Bernie Sanders has no chance to win the Democratic presidential nomination, but the breathtaking details of the climate-change plan he released this week are still worth noting. They show where the Democratic Party is headed.

The Vermont Senator calls climate change “the single greatest threat facing our planet,” and he seems to mean it. He is proposing a 40% reduction in carbon emissions by 2030, and an 80% reduction by 2050, which is significantly more than the up-to 28% cut by 2025 that President Obama has pledged at the...