Lamb- least sustinable food!?

Here's an interesting article I read on yahoo today about sustainable foods.  I always thought that beef was the least sustainable meat, and eat it only sparingly, but it turns out lamb is even worse for the environment!  It also says cheese is a bad choice when practicing sustainability.  Well shoot me, I could never give up a good shaving of parm or extra sharp cheddar.

http://shine.yahoo.com/channel/food/the-1-best-protein-for-your-health-and-the-worst-to-avoid-2512800/