A Google search for "Earth Day" lists the Earth Day Network as the top (non-sponsored) link. A "Call for Climate" is the call to action listed front and center on that site.

A great site for a variety of content, especially environmental and climate data, is NASA's Earth Observations site. NEO's mission (as stated on the site) is to help you picture climate change and environmental changes happening on our home planet. So it seems like a great site to visit with Explorer on Earth Day.

Here we've added a couple of KMZ files from the site, one showing land surface temperature at night for the period between February 1 and March 1, 2000, and the other for the same dates in 2008. We've used the swipe tool to show the differences (you'll see the swipe lining splitting the globe at its center). The 2008 data is on the right, with the 2000 data on the left. You'll see that things were a bit warmer (blue is cooler, red is warmer, hottest is yellow) in the central part of the US during that time period 8 years ago.

Here's the swipe tool being used again on content from the same February to March time period, but this time for daytime land surface temperatures. Content for 2000 is on the left and 2008 on the right. We've also added the February through March 2008 ocean temperatures.

And what would Earth Day be without a look at the earth's population? Here's the 2000 world population data, also from the NEO site. Try using the swipe tool and other content from the Explorer Resource Center and Geography Network to consider why there are distinct population lines in the middle of the US and along the northern border of India and Nepal.

If you've done some interesting things with Explorer for Earth Day, let us know about it or share what you've done on the ArcGIS Explorer Showcase.