I recently installed Ubuntu 14.04.3 LTS on my Thinkpad X1 carbon (3rd Edition). Most of the stuff worked out of the box, yet the screen brightness adjustment key has no effect.
After Googling around, this solution works.
Create a file named /usr/share/X11/xorg.conf.d/20-intel.conf, with the following content.
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Section "Device" Identifier "card0" Driver "intel" Option "Backlight" "intel_backlight" BusID "PCI:0:2:0" EndSection
Thanks to this post.
In fact, this issue is probably not related to this particular hardware. I found another thread in askubuntu.com. Yet the solution there did not work for me.