Here is an excerpt from a website talking about leaving animals in cars:
"On a warm, sunny day windows collect light, trapping heat inside the vehicle, and pushing the temperature inside to dangerous levels. On an 85-degree Fahrenheit day, for example, the temperature inside a car with the windows opened slightly can reach 102 degrees within ten minutes. After 30 minutes, the temperature will reach 120 degrees."
http://www.hsus.org/pets/pet_care/co..._your_pet.html
That being said, I generally keep my clubs inside. I have, however, had no problems from the occasional times that I have left them in a car.