In mountainous regions, the timberline is the highest altitude at which trees grow  In the Rocky Mountains, the current timberline is at the altitude above which growing season temperatures remain cooler than 10 degrees centigrade  Fossilized remains of trees that grew 10, 000 years ago have been found 100 meters above the current Rocky Mountain timberline
Clearly, ttherefore, the climate of the Rocky Mountains is cooler now than it was 10, 000 years ago
The climate of the Rocky Mountains has never been significantly warmer than during the lifetime of the trees whose fossilized remains have been found.