Environment
For now, we all share one earth. The natural systems of the earth (air and water circulation) and manmade systems (such as transport) mean that harms to our environment may not be localized, and may impact the well being of all of us. We all breathe the same air, and many of us share water supplies (and generally over time we all share water supplies). Diseases started in one area can be transmitted to another, and contaminants produced in area may be transported to another. Since the earth is inherently common to all humans, it requires that we reach consensus on the common good in order to safeguard it.
Below are thoughts on some of the challenges commonly understood to be facing us all at a global level.
Air and Water
The natural air and water cycles and circulation on this planet result in the our air and water being some of our most commonly shared resources. The movement of both of these essential elements to life mean that damage or harm done to them in one area may rapidly spread across the globe. Therefore our air and water are perhaps some of the most pressing concerns of the common good.
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Habitat
Ashes to ashes, dust to dust, as the saying goes. We are what we eat, and what we eat is comprised of the soil (and air and water) that it in turn is derived from. Destruction of this habitat holds the potential to be our own destruction. Though on longer cycles, our local habitats have the potential to become unfit for our use due to contamination or denourishment.
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Global Warming
The evidence for global warming may be difficult to understand and uncertain, but the risk of substantial damage to the only planet we have cannot be ignored. While more exploration is needed on the evidence (and degree) of the potential problem, possible solutions to it (especially those that are beneficial in other ways) should concurrently be explored as well.