It's more then likely dust buildup, remove the GPU from the system and carefully clean it out with canned air or a oil less air compressor, you can also go a step further and reapply the thermal paste. Once this is done I would reinstall the GPU and use a program such as MSI Afterburner to create a custom fan profile that would turn up the fans to 100% but 80C. Doing all these steps should bring down that temp considerably. Also using Vsync to cap the FPS at the monitor refresh rate should help keep the temps down but you'll really want to clean the card out as that will show the biggest improvement in temps.
Fun game btw, I play it casually every once in awhile.