War: Monument to war and the purpose of war, rather than the lives lost. It would incorporate reasons for fighting wars like religion, oil, and land. Not sure how it would be conceptualized.
Recyclables: either an ode to recyclables or an impermanent monument made of all compostable materials that would deteriorate within a few years and absorb back into the natural environment.
But the idea I think I'll run with is a monument to Garbage. This monument would be dedicated to remembering the continually growing heaps that are essentially out of sight, out of mind.
Great Pacific Garbage Patch, a enormous region where garbage has accumulated due to ocean currents. The current rotational pattern keeps the debris in a specific area of the Pacific Ocean. Recent research done by the National Science Foundation suggest that the effected area is roughly double the size of Texas. The debris consists mostly of plastics, chemical sludge, and other random debris.
Though the patch is enormous, it cannot be seen through satellites and in programs like Google Earth because the majority of the debris consists of plastics which break down into small polymers that cannot be seen from space. Also, much of the debris is within what is referred to as a water column which is a conceptual column of water reaching from ocean floor to surface in any specific area.
This monument effects the public by keeping them aware of how their consumption effects the planet. It's a constant reminder of what we are all doing and possibly an incentive to change for the better. Obviously it wouldn't be feasable to create a monument in the actual size of ocean effected by the garbage, but if it were a large enough monument to say place within the San Francisco Bay and visible from Oakland, Berkeley, San Francisco, and Marin the visual effect off of our shores would be fairly dramatic. It would ideally be made of formed concrete, wood, or other natural materials to avoid the possible leaching of more plastics into the ocean. If it could be placed on a large pillar stationed on the bay floor for permanency, the actual monument would be above water level and the types of materials available for use are greatly expanded.