Are Polar Bears scared of big shiny pipes?

Does anyone know exactly how it is that, building a natural gas pipeline will cause the extinction of the polar bears. Wouldn't switching cars to run on natural gas and propane reduce the green house gases that cause global warming? So if putting the bears on the endangered species list prevents the pipeline from being built and natural gas wells from being drilled, then isn't it the endangered species list itself that is speeding up their demise?