High-altitude clouds of ice crystals were spotted on Tuesday by scientists in Colorado and Utah, the farthest south such clouds have ever been seen in the Northern Hemisphere, suggesting that the same greenhouse gases that are warming Earth’s lower atmosphere are also chilling and adding water to the upper atmosphere. In other climate news, jet contrails, ice clouds that streak behind planes in long, thin lines, are expected to contribute significantly to climate change within the next 50 years, U.S. and German scientists write in the journal Geophysical Research Letters. Also, another group of scientists — this time, American and British researchers publishing in the Review of Geophysics — has concluded that the warmest years on record all occurred in the 1990s.