I think he was sending a report home or communicating with another alien, and the autumn leaves falling from trees etc convinced him the Earth was dying.
Likely a short story written before 2000.
This is "That Strain Again" by Charles Sheffield.
All around us the great blight spreads. Everywhere we look the Earth is dying. We are contagion and bear guilt for the death of this world.
.....
Somewhere out there we are going to run into a planet with as big a shock in store for us as our seasons were to the Vegans.