Two provocative ways to see long-term changes on earth are currently being promoted in honor of Earth Week. A Web site by NASA, and an app from HarperCollins, both show striking side-by-side satellite images of locations that have changed dramatically over time spans of up to 30 years or more. The primary intent is to show how we humans are altering the planets surface.

NASA has posted more than 160 comparative views on a Web site it calls State of Flux. The sharp images are grouped into categories, such as human impact (see below).

mining in Chile

The Atacama Desert in northern Chile (left, 1975) has been scraped away and artificially tinted by open-pit copper mining (right, 2008). Images courtesy U.N. Environmental Program

Other categories include cities, extreme events such as fires, floods and tsunamis, and land cover (below). The images can be downloaded and used for free.

agriculture in Saudi Arabia

Massive irrigation in the Wadi As-Sirhan region of Saudi Arabia has turned a barren desert (left, 1986) into crop-producing fields (right, green dots, 2004). Images courtesy of U.N. Environmental Program

The second option for viewing earth from above is a nice app called Fragile Earth. It focuses more on how climate change, natural disasters and human actions have altered the environment. The app is also organized into categories, including mans impact, the warming world, and deserts and drought (left and below). lake mead 1985

lake mead 2010r

Lake Mead (above left, 1985), which straddles Arizona and Nevada, is the largest reservoir in the U.S., but it has dropped precipitously (immediately above, 2010) because of human demand for water and persistent drought. Images courtesy NASA/Earth Observatory

The goal of Fragile Earth is to impress upon people how drastically we can transform our surroundings. The app, for iPad and iPhone, is compelling because a user can see the comparison images by swiping a finger back and forth across the display. Text explaining the images, dates they were taken and causes for the changes depicted scrolls up from the bottom of the screen when summoned. HarperCollins has dropped the regular $2.99 price to $0.99 through Sunday, April 29, the last day of Earth Week.