[This is not my own bug, but I decided that it would be better to repost it upstream from https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/825989. I have the same problem with my laptop GPU.]
The clock frequency of the GPU is set to maximum by default and it is never throttled back. This leads to a laptop battery that is drained quickly, an unneccesary amount of heat produced, an annoying fan, etc etc.
Users can set the frequency through the command line like with
echo dynpm > /sys/class/drm/card0/device/power_method
but most users don't know about this.
Does this change anything?: http://www.botchco.com/agd5f/?p=57
Available in kernel 3.11
Fix released downstream.