In 1976 we celebrated the bicentennial of American independence. That independence was an interesting event, but not nearly as important even to Americans as something else that was happening at the same time. That was the Industrial Revolution. At first it promised humanity endless leisure and abundance. The simple reality is that a trivial two hundred years of the Industrial Revolution have brought the human species to the brink of self-extinction.
Nuclear war is the most immediate threat.
There are just over four billion persons on the Earth, so about 4 tons of TNT for every man, woman and child in the world. So I had better say what a tactical nuclear weapon is. For comparison the Titan missile whose fuel recently blew up in its silo in Arkansas had perhaps 100 times that explosive power. There is also the mushroom cloud of radioactive fallout that enters the atmosphere and stratosphere and eventually covers the entire globe. This goes on showering the Earth with potentially lethal ionizing radiation, and every rain and snowfall brings down radioactive elements to be inhaled, and by entering the food chain, ingested.