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It's 95% hydro in Norway, but that has impacts too. I doubt few countries are 100% coal either. Lots of factors involved, but it illustrates potential. Biofuels are not ideal, but could play a part.

Reality in the middle

100% hydroelectric Break-even: 8,400 miles

U.S. average energy mix (23% coal-fired, plus other fossil fuels and renewables) Break-even: 13,500 miles

Power scenario 3: 100% coal-fired Break-even: 78,700 miles