Houston Chronicle: Some cite global warming as the culprit. Others point to a natural cycle in world climatory conditions. This much is certain: Glacier Bay National Park is changing. I’ve seen it myself. During six visits over the past quarter century, I’ve witnessed the gradual erosion of some of the world’s most magnificent rivers of ice. The lesson for travelers: Go now if your time and budget permits. Glacier Bay is home to 12 tidewater glaciers — ice sheets strong enough to …