Ancient city's demise point to dangers of climate change

The smoke came down out of Canada last week. It turned the skies in Chicago and throughout the Midwest the color of dishwater. Then it kept coming south, until it drifted over a field across the river from St. Louis where the largest city north of what is now Mexico once stood. Most of us were never taught it was there. A thousand years ago, 10,000 to 20,000 people lived at Cahokia. They farmed corn on the bottomland, traded copper down from the Great Lakes and shell up from the Gulf, and…