></b>Its population grew in these years from about a half million to over 5 million. Saudi Arabia experienced urbanization later than many other countries; in the early 1970s, its urban-rural ratio was still about 1:3. By 1990, that had reversed to about 3:1. (USGS)

<b>Oct 12, 2011, 2003 -- Riyadh, Saudi Arabia<br></b>The dark red squiggly line that winds through the western part of Riyadh is called the Wadi Hanifa, or the Hanifa Valley. (USGS)

<b>Sept. 23, 1977 -- Southern Aral Sea<br></b>Before the 1960s, fishing in Central Asia's Aral Sea was an important resource for surrounding communities. When the rivers flowing into the Aral Sea were diverted to irrigate cotton and other crops, many more consequences than the loss of fishing resulted. (USGS)

<b>Sept. 18, 1998 -- Southern Aral Sea<br></b>Beginning in the 1960s, the Soviet Union began to divert water from the two major rivers in Central Asia -- Amu Darya and Syr Darya -- that flow into the Aral Sea to irrigate millions of acres of cotton and rice farms. Consequently, the volume of the Aral Sea has reduced from more than 700 cubic kilometers in 1960 to 75 cubic kilometers in 2007. (USGS)

<b>July 24, 2010 -- Southern Aral Sea<br></b>As the water levels dropped, the water salinity increased. One liter of Aral water once had 14 grams of salt, but in 2007, the same volume had more than 100 grams -- twice the salinity of the ocean. (USGS)