Probably from the 80s or 90s. USA paperback. Cover has a human and a humanoid alien wearing back packs (hiking across a desert type terrain) who are standing on a pile of rocks or rocky prominence. Human on the top, alien a couple feet behind him. (clearly looking for the path to get out of desert)
Humans have landed on planet and made contact with the aliens, learning about them and their culture.
The planet is in either a highly elliptical orbit or has a companion star in an elliptical orbit. Pretty sure it's a companion star, as if I recall correctly the planet is going to experience a hot period for decades. The humans are trying to figure out how the current plant and animal life could possibly survive, as the surface temperature is going to increase by about 200-300 degrees. The companion star has an orbit of centuries and the humans can't figure out how aliens have progressed to their current level (medieval or 18th century equivalent) in only a few hundred years.
Eventually the human scientists put samples of plant and animal life in an aquarium and raise the temperature. Everything dies, all water boils off and even the atmosphere in the aquarium changes, but from the plants and animal remains, spores? (seeds? pods?) erupt and new forms of life capable of handling the high temperature emerge.
The humans discover there are deep ice caves that some of the aliens will seal themselves in and after the companion star recedes, will emerge and start training the next generation of humanoid aliens.