Melting and shrinking glaciers as well as various temperature records from the earth's surface, weather ballons and satellites, IIRC.
NO climate model has ever been validated.
Not all Glaciers are melting and shrinking, some are growing. Global warming has been proven to be BS, several times. The alarmists however don't care about facts, they will make their own and scream and cry to the people so they can get funding. It is all about the money and control. If you can keep people scared you can control them, it is that simple.
In the last couple weeks I read a story lamenting the melting glaciers in Austria, and how they were covering parts of the glaciers with some material to help protect them from the heat.
It went on to say that in 2003 it was so bad they could see "a forest of tree stumps" under the ice.
This tells me that at one time the mountain was a lot warmer than it is now, and it was warmer for a long enough time to grow a forest.