Computer desks with small, enclosures that just fit a tower are great for office computers or low draw devices, not performance minded machines. While you can certainly cram performance equipment in there, you are seeing the direct result of why I would personally recommend against it.
If the environment is otherwise temperature controlled, so the air around the enclosure is cooler than normal, you might get away with it, however, if you're in a home environment where the room is over say 20°C or 68°F, your equipment is starting out warm.
If you want to continue housing the equipment in the enclosed space, I recommend as others have, to modify the enclosed space to facilitate better airflow.
In the long run, it's likely that you will see a diminished lifespan for the hot running equipment, but I'm not sure anybody can accurately quantify by how much.