Unit 2: Negative externalities: The ski resort with no snow

As global warming accelerates, low level ski resorts are feeling the consequences says this BBC clip

< http://www.bbc.co.uk/mediaselector/check/player/nol/newsid_6920000/newsid_6922700?redirect=6922777.stm&news=1&bbram=1&bbwm=1&nbram=1&nbwm=1&asb=1 >

a) Why is global warming a negative externality for this ski resort?