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 planet’s 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).

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.

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 man’s impact, the warming world, and deserts and drought (left and below).

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.