no suitable alternative power sources (e.g. jet planes)...no one is advocating a "guillotine break" with FFs...the transition will take time and it won't be absolute...but the sooner we all come together on a plan, the better off we'll be.
I'll take this opportunity to again recommend a Google search for "David J.C. MacKay" and his free, downloadable book "Sustainable Energy - Without The Hot Air"...IMO it provides an excellent overview (albeit with a need to accept numbers) of Why and How we need to shift our current energy policy...