It's not natural.
I can point to one fairly easy-to-understand paper which completely proves the case.
Meehl, Gerald A., Warren M. Washington, Caspar M. Ammann, Julie M. Arblaster, T. M. L. Wigley, Claudia Tebaldi, 2004: Combinations of Natural and Anthropogenic Forcings in Twentieth-Century Climate. J. Climate, 17, 3721–3727.
Should be fairly easy to find from that.
~Hélios~
03-Mar-2011 22:03:54