Looking back one, two and five years ago today (give or take) on Worldchanging:

2009
The Relative Merits of Plastic Bottles and Concrete Slabs
As a result of two experiences with 'green memes' that he had as a keynote speaker at a fundraiser for the Ecology Action Center, Chris Turner argues that his use of a plastic water bottle to keep hydrated during his lecture was personal, symbolic, highly visible and statistically meaningless; while the work being done at the concrete factory he visited, where they're attempting to turn the world’s concrete factories into carbon sinks, is universal, practical, invisible and statistically huge. This is a good read with valuable information on the embedded energy of concrete, and stimulated a great discussion in the comments section...

2008
New Fish Farms Move from Ocean to Warehouse
Ben Block reports on a land-based fish farming technique under development at the University of Maryland Institute's Center for Marine Biotechnology where researchers are not only altering nature, they are creating what may be the next generation of seafood...

2005
The Catastrophist's Dilemma
Jamais Cascio reflects on the movie The Day After Tomorrow and wonders how it foreshadows disaster movies to come...

Other recent "look backs":
April 23
April 27
April 28