AP Photo

The goal of locking down tar-sands oil and stopping other forms of fossil fuel production such as fracking — as many protesters demanded in Sunday's demonstration — would be more compelling if the U.S. were ready to shift to renewable fuels such as solar, wind and biomass to power vehicles, heat homes and run factories. Last year, though, renewables supplied just 9.4% of all U.S. energy needs, despite robust tax incentives for wind power and electric cars. Shutting down conventional sources of energy at this point is naive and economically destructive. Demand might be further reduced by making vehicles and buildings more efficient. A carbon tax or a cap-and-trade system could do the same by making the price of conventional fuels better reflect their cost to the environment. Until...